Physics and Basic Electrical Equipment (Electrotherapy Details)
The therapies that are produced by electrical equipment or electrical devices are called electrotherapy. Basic electrical equipments are TENS, SWT, IRR, and
Physics and Basic Electrical Equipment
The Atom and atomic structure-
All matter is composed of atoms and some understanding of these basic units is necessary for a working knowledge of physics and chemistry. Historically, atoms were described as minute indivisible particles rather like billiard balls, but the advent of quantum physics has demonstrated the existence of many subatomic particles that make up the atom.
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The atom– The atom can be described as having a central nucleus surrounded by a cloud of electrons revolving in definite orbits.
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The nucleus– This is the central part of the atom, made up of particles held together by immensely strong nuclear forces. The two most important nuclear particles are the proton and the neutron. The proton– This is a comparatively large nuclear particle that possesses a positive charge exactly opposite to the negative charge of an electron. It is the positive charge of the protons that gives the nucleus of the atom its overall positive charge. Normally, atoms are electrically neutral, which means that for every proton (positive) there is a revolving electron (negative) so that the charges cancel one another out. The number of protons in the nucleus determines the element of which it is an atom, and is called the atomic number. For example, hydrogen is the first and smallest element and has one proton, thus its atomic number is 1. Uranium has 92 protons so it has an atomic number 92.
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The neutron– This is a nuclear particle with a mass almost equal to that of a proton, but electrically neutral neither positive or negative. Usually the number of neutrons approximately equals the number of protons. Although the number of neutrons does not affect the overall electrical charge of the atom, these quite large particles do affects its weight.
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Isotopes-with certain elements it is possible for different atoms (each atom having the same number of protons). To have different numbers of neutrons in their nuclei, so that different atoms have differing atomic masses. These atoms are examples of isotopes. For example carbon with an atomic number of 6 (possessing 6 protons) may have an atomic mass (proton plus neutrons) of 12,13,or 14 these atom having 6,7 and 8 neutrons respectively. To summarize an isotope is an atom of an element which contains the standard number of protons but a nonstandard number of neutrons.
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Electrons- These are negatively â charged particles found revolving in orbits around the nucleus and in the neutral atom their number equals the number of protons. Consequently the atomic number also gives the number of electrons found in the atom. Although electrons are very small they are important in determining the chemical and physical activity of the atom. The electrons are arranged in definite energy shells or orbits around the nucleus called principal quantum shells, of which there are seven. Each of these principal shells can be sub-divided into a maximum of four sub-shells, labelled s,p,d and f. Some general, although not inviolable, rules have been formulated as to the way electrons behave. They fill the lowest energy shells first, i.e. those nearest the nucleus, and will not start filling another shell until the previous one is full.
āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨- āĻāĻā§āĻ˛āĻŋ āĻ¨ā§āĻ¤āĻŋāĻŦāĻžāĻāĻ - āĻāĻžāĻ°ā§āĻāĻ¯ā§āĻā§āĻ¤ āĻāĻŖāĻžāĻā§āĻ˛āĻŋ āĻ¨āĻŋāĻāĻā§āĻ˛āĻŋāĻ¯āĻŧāĻžāĻ¸ā§āĻ° āĻāĻžāĻ°āĻĒāĻžāĻļā§ āĻāĻā§āĻˇāĻĒāĻĨā§ āĻā§āĻ°ā§āĻŖāĻžāĻ¯āĻŧāĻŽāĻžāĻ¨ āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻ¨āĻŋāĻ°āĻĒā§āĻā§āĻˇ āĻĒāĻ°āĻŽāĻžāĻŖā§āĻ¤ā§ āĻ¤āĻžāĻĻā§āĻ° āĻ¸āĻāĻā§āĻ¯āĻž āĻĒā§āĻ°ā§āĻāĻ¨ā§āĻ° āĻ¸āĻāĻā§āĻ¯āĻžāĻ° āĻ¸āĻŽāĻžāĻ¨āĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ āĻĒāĻžāĻ°āĻŽāĻžāĻŖāĻŦāĻŋāĻ āĻ¸āĻāĻā§āĻ¯āĻžāĻāĻŋ āĻĒāĻ°āĻŽāĻžāĻŖā§āĻ¤ā§ āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ā§āĻ° āĻ¸āĻāĻā§āĻ¯āĻžāĻ āĻĻā§āĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋāĻ āĻāĻ˛ā§āĻā§āĻā§āĻ°āĻ¨ āĻā§āĻŦ āĻā§āĻ āĻ¤āĻžāĻ°āĻž āĻĒāĻ°āĻŽāĻžāĻŖā§āĻ° āĻ°āĻžāĻ¸āĻžāĻ¯āĻŧāĻ¨āĻŋāĻ āĻāĻŦāĻ āĻļāĻžāĻ°ā§āĻ°āĻŋāĻ āĻāĻžāĻ°ā§āĻ¯āĻāĻ˛āĻžāĻĒ āĻ¨āĻŋāĻ°ā§āĻ§āĻžāĻ°āĻŖā§ āĻā§āĻ°ā§āĻ¤ā§āĻŦāĻĒā§āĻ°ā§āĻŖāĨ¤ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§āĻ˛āĻŋ āĻ¨āĻŋāĻ°ā§āĻĻāĻŋāĻˇā§āĻ āĻļāĻā§āĻ¤āĻŋāĻ° āĻļā§āĻ˛ āĻŦāĻž āĻ¨āĻŋāĻāĻā§āĻ˛āĻŋāĻ¯āĻŧāĻžāĻ¸ā§āĻ° āĻāĻžāĻ°āĻĒāĻžāĻļā§ āĻāĻā§āĻˇāĻĒāĻĨā§ āĻ¸āĻžāĻāĻžāĻ¨ā§ āĻĨāĻžāĻā§ āĻ¯āĻžāĻā§ āĻĒā§āĻ°āĻ§āĻžāĻ¨ āĻā§āĻ¯āĻŧāĻžāĻ¨ā§āĻāĻžāĻŽ āĻļā§āĻ˛ āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧ, āĻ¯āĻžāĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻ¸āĻžāĻ¤āĻāĻŋ āĻ°āĻ¯āĻŧā§āĻā§āĨ¤ āĻāĻ āĻĒā§āĻ°āĻ§āĻžāĻ¨ āĻļā§āĻ˛āĻā§āĻ˛āĻŋāĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋāĻā§ s, p, d āĻāĻŦāĻ f āĻ˛ā§āĻŦā§āĻ˛āĻ¯ā§āĻā§āĻ¤ āĻ¸āĻ°ā§āĻŦāĻžāĻ§āĻŋāĻ āĻāĻžāĻ°āĻāĻŋ āĻ¸āĻžāĻŦ-āĻļā§āĻ˛ā§ āĻāĻĒ-āĻŦāĻŋāĻāĻā§āĻ¤ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻŋāĻā§ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖ, āĻ¯āĻĻāĻŋāĻ āĻ āĻ˛āĻā§āĻāĻ¨ā§āĻ¯āĻŧ āĻ¨āĻ¯āĻŧ, āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻā§āĻāĻžāĻŦā§ āĻāĻāĻ°āĻŖ āĻāĻ°ā§ āĻ¸ā§ āĻ¸āĻŽā§āĻĒāĻ°ā§āĻā§ āĻ¨āĻŋāĻ¯āĻŧāĻŽ āĻĒā§āĻ°āĻŖāĻ¯āĻŧāĻ¨ āĻāĻ°āĻž āĻšāĻ¯āĻŧā§āĻā§āĨ¤ āĻ¤āĻžāĻ°āĻž āĻĒā§āĻ°āĻĨāĻŽā§ āĻ¸āĻ°ā§āĻŦāĻ¨āĻŋāĻŽā§āĻ¨ āĻļāĻā§āĻ¤āĻŋāĻ° āĻļā§āĻ˛āĻā§āĻ˛āĻŋ āĻĒā§āĻ°āĻŖ āĻāĻ°ā§, āĻ āĻ°ā§āĻĨāĻžā§ āĻ¨āĻŋāĻāĻā§āĻ˛āĻŋāĻ¯āĻŧāĻžāĻ¸ā§āĻ° āĻ¨āĻŋāĻāĻāĻŦāĻ°ā§āĻ¤ā§āĻā§āĻ˛āĻŋ, āĻāĻŦāĻ āĻāĻā§āĻ°āĻāĻŋ āĻĒā§āĻ°ā§āĻŖ āĻ¨āĻž āĻšāĻāĻ¯āĻŧāĻž āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻ āĻ¨ā§āĻ¯ āĻļā§āĻ˛āĻā§āĻ˛āĻŋ āĻĒā§āĻ°āĻŖ āĻāĻ°āĻž āĻļā§āĻ°ā§ āĻāĻ°āĻŦā§ āĻ¨āĻžāĨ¤
Atoms tend to seek the condition in which their outer electron shell is full, and to achieve this they may gain or lose electrons. Normally in a neutral atom the number of protons (positive) equals the number of electrons (negative) .However, if an atom gains an electron then it has an excess of negative charge and becomes a negative ion (anion).For example, an atom of chlorine normally has 17 protons and 17 electrons. If an extra electron joins the outer shell then the atom becomes a negative chlorine ion (Cl-).The converse is also true. If an atom loses an electron it then has and excess of positive charge and becomes a positive ion (cation).For example, sodium normally has 11 Protons and 11 Electrons. If it loses an electron it becomes Na+, a positive sodium ion.
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Chemical activity-The number of electrons in the outer shell of an atom has an effort on the reactions of that atom with others. For example, the inert or noble gases such as argon, krypton and neon have a complete outer principal quantum shell and are thus reluctant to enter into chemical combination with other atoms. Other elements, such as sodium, have only one electron in their outer shell and are thus highly reactive in terms of joining with other atoms.
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The formation of compounds-
A compound is a substance formed by the union of two or more elements, the union taking place via the electrons of the atoms involved to form molecule of the compound. Compounds may be either electrovalent or covalent. An Electrovalent compound occurs where an atom of one element gives an electron to the atom of another element.
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Electromagnetic radiation
Electromagnetic radiation is produced by the movement of electrons within the atom. If energy is added to an atom, e.g. by heat, this can cause an electron to move out to a higher-energy electron shell. It is then said to be in an excited state. When the electron returns to its normal level, energy is released as a pulse of electromagnetic energy (aphoton). The type of electromagnetic wave produced depends upon which electron shells are involved in the electron’ jump’, and it is this which gives rise to the characteristic spectra seen when certain elements are heated, e.g. yellow for sodium. Tungsten illustrates the phenomena well. When it is first heated only infra-red electromagnetic waves are emitted, and these can be felt as heat. As more energy is added, the energy ‘jumps’ between electron shells become bigger and the photons of electromagnetic energy eventually reach the visible spectrum so that the metal glows red and then white as the temperature increases.
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Conductors and non-conductors of electricity-
Conductors are elements whose atoms have few electrons in their outer orbit. For example, copper has a loosely-held single electron in its outer orbit which in a copper bar or wire may be allowed to drift away from the parent atom. It is such conducting electrons which facilitate the passage of an electric current. Non-conductors (insulators) are materials made of atoms in which the electrons in the outer shell are firmly held in their orbits and will not leave the atom in order to conduct a current.
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States of matter
Matter can be solid, liquid or gaseous, e.g. water may exist as ice, water or steam. The molecules of a substance are usually influenced by at least two forces: a cohesive force, which attracts the molecules of the substance to one another, and a kinetic force-the force of movement of the molecules — which is dependent on the thermal energy contained by the mass of molecules.
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In the solid state there is a strong cohesive force between the molecules, which holds them in a rigid lattice formation so that the shape of the mass remains constant. The kinetic energy produces only a vibration of the molecules about a mean position.
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As more energy (e.g. heat) is added to the solid, the kinetic energy increases and the movement of the molecules eventually becomes such that the rigid structure collapses so that the liquid state is reached. In this state the molecules are in contact but can move freely past one another: the liquid thus maintains its volume but takes on the shape of its container.
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If even more heat is applied, there comes a point when the kinetic energy is so much greater than the cohesive forces that the molecules fly apart to form a gas. The molecules of the gas are continually colliding with one another and with the walls of the container, so that the gas exerts pressure. This pressure increases with any further rise in temperature.
āĻ¯āĻĻāĻŋ āĻāĻ°āĻ āĻŦā§āĻļāĻŋ āĻ¤āĻžāĻĒ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻšāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻāĻŽāĻ¨ āĻāĻāĻāĻŋ āĻŦāĻŋāĻ¨ā§āĻĻā§ āĻāĻ¸ā§ āĻ¯āĻāĻ¨ āĻāĻ¤āĻŋāĻļāĻā§āĻ¤āĻŋ āĻ¸āĻŽāĻ¨ā§āĻŦāĻŋāĻ¤ āĻļāĻā§āĻ¤āĻŋāĻ° āĻā§āĻ¯āĻŧā§ āĻāĻ¤ āĻŦā§āĻļāĻŋ āĻšāĻ¯āĻŧ āĻ¯ā§ āĻ āĻŖā§āĻā§āĻ˛āĻŋ āĻāĻĄāĻŧā§ āĻāĻŋāĻ¯āĻŧā§ āĻā§āĻ¯āĻžāĻ¸ āĻ¤ā§āĻ°āĻŋ āĻāĻ°ā§āĨ¤ āĻā§āĻ¯āĻžāĻ¸ā§āĻ° āĻ āĻŖā§āĻā§āĻ˛āĻŋ āĻā§āĻ°āĻŽāĻžāĻāĻ¤ āĻāĻā§ āĻ āĻĒāĻ°ā§āĻ° āĻ¸āĻžāĻĨā§ āĻāĻŦāĻ āĻĒāĻžāĻ¤ā§āĻ°ā§āĻ° āĻĻā§āĻ¯āĻŧāĻžāĻ˛ā§āĻ° āĻ¸āĻžāĻĨā§ āĻ¸āĻāĻāĻ°ā§āĻˇā§ āĻ˛āĻŋāĻĒā§āĻ¤ āĻšāĻ¯āĻŧ, āĻ¯āĻžāĻ¤ā§ āĻā§āĻ¯āĻžāĻ¸āĻāĻŋ āĻāĻžāĻĒ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°ā§āĨ¤ āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ° āĻāĻ°āĻ āĻŦā§āĻĻā§āĻ§āĻŋāĻ° āĻ¸āĻžāĻĨā§ āĻāĻ āĻāĻžāĻĒ āĻŦā§āĻĻā§āĻ§āĻŋ āĻĒāĻžāĻ¯āĻŧāĨ¤
Latent heat
A specific amount of energy is required to change the solid form of a particular substance into a liquid, or the liquid into a gas. This energy is called latent heat and is the energy required for (or released by) a change of state. In the case of water, I gram of ice at o°C requires 336joules of energy to convert it to I g of water at o°C (latent heat of fusion),and I gram of water at I00°C requires 2268 joules of energy to convert it to I g of steam at Ioo°C (latent heat of vaporization). As matter changes from a state of high kinetic energy to one of lower kinetic energy (e.g. steam to water, liquid to solid), this latent energy is released.
āĻāĻāĻāĻŋ āĻ¨āĻŋāĻ°ā§āĻĻāĻŋāĻˇā§āĻ āĻĒāĻĻāĻžāĻ°ā§āĻĨā§āĻ° āĻāĻ āĻŋāĻ¨ āĻ°ā§āĻĒāĻā§ āĻ¤āĻ°āĻ˛ āĻŦāĻž āĻ¤āĻ°āĻ˛āĻā§ āĻā§āĻ¯āĻžāĻ¸ā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻ°āĻ¤ā§ āĻāĻāĻāĻŋ āĻ¨āĻŋāĻ°ā§āĻĻāĻŋāĻˇā§āĻ āĻĒāĻ°āĻŋāĻŽāĻžāĻŖ āĻļāĻā§āĻ¤āĻŋāĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻļāĻā§āĻ¤āĻŋāĻā§ āĻ¸ā§āĻĒā§āĻ¤ āĻ¤āĻžāĻĒ āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ° āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ā§āĻ° āĻāĻ¨ā§āĻ¯ (āĻŦāĻž āĻĻā§āĻŦāĻžāĻ°āĻž āĻŽā§āĻā§āĻ¤āĻŋ) āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ā§āĻ¯āĻŧ āĻļāĻā§āĻ¤āĻŋāĨ¤ āĻĒāĻžāĻ¨āĻŋāĻ° āĻā§āĻˇā§āĻ¤ā§āĻ°ā§, o°C āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ¯āĻŧ I āĻā§āĻ°āĻžāĻŽ āĻŦāĻ°āĻĢā§āĻ° āĻāĻ¨ā§āĻ¯ 336 āĻā§āĻ˛ āĻļāĻā§āĻ¤āĻŋāĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧ āĻ¯āĻžāĻ¤ā§ āĻāĻāĻŋāĻā§ o°C (āĻĢāĻŋāĻāĻļāĻ¨ā§āĻ° āĻ¸ā§āĻĒā§āĻ¤ āĻ¤āĻžāĻĒ) Ig āĻāĻ˛ā§ āĻ°ā§āĻĒāĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻāĻ°āĻ¤ā§ āĻšāĻ¯āĻŧ āĻāĻŦāĻ I00°C āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ¯āĻŧ I āĻā§āĻ°āĻžāĻŽ āĻāĻ˛ā§āĻ° āĻāĻ¨ā§āĻ¯ 2268 āĻā§āĻ˛ āĻļāĻā§āĻ¤āĻŋāĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧ Ioo°C (āĻŦāĻžāĻˇā§āĻĒā§āĻāĻŦāĻ¨ā§āĻ° āĻ¸ā§āĻĒā§āĻ¤ āĻ¤āĻžāĻĒ) āĻ āĻāĻāĻŋāĻā§ Ig āĻŦāĻžāĻˇā§āĻĒā§ āĻ°ā§āĻĒāĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻļāĻā§āĻ¤āĻŋāĨ¤ āĻ¯ā§āĻšā§āĻ¤ā§ āĻĒāĻĻāĻžāĻ°ā§āĻĨ āĻāĻā§āĻ āĻāĻ¤āĻŋāĻļāĻā§āĻ¤āĻŋāĻ° āĻ āĻŦāĻ¸ā§āĻĨāĻž āĻĨā§āĻā§ āĻ¨āĻŋāĻŽā§āĻ¨ āĻāĻ¤āĻŋāĻļāĻā§āĻ¤āĻŋāĻ° āĻāĻāĻāĻŋāĻ¤ā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻšāĻ¯āĻŧ (āĻ¯ā§āĻŽāĻ¨ āĻŦāĻžāĻˇā§āĻĒ āĻĨā§āĻā§ āĻāĻ˛, āĻ¤āĻ°āĻ˛ āĻĨā§āĻā§ āĻāĻ āĻŋāĻ¨), āĻāĻ āĻ¸ā§āĻĒā§āĻ¤ āĻļāĻā§āĻ¤āĻŋ āĻ¨āĻŋāĻ°ā§āĻāĻ¤ āĻšāĻ¯āĻŧāĨ¤ Transmission of heat
Conduction
If one end of a solid metal rod is heated, the energy added causes an increased vibration of molecules. This vibration is transmitted to adjacent molecules and in this way heat is conducted along the bar from the area of high temperature to areas of lower temperature. Some materials are good conductors of heat ,e.g. metals, while others are not ,e.g. wood and some plastics.
āĻāĻāĻāĻŋ āĻāĻ āĻŋāĻ¨ āĻ§āĻžāĻ¤āĻŦ āĻ°āĻĄā§āĻ° āĻāĻ āĻĒā§āĻ°āĻžāĻ¨ā§āĻ¤ āĻāĻ¤ā§āĻ¤āĻĒā§āĻ¤ āĻšāĻ˛ā§, āĻ¯ā§āĻ āĻāĻ°āĻž āĻļāĻā§āĻ¤āĻŋ āĻ āĻŖā§āĻā§āĻ˛āĻŋāĻ° āĻāĻŽā§āĻĒāĻ¨ āĻŦā§āĻĻā§āĻ§āĻŋ āĻāĻ°ā§āĨ¤ āĻāĻ āĻāĻŽā§āĻĒāĻ¨ āĻ¸āĻāĻ˛āĻā§āĻ¨ āĻ āĻŖā§āĻ¤ā§ āĻĒā§āĻ°ā§āĻ°āĻŖ āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻāĻāĻžāĻŦā§ āĻāĻā§āĻ āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ° āĻāĻ˛āĻžāĻāĻž āĻĨā§āĻā§ āĻ¨āĻŋāĻŽā§āĻ¨ āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ° āĻ āĻā§āĻāĻ˛ā§ āĻŦāĻžāĻ° āĻŦāĻ°āĻžāĻŦāĻ° āĻ¤āĻžāĻĒ āĻ¸āĻā§āĻāĻžāĻ˛āĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻŋāĻā§ āĻāĻĒāĻžāĻĻāĻžāĻ¨ āĻāĻ¤ā§āĻ¤āĻžāĻĒā§āĻ° āĻāĻžāĻ˛ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§, āĻ¯ā§āĻŽāĻ¨ āĻ§āĻžāĻ¤ā§, āĻ āĻ¨ā§āĻ¯āĻ°āĻž āĻ¨āĻ¯āĻŧ, āĻ¯ā§āĻŽāĻ¨ āĻāĻžāĻ āĻāĻŦāĻ āĻāĻŋāĻā§ āĻĒā§āĻ˛āĻžāĻ¸ā§āĻāĻŋāĻāĨ¤
Convection
Convection takes place in a liquid or a gas (i.e. in fluids). If one part of the fluid is heated, the kinetic energy of the molecules in that part is increased, they move further apart and this part becomes less dense. Consequently it rises, displacing the more dense fluid above which descends to take its place. The currents so produced are called convection currents.
āĻĒāĻ°āĻŋāĻāĻ˛āĻ¨ āĻ¤āĻ°āĻ˛ āĻŦāĻž āĻā§āĻ¯āĻžāĻ¸ā§ (āĻ āĻ°ā§āĻĨāĻžā§ āĻ¤āĻ°āĻ˛ āĻĒāĻĻāĻžāĻ°ā§āĻĨā§) āĻšāĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋ āĻ¤āĻ°āĻ˛ā§āĻ° āĻāĻāĻāĻŋ āĻ āĻāĻļ āĻāĻ¤ā§āĻ¤āĻĒā§āĻ¤ āĻšāĻ¯āĻŧ, āĻ¸ā§āĻ āĻ āĻāĻļā§āĻ° āĻ āĻŖā§āĻā§āĻ˛āĻŋāĻ° āĻāĻ¤āĻŋāĻļāĻā§āĻ¤āĻŋ āĻŦā§āĻĻā§āĻ§āĻŋ āĻĒāĻžāĻ¯āĻŧ, āĻ¤āĻžāĻ°āĻž āĻāĻ°āĻ āĻĻā§āĻ°ā§ āĻ¸āĻ°ā§ āĻ¯āĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻ āĻ āĻāĻļāĻāĻŋ āĻāĻŽ āĻāĻ¨ āĻšāĻ¯āĻŧāĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻāĻāĻŋ āĻāĻ¤ā§āĻĨāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻāĻĒāĻ°ā§ āĻāĻ°āĻ āĻāĻ¨ āĻ¤āĻ°āĻ˛ āĻ¸ā§āĻĨāĻžāĻ¨āĻā§āĻ¯ā§āĻ¤ āĻāĻ°ā§ āĻ¯āĻž āĻ¤āĻžāĻ° āĻāĻžāĻ¯āĻŧāĻāĻž āĻ¨āĻŋāĻ¤ā§ āĻ¨ā§āĻŽā§ āĻāĻ¸ā§āĨ¤ āĻāĻāĻāĻžāĻŦā§ āĻā§āĻĒāĻ¨ā§āĻ¨ āĻ¸ā§āĻ°ā§āĻ¤āĻā§ āĻĒāĻ°āĻŋāĻāĻ˛āĻ¨ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻŦāĻ˛ā§āĨ¤
Radiation
Heat may be transmitted by infra-red electromagnetic radiation. As described previously (p. 4), the heating of certain atoms causes an electron to move to a higher-energy electron shell. As it returns to its normal shell, the energy is released as a pulse of infra-red electromagnetic energy.
āĻ¤āĻžāĻĒ āĻāĻ¨āĻĢā§āĻ°āĻž-āĻ˛āĻžāĻ˛ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻŦāĻŋāĻāĻŋāĻ°āĻŖ āĻĻā§āĻŦāĻžāĻ°āĻž āĻ¸āĻā§āĻāĻžāĻ°āĻŋāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻĒā§āĻ°ā§āĻŦā§ āĻŦāĻ°ā§āĻŖāĻŋāĻ¤ (āĻĒā§. 4), āĻ¨āĻŋāĻ°ā§āĻĻāĻŋāĻˇā§āĻ āĻĒāĻ°āĻŽāĻžāĻŖā§āĻ° āĻāĻ¤ā§āĻ¤āĻžāĻĒā§āĻ° āĻĢāĻ˛ā§ āĻāĻāĻāĻŋ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻāĻāĻāĻŋ āĻāĻā§āĻ-āĻļāĻā§āĻ¤āĻŋāĻ° āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻļā§āĻ˛ā§ āĻāĻ˛ā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻ¯āĻāĻ¨ āĻāĻāĻŋ āĻ¤āĻžāĻ° āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻļā§āĻ˛ā§ āĻĢāĻŋāĻ°ā§ āĻāĻ¸ā§, āĻ¤āĻāĻ¨ āĻļāĻā§āĻ¤āĻŋāĻāĻŋ āĻāĻ¨āĻĢā§āĻ°āĻž-āĻ°ā§āĻĄ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻļāĻā§āĻ¤āĻŋāĻ° āĻ¸ā§āĻĒāĻ¨ā§āĻĻāĻ¨ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻŽā§āĻā§āĻ¤āĻŋ āĻĒāĻžāĻ¯āĻŧāĨ¤
Physical effects of heat
Expansion is the result of increased kinetic energy producing a greater vibration of molecules, which thus move further apart.
āĻ¸āĻŽā§āĻĒā§āĻ°āĻ¸āĻžāĻ°āĻŖ āĻšāĻ˛ āĻŦāĻ°ā§āĻ§āĻŋāĻ¤ āĻāĻ¤āĻŋāĻļāĻā§āĻ¤āĻŋāĻ° āĻĢāĻ˛āĻžāĻĢāĻ˛ āĻ¯āĻž āĻ āĻŖā§āĻā§āĻ˛āĻŋāĻ° āĻāĻāĻāĻŋ āĻŦā§āĻšāĻ¤ā§āĻ¤āĻ° āĻāĻŽā§āĻĒāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°ā§, āĻ¯āĻž āĻāĻāĻāĻžāĻŦā§ āĻāĻ°āĻ āĻĻā§āĻ°ā§ āĻ¸āĻ°ā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤
Change of state See section on states of matter.
Acceleration of chemical action can’t Hoff’s Law says that any chemical action capable of being accelerated is accelerated by a rise in temperature. The converse, that cooling slows the rate of reaction, is also true.
āĻ°āĻžāĻ¸āĻžāĻ¯āĻŧāĻ¨āĻŋāĻ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ° āĻ¤ā§āĻŦāĻ°āĻŖ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¨āĻž āĻšāĻĢā§āĻ° āĻāĻāĻ¨ āĻŦāĻ˛ā§ āĻ¯ā§ āĻā§āĻ¨ā§ āĻ°āĻžāĻ¸āĻžāĻ¯āĻŧāĻ¨āĻŋāĻ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¤ā§āĻŦāĻ°āĻžāĻ¨ā§āĻŦāĻŋāĻ¤ āĻšāĻ¤ā§ āĻ¸āĻā§āĻˇāĻŽ āĻ¤āĻž āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻž āĻŦā§āĻĻā§āĻ§āĻŋāĻ° āĻĻā§āĻŦāĻžāĻ°āĻž āĻ¤ā§āĻŦāĻ°āĻžāĻ¨ā§āĻŦāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻĨā§āĻĒāĻāĻĨāĻ¨, āĻ¯ā§ āĻļā§āĻ¤āĻ˛ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ° āĻšāĻžāĻ°āĻā§ āĻ§ā§āĻ° āĻāĻ°ā§ āĻĻā§āĻ¯āĻŧ, āĻāĻāĻŋāĻ āĻ¸āĻ¤ā§āĻ¯āĨ¤
Production of a potential difference If the junction of two dissimilar metals, e.g. bismuth and antimony, is heated, a potential difference is produced between their free ends (the thermocouple principle).Production of electromagnetic waves See section on electromagnetic radiation
āĻāĻāĻāĻŋ āĻ¸āĻŽā§āĻāĻžāĻŦā§āĻ¯ āĻĒāĻžāĻ°ā§āĻĨāĻā§āĻ¯ āĻā§āĻĒāĻžāĻĻāĻ¨ āĻ¯āĻĻāĻŋ āĻĻā§āĻāĻŋ āĻāĻŋāĻ¨ā§āĻ¨ āĻ§āĻžāĻ¤ā§āĻ° āĻ¸āĻāĻ¯ā§āĻāĻ¸ā§āĻĨāĻ˛, āĻ¯ā§āĻŽāĻ¨ āĻŦāĻŋāĻ¸āĻŽāĻžāĻĨ āĻāĻŦāĻ āĻ ā§āĻ¯āĻžāĻ¨ā§āĻāĻŋāĻŽāĻ¨āĻŋ, āĻāĻ¤ā§āĻ¤āĻĒā§āĻ¤ āĻšāĻ¯āĻŧ, āĻ¤āĻžāĻĻā§āĻ° āĻŽā§āĻā§āĻ¤ āĻĒā§āĻ°āĻžāĻ¨ā§āĻ¤ā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻāĻāĻāĻŋ āĻ¸āĻŽā§āĻāĻžāĻŦā§āĻ¯ āĻĒāĻžāĻ°ā§āĻĨāĻā§āĻ¯ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻšāĻ¯āĻŧ (āĻĨāĻžāĻ°ā§āĻŽā§āĻāĻ˛ āĻ¨ā§āĻ¤āĻŋ)āĨ¤ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻ¤āĻ°āĻā§āĻā§āĻ° āĻāĻ¤ā§āĻĒāĻžāĻĻāĻ¨ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻŦāĻŋāĻāĻŋāĻ°āĻŖ āĻŦāĻŋāĻāĻžāĻ āĻĻā§āĻā§āĻ¨
Thermionic emission- The heating of molecules of some materials ,e.g. tungsten, may cause such molecular agitation that some electrons leave their atoms and may even break free of the surface of the metal. This leaves a positive charge which tends to attract the negative electrons back. However, a point is reached where the rate of loss of electrons equals the rate of return and a cloud of electrons then exists as a space charge around the object. This process is called thermionic emission and is the principle upon which electric valves work.
āĻĨāĻžāĻ°ā§āĻŽāĻŋāĻāĻ¨āĻŋāĻ āĻ¨āĻŋāĻ°ā§āĻāĻŽāĻ¨- āĻāĻŋāĻā§ āĻĒāĻĻāĻžāĻ°ā§āĻĨā§āĻ° āĻ āĻŖā§ āĻāĻ°āĻŽ āĻāĻ°āĻž, āĻ¯ā§āĻŽāĻ¨ āĻāĻāĻ¸ā§āĻā§āĻ¨, āĻāĻŽāĻ¨ āĻāĻŖāĻŦāĻŋāĻ āĻāĻ¨ā§āĻĻā§āĻ˛āĻ¨ā§āĻ° āĻāĻžāĻ°āĻŖ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¯ā§ āĻāĻŋāĻā§ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻ¤āĻžāĻĻā§āĻ° āĻĒāĻ°āĻŽāĻžāĻŖā§ āĻā§āĻĄāĻŧā§ āĻ¯āĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻŽāĻ¨āĻāĻŋ āĻ§āĻžāĻ¤ā§āĻ° āĻĒā§āĻˇā§āĻ āĻĨā§āĻā§ āĻŽā§āĻā§āĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻāĻŋ āĻāĻāĻāĻŋ āĻāĻ¤āĻŋāĻŦāĻžāĻāĻ āĻāĻžāĻ°ā§āĻ āĻā§āĻĄāĻŧā§ āĻĻā§āĻ¯āĻŧ āĻ¯āĻž āĻ¨ā§āĻ¤āĻŋāĻŦāĻžāĻāĻ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§āĻ˛āĻŋāĻā§ āĻĢāĻŋāĻ°ā§ āĻāĻāĻ°ā§āĻˇāĻŖ āĻāĻ°ā§āĨ¤ āĻ¯āĻžāĻāĻšā§āĻ, āĻāĻāĻāĻŋ āĻŦāĻŋāĻ¨ā§āĻĻā§āĻ¤ā§ āĻĒā§āĻāĻāĻžāĻ¨ā§ āĻšāĻ¯āĻŧ āĻ¯ā§āĻāĻžāĻ¨ā§ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻšāĻžāĻ°āĻžāĻ¨ā§āĻ° āĻšāĻžāĻ° āĻ°āĻŋāĻāĻžāĻ°ā§āĻ¨ā§āĻ° āĻšāĻžāĻ°ā§āĻ° āĻ¸āĻŽāĻžāĻ¨ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ā§āĻ° āĻŽā§āĻ āĻ¤āĻāĻ¨ āĻŦāĻ¸ā§āĻ¤ā§āĻ° āĻāĻžāĻ°āĻĒāĻžāĻļā§ āĻ¸ā§āĻĒā§āĻ¸ āĻāĻžāĻ°ā§āĻ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻŦāĻŋāĻĻā§āĻ¯āĻŽāĻžāĻ¨ āĻĨāĻžāĻā§āĨ¤ āĻāĻ āĻĒā§āĻ°āĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻŋāĻā§ āĻĨāĻžāĻ°ā§āĻŽāĻŋāĻāĻ¨āĻŋāĻ āĻ¨āĻŋāĻ°ā§āĻāĻŽāĻ¨ āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻāĻŋ āĻ¸ā§āĻ āĻ¨ā§āĻ¤āĻŋ āĻ¯āĻžāĻ° āĻāĻĒāĻ° āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻžāĻ˛āĻ āĻāĻžāĻ āĻāĻ°ā§āĨ¤
Reduced viscosity of fluids- The molecules in viscous fluids are fairly strongly attracted to one another. Heating increases the kinetic movement of these molecules and reduces their cohesive mutual attraction: this makes the fluid less viscous.
āĻ¤āĻ°āĻ˛ āĻĒāĻĻāĻžāĻ°ā§āĻĨā§āĻ° āĻ¸āĻžāĻ¨ā§āĻĻā§āĻ°āĻ¤āĻž āĻšā§āĻ°āĻžāĻ¸- āĻ¸āĻžāĻ¨ā§āĻĻā§āĻ° āĻ¤āĻ°āĻ˛ā§āĻ° āĻ āĻŖā§āĻā§āĻ˛āĻŋ āĻāĻā§ āĻ āĻĒāĻ°ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋ āĻŽā§āĻāĻžāĻŽā§āĻāĻŋāĻāĻžāĻŦā§ āĻāĻā§āĻˇā§āĻ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ¤ā§āĻ¤āĻžāĻĒ āĻāĻ āĻ āĻŖā§āĻā§āĻ˛āĻŋāĻ° āĻāĻ¤āĻŋāĻāĻ¤ āĻāĻ¤āĻŋ āĻŦāĻžāĻĄāĻŧāĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻ¤āĻžāĻĻā§āĻ° āĻ¸āĻāĻšāĻ¤ āĻĒāĻžāĻ°āĻ¸ā§āĻĒāĻ°āĻŋāĻ āĻāĻāĻ°ā§āĻˇāĻŖ āĻšā§āĻ°āĻžāĻ¸ āĻāĻ°ā§: āĻāĻāĻŋ āĻ¤āĻ°āĻ˛āĻā§ āĻāĻŽ āĻ¸āĻžāĻ¨ā§āĻĻā§āĻ° āĻāĻ°ā§ āĻ¤ā§āĻ˛ā§āĨ¤
Static electricity
The simplest way of producing a static electric charge is to rub two suitable materials together. If two insulators such as glass and flannel are rubbed together, a positive charge is produced on the flannel and a negative charge on the glass. This is because electrons are transferred from the superficial atoms of the flannel to the surface of the glass.
āĻāĻāĻāĻŋ āĻ¸ā§āĻĨāĻŋāĻ° āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻžāĻ°ā§āĻ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻ¨ āĻāĻ°āĻžāĻ° āĻ¸āĻŦāĻā§āĻ¯āĻŧā§ āĻ¸āĻšāĻ āĻāĻĒāĻžāĻ¯āĻŧ āĻšāĻ˛ āĻĻā§āĻāĻŋ āĻāĻĒāĻ¯ā§āĻā§āĻ¤ āĻāĻĒāĻāĻ°āĻŖ āĻāĻāĻ¸āĻžāĻĨā§ āĻāĻˇāĻžāĨ¤ āĻā§āĻ˛āĻžāĻ¸ āĻāĻŦāĻ āĻĢā§āĻ˛ā§āĻ¯āĻžāĻ¨ā§āĻ˛ā§āĻ° āĻŽāĻ¤ā§ āĻĻā§āĻāĻŋ āĻāĻ¨āĻ¸ā§āĻ˛ā§āĻāĻ°āĻā§ āĻāĻāĻ¸āĻžāĻĨā§ āĻāĻˇāĻ˛ā§ āĻĢā§āĻ˛ā§āĻ¯āĻžāĻ¨ā§āĻ˛ā§āĻ° āĻāĻĒāĻ° āĻāĻāĻāĻŋ āĻ§āĻ¨āĻžāĻ¤ā§āĻŽāĻ āĻāĻžāĻ°ā§āĻ āĻāĻŦāĻ āĻāĻžāĻā§āĻ° āĻāĻĒāĻ° āĻāĻāĻāĻŋ āĻāĻŖāĻžāĻ¤ā§āĻŽāĻ āĻāĻžāĻ°ā§āĻ āĻ¤ā§āĻ°āĻŋ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ° āĻāĻžāĻ°āĻŖ āĻšāĻ˛ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§āĻ˛āĻŋ āĻĢā§āĻ˛ā§āĻ¯āĻžāĻ¨ā§āĻ˛ā§āĻ° āĻ¸ā§āĻĒāĻžāĻ°āĻĢāĻŋāĻļāĻŋāĻ¯āĻŧāĻžāĻ˛ āĻĒāĻ°āĻŽāĻžāĻŖā§ āĻĨā§āĻā§ āĻāĻžāĻā§āĻ° āĻĒā§āĻˇā§āĻ ā§ āĻ¸ā§āĻĨāĻžāĻ¨āĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤
As the materials involved are insulators, the charges are held on the surfaces of the objects and spread themselves evenly over the surface sunless there are points or corners, at which the charges tend to concentrate.
āĻ¯ā§āĻšā§āĻ¤ā§ āĻāĻĄāĻŧāĻŋāĻ¤ āĻāĻĒāĻžāĻĻāĻžāĻ¨āĻā§āĻ˛āĻŋ āĻ āĻ¨ā§āĻ¤āĻ°āĻ, āĻ¤āĻžāĻ āĻāĻžāĻ°ā§āĻāĻā§āĻ˛āĻŋ āĻŦāĻ¸ā§āĻ¤ā§āĻ° āĻĒā§āĻˇā§āĻ ā§āĻ° āĻāĻĒāĻ° āĻ§āĻžāĻ°āĻŖ āĻāĻ°ā§ āĻāĻŦāĻ āĻ¸ā§āĻ°ā§āĻ¯ā§āĻ° āĻāĻĒāĻ°āĻŋāĻāĻžāĻā§ āĻ¸āĻŽāĻžāĻ¨āĻāĻžāĻŦā§ āĻŦāĻŋāĻ¸ā§āĻ¤ā§āĻ¤ āĻšāĻ¯āĻŧ āĻ¯āĻĻāĻŋ āĻ¨āĻž āĻ¸ā§āĻāĻžāĻ¨ā§ āĻŦāĻŋāĻ¨ā§āĻĻā§ āĻŦāĻž āĻā§āĻŖ āĻĨāĻžāĻā§, āĻ¯ā§āĻāĻžāĻ¨ā§ āĻāĻžāĻ°ā§āĻāĻā§āĻ˛āĻŋ āĻāĻ¨ā§āĻā§āĻ¤ āĻšāĻ¤ā§ āĻĨāĻžāĻā§āĨ¤
Electric lines of force.
A charged body and its electric field
The charged body is continually seeking to attain its neutral state: if negative, by losing electrons; if positive, by gaining electrons. This phenomenon creates a zone of influence (an electric field), which is the area in which the charged body has an effect. This field may be considered to be made up of lines of force surrounding the body. Lines of force are by convention paths along which a free positive charge would travel.
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They show certain properties:
I. The lines of force surrounding an isolated charged body are straight.
2. Lines of force repel one another (as do like charges).
3. Lines of force pass more easily through conductors than through insulators.
4. Lines of force concentrate on that part of the surface of a charged body nearest to another object over which they can exert an influence. If two bodies with opposing charges are placed opposite one another there is a force of attraction between them. If two similarly charged objects are placed near one another, they repel one another. If a charged body is placed in contact with another body (charged or uncharged), then electrons flow between them until they are at the same potential.
āĻāĻāĻāĻŋ āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻāĻžāĻ°ā§āĻāĻ¯ā§āĻā§āĻ¤ āĻĻā§āĻšāĻā§ āĻāĻŋāĻ°ā§ āĻĨāĻžāĻāĻž āĻŦāĻ˛ āĻ°ā§āĻāĻž āĻ¸ā§āĻāĻžāĨ¤
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Potential and capacitance-
The electrical potential of a body is the electrical condition of that body when compared to the neutral potential of the Earth. Bodies with an excess of electrons are called negative, bodies deficient in electrons are called positive.
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The unit of potential is the volt (see Glossary for definition), conveniently considered as indicating the repelling power between like charges: a high potential means a strong repelling power. The magnitude of the potential depends on the quantity of electricity with which the object is charged, i.e. the number of electrons gained or lost, and the capacitance of the object. If two similar objects are charged with different quantities of electricity, the one with the most will have the greater repelling power or potential. The quantity of electricity is measured in coulombs, a coulomb being equivalent to6.26×1018 electron charges. There is a direct relationship between potential and electrical charge.
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The capacitance of an object is the ability of the body to hold an electrical charge, and depends upon the material and the surface area of the body. Some materials hold a charge better than others. As the charge is always held on the surface, the greater the surface area the greater is the capacitance of the body. There is an inverse relationship between capacitance and potential; the larger the capacitance of the body, the smaller the potential or repelling power developed by a given charge. For example, if coulombs is used to charge each of two objects which have different capacitances, the repelling power or potential developed will be greater for the smaller object.
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Difference of potential
A difference of potential exists between similar bodies charged with different quantities of electricity. This is demonstrated in any of the situations shown, if a conducting connection is made between the two bodies, electrons will flow from the more negative body to the less negative one. The force producing this movement is called an electromotive force(EMF). Electron flow continues until both objects are at the same potential. EMF (like potential) is measured in volts.
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Current electricity
An electric current occurs when there is a flow of charged particles (generally electrons) in a conductor. By historical convention, current has always been envisaged as flowing from positive to negative e,i,e. in precisely the reverse direction to the actual flow of electrons. To avoid confusion, it is essential to distinguish carefully, therefore, between electron flow and conventional current flow. The factors essential for the production of an electric current are a difference of potential (PD), and a conducting pathway between the points of potential difference. Electrons will flow only for as long as the potential difference and the pathway exist. To sustain a current flow some means of maintaining the PD between the ends of the circuit is essential. This is achieved by chemical action, using a battery, or by electromagnetic induction with a dynamo.
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Electromotive force (EMF)
This is the force which causes electrons to move along a conductor connecting points of different potential. The greater the potential difference, the greater the EMF. Both are measured in volts.
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Resistance
The conductor through which electrons have to flow offers some resistance to their flow. The unit of electrical resistance is the ohm (symbol Ί: see Glossary for definition). The electrical resistance of a conductor made of a certain material, of a certain length and cross-sectional area, and at a certain temperature, will always be the same.
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The material of the conductor Copper, for example, has a single electron in its outer shell. At room temperature the kinetic energy of the atoms displaces some of these electrons, which are then free to act as conduction electrons, carrying electric charge from one end of the conductor to the other. Most metals are good conductors. A good conductor is said to have a low resistance, a poor conductor a high resistance.
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The length of the pathway At normal temperatures, even good conductors offer some resistance to electron flow. Consequently, the longer the pathway the greater is the electrical resistance.
āĻĒāĻĨā§āĻ° āĻĻā§āĻ°ā§āĻā§āĻ¯ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ¯āĻŧ, āĻāĻŽāĻ¨āĻāĻŋ āĻāĻžāĻ˛ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§ āĻāĻ˛ā§āĻā§āĻā§āĻ°āĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻāĻŋāĻā§ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻĒā§āĻ°āĻ¸ā§āĻ¤āĻžāĻŦ āĻĻā§āĻ¯āĻŧāĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻĒāĻĨ āĻ¯āĻ¤ āĻĻā§āĻ°ā§āĻ āĻšāĻŦā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻā§āĻˇāĻŽāĻ¤āĻž āĻ¤āĻ¤ āĻŦā§āĻļāĻŋāĨ¤
The cross-sectional area of the conductor greater the cross-sectional area is the more room there is for electrons to pass, therefore the resistance is lower. If a high resistance is required, thin wire is used. Temperature As the temperature of a conductor increases so does the kinetic movement of the molecules. This increased movement impedes the passage of electrons and so increases the resistance.
āĻāĻ¨ā§āĻĄāĻžāĻā§āĻāĻ°ā§āĻ° āĻā§āĻ°āĻ¸-āĻŦāĻŋāĻāĻžāĻā§āĻ¯āĻŧ āĻā§āĻˇā§āĻ¤ā§āĻ°āĻāĻŋ āĻā§āĻ°āĻ¸-āĻŦāĻŋāĻāĻžāĻā§āĻ¯āĻŧ āĻāĻ˛āĻžāĻāĻž āĻ¯āĻ¤ āĻŦā§āĻļāĻŋ āĻ¸ā§āĻāĻžāĻ¨ā§ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§āĻ˛āĻŋ āĻ āĻ¤āĻŋāĻā§āĻ°āĻŽ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ¤āĻ¤ āĻŦā§āĻļāĻŋ āĻāĻžāĻ¯āĻŧāĻāĻž āĻĨāĻžāĻā§, āĻ¤āĻžāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻāĻŽ āĻšāĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋ āĻāĻāĻāĻŋ āĻāĻā§āĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧ, āĻĒāĻžāĻ¤āĻ˛āĻž āĻ¤āĻžāĻ° āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°āĻž āĻšāĻ¯āĻŧāĨ¤ āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻž āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ° āĻ¤āĻžāĻĒāĻŽāĻžāĻ¤ā§āĻ°āĻž āĻŦāĻžāĻĄāĻŧāĻžāĻ° āĻ¸āĻžāĻĨā§ āĻ¸āĻžāĻĨā§ āĻ āĻŖā§āĻ° āĻāĻ¤āĻŋāĻāĻ¤ āĻāĻ¤āĻŋāĻ āĻŦā§āĻĻā§āĻ§āĻŋ āĻĒāĻžāĻ¯āĻŧāĨ¤ āĻāĻ āĻŦāĻ°ā§āĻ§āĻŋāĻ¤ āĻ¨āĻĄāĻŧāĻžāĻāĻĄāĻŧāĻž āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ā§āĻ° āĻāĻ¤ā§āĻ¤āĻ°āĻŖā§ āĻŦāĻžāĻ§āĻž āĻĻā§āĻ¯āĻŧ āĻāĻŦāĻ āĻ¤āĻžāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻā§āĻˇāĻŽāĻ¤āĻž āĻŦāĻžāĻĄāĻŧāĻžāĻ¯āĻŧāĨ¤
Magnitude of current
The intensity, or magnitude, of current (I) is the rate of flow of electrons through the conductor per second. Electric current is measured in amperes, one ampere being a rate of flow of one coulomb (6.26×1018 electrons) per second. There is a more complex definition of the ampere (see Glossary) based on the magnetic effect produced by the flow of electrons along a wire. In medical electricity the unit of current most used is the milliampere, which is I/I000 of an ampere.
āĻāĻžāĻ°ā§āĻ¨ā§āĻ (I) āĻāĻ° āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻŦāĻž āĻŽāĻžāĻ¤ā§āĻ°āĻž āĻšāĻ˛ āĻĒā§āĻ°āĻ¤āĻŋ āĻ¸ā§āĻā§āĻ¨ā§āĻĄā§ āĻāĻ¨ā§āĻĄāĻžāĻāĻāĻ°ā§āĻ° āĻŽāĻžāĻ§ā§āĻ¯āĻŽā§ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻšāĻžāĻ°āĨ¤ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻŦāĻžāĻš āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°ā§ āĻĒāĻ°āĻŋāĻŽāĻžāĻĒ āĻāĻ°āĻž āĻšāĻ¯āĻŧ, āĻāĻ āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ° āĻĒā§āĻ°āĻ¤āĻŋ āĻ¸ā§āĻā§āĻ¨ā§āĻĄā§ āĻāĻ āĻā§āĻ˛āĻŽā§āĻŦ (6.26x1018 āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨) āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻšāĻžāĻ°āĨ¤ āĻāĻāĻāĻŋ āĻ¤āĻžāĻ° āĻŦāĻ°āĻžāĻŦāĻ° āĻāĻ˛ā§āĻā§āĻā§āĻ°āĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻš āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻā§āĻŽā§āĻŦāĻā§āĻ¯āĻŧ āĻĒā§āĻ°āĻāĻžāĻŦā§āĻ° āĻāĻĒāĻ° āĻāĻŋāĻ¤ā§āĻ¤āĻŋ āĻāĻ°ā§ āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°ā§āĻ° āĻāĻ°āĻ āĻāĻāĻŋāĻ˛ āĻ¸āĻāĻā§āĻāĻž āĻ°āĻ¯āĻŧā§āĻā§ (āĻā§āĻ˛ā§āĻ¸āĻžāĻ°āĻŋ āĻĻā§āĻā§āĻ¨)āĨ¤ āĻāĻŋāĻāĻŋā§āĻ¸āĻž āĻŦāĻŋāĻĻā§āĻ¯ā§āĻ¤ā§ āĻ¸āĻ°ā§āĻŦāĻžāĻ§āĻŋāĻ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻāĻāĻ āĻšāĻ˛ āĻŽāĻŋāĻ˛āĻŋāĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°, āĻ¯āĻž āĻāĻāĻāĻŋ āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°ā§āĻ° 1/1000āĨ¤
The magnitude of current through a conductor depends upon the applied EMF and the resistance of the conductor. The greater the EMF applied, the greater is the flow of electrons. There is therefore a direct relationship between current and applied EMF. The greater the resistance, the more difficult it is for electrons to move through the conductor, so a high resistance tends to produce a low current this is an inverse relationship.
āĻāĻāĻāĻŋ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ° āĻŽāĻžāĻ§ā§āĻ¯āĻŽā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻŽāĻžāĻ¤ā§āĻ°āĻž āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻā§āĻ¤ EMF āĻāĻŦāĻ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻāĻĒāĻ° āĻ¨āĻŋāĻ°ā§āĻāĻ° āĻāĻ°ā§āĨ¤ EMF āĻ¯āĻ¤ āĻŦā§āĻļāĻŋ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻšāĻŦā§, āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ā§āĻ° āĻĒā§āĻ°āĻŦāĻžāĻš āĻ¤āĻ¤ āĻŦā§āĻļāĻŋ āĻšāĻŦā§āĨ¤ āĻ¤āĻžāĻ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻāĻŦāĻ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻā§āĻ¤ EMF āĻāĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻāĻāĻāĻŋ āĻ¸āĻ°āĻžāĻ¸āĻ°āĻŋ āĻ¸āĻŽā§āĻĒāĻ°ā§āĻ āĻ°āĻ¯āĻŧā§āĻā§āĨ¤ āĻ°ā§āĻāĻŋāĻ¸ā§āĻā§āĻ¯āĻžāĻ¨ā§āĻ¸ āĻ¯āĻ¤ āĻŦā§āĻļāĻŋ āĻšāĻŦā§, āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨ā§āĻ° āĻĒāĻā§āĻˇā§ āĻāĻ¨ā§āĻĄāĻžāĻāĻāĻ°ā§āĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻāĻ˛āĻžāĻāĻ˛ āĻāĻ°āĻž āĻ¤āĻ¤ āĻŦā§āĻļāĻŋ āĻāĻ āĻŋāĻ¨, āĻ¤āĻžāĻ āĻāĻāĻāĻŋ āĻāĻā§āĻ āĻ°ā§āĻāĻŋāĻ¸ā§āĻā§āĻ¯āĻžāĻ¨ā§āĻ¸ āĻāĻŽ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻ¤ā§āĻ°āĻŋ āĻāĻ°āĻ¤ā§ āĻĨāĻžāĻā§ āĻāĻāĻŋ āĻāĻāĻāĻŋ āĻŦāĻŋāĻĒāĻ°ā§āĻ¤ āĻ¸āĻŽā§āĻĒāĻ°ā§āĻāĨ¤
Ohm’s law
A constant relationship exists between the magnitude of current in a conductor, the applied EMF, and the resistance of the conductor. This relationship is called ohms law. Simply stated
āĻāĻāĻāĻŋ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ¤ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻŽāĻžāĻ¤ā§āĻ°āĻž, āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻā§āĻ¤ EMF āĻāĻŦāĻ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻāĻāĻāĻŋ āĻ§ā§āĻ°ā§āĻŦāĻ āĻ¸āĻŽā§āĻĒāĻ°ā§āĻ āĻŦāĻŋāĻĻā§āĻ¯āĻŽāĻžāĻ¨āĨ¤ āĻāĻ āĻ¸āĻŽā§āĻĒāĻ°ā§āĻāĻā§ ohms āĻāĻāĻ¨ āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧāĨ¤ āĻ¸āĻšāĻāĻāĻžāĻŦā§ āĻŦāĻ˛āĻž
Ohms law said that
Resistance in series.2 (Ί denotes ohms). Total resistance=R1+R2+R3=35Ί.the magnitude of an electric current varies directly with the EMF and inversely with the
āĻ¸āĻŋāĻ°āĻŋāĻā§ āĻ°ā§āĻ§āĨ¤ 2 (Ί āĻŦā§āĻāĻžāĻ¯āĻŧ āĻāĻšāĻŽ)āĨ¤ āĻŽā§āĻ āĻ°ā§āĻ§=R1+R2+R3=35Ί. āĻāĻāĻāĻŋ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻŽāĻžāĻ¤ā§āĻ°āĻž āĻ¸āĻ°āĻžāĻ¸āĻ°āĻŋ EMF āĻāĻ° āĻ¸āĻžāĻĨā§ āĻāĻŦāĻ āĻŦāĻŋāĻĒāĻ°ā§āĻ¤āĻāĻžāĻŦā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻšāĻ¯āĻŧ
Resistance:
y/=1
Where I=Current in amperes
E=EMF in volts
R=Resistance in ohms
āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§:
y/=1
āĻ¯ā§āĻāĻžāĻ¨ā§ I=āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻ
E = EMF āĻā§āĻ˛ā§āĻā§
R=āĻāĻšāĻŽā§ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§
If two of these quantities are known, the third can always be calculated.
āĻāĻ āĻĒāĻ°āĻŋāĻŽāĻžāĻŖā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻĻā§āĻāĻŋ āĻāĻžāĻ¨āĻž āĻĨāĻžāĻāĻ˛ā§, āĻ¤ā§āĻ¤ā§āĻ¯āĻŧāĻāĻŋ āĻ¸āĻ°ā§āĻŦāĻĻāĻž āĻāĻŖāĻ¨āĻž āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤
Resistance in series
If the components of an electrical circuit are connected in series (i.e consecutively, there is only one possible pathway for a current. As the current has to pass through each resistance in turn, the total resistance equals the sum of the individual resistances.
āĻ¯āĻĻāĻŋ āĻāĻāĻāĻŋ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻ¸āĻžāĻ°ā§āĻāĻŋāĻā§āĻ° āĻāĻĒāĻžāĻĻāĻžāĻ¨āĻā§āĻ˛āĻŋ āĻ¸āĻŋāĻ°āĻŋāĻā§ āĻ¸āĻāĻ¯ā§āĻā§āĻ¤ āĻĨāĻžāĻā§ (āĻ āĻ°ā§āĻĨāĻžā§ āĻ§āĻžāĻ°āĻžāĻŦāĻžāĻšāĻŋāĻāĻāĻžāĻŦā§, āĻāĻāĻāĻŋ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻāĻ¨ā§āĻ¯ āĻļā§āĻ§ā§āĻŽāĻžāĻ¤ā§āĻ° āĻāĻāĻāĻŋ āĻ¸āĻŽā§āĻāĻžāĻŦā§āĻ¯ āĻĒāĻĨ āĻ°āĻ¯āĻŧā§āĻā§āĨ¤ āĻ¯ā§āĻšā§āĻ¤ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻāĻā§ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¯ā§āĻ¤ā§ āĻšāĻ¯āĻŧ, āĻŽā§āĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻĒā§āĻĨāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻ¸āĻŽāĻˇā§āĻāĻŋāĻ° āĻ¸āĻŽāĻžāĻ¨ āĻšāĻ¯āĻŧāĨ¤
Resistance in parallel
In this situation, the current is offered a number of alternative routes. As shown in the effect is that the current divides into three parts at A and unites again at B.
The proportion of the current in each resistance depends upon the relative magnitudes of the resistances. By applying Ohm’s law we find that the largest resistance carries the smallest current and the smallest. resistance the largest current. The formula used to calculate the total resistance:
āĻāĻ āĻĒāĻ°āĻŋāĻ¸ā§āĻĨāĻŋāĻ¤āĻŋāĻ¤ā§, āĻ¸ā§āĻ°ā§āĻ¤āĻā§ āĻŦā§āĻļ āĻāĻ¯āĻŧā§āĻāĻāĻŋ āĻŦāĻŋāĻāĻ˛ā§āĻĒ āĻ°ā§āĻā§āĻ° āĻĒā§āĻ°āĻ¸ā§āĻ¤āĻžāĻŦ āĻĻā§āĻāĻ¯āĻŧāĻž āĻšāĻ¯āĻŧāĨ¤ āĻĒā§āĻ°āĻāĻžāĻŦ āĻšāĻ˛ āĻ¯ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻ A-āĻ¤ā§ āĻ¤āĻŋāĻ¨āĻāĻŋ āĻāĻžāĻā§ āĻŦāĻŋāĻāĻā§āĻ¤ āĻšāĻ¯āĻŧā§ āĻāĻŦāĻžāĻ° B-āĻ¤ā§ āĻāĻāĻ¤ā§āĻ°āĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤
āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ā§āĻ° āĻ āĻ¨ā§āĻĒāĻžāĻ¤ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻāĻĒā§āĻā§āĻˇāĻŋāĻ āĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ° āĻāĻĒāĻ° āĻ¨āĻŋāĻ°ā§āĻāĻ° āĻāĻ°ā§āĨ¤ āĻāĻšāĻŽā§āĻ° āĻ¸ā§āĻ¤ā§āĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°ā§ āĻāĻŽāĻ°āĻž āĻĻā§āĻāĻ¤ā§ āĻĒāĻžāĻ āĻ¯ā§ āĻŦā§āĻšāĻ¤ā§āĻ¤āĻŽ āĻ°ā§āĻ§ āĻā§āĻˇā§āĻĻā§āĻ°āĻ¤āĻŽ āĻ¤āĻĄāĻŧāĻŋā§ āĻāĻŦāĻ āĻā§āĻˇā§āĻĻā§āĻ°āĻ¤āĻŽ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻŦāĻšāĻ¨ āĻāĻ°ā§āĨ¤ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻŦā§āĻšāĻ¤ā§āĻ¤āĻŽ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨. āĻŽā§āĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻāĻŖāĻ¨āĻž āĻāĻ°āĻ¤ā§ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻ¸ā§āĻ¤ā§āĻ°:
R
R=R/1+1/12+1/3
Substituting the values from-
R/=/5+1/0+2/20=0.35
Resistance in parallel
Total resistance =R/1+1/1+1/
Hence R=2.85Ί.
The total resistance for the circuit,2.85 Ί,is less than any one of the individual resistances. The reason for this is that connecting resistances in parallel has the effect of increasing the cross-sectional area of the pathway.
āĻ¸āĻžāĻ°ā§āĻāĻŋāĻā§āĻ° āĻŽā§āĻ āĻ°ā§āĻ§, 2.85 Ί, āĻ¯ā§ āĻā§āĻ¨ā§ āĻāĻāĻāĻŋ āĻĒā§āĻĨāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻĨā§āĻā§ āĻāĻŽāĨ¤ āĻāĻ° āĻāĻžāĻ°āĻŖ āĻšāĻ˛ āĻ¯ā§ āĻ¸āĻŽāĻžāĻ¨ā§āĻ¤āĻ°āĻžāĻ˛āĻāĻžāĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻ¸āĻāĻ¯ā§āĻ āĻāĻ°āĻžāĻ° āĻĢāĻ˛ā§ āĻĒāĻĨā§āĻ° āĻā§āĻ°āĻ¸-āĻŦāĻŋāĻāĻžāĻā§āĻ¯āĻŧ āĻāĻ˛āĻžāĻāĻž āĻŦā§āĻĻā§āĻ§āĻŋāĻ° āĻĒā§āĻ°āĻāĻžāĻŦ āĻ°āĻ¯āĻŧā§āĻā§āĨ¤
Thermal effect of an electric current
When a current passes through a conductor, some of its energy is converted into thermal energy (heat). The amount of heat produced can be calculated using Joule’s Law ,which states that the amount of heat produced in a conductor is proportional to the square of the current(I2),the resistance(R), and the time (t) for which the current flows. This may be expressed as:
āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻŦāĻŋāĻĻā§āĻ¯ā§ā§ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻ¤āĻžāĻ° āĻāĻŋāĻā§ āĻļāĻā§āĻ¤āĻŋ āĻ¤āĻžāĻĒ āĻļāĻā§āĻ¤āĻŋāĻ¤ā§ (āĻ¤āĻžāĻĒ) āĻ°ā§āĻĒāĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻ¤āĻžāĻĒā§āĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖ āĻā§āĻ˛ā§āĻ° āĻ¸ā§āĻ¤ā§āĻ° āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°ā§ āĻāĻŖāĻ¨āĻž āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯āĻž āĻŦāĻ˛ā§ āĻ¯ā§ āĻāĻāĻāĻŋ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ¤ā§ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻ¤āĻžāĻĒā§āĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ (I2), āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° (R) āĻŦāĻ°ā§āĻāĻā§āĻˇā§āĻ¤ā§āĻ°ā§āĻ° āĻ¸āĻŽāĻžāĻ¨ā§āĻĒāĻžāĻ¤āĻŋāĻ āĻāĻŦāĻ āĻ¸āĻŽāĻ¯āĻŧ (āĻāĻŋ) āĻ¯āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻšāĨ¤ āĻāĻ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻĒā§āĻ°āĻāĻžāĻļ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§:
Q=I2Rt
Where,
I=current in amperes
R=resistance in ohms
t=time in seconds.
This formula gives the thermal energy produced by current flow in units called joules (see Glossary for definition).
Until recently joules were converted to units of heat called calories, by dividing by 4.2, but in the SI system this is no longer necessary.
āĻāĻ āĻ¸ā§āĻ¤ā§āĻ°āĻāĻŋ āĻā§āĻ˛ āĻ¨āĻžāĻŽāĻ āĻāĻāĻ¨āĻŋāĻā§ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻš āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻ¤āĻžāĻĒ āĻļāĻā§āĻ¤āĻŋ āĻĻā§āĻ¯āĻŧ (āĻ¸āĻāĻā§āĻāĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻļāĻŦā§āĻĻāĻā§āĻˇ āĻĻā§āĻā§āĻ¨)āĨ¤ āĻ¸āĻŽā§āĻĒā§āĻ°āĻ¤āĻŋ āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻā§āĻ˛āĻā§āĻ˛āĻŋāĻā§ 4.2 āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻžāĻ āĻāĻ°ā§ āĻā§āĻ¯āĻžāĻ˛ā§āĻ°āĻŋ āĻ¨āĻžāĻŽāĻ āĻ¤āĻžāĻĒā§āĻ° āĻāĻāĻā§ āĻ°ā§āĻĒāĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻāĻ°āĻž āĻšāĻ¯āĻŧā§āĻāĻŋāĻ˛, āĻāĻŋāĻ¨ā§āĻ¤ā§ SI āĻ¸āĻŋāĻ¸ā§āĻā§āĻŽā§ āĻāĻāĻŋ āĻāĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻ¨ā§āĻāĨ¤
Electrical energy and power
Energy in any system is the ability to do work. Energy may exist in many forms, e.g. heat, sound, magnetic, electromagnetic, mechanical, chemical and nuclear energy. Although energy cannot be created or destroyed (except in nuclear reactions), it can be converted from one form to another ,e.g. mechanical to electrical in a dynamo or electric alto mechanical in an electric motor.
The amount of work done in a system depends upon the magnitude of the force applied and the effect it has. With an electric current, the electrons moved, measured in coulombs:
āĻ¯ā§ āĻā§āĻ¨ āĻ¸āĻŋāĻ¸ā§āĻā§āĻŽā§ āĻļāĻā§āĻ¤āĻŋ āĻšāĻ˛ āĻāĻžāĻ āĻāĻ°āĻžāĻ° āĻā§āĻˇāĻŽāĻ¤āĻžāĨ¤ āĻļāĻā§āĻ¤āĻŋ āĻ āĻ¨ā§āĻ āĻ°ā§āĻĒā§ āĻŦāĻŋāĻĻā§āĻ¯āĻŽāĻžāĻ¨ āĻĨāĻžāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯ā§āĻŽāĻ¨ āĻ¤āĻžāĻĒ, āĻļāĻŦā§āĻĻ, āĻā§āĻŽā§āĻŦāĻ, āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ, āĻ¯āĻžāĻ¨ā§āĻ¤ā§āĻ°āĻŋāĻ, āĻ°āĻžāĻ¸āĻžāĻ¯āĻŧāĻ¨āĻŋāĻ āĻāĻŦāĻ āĻĒāĻžāĻ°āĻŽāĻžāĻŖāĻŦāĻŋāĻ āĻļāĻā§āĻ¤āĻŋāĨ¤ āĻ¯āĻĻāĻŋāĻ āĻļāĻā§āĻ¤āĻŋ āĻ¤ā§āĻ°āĻŋ āĻŦāĻž āĻ§ā§āĻŦāĻāĻ¸ āĻāĻ°āĻž āĻ¯āĻžāĻ¯āĻŧ āĻ¨āĻž (āĻĒāĻžāĻ°āĻŽāĻžāĻŖāĻŦāĻŋāĻ āĻŦāĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻŦā§āĻ¯āĻ¤ā§āĻ¤), āĻāĻāĻŋ āĻāĻ āĻĢāĻ°ā§āĻŽ āĻĨā§āĻā§ āĻ āĻ¨ā§āĻ¯ āĻĢāĻ°ā§āĻŽā§ āĻ°ā§āĻĒāĻžāĻ¨ā§āĻ¤āĻ°āĻŋāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯ā§āĻŽāĻ¨ āĻāĻāĻāĻŋ āĻĄāĻžāĻ¯āĻŧāĻ¨āĻžāĻŽā§āĻ¤ā§ āĻ¯āĻžāĻ¨ā§āĻ¤ā§āĻ°āĻŋāĻ āĻĨā§āĻā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻŦāĻž āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻŽā§āĻāĻ°ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻ āĻ˛ā§āĻā§ āĻ¯āĻžāĻ¨ā§āĻ¤ā§āĻ°āĻŋāĻāĨ¤ āĻāĻāĻāĻŋ āĻ¸āĻŋāĻ¸ā§āĻā§āĻŽā§ āĻ¸āĻŽā§āĻĒāĻ¨ā§āĻ¨ āĻāĻžāĻā§āĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻļāĻā§āĻ¤āĻŋ āĻāĻŦāĻ āĻāĻ° āĻĒā§āĻ°āĻāĻžāĻŦā§āĻ° āĻāĻĒāĻ° āĻ¨āĻŋāĻ°ā§āĻāĻ° āĻāĻ°ā§āĨ¤ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻ¸āĻžāĻĨā§, āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§āĻ˛āĻŋ āĻ¸āĻ°āĻžāĻ¨ā§ āĻšāĻ¯āĻŧ, āĻā§āĻ˛āĻŽā§āĻŦā§ āĻĒāĻ°āĻŋāĻŽāĻžāĻĒ āĻāĻ°āĻž āĻšāĻ¯āĻŧ:
W=ExC
Where W=work done (joules)
E=EMF (volts)
C=Quantity of electricity (coulombs).
Power is rate of doing work: to calculate this, time has to be considered. If an EMF of I volt moves I coulomb of electrons in I second, then the power of the system is I watt. A rate of flow of electric charge of coulomb in I second is I ampere.
Therefore the electrical power in a circuit can be calculated by multiplying the EMF and the current:
Power (watts)=EMF (volts) x current (amps)
The kilowatt-hour is the British unit of electrical energy. It is the energy needed to maintain an output of 1000 watts of power for I hour, and is used when calculating electricity bills.
āĻā§āĻˇāĻŽāĻ¤āĻž āĻšāĻ˛ āĻāĻžāĻā§āĻ° āĻāĻ°āĻžāĻ° āĻšāĻžāĻ°: āĻāĻāĻŋ āĻāĻŖāĻ¨āĻž āĻāĻ°āĻ¤ā§, āĻ¸āĻŽāĻ¯āĻŧ āĻŦāĻŋāĻŦā§āĻāĻ¨āĻž āĻāĻ°āĻ¤ā§ āĻšāĻŦā§āĨ¤ āĻ¯āĻĻāĻŋ I āĻā§āĻ˛ā§āĻā§āĻ° āĻāĻāĻāĻŋ EMF I āĻ¸ā§āĻā§āĻ¨ā§āĻĄā§ I āĻā§āĻ˛āĻŽā§āĻŦ āĻāĻ˛ā§āĻāĻā§āĻ°āĻ¨āĻā§ āĻ¸āĻ°āĻŋāĻ¯āĻŧā§ āĻĻā§āĻ¯āĻŧ, āĻ¤āĻžāĻšāĻ˛ā§ āĻ¸āĻŋāĻ¸ā§āĻā§āĻŽā§āĻ° āĻļāĻā§āĻ¤āĻŋ āĻšāĻ˛ I āĻāĻ¯āĻŧāĻžāĻāĨ¤ I āĻ¸ā§āĻā§āĻ¨ā§āĻĄā§ āĻā§āĻ˛āĻŽā§āĻŦā§āĻ° āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻžāĻ°ā§āĻā§āĻ° āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻšāĻžāĻ° āĻšāĻ˛ I āĻ ā§āĻ¯āĻžāĻŽā§āĻĒāĻŋāĻ¯āĻŧāĻžāĻ°āĨ¤ āĻ¸ā§āĻ¤āĻ°āĻžāĻ āĻāĻāĻāĻŋ āĻ¸āĻžāĻ°ā§āĻāĻŋāĻā§āĻ° āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻļāĻā§āĻ¤āĻŋ EMF āĻāĻŦāĻ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨āĻā§ āĻā§āĻŖ āĻāĻ°ā§ āĻāĻŖāĻ¨āĻž āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§: āĻĒāĻžāĻāĻ¯āĻŧāĻžāĻ° (āĻāĻ¯āĻŧāĻžāĻ) = EMF (āĻā§āĻ˛ā§āĻ) x āĻāĻžāĻ°ā§āĻ¨ā§āĻ (amps) āĻāĻŋāĻ˛ā§āĻāĻ¯āĻŧāĻžāĻ-āĻāĻ¨ā§āĻāĻž āĻšāĻ˛ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻļāĻā§āĻ¤āĻŋāĻ° āĻŦā§āĻ°āĻŋāĻāĻŋāĻļ āĻāĻāĻāĨ¤ āĻāĻāĻŋ I āĻāĻ¨ā§āĻāĻžāĻ° āĻāĻ¨ā§āĻ¯ 1000 āĻāĻ¯āĻŧāĻžāĻ āĻĒāĻžāĻāĻ¯āĻŧāĻžāĻ°ā§āĻ° āĻāĻāĻāĻĒā§āĻ āĻŦāĻāĻžāĻ¯āĻŧ āĻ°āĻžāĻāĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ā§āĻ¯āĻŧ āĻļāĻā§āĻ¤āĻŋ, āĻāĻŦāĻ āĻŦāĻŋāĻĻā§āĻ¯ā§ā§ āĻŦāĻŋāĻ˛ āĻāĻŖāĻ¨āĻž āĻāĻ°āĻžāĻ° āĻ¸āĻŽāĻ¯āĻŧ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤
Magnetism
A magnet is an object which exhibits certain properties. For example, when free to rotate, it will align itself in the North-South direction. It also has the power to attract, and produce magnetism in, certain other materials.
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The molecular theory of magnetism
No matter how many times a magnet is divided, it will always present a North and a South pole. This phenomenon could conceivably be carried on down to molecular level, where it is thought that the revolving electrons produce a North and a South pole for each molecule, giving so-called ‘molecular magnets’. In a non-magnetized state, these molecular magnets are arranged in a haphazard way and cancel out one another’s effects. In the magnetized state, the molecular magnet, are ordered so that one end of the piece of metal exhibits a North pole and the other a South.
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In magnetized materials such as steel, the friction between the molecule is great and the ordered magnetic effect is retained, giving a permanent magnet. Heating or banging will, however, disrupt the order and so magnetism will be lost. In a material such as soft iron there is little friction between the molecules, although they can easily be influenced into an order pattern, this Pattern will be also lost very easily. Thus soft iron only forms temporary magnets.
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The magnetic effect of a wire carrying an electric current can be used to create an electro magnet, which exists only for as long as current flows.
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Properties of a magnet
1. Setting in a North-South direction As the Earth itself is a giant magnet, the Earth’s magnetic field will influence a suspended magnet North Pole.
2. Like magnetic poles repel one another North repels North and South repels South. Unlike magnetic poles attract one another, i.e. North attracts South and South attracts North.
3. Transmission of properties A magnet can produce properties of magnetism in suitable materials. As one pole of a bar magnet is stroked along the material all the opposite poles of the molecular magnets are attracted towards it so that the object is magnetized. The end that the magnet leaves will have the pole opposite to that used to induce the effect.
A magnet may also produce a magnetic effect in an object without influence of the magnet over the molecular magnets of the susceptible materials which produces the magnetic effect.
4. Attraction of suitable materials Magnets attract certain materials. This effect is produced by magnetic induction.
5. A magnetic field this is the area or zone of influence around a magnet in which its magnetic forces are apparent. This field may be considered as being made up of magnetic lines of force which have the following properties:
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a) They travel from North to South, which is the path a free North Pole would take.
b) They attempt to take the shortest route possible but repel one another so then
c) They travel more easily through some materials, e.g. metals, than through others.
If traced using iron filings, the magnetic field of a bar magnet looks similar. The field between two unlike poles is concentrated.
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Magnetic effect of an electric current
The fact that an electric current flowing along a wire sets up a magnetic field can be shown by placing the wire close to a magnetic compass needle and watching the deflection produced as the current is turned on. The magnetic lines of force are arranged in a definite and constant direction: when looking along the wire from the positive (+) end towards the negative(-) end, i.e. in the direction of the conventional electric current, the lines of force go clockwise
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If the wire carrying a current is wound into a coil, the magnetic effect becomes concentrated so that one end of the coil presents a North Pole and the other a South . An electromagnet consists of a coil of wire wound onto a soft iron bar. When a current passes through the wire it magnetizes the bar by induction. The magnetic field produced reinforces that of the coil and the resultant field is very strong. As soon as the current is switched off, the magnetic effect is lost.
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Electromagnetic induction
Electromagnetic induction is the means by which electricity is produced from magnetism (and vice-versa). It is the result of interaction between a conductor and magnetic lines of force an EMF is produced in the conductor by the magnetic lines of force surrounding a magnet, without contact between the magnet and the conductor.
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The factors essential to electromagnetic induction are-
I. A conductor.
2. Magnetic lines of force.
3. Relative movement of I and 2.
If the conductor is part of a closed circuit, the magnetic lines of force produce an EMF which causes movement of the electrons in the conductor. This can be shown with an ammeter connected across a coil of wire. When a magnet is moved into the coil, the magnetic lines of force cut across the conducting wire of the coil and cause movement of electrons in the coil. These electrons repel adjacent electrons and so on, and a current is set up in the circuit. Movement of the ammeter need, Indicating current flow, will be seen only when either the magnet or the coil is moving. If the magnetic lines of force are stationary relative to the coil of wire, there is no induction. Electromagnetic induction also occurs if the magnetic field used is that surrounding a coil of wire. The principles are the same; there must be movement of the magnetic field relative to the conductor. This maybe achieved by using an alternating current in the primary coil which causes the magnetic field to build up, fall, then build up in the opposite direction, then fall, etc. An alternating current is represented. The current builds up to a maximum positive value and then falls to zero. It then drops to a maximum negative value before returning to zero. This rise and fall of current produces movement of the magnetic lines of force.
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In practice, the conductor in which the EMF is induced is usually a coil of wire, while the magnetic field used to induce the EMF is that of a permanent magnet or a current-carrying coil of wire. Movement of one of these relative to the other is achieved either by spinning the conductor in the magnetic field, as in a dynamo, or by varying the intensity of current in the coil of wire, as in a transformer.
The direction of the induced EMF
The direction in which the magnetic lines of force move relative to the conductor affects the direction in which the induced current flows. This can again be seen by using the bar magnet and coil shown as the magnet is moved into the coil ,the ammeter needle is deflected in one direction. As it is with drawn, deflection occurs in the opposite direction, thus demonstrating that the direction of current flow changes with a reversal of movement of the magnetic field.
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The same is true when the inducing magnetic field is that surrounding a current-carrying coil of wire. As the current rises and the magnetic lines of force move out, thus cutting the conductor, deflection of the ammeter needle occurs in one direction. As the current drops to zero, the magnetic lines of force move back in force is now reversed, and so is the direction of the induced current indicated by the ammeter. This result is often quoted as Lenz’s Law, which states that the direction of the induced EMF is such that it tends to oppose the force producing it.
The strength of the induced EMF
This depends upon two factors the rate of change of the magnetic field and the inductance of the conductor.
I. The rate of change of the magnetic field The more rapid the movement of the permanent magnet and the stronger the magnet used, the greater is the rate at which the magnetic lines of force cut the conductor and the greater the induced EMF. In the case of a current-carrying coil of wire, if the frequency of current is increased (and hence the rate of rise and collapse of the magnetic field),a stronger EMF is induced.
2. The inductance of the conductor Inductance is the ability of a conductor to have a current induced in it. Inductance is measured inhenries (see Glossary). Inductance is constant for any particular conductor, but high inductance can be designed into a conducting coil by incorporating the following principles.
a. Using many turns of wire in the coil.
b. Placing the turns close together.
c. Winding the coil onto a soft iron core.
This ensures that the magnetic lines of force cut the maximum number of coils in the conductor and thus induce a strong EMF into it.
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Mutual induction
Mutual induction is said to occur when an EMF is induced in an adjacent conductor by the magnetic field set up around a coil of wire carrying a varying current.
Self-induction
Self-induction occurs within a coil carrying a varying current. A magnetic field is generated around each turn of wire, As the current increases, the magnetic lines of force move out, cutting adjacent turns of wire and thus inducing an EMF in them.
Following Lenzâs law, the direction of this induced EMF will be opposite to the force(or current)producing it. Therefore the induced EMF is in the opposite direction the main current and so opposes its rise. Self-induced EMFs of this type are therefore called ‘back EMFs’.
A similar sequence of events occurs when the primary current starts to fall. The magnetic field now collapses and the lines of force move back in, cutting adjacent turns of wire but in the opposite direction from before. Consequently the induced EMF is also in the opposite direction and flows forwards as a ‘forward EMF’.
The overall effect of back and forward EMFs is to retard the rate of current rise and prolong its fall. With alternating currents, especially at high frequencies, inductance acts in some respects lke electrical resistance in determining the current flow for a particular applied voltage.
āĻĒāĻžāĻ°āĻ¸ā§āĻĒāĻ°āĻŋāĻ āĻāĻ¨āĻ¯āĻŧāĻ¨ āĻĒāĻžāĻ°āĻ¸ā§āĻĒāĻ°āĻŋāĻ āĻāĻŦā§āĻļ āĻāĻāĻ¤ā§ āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧ āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ EMF āĻāĻāĻāĻŋ āĻ¸āĻāĻ˛āĻā§āĻ¨ āĻĒāĻ°āĻŋāĻŦāĻžāĻšā§āĻ¤ā§ āĻĒā§āĻ°āĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻā§āĻŽā§āĻŦāĻ āĻā§āĻˇā§āĻ¤ā§āĻ° āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻāĻāĻŋ āĻ¤āĻžāĻ°ā§āĻ° āĻāĻāĻāĻŋ āĻā§āĻŖā§āĻĄāĻ˛ā§āĻ° āĻāĻžāĻ°āĻĒāĻžāĻļā§ āĻ¸ā§āĻ āĻāĻ°āĻž āĻāĻāĻāĻŋ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻŦāĻšāĻ¨ āĻāĻ°ā§āĨ¤ āĻ¸ā§āĻŦ-āĻāĻŦā§āĻļ āĻ¸ā§āĻŦ-āĻāĻ¨ā§āĻĄāĻžāĻāĻļāĻ¨ āĻāĻāĻāĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻāĻā§ āĻ¯āĻž āĻāĻāĻāĻŋ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻŦāĻšāĻ¨ āĻāĻ°ā§āĨ¤ āĻ¤āĻžāĻ°ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻŦāĻžāĻāĻā§āĻ° āĻāĻžāĻ°āĻĒāĻžāĻļā§ āĻāĻāĻāĻŋ āĻā§āĻŽā§āĻŦāĻ āĻā§āĻˇā§āĻ¤ā§āĻ° āĻ¤ā§āĻ°āĻŋ āĻšāĻ¯āĻŧ, āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻŦāĻžāĻĄāĻŧāĻžāĻ° āĻ¸āĻžāĻĨā§ āĻ¸āĻžāĻĨā§ āĻŦāĻ˛ā§āĻ° āĻā§āĻŽā§āĻŦāĻ āĻ°ā§āĻāĻžāĻā§āĻ˛āĻŋ āĻ¸āĻ°ā§ āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻžāĻ°ā§āĻ° āĻ¸āĻāĻ˛āĻā§āĻ¨ āĻŦāĻžāĻāĻāĻā§āĻ˛āĻŋ āĻā§āĻā§ āĻĻā§āĻ¯āĻŧ āĻāĻŦāĻ āĻāĻāĻāĻžāĻŦā§ āĻ¤āĻžāĻĻā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻāĻāĻāĻŋ EMF āĻĒā§āĻ°āĻ°ā§āĻāĻŋāĻ¤ āĻāĻ°ā§āĨ¤ Lenz-āĻāĻ° āĻāĻāĻ¨ āĻ āĻ¨ā§āĻ¸āĻ°āĻŖ āĻāĻ°ā§, āĻāĻ āĻĒā§āĻ°āĻ°ā§āĻāĻŋāĻ¤ EMF-āĻāĻ° āĻĻāĻŋāĻāĻ¨āĻŋāĻ°ā§āĻĻā§āĻļ āĻšāĻŦā§ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻŦāĻ˛ (āĻŦāĻž āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨) āĻāĻ° āĻŦāĻŋāĻĒāĻ°ā§āĻ¤āĨ¤ āĻ¤āĻžāĻ āĻĒā§āĻ°āĻ°ā§āĻāĻŋāĻ¤ EMF āĻŽā§āĻ˛ āĻ¸ā§āĻ°ā§āĻ¤ā§āĻ° āĻŦāĻŋāĻĒāĻ°ā§āĻ¤ āĻĻāĻŋāĻā§ āĻĨāĻžāĻā§ āĻāĻŦāĻ āĻ¤āĻžāĻ āĻāĻ° āĻāĻ¤ā§āĻĨāĻžāĻ¨ā§āĻ° āĻŦāĻŋāĻ°ā§āĻ§āĻŋāĻ¤āĻž āĻāĻ°ā§āĨ¤ āĻāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻ¸ā§āĻŦ-āĻĒā§āĻ°āĻ°ā§āĻāĻŋāĻ¤ āĻāĻāĻŽāĻāĻĢāĻā§ āĻ¤āĻžāĻ 'āĻŦā§āĻ¯āĻžāĻ āĻāĻāĻŽāĻāĻĢ' āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧāĨ¤ āĻāĻāĻ¨āĻžāĻ° āĻ āĻ¨ā§āĻ°ā§āĻĒ āĻā§āĻ°āĻŽ āĻāĻā§ āĻ¯āĻāĻ¨ āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻĒāĻĄāĻŧāĻž āĻļā§āĻ°ā§ āĻšāĻ¯āĻŧāĨ¤ āĻā§āĻŽā§āĻŦāĻ āĻā§āĻˇā§āĻ¤ā§āĻ°āĻāĻŋ āĻāĻāĻ¨ āĻā§āĻā§ āĻĒāĻĄāĻŧā§āĻā§ āĻāĻŦāĻ āĻŦāĻ˛ āĻ°ā§āĻāĻžāĻā§āĻ˛āĻŋ āĻĒāĻŋāĻāĻ¨ā§ āĻāĻ˛ā§ āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻžāĻ°ā§āĻ° āĻ¸āĻāĻ˛āĻā§āĻ¨ āĻŦāĻžāĻāĻāĻā§āĻ˛āĻŋ āĻā§āĻā§ āĻĻā§āĻ¯āĻŧ āĻ¤āĻŦā§ āĻāĻā§āĻ° āĻĨā§āĻā§ āĻŦāĻŋāĻĒāĻ°ā§āĻ¤ āĻĻāĻŋāĻā§āĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻĒā§āĻ°āĻ°ā§āĻāĻŋāĻ¤ EMF āĻŦāĻŋāĻĒāĻ°ā§āĻ¤ āĻĻāĻŋāĻā§āĻ āĻĨāĻžāĻā§ āĻāĻŦāĻ 'āĻĢāĻ°ā§āĻ¯āĻŧāĻžāĻ°ā§āĻĄ āĻāĻāĻŽāĻāĻĢ' āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻ¸āĻžāĻŽāĻ¨ā§āĻ° āĻĻāĻŋāĻā§ āĻĒā§āĻ°āĻŦāĻžāĻšāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻŦā§āĻ¯āĻžāĻ āĻāĻŦāĻ āĻĢāĻ°āĻāĻ¯āĻŧāĻžāĻ°ā§āĻĄ EMF-āĻāĻ° āĻ¸āĻžāĻŽāĻā§āĻ°āĻŋāĻ āĻĒā§āĻ°āĻāĻžāĻŦ āĻšāĻ˛ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻŦā§āĻĻā§āĻ§āĻŋāĻ° āĻšāĻžāĻ°āĻā§ āĻ¸ā§āĻĨāĻāĻŋāĻ¤ āĻāĻ°āĻž āĻāĻŦāĻ āĻāĻ° āĻĒāĻ¤āĻ¨āĻā§ āĻĻā§āĻ°ā§āĻāĻžāĻ¯āĻŧāĻŋāĻ¤ āĻāĻ°āĻžāĨ¤ āĻŦāĻŋāĻāĻ˛ā§āĻĒ āĻ¸ā§āĻ°ā§āĻ¤ā§āĻ° āĻ¸āĻžāĻĨā§, āĻŦāĻŋāĻļā§āĻˇ āĻāĻ°ā§ āĻāĻā§āĻ āĻĢā§āĻ°āĻŋāĻā§āĻ¯āĻŧā§āĻ¨ā§āĻ¸āĻŋāĻ¤ā§, āĻāĻ¨ā§āĻĄāĻžāĻāĻā§āĻ¯āĻžāĻ¨ā§āĻ¸ āĻāĻŋāĻā§ āĻā§āĻˇā§āĻ¤ā§āĻ°ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ā§āĻ° āĻŽāĻ¤ā§ āĻāĻžāĻ āĻāĻ°ā§ āĻāĻāĻāĻŋ āĻ¨āĻŋāĻ°ā§āĻĻāĻŋāĻˇā§āĻ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻā§āĻ¤ āĻā§āĻ˛ā§āĻā§āĻā§āĻ° āĻāĻ¨ā§āĻ¯ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻ°āĻŦāĻžāĻš āĻ¨āĻŋāĻ°ā§āĻ§āĻžāĻ°āĻŖā§āĨ¤
Eddy currents
Any conductor lying within a varying magnetic field has an EMF induced in 1t. If the conductor is solid, the magnetic lines of force passing through it set up circular currents called eddy currents. The seedy currents are at right angles to the magnetic lines of force and will produce a hearing effect in accordance with Joule’s law (see p. I4). In most electrical apparatus eddy currents are unwanted, and are prevented by laminating the conductor, i.e. cutting it into layers and insulating each layer from the others. However, eddy currents can be used to produce a heating effect in the patient’s tissues, using the magnetic field surrounding an inductothermy cable .
The electromagnetic spectrum
The spectrum contains the following kinds of radiation, which are distinguished by their different wavelengths (I nm=10=10-9I pm8U=10-12m):
Radio waves o.1 mm-10o km Ultra-violet Io nm-400 nm
Infra-red 750 nm-0.4 mm X-rays Gamma rays o.o1 pm-1oo nm
Visible light 400 nm-750 nm
Wavelength
Wavelength is the distance between a point on one electromagnetic wave and exactly the same point on the next wave. This maybe very long, wireless waves being measured in hundreds of metres, or very short, very small ultra-violet waves being measured in nanometres.
Velocity
Velocity is constant for all forms of electromagnetic waves, being 3x108m (300000 km) per second, i.e. the speed of light.
Frequency
Frequency is the number of complete waves passing any fixed point in one second. A mathematical relationship exists between wavelength, velocity and frequency as velocity is constant for all electromagnetic waves. For example: if we have a set distance across which two different electromagnetic waves have to pass, the time taken for each wave to cross will be the same. If one wave has a long wavelength it will require only a low frequency, where as a wave with a short wavelength will require a high frequency . Hence there is an inverse relationship between wavelength and frequency for electromagnetic waves.
Laws governing radiation
Infra-red, visible and ultra-violet waves travel in straight lines until they encounter a different medium, when they may be transmitted, reflected or absorbed.
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Reflection
A normal is a line drawn perpendicular to the surface of a medium at the point where an electromagnetic wave strikes. Angles of reflection or refraction are measured between the electromagnetic ray and the normal.
Reflection occurs when an electromagnetic wave encounters a medium which will not transmit it. In this case the ray is reflected back in the same plane such that the angle between the incident ray and the normal equals the angle between the reflected ray and the normal .
If the incident angle is o° (i,e. the radiation strikes the surface at right-angles) then the angle of reflection is also o° (the incident ray ,normal and reflected ray all coincide).
The laws of reflection are employed in the design of reflectors used for the re-direction of rays towards an appropriate target. In infra-red and ultra-violet lamps a parabolic reflector is normally used, as this avoids the danger of the concentration of rays which occurs with some shapes of reflector. A parabolic reflector collects all the rays travelling in an inappropriate direction and reflects them from its surface so that they eventually all emerge parallel It should be remembered however, that the majority of rays emitted forwards from these lamps come directly from the source and so diverge; only a small proportion are reflected in the above way.
Internal reflection
Internal reflection occurs when the angle of incidence of a ray as it strikes an interface between two media is such that instead of being transmitted it is reflected. This happens at angles of incidence above a certain critical angle. Internal reflection in quartz is used to cause ultra-violet light to pass down a specially cut quartz rod and be emitted only from the end. This method of application is used when ultra-violet is applied to an internal cavity or an infected sinus (seepp.170=I7I).
Refraction
Refraction occurs when electromagnetic rays are transmitted from one medium to another with an angle of incidence greater than zero. Rays with a zero angle of incidence, i.e. striking the surface at right-angles, continue in the same straight line.
Refraction causes the ray to be deflected from its original course by an amount depending on the media involved and the angle of in cadence (Snell’s law). When passing into an optically denser medium, the ray is refracted towards the normal. When passing into a less dense medium, it is refracted away from the normal. Refraction is important when using hydrotherapy as a form of treatment, as the refraction of rays passing from water to air makes the position of objects in the water (e.g. steps) difficult to assess. The same is true when using water as a coupling medium for ultra-sound.
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Absorption
When electromagnetic rays strike a new medium they may be absorbed and thus produce an effect (law of Grotthus).The proportion of rays absorbed depends upon the wavelength of the rays, the nature of the medium and the angle of incidence.
A filter is a medium which will absorb some electromagnetic waves whilst allowing others to pass. Window glass allows visible light and infra-red rays to pass while absorbing (filtering out) ultra-violet rays. Water absorbs infra-red but allows visible and ultra-violet to pass. X-rays are passed through soft tissues onto a photographic plate, but area bsorbed to a greater extent by bone. Cellophane absorbs short ultra-violet rays while allowing long ones to pass.
The angle at which the rays strike the surface also affects the proportion absorbed. The way in which the angle affects the intensity of radiation at the surface is often quoted as the cosine law, which states that the intensity of rays at a surface varies with the cosine of the angle between the incident ray and the normal.
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Consultation of cosine tables will show that:
Cosine of 90=0
Cosine of o=I
In practical terms this means that if the angle of incidence is 90then no rays will be absorbed, as they will be travelling parallel to the surface. If the angle of incidence is o° then the rays are striking the surface so as to make a right angle with it, and the maximum number will be absorbed. The closer to zero the angle of incidence is, the more rays will be absorbed. Therefore, when applying ultra-violet and infra-red radiation, great efforts should be made to ensure that the maximum number of rays strike the surface at 90(angle of incidence=o) for the most effective treatment.
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Electromagnetic waves being produced from a point source also obey the law of inverse squares. This states that the intensity of rays from a point source varies inversely with the square of the distance from that point source. Rays produced from a point source diverge from one another at a uniform rate. If, for example, a set number of rays cover a square with sides of 1 m at a distance of 1m from the source, at 2 m they will cover a square with sides 2 m long the first square has an area of I ãĄ, whereas the second has an area of 4ãĄ. As the same number of rays are striking both squares (ignoring atmospheric absorption), then the intensity of radiation on the smaller square will be four times that on the larger at any point.
As ultra-violet lamps and some infra-red lamps act almost as point sources, the rays they generate obey the law of inverse squares. In practical terms this means that the closer a patient is to the source, the greater is the intensity of the radiation being received at any one point on the skin; the further away, the less the intensity. In certain situations moving the lamp closer will allow a shorter dose to be given. if a dose of ultra-violet of 60 sec at 1oo cm produces a certain effect ,the same effect could be obtained in I5 sec at a distance of 50 cm, i.e.one quarter of the time at half the distance.
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Basic electrical components
The static transformer
An electrical transformer works on the principles of electro magnetic induction and is used to alter voltage or to render a current earth-free.
Construction the transformer consists of two coils of insulated wire wound onto a laminated soft-iron frame. The two coils may be wound on top of one another or on opposite sides of the frame.
Working an alternating current is passed through the primary coil and this sets up a varying magnetic field which cuts the secondary coil. By electromagnetic induction an EMF is induced into the secondary circuit
Functions of the transformer
To alter the voltage of an alternating current The EMF induced in the secondary coil depends upon the number of turns of wire it has relative to the primary coil:
I. If both primary and secondary coils have the same number of turns, then the voltage in each will be the same. This is an example of an even-ratio transformer.
2. If the secondary coil has fewer turns than the primary then the EMF or voltage in the secondary will be less than that in the primary. it is stepped down. Such an arrangement produces a step-down transformer. For example, if the primary coil has 120 turns and an applied voltage of root volts, and the secondary has 60 turns, then the voltage in the secondary will be stepped down to 50 volts.
3. If the secondary coil has more turns than the primary, the voltage developed in the secondary will be increased or stepped up. We then have a step-up transformer. For example, if the primary, coil still has 120turns and an EMF of 1oo volts, and the secondary has 240 turns, then the EMF developed in the secondary coil will be 200 volts.
āĻ¸ā§āĻā§āĻ¯āĻžāĻāĻŋāĻ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ° āĻāĻāĻāĻŋ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ° āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§ āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻāĻ¨ā§āĻĄāĻžāĻāĻļāĻ¨ā§āĻ° āĻ¨ā§āĻ¤āĻŋāĻ° āĻāĻĒāĻ° āĻāĻžāĻ āĻāĻ°ā§ āĻāĻŦāĻ āĻā§āĻ˛ā§āĻā§āĻ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻ°āĻ¤ā§ āĻŦāĻž āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻĨāĻŋāĻŦā§-āĻŽā§āĻā§āĻ¤ āĻ°ā§āĻ¨ā§āĻĄāĻžāĻ° āĻāĻ°āĻ¤ā§ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻ¨āĻŋāĻ°ā§āĻŽāĻžāĻŖ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°āĻāĻŋ āĻāĻāĻāĻŋ āĻ¸ā§āĻ¤āĻ°āĻŋāĻ¤ āĻ¨āĻ°āĻŽ-āĻ˛ā§āĻšāĻžāĻ° āĻĢā§āĻ°ā§āĻŽā§āĻ° āĻāĻĒāĻ° āĻāĻ¤ā§āĻ¤āĻžāĻĒāĻ¯ā§āĻā§āĻ¤ āĻ¤āĻžāĻ°ā§āĻ° āĻā§āĻˇāĻ¤ āĻĻā§āĻāĻŋ āĻāĻ¯āĻŧā§āĻ˛ āĻ¨āĻŋāĻ¯āĻŧā§ āĻāĻ āĻŋāĻ¤āĨ¤ āĻĻā§āĻāĻŋ āĻāĻ¯āĻŧā§āĻ˛ āĻāĻā§ āĻ āĻĒāĻ°ā§āĻ° āĻāĻĒāĻ°ā§ āĻŦāĻž āĻĢā§āĻ°ā§āĻŽā§āĻ° āĻŦāĻŋāĻĒāĻ°ā§āĻ¤ āĻĻāĻŋāĻā§ āĻā§āĻˇāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻāĻāĻŋ āĻŦāĻŋāĻāĻ˛ā§āĻĒ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻāĻžāĻ āĻāĻ°āĻž āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻ¯āĻŧā§āĻ˛ā§āĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻāĻŋ āĻāĻāĻāĻŋ āĻāĻŋāĻ¨ā§āĻ¨ āĻā§āĻŽā§āĻŦāĻ āĻā§āĻˇā§āĻ¤ā§āĻ° āĻ¸ā§āĻ āĻāĻĒ āĻāĻ°ā§ āĻ¯āĻž āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛āĻāĻŋāĻā§ āĻā§āĻā§ āĻĻā§āĻ¯āĻŧāĨ¤ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻŽā§āĻ¯āĻžāĻāĻ¨ā§āĻāĻŋāĻ āĻāĻ¨ā§āĻĄāĻžāĻāĻļāĻ¨ āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻāĻāĻŋ EMF āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻ¸āĻžāĻ°ā§āĻāĻŋāĻā§ āĻĒā§āĻ°āĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°ā§āĻ° āĻāĻžāĻ°ā§āĻ¯āĻžāĻŦāĻ˛ā§ āĻāĻāĻāĻŋ āĻŦāĻŋāĻāĻ˛ā§āĻĒ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻā§āĻ˛ā§āĻā§āĻ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻ°āĻ¤ā§ āĻā§āĻŖ āĻā§āĻŖā§āĻĄāĻ˛ā§āĻ¤ā§ āĻĒā§āĻ°āĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ EMF āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻā§āĻŖā§āĻĄāĻ˛ā§āĻ° āĻ¸āĻžāĻĒā§āĻā§āĻˇā§ āĻ¤āĻžāĻ°ā§āĻ° āĻŦāĻžāĻāĻāĻā§āĻ˛āĻŋāĻ° āĻ¸āĻāĻā§āĻ¯āĻžāĻ° āĻāĻĒāĻ° āĻ¨āĻŋāĻ°ā§āĻāĻ° āĻāĻ°ā§: I. āĻ¯āĻĻāĻŋ āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻŦāĻ āĻŽāĻžāĻ§ā§āĻ¯āĻŽāĻŋāĻ āĻāĻāĻ¯āĻŧ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻāĻāĻ āĻ¸āĻāĻā§āĻ¯āĻ āĻŦāĻžāĻāĻ āĻĨāĻžāĻā§, āĻ¤āĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋāĻ¤ā§ āĻā§āĻ˛ā§āĻā§āĻ āĻāĻāĻ āĻšāĻŦā§āĨ¤ āĻāĻāĻŋ āĻāĻāĻāĻŋ āĻ¸āĻŽāĻžāĻ¨-āĻ āĻ¨ā§āĻĒāĻžāĻ¤ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°ā§āĻ° āĻāĻĻāĻžāĻšāĻ°āĻŖāĨ¤ 2. āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻ¯āĻĻāĻŋ āĻĒā§āĻ°āĻžāĻāĻŽāĻžāĻ°āĻŋāĻ° āĻĨā§āĻā§ āĻāĻŽ āĻŦāĻžāĻāĻ āĻĨāĻžāĻā§ āĻ¤āĻžāĻšāĻ˛ā§ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋāĻ¤ā§ EMF āĻŦāĻž āĻā§āĻ˛ā§āĻā§āĻ āĻĒā§āĻ°āĻžāĻāĻŽāĻžāĻ°āĻŋāĻ° āĻĨā§āĻā§ āĻāĻŽ āĻšāĻŦā§āĨ¤ āĻāĻāĻž āĻĒāĻĻāĻ¤ā§āĻ¯āĻžāĻ āĻāĻ°āĻž āĻšāĻ¯āĻŧ. āĻāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻŦā§āĻ¯āĻŦāĻ¸ā§āĻĨāĻž āĻāĻāĻāĻŋ āĻ¸ā§āĻā§āĻĒ-āĻĄāĻžāĻāĻ¨ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ° āĻāĻ¤ā§āĻĒāĻžāĻĻāĻ¨ āĻāĻ°ā§āĨ¤ āĻāĻĻāĻžāĻšāĻ°āĻŖāĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻ¯āĻĻāĻŋ āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻ¯āĻŧā§āĻ˛ā§ 120āĻāĻŋ āĻāĻžāĻ°ā§āĻ¨ āĻĨāĻžāĻā§ āĻāĻŦāĻ āĻ°ā§āĻ āĻā§āĻ˛ā§āĻā§āĻ° āĻāĻāĻāĻŋ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻā§āĻ¤ āĻā§āĻ˛ā§āĻā§āĻ āĻĨāĻžāĻā§ āĻāĻŦāĻ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋāĻ¤ā§ 60āĻāĻŋ āĻāĻžāĻ°ā§āĻ¨ āĻĨāĻžāĻā§, āĻ¤āĻžāĻšāĻ˛ā§ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋāĻ¤ā§ āĻā§āĻ˛ā§āĻā§āĻāĻāĻŋ 50 āĻā§āĻ˛ā§āĻā§ āĻ¨ā§āĻŽā§ āĻāĻ¸āĻŦā§āĨ¤ 3. āĻ¯āĻĻāĻŋ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻĒā§āĻ°āĻžāĻāĻŽāĻžāĻ°ā§āĻ° āĻĨā§āĻā§ āĻŦā§āĻļāĻŋ āĻŦāĻžāĻāĻ āĻĨāĻžāĻā§, āĻ¤āĻžāĻšāĻ˛ā§ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋāĻ¤ā§ āĻŦāĻŋāĻāĻļāĻŋāĻ¤ āĻā§āĻ˛ā§āĻā§āĻ āĻŦāĻžāĻĄāĻŧāĻžāĻ¨ā§ āĻšāĻŦā§ āĻŦāĻž āĻ§āĻžāĻĒā§ āĻ§āĻžāĻĒā§ āĻŦā§āĻĻā§āĻ§āĻŋ āĻĒāĻžāĻŦā§āĨ¤ āĻāĻŽāĻ°āĻž āĻ¤āĻžāĻ°āĻĒāĻ° āĻāĻāĻāĻŋ āĻ¸ā§āĻā§āĻĒ āĻāĻĒ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ° āĻāĻā§. āĻāĻĻāĻžāĻšāĻ°āĻŖāĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻ¯āĻĻāĻŋ āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻāĻāĻ¨āĻ 120 āĻāĻžāĻ°ā§āĻ¨ āĻāĻŦāĻ 1oo āĻā§āĻ˛ā§āĻā§āĻ° āĻāĻāĻāĻŋ EMF āĻĨāĻžāĻā§ āĻāĻŦāĻ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋāĻ¤ā§ 240 āĻāĻžāĻ°ā§āĻ¨ āĻĨāĻžāĻā§, āĻ¤āĻžāĻšāĻ˛ā§ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻŦāĻŋāĻāĻļāĻŋāĻ¤ EMF āĻšāĻŦā§ 200 āĻā§āĻ˛ā§āĻāĨ¤
It is important to note that the electrical power in both primary and secondary coils is the¡ same. Power is measured in watts (watts=voIts x amps), so the quantity watts x amps must be the same for the primary and the secondary coils, i.e. any change in voltage must be accompanied by a change in current. For example, Which shows a step-down transformer, if the voltage is halved in the secondary coil, the current. must be doubled. For the step-up transformer shown where the voltage in the secondary coil is doubled, the current is halved. However, these are idealized situations which ignore power loss in the transformer.
āĻāĻāĻŋ āĻ˛āĻā§āĻˇ āĻāĻ°āĻž āĻā§āĻ°ā§āĻ¤ā§āĻŦāĻĒā§āĻ°ā§āĻŖ āĻ¯ā§ āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻŦāĻ āĻŽāĻžāĻ§ā§āĻ¯āĻŽāĻŋāĻ āĻāĻāĻ¯āĻŧ āĻāĻ¯āĻŧā§āĻ˛ā§āĻ° āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻļāĻā§āĻ¤āĻŋ āĻāĻāĻāĨ¤ āĻĒāĻžāĻāĻ¯āĻŧāĻžāĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻĒ āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻāĻ¯āĻŧāĻžāĻā§ (watts=voIts x amps), āĻ¤āĻžāĻ āĻĒā§āĻ°āĻžāĻāĻŽāĻžāĻ°āĻŋ āĻāĻŦāĻ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§āĻ° āĻāĻ¨ā§āĻ¯ āĻāĻ¯āĻŧāĻžāĻ x amps āĻāĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖ āĻ āĻŦāĻļā§āĻ¯āĻ āĻāĻāĻ āĻšāĻ¤ā§ āĻšāĻŦā§, āĻ āĻ°ā§āĻĨāĻžā§ āĻā§āĻ˛ā§āĻā§āĻā§āĻ° āĻ¯ā§āĻā§āĻ¨ā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻ āĻŦāĻļā§āĻ¯āĻ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ā§āĻ° āĻ¸āĻžāĻĨā§ āĻšāĻ¤ā§ āĻšāĻŦā§āĨ¤ āĻāĻĻāĻžāĻšāĻ°āĻŖāĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻ, āĻ¯āĻž āĻāĻāĻāĻŋ āĻ¸ā§āĻā§āĻĒ-āĻĄāĻžāĻāĻ¨ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ° āĻĻā§āĻāĻžāĻ¯āĻŧ, āĻ¯āĻĻāĻŋ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§ āĻā§āĻ˛ā§āĻā§āĻ āĻ āĻ°ā§āĻ§ā§āĻ āĻšāĻ¯āĻŧ, āĻāĻžāĻ°ā§āĻ¨ā§āĻāĨ¤ āĻĻā§āĻŦāĻŋāĻā§āĻŖ āĻāĻ°āĻž āĻāĻŦāĻļā§āĻ¯āĻāĨ¤ āĻ āĻĻā§āĻāĻžāĻ¨ā§ āĻ¸ā§āĻā§āĻĒ-āĻāĻĒ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°ā§āĻ° āĻāĻ¨ā§āĻ¯, āĻ¯ā§āĻāĻžāĻ¨ā§ āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛ā§āĻ° āĻā§āĻ˛ā§āĻā§āĻ āĻĻā§āĻŦāĻŋāĻā§āĻŖ āĻšāĻ¯āĻŧ, āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻ āĻ°ā§āĻ§ā§āĻ āĻšāĻ¯āĻŧāĨ¤ āĻ¯āĻžāĻāĻšā§āĻ, āĻāĻā§āĻ˛āĻŋ āĻāĻĻāĻ°ā§āĻļāĻŋāĻ āĻĒāĻ°āĻŋāĻ¸ā§āĻĨāĻŋāĻ¤āĻŋ āĻ¯āĻž āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°ā§ āĻŦāĻŋāĻĻā§āĻ¯ā§āĻ¤ā§āĻ° āĻā§āĻˇāĻ¤āĻŋ āĻāĻĒā§āĻā§āĻˇāĻž āĻāĻ°ā§āĨ¤
Step-up and step-down transformers thus allow the mains voltage of 240 V to be changed to an appropriate level for different pieces of equipment.
āĻ¸ā§āĻā§āĻĒ-āĻāĻĒ āĻāĻŦāĻ āĻ¸ā§āĻā§āĻĒ-āĻĄāĻžāĻāĻ¨ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĢāĻ°āĻŽāĻžāĻ°āĻā§āĻ˛āĻŋ āĻāĻāĻāĻžāĻŦā§ 240 V-āĻāĻ° āĻŽā§āĻāĻ¨ āĻā§āĻ˛ā§āĻā§āĻāĻā§ āĻŦāĻŋāĻāĻŋāĻ¨ā§āĻ¨ āĻ¸āĻ°āĻā§āĻāĻžāĻŽā§āĻ° āĻā§āĻāĻ°ā§āĻā§āĻ˛āĻŋāĻ° āĻāĻ¨ā§āĻ¯ āĻāĻāĻāĻŋ āĻāĻĒāĻ¯ā§āĻā§āĻ¤ āĻ¸ā§āĻ¤āĻ°ā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻ°āĻžāĻ° āĻ āĻ¨ā§āĻŽāĻ¤āĻŋ āĻĻā§āĻ¯āĻŧāĨ¤
Primary coil
Secondary coil
Voltage=100V
Voltage=200V
Current=1A
Power =100W
Power =VxI
Current=Power ÃˇV
=0.5A
=100W
A step-up transformer: the number of turns in the secondary coil is twice that in the primary, so the voltage across the secondary coil is twice that across the primary, the current through it half that through the primary. The power (volts x amps) is the same in each case.
To render a current earth-free Mains electricity is produced by a dynamo and the consumer is supplied with a wire at high potential, called the live wire, and a wire at zero potential connected to earth, called the neutral wire. Most electrical apparatus works on a current which flows from the live wire, through the apparatus, to the neutral wire and earth.
If an accidental connection is made between the live wire and earth, current will flow along it: if this connection were made by a person ,they would then receive an ‘earth shock’ as the current flowed through them to earth. The static transformer reduces this danger by using electromagnetic induction to transfer the electrical energy into the secondary coil where earth plays no part in the circuit.The effect on the secondary coil of the magnetic field around the primary is to cause electrons to move around the secondary circuit, but not to leave it. Earth plays no part in the secondary circuit because even if an earth connection is made with it, electrons will not leave the circuit but will continue to flow around it. This is an important safety factor, and all currents applied to patients are rendered earth-free by using a static transformer.
āĻĒā§āĻ°āĻžāĻĨāĻŽāĻŋāĻ āĻāĻ¯āĻŧā§āĻ˛
āĻ¸ā§āĻā§āĻ¨ā§āĻĄāĻžāĻ°āĻŋ āĻāĻ¯āĻŧā§āĻ˛
āĻā§āĻ˛ā§āĻā§āĻ = 100V
āĻā§āĻ˛ā§āĻā§āĻ = 200V
āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨=1A
āĻļāĻā§āĻ¤āĻŋ = 100W
āĻļāĻā§āĻ¤āĻŋ = VxI
āĻāĻžāĻ°ā§āĻ¨ā§āĻ = āĻĒāĻžāĻāĻ¯āĻŧāĻžāĻ° Ãˇ āĻāĻŋ
=0.5A
=100W
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Other types of transformer
The variable transformer
This consists of a primary and a secondary coil, but is constructed so that one of them can be altered in length (Fig. I.38). The primary coil has a number of tappings taken from it and a movable contact can be placed on any one of these by turning a knob. The effect of decreasing the number of turns in the primary coil relative to the secondary is to cause a step-up of voltage in the secondary coil. In this way a very crude control of voltage is obtained.
The autotransformer
An autotransformer consists of a single coil of wire with four contact points coming from it (Fig. I.39). When it is used as a step-up transformer, CD is the primary coil and AB the secondary. Although the autotransformer works on the principles of electromagnetic induction, it has the disadvantage that it allows only a small step-up and does not render the current earth-free. It is found in the starter-circuit of ultra-violet lamps and is introduced into the circuit to strike the arc in the lamp.
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The capacitor (condenser)
The capacitor (also known as a condenser) is a device for storing an electric charge. In its simplest form it consists of two metal plates separated by an insulator called the dielectric. If the plates are given opposite static electric charges, the electric lines of force concentrate between the plates.
The electric field between the plates has an effect on the atoms of the dielectric, causing their electron orbits to distort as they are attracted towards the positive plate. The atoms remain in this state of tension until the potential difference across the capacitor is removed, when the energy is released.
Capacitance of a capacitor
Capacitance is the ability to hold an electric charge and is measured in farads, although for practical purposes the microfarad (I0-6 farad)is used.
The potential difference developed between the plates of the capacitor depends upon its capacitance and the amount of electricity with which it is charged. The relationship between potential (in volts) and charge (in coulombs) is a direct one more electrons result in a greater potential.
If the capacitor has a large capacitance (ability to hold a charge), then for a given quantity of electrons only a relatively small potential difference will be developed. Thus the relationship between capacitance and potential is in verse.
The capacitance of a capacitor is affected by such factors as the size of the plates, the material of the plates, the width of the dielectric and the material of the dielectric.
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Types of capacitor
All types of capacitor have the same basic construction: two metal plates separated by an insulator a the plates have been increased in area by projecting interleaving fins from the plates thus increasing the capacitance. Alternatively, plates of tinfoil can be used separated by a dielectric of waxed paper and the whole wrapped into a compact roll.
A variable capacitor is shown in It that one set of plates can be moved relative to the other, thus varying the surface area of plates facing one another. When all the surfaces of both sets of plates are fully interleaved, the capacitance is at its maximum. As one set is withdrawn by turning a knob, the capacitance is gradually reduced. Variable capacitors are found in radio sets and short-wave diathermy machines, controlled by the ‘tuning’ knob: varying the capacitance allows a circuit to be tuned to match the frequency of another oscillating circuit, thereby facilitating maximum transfer of energy between the two circuits.
Electric field of a capacitor
The electric field between the plates of a charged capacitor consists of electric lines of force which tend to take the shortest possible route between the plates. However, they repel one another and pass more easily through some materials than through others so that they are very rarely straight.
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Charging and discharging a capacitor
A capacitor can be charged using electrostatic induction, where a static electric charge is allowed to build up on the plates of the capacitor, or by applying a potential difference across the plates from either the mains or a battery.
A capacitor discharges when the accumulated charge is allowed to flow off the plates. If the two plates with opposite charges are connected, electrons flow from the negative to the positive plate until their charges are equal. The time taken for this discharge depends on the capacitance of the condenser, the resistance (or inductance) of the pathway and the quantity of electricity involved.
Capacitor discharge through an inductance or oscillator circuit If the charged capacitor is discharged through a circuit of low ohmicresistance which includes an inductance (a coil of wire) (Fig. I.44a), electrons flow forwards then back between the plates in an oscillating manner. The reason for this sequence of events is that as current flows through the inductance, self-induced EMFs are produced. These back EMFs impede electron flow, but when both plates reach the same potential the forward EMF causes an electron flow on to one plate with the result that it becomes negatively charged. This sequence continues as a series of damped oscillations until all the energy in the system is exhausted The frequency of oscillation is often many millions per second and the oscillator circuit forms the basis of machines such as the short-wave diathermy and ultrasonic apparatus which require a high-frequency current to operate.-
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Thermionic valves
As the name implies, these are devices which allow electron flow in one direction only and work using heat.
Diode valves
A diode consists of an evacuated glass tube into which are sealed two separate. eletrodes. The cathode, or filament, is constructed so that as current flows through it a space charge of electrons develops around it as a result of the thermal effect of the current (thermionic emission). The anode, or plate, is the other electrode. When positive it attracts electrons across the valve. Electrons can pass only from cathode to anode, as there is n0 space charge around the cold anode. Consequently the thermionic valve is a device which allows electrons to flow in one direction but not in there verse direction.
In order to reduce the time lag prior to thermionic emission taking place ,the cathode or filament may be heated by a separate heating circuit or coated with thorium oxide which releases electrons at a comparatively low temperature.
Triode valves
The triode valve works on exactly the same principle as the diode valve but has a third electrode (the grid) placed between the cathode and the anod. It is possible, using an external circuit, to make the grid negative, positive or neutral. If neutral, the grid will not affect electron flow across the valve. If positive, it will attract electrons away from the cathode and thus amplify the electron flow through the valve. If negative, the grid will repel electrons and reduce or even stop the electron flow. In this third case the valve can act as a switch or regulator.
Semiconductors
Semiconductors are usually metals which because of thermal agitation or the addition of impurities have electrons free to conduct current. They are either n’ type, with an excess of electrons, or ‘p’ type, where a deficiency of electrons gives rise to positive ‘holes’. If a ‘p’ and an’n’ type of semiconductor are fused together, current can only pass in the ‘n’â’p’ direction and the semiconductor therefore acts as a valve.
Rectification of an alternating current
Rectification is the conversion of an alternating current to a direct current. This is achieved using a circuit with two diode valves or two semiconductors in it the direct current still varies considerably in intensity, having large peaks and troughs. In order to depress the peaks and elevate the troughs a smoothing circuit is used which includes choke coils (see Glossary) and capacitors. The forward and back EMFs of the choke coil and the charging and discharging of the capacitor at appropriate points smooth the current.
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Devices for regulation of current
The rheostat
The rheostat can be used to regulate current by altering either the resistance of the circuit or the potential in part of the circuit.
Construction of the rheostat A rheostat consists of a coil of high-resistance wire wound onto an insulating block with each turn insulated from adjacent turns. Contacts is made via a strip from which the insulation has been removed. The contact is mounted either on a straight sliding bar or on a pivot turned by a knob.
Variable resistance or series rheostat . In this device the rheostat is wired series with the apparatus. Ohms law states that the current in a circuit inversely proportional to the resistance of the circuit. If all the coils of wire in the rheostat are included in the circuit, resistance is at its maximum current at its lowest. As the contact is moved, reducing the number of turns of wire included in the circuit , the current increases.
This arrangement is not suitable for currents applied directly to patients, as it never reduces current flow to zero. It is usually found in apparatus where an effect on the degree of heating is required, e.g. for wax baths. There is a stabilizing rheostat on ultra-violet lamps.
Potentiometer or shunt rheostat The shunt rheostat is wired across a source of potential difference and any other circuit has to be taken off in parallel to it. This piece of apparatus works by varying the potential between the ends of the circuit XB. According to Ohm’s law, the greater the potential difference across a resistance the greater the current produced. When contact B is at point X there is no potential difference between the ends of the circuit and no current flows. As contact B moves towards Y, the potential difference gradually increases until at Y it is the same as the potential applied at A. Consequently at point Y maximum current is flowing through the parallel circuit.
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This is the type of current-regulating mechanism found in apparatus where an electric current has to be applied directly to a patient, as the current intensity can be increased gradually from zero up to maximum.
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Electrical Stimulation of Nerve and Muscle
A current which varies sufficiently in magnitude can stimulate a motor while in the absence of a motor nerve the muscle fibres can be stimulated directly by a suitable current. Intermittent currents are used in both cases, and a considerable range of such currents is available. The duration of the current used ranges from o.oI milliseconds (ms) to 3 seconds. The equipment commonly provides durations of o.0I, 0.03, O,I, 0.3, I, 3, I0, 30, 1oo and 300 ms.
Impulses with a duration of less than Io ms may be classed as having a short duration, and are used for stimulating normal (innervated)muscles. Such impulses are said to be of the faradic type. There petition rate of the impulses is great, usually 50-100 per second.
Impulses with a duration of more than Io ms may be classed a shaving a long duration and are used for the stimulation of denervated muscles. They are often termed interrupted (or modified) direct current. The impulses are repeated less frequently than those of short duration for impulses lasting 1oo ms each a frequency of 30 per minute is usual.
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āĻāĻāĻāĻŋ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻ¯āĻžāĻ° āĻŽāĻžāĻ¤ā§āĻ°āĻž āĻ¯āĻĨā§āĻˇā§āĻ āĻĒāĻ°āĻŋāĻŽāĻžāĻŖā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻāĻāĻāĻŋ āĻŽā§āĻāĻ°āĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻ āĻ¨ā§āĻĒāĻ¸ā§āĻĨāĻŋāĻ¤āĻŋāĻ¤ā§ āĻĒā§āĻļā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋ āĻāĻāĻāĻŋ āĻāĻĒāĻ¯ā§āĻā§āĻ¤ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻĻā§āĻŦāĻžāĻ°āĻž āĻ¸āĻ°āĻžāĻ¸āĻ°āĻŋ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻŦāĻŋāĻ°āĻ¤āĻŋāĻšā§āĻ¨ āĻ¸ā§āĻ°ā§āĻ¤ āĻāĻāĻ¯āĻŧ āĻā§āĻˇā§āĻ¤ā§āĻ°ā§āĻ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻ¸ā§āĻ°ā§āĻ¤ā§āĻ° āĻāĻāĻāĻŋ āĻāĻ˛ā§āĻ˛ā§āĻāĻ¯ā§āĻā§āĻ¯ āĻĒāĻ°āĻŋāĻ¸āĻ° āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻŦā§āĻ¯āĻžāĻĒā§āĻ¤āĻŋāĻ° āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ o.oI āĻŽāĻŋāĻ˛āĻŋāĻ¸ā§āĻā§āĻ¨ā§āĻĄ (ms) āĻĨā§āĻā§ 3 āĻ¸ā§āĻā§āĻ¨ā§āĻĄ āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤āĨ¤ āĻ¸āĻ°āĻā§āĻāĻžāĻŽāĻā§āĻ˛āĻŋ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ o.01, 0.03, O,1, 0.3, 1, 3, 10, 30, 1oo āĻāĻŦāĻ 300 ms āĻāĻ° āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ āĻĒā§āĻ°āĻĻāĻžāĻ¨ āĻāĻ°ā§āĨ¤
1o ms-āĻāĻ° āĻā§āĻ¯āĻŧā§ āĻāĻŽ āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ā§āĻ° āĻāĻŽāĻĒāĻžāĻ˛āĻ¸āĻā§āĻ˛āĻŋāĻā§ āĻ¸ā§āĻŦāĻ˛ā§āĻĒ āĻŽā§āĻ¯āĻŧāĻžāĻĻā§ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻļā§āĻ°ā§āĻŖā§āĻŦāĻĻā§āĻ§ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŦāĻ āĻāĻāĻŋ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ (āĻāĻ¨āĻ°ā§āĻā§āĻā§āĻĄ) āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻāĻŦā§āĻāĻā§ āĻĢā§āĻ¯āĻžāĻ°āĻžāĻĄāĻŋāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧāĨ¤ āĻ¸ā§āĻāĻžāĻ¨ā§ āĻāĻŦā§āĻā§āĻ° āĻĒāĻŋāĻāĻŋāĻļāĻ¨ āĻ°ā§āĻ āĻĻā§āĻ°ā§āĻĻāĻžāĻ¨ā§āĻ¤, āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻĒā§āĻ°āĻ¤āĻŋ āĻ¸ā§āĻā§āĻ¨ā§āĻĄā§ 50-100āĨ¤
Io ms-āĻāĻ° āĻŦā§āĻļāĻŋ āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ā§āĻ° āĻāĻŽāĻĒāĻžāĻ˛āĻ¸āĻā§āĻ˛āĻŋāĻā§ āĻāĻāĻāĻŋ āĻĻā§āĻ°ā§āĻ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻļā§āĻāĻŋāĻ āĻļā§āĻ°ā§āĻŖā§āĻŦāĻĻā§āĻ§ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŦāĻ āĻāĻāĻŋ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻŦā§āĻ¯āĻŦāĻšā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻā§āĻ˛āĻŋāĻā§ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻŦāĻŋāĻā§āĻ¨āĻŋāĻ¤ (āĻŦāĻž āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤) āĻ¸āĻ°āĻžāĻ¸āĻ°āĻŋ āĻĒā§āĻ°āĻŦāĻžāĻš āĻŦāĻ˛āĻž āĻšāĻ¯āĻŧāĨ¤ āĻĒā§āĻ°āĻ¤āĻŋ āĻŽāĻŋāĻ¨āĻŋāĻā§ 30 āĻāĻ° āĻĢā§āĻ°āĻŋāĻā§āĻ¯āĻŧā§āĻ¨ā§āĻ¸āĻŋ 1oo ms āĻ¸ā§āĻĨāĻžāĻ¯āĻŧā§ āĻšāĻāĻ¯āĻŧāĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ¸āĻāĻā§āĻˇāĻŋāĻĒā§āĻ¤ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻā§āĻ¯āĻŧā§ āĻāĻŽ āĻāĻ¨ āĻāĻ¨ āĻĒā§āĻ¨āĻ°āĻžāĻŦā§āĻ¤ā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤
Faradic-type current
A faradic-type current is a short-duration interrupted direct current with a pulse duration of o1-1 ms and a frequency of 50-Ioo Hz (hertz). The term faradism was originally used to signify the type of current produced by a faradic coil, which is a type of induction coil. The current provided by the first faradic coils was an unevenly alternating current, each cycle consisting of two unequal phases, the first of low intensity and long duration, the second of high intensity and short duration. The frequency was approximately 50 Hz and the duration of the second phase, which was the effective one, about 1 ms. A graph of this current is shown.
Faradic coils have now been superseded by electronic stimulators. These supply currents which produce the same physiological effects as the original faradic current, although often differing considerably from them in wave form The features essential for the production of these physiological effects are that impulses with a duration of between O.I and1 ms are repeated 50-100 times per second. The electronic stimulator for the production of the faradic-type current works on the same principles as that for the interrupted .c., but the resistances controlling the duration of the impulses and the intervals between them have a very low value to give the required duration and repetition rate.
Modified faradic currents – Faradic-type currents are always surged for treatment purposes to produce a near-normal tetanic-like contraction and relaxation of muscle. The unmodified current is shown in Fig.2.3a. The current is’ surged’ so that the intensity of successive impulses increases gradually, each impulse reaching a peak value greater than the preceding one, as in then falls, either suddenly or gradually. In the original faradic coils the current was surged by hand, but in modern stimulators an electronic device is used. The circuit can be modified to give surges of various durations frequencies and wave forms. It is desirable that the durations
of the surges and the intervals between them should be regulated by separate controls in order that the most satisfactory muscle contractions and rest periods can be obtained for each patient. Various forms of surge may be available, corresponding to trapezoidal, triangular and saw-tooth impulses, and that most suitable for each patient must be selected. These various types of surge are shown.
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Electrical activity of nerves
Nerve transmission
Owing to the difference in concentration of ions inside and outside the plasma membrane, there is a difference of potential (PD) between the inside and outside of a nerve. The resting nerve is positive outside and negative inside and the plasma membrane is not permeable to sodium ions. This is described as the polarized state of the membrane.
When a nerve is stimulated, the stimulus causes a fall in the potential difference across the plasma membrane. When this fall reaches a certain critical level it causes an alteration in the permeability of the membrane to sodium ions. This results in an alteration in the concentration of ions inside and outside and a further fall of PD until a reversal of polarity occurs: the membrane is now positive inside and negative outside. Immediately after this activity the sodium ions are pumped out again and that part of the nerve returns to its resting state. The difference of potential between the active and resting part of the nerve causes local electron flow between the active and the adjacent parts of the nerve.The current flows through the membrane in the opposite direction to the potential difference (PD) across the fibre. The fibre acts as a resistance to the current, so that the current flow lowers the PD, thus making the membrane permeable to sodium ions, which reverses the PD as before. These changes are then propagated along the length of the nerve fibre. This wave of change of the polarized state constitutes the passage of an impulse along the nerve.
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Electrical stimulation of nerves
A nerve impulse can be initiated by an electrical stimulus. To achieve this, a varying current of adequate intensity must be applied. The plasma membrane of the nerve fibre forms a resistance which lies in series with the other tissues, so a PD is set up across it as the current flows. The surface of the membrane nearer to the cathode becomes negative in relation to the opposite surface. In the surfaces of the plasma membranes marked ‘n’ lie nearest to the cathode and so become more negative, while the surfaces marked ‘p’, lying nearer to the anode, become more positive. On the side of the nerve nearer to the anode this increases the resting PD across the membrane, but on the side of the nerve nearer to the cathode (B)the additional charges are of opposite polarity to those present on the resting membrane and so reduce thÊ PD across it. If the PD falls below the level at which the membrane becomes permeable to sodium ions ,these ions begin to enter the axon and initiate the series of events described above so that a nerve impulse is initiated.
An impulse is initiated if the PD falls sufficiently across any part of the plasma membrane of the nerve cell or fibre. If the cathode is applied over a superficial nerve, the side of the nerve nearest to the cathode is activated, but the anode can equally cause the initiation of a nerve impulse. In this case it is the aspect of the nerve further from the anode that is activated. The current spreads in the tissues, so the current density is rather less on the further surface of the nerve fibre than on the nearer one, and inconsequence the anode is less effective than the cathode in initiating a nimpulse. On some types of electronic apparatus the polarity of the terminals is marked. This refers to the polarity during the high peak of current, which is the effective stimulus. The active electrode should be connected to the cathode which produces a contraction of innervated muscle with less current than is required at the anode.
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Accommodation
When a constant current flows the nerve adapts itself, by a mechanism not fully understood, to the altered conditions. This effect is known as accommodation. As a consequence an unvarying current is not effective in initiating an impulse.
When the current rises, the impulse is initiated as described above, but a fall in current can also initiate an impulse. While the current flows¡ at a constant level accommodation of the nerve takes place and the PD resulting from the current flow no longer affects the excitability of the nerve fibre, which has adapted itself to the altered conditions. When the current ceases, the PD which it caused across the plasma membrane suddenly disappears, so altering the total PD across the membrane. On the aspect of the nerve nearer to the anode the applied PD was augmenting that across the resting membrane and its sudden loss causes a fall in the PD. If this fall goes as far as the level at which the membrane becomes permeable to sodium ions, an impulse is initiated. However a fall in current is less effective than a rise in initiating an impulse. It is the side of the nerve nearer to the anode that is affected, and so the anode produces a greater stimulation than the cathode.
Because the nerve has the power of accommodation a current which rises or falls suddenly is more effective in initiating an impulse than one which changes slowly. If the variation of current is gradual there is time for accommodation to take place, and so a greater current is changes ‘very slowly does not initiate a nerve impulse at all.
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Effects of nerve stimulation
When a nerve impulse is initiated at a nerve cell or an end organ, there simultaneously in both directions from the point of stimulation.
impulse has no effect, but the upward travelling impulse is appreciated durations are applied, using the same current for each, it is found that impulse. Impulses of long duration produce an uncomfortable, stabbing sensation, but this becomes less as the duration of the impulses is reduced until with impulses of 1 ms and less only a mild prickling sensation is experienced.
When a motor nerve is stimulated, the upward-travelling impulse is unable to pass the first synapse, as it is travelling in the wrong direction, but the downward-travelling impulse passes to the muscles supplied by the nerve, causing them to contract.
When a stimulus is applied to a motor nerve trunk, impulses pass to all the muscles that the nerve supplies below the point at which it is stimulated, causing them to contract.
When the current is applied directly over an innervated muscle, the nerve fibres in the muscle are stimulated in the same way. The maximum response is obtained either from stimulation at the motor point, which is the point at which the main nerve enwrs the muscle or ,in the case of deeply placed muscles, at the point where the muscle emerges from under cover of the more superficial ones.
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Effects of frequency of stimulation
When a single stimulus is applied, impulses pass simultaneously to a number of motor units so that in normal circumstances there is a sudden brisk contraction, followed by immediate relaxation. If a succession of stimuli are applied at rather long intervals, e.g. one stimulus per second, each produces an isolated muscle contraction and there is time for complete relaxation between the impulses. Increasing the frequency of the stimuli shortens the periods of relaxation until at frequencies exceeding 20 Hz there is not time for complete relaxation between the contractions, so that partial tetany results. Further increase in the frequency reduces the amount of relaxation still further until, at frequencies over 60 Hz , there is no perceptible relaxation and the contraction is fully tetanic.
Strength of contra
This depends on the number of motor units activated (which in turn depends on the intensity of the current applied) and the rate of change of current. If the intensity of current rises suddenly there is no time for accommodation to take place and a muscle contraction results. If the current rises more slowly, as with the trapezoidal, triangular and saw-tooth impulses, there is some accommodation and a greater intensity of current is needed to produce a contraction.
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Physiological effects of faradic-type current
The tissues of the body are capable of transmitting an electric current because the tissue fluids contain ions and so are conductors. Consequently the current passing through the body consists of a two-way migration of ions, and the conductivity of the different tissues varies according to the amount of fluid that they contain. Muscle, for example, has a good blood supply and so is a good conductor, while fatis a poor conductor. The current tends to travel through those tissues which have a low resistance, although it is not always possible for it to avoid the high resistance layers. The epidermis has a high resistance, 1ooo Ί or more, as it contains little fluid and the superficial layers do not readily absorb moisture. The current must pass through the epidermis and appropriate measures are used to reduce its resistance when applying electrical treatments. Passage of current may result in chemical changes, which can constitute a danger in some treatments.
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Stimulation of sensory nerves
When a current of the faradic type is applied to the body, a mild prickling sensation is experienced. This is due to stimulation of the sensory nerves, and is not very marked because the stimuli are of fairly short duration. The sensory stimulation causes a reflex vasodilatation of the superficial blood vessels, so that there is slight reddening of the skin (erythema). The vasodilatation is generally confined to the superficial tissues, and is of little practical importance.
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Stimulation of motor nerves
A current of the faradic type stimulates the motor nerves and, if the current is of sufficient intensity, causes contraction of the muscles which they supply. Because the stimuli are repeated 50 times per second or more, the contraction is tetanic. If this type of contraction is maintained for more than a short period of time, muscle fatigue is produced, so the current is commonly surged to allow for muscle relaxation. When the current is surged the contraction gradually increases and decreases in strength, in a manner similar to a voluntary contraction.
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Effects of muscle contraction When a muscle contracts as a result of electrical stimulation, the with voluntary contraction. There is increased metabolism ,with a increased output of waste products, including metabolites. The metabolites cause dilatation of
capillaries and arterioles, and there is a considerable increase in the blood supply to the muscle.
As the muscles contract and relax they exert a pumping action on the veins and lymphatic vessels lying within and around them. The valve sin these vessels ensure that the fluid they contain is moved towards the heart. If the muscle contractions are sufficiently strong to cause joint movement this also exerts a pumping effect. There is thus increased venous ÃĄnd lymphatic return.
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Stimulation of denervated muscle
The current required to produce a contraction of denervated muscle with an impulse lasting for 1 ms is usually too great to be tolerable for treatment purposes. The faradic type of current is therefore not satisfactory for the stimulation of denervated muscles.
Chemical effects of faradic-type current
When a direct current is passed through an electrolyte, chemical changes take place at the electrodes. If the chemicals formed come in contact with the tissues there is a danger of electrolytic burns, although the danger is appreciably less with an intermittent than with a constant direct current. When the current is alternating, the ions move one way during one phase of current and in the reverse direction during the other phase, and if the two phases are equal the chemicals formed during one phase are neutralized during the next phase. If, however, the reverse wave of current is not similar to the forward wave, there are danger is not so great as with a direct current). The current obtained than the other, but if the impulses are of very short duration the danger of burns. It is, however, advisable to take appropriate precautions.
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Indications for use of faradic-type currents
Facilitation of muscle contraction
When a patient is unable to produce a muscle contraction ,or finds difficulty in doing so, electrical stimulation may be of use in assisting voluntary contraction. A muscle contraction is the result of a complex integration of the neuronal circuits both at spinal level and from the higher centres. Thought to include the following events-
I .Excitation of the small (fusimotor) efferent fibres, which causes contraction of the
intrafusal muscle fibres.
2. Stretching of the muscle spindle which stimulates the primary nerve endings and therefore sends information to the large anterior horn cells causing an excitation of the extrafusal muscle fibres.
3. Inhibition of the anterior horn cells supplying the antagonistic muscle group.
Pain has an inhibitory effect on the large anterior horn cells, so impeding the transmission of impulses to the motor units. Electrical stimulation of the motor neurones should reduce the inhibition, so facilitating the transmission of voluntary impulses to the muscle and also inducing relaxation of its antagonists. When muscle contraction is inhibited by pain or recent injury, for instance when active contraction of the quadriceps is impossible in rheumatoid arthritis of the knee joint or after meniscectomy, electrical stimulation may be of assistance in establishing voluntary contraction.
Details of the technique can be found on pp. 52-64. The treatment must be arranged so that the part is in a pain-free position and so that no movement causing pain is produced, as this would inhibit the discharge from the large anterior horn cells. Voluntary contraction should be attempted at the same time as the electrical stimulation, which is required only until a good voluntary contraction can be performed unaided.
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Re-education of muscle-action
Inability to contract a muscle voluntarily may be the result of prolonged disuse, as in the intrinsic foot muscles in a longstanding flat foot, or of incorrect use, as with the abductor hallucis in hallux valgus. In these circumstances, faradic stimulation may be used to produce contractions and so help to restore the sense of movement. The brain appreciates movements, not muscle actions, so the current should be applied in such a way that it causes the movement that the patient is unable to perform .Active contractions should be attempted at the same time as the electrical stimulation, the treatment being a preliminary to active exercise. It will probably take longer to establish a voluntary contraction than in those cases where inhibition is due to pain or injury but once a satisfactory contraction can be performed electrical stimulation should be discontinued.
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Training a new muscle-action
After tendon transplantation or other reconstruction operations a muscle may be required to perform a different action from that which it previously carried out. A new movement pattern has to be established. The muscle is stimulated with the faradic-type current, so that its new action is performed, and the patient must concentrate on the movement and attempt to assist with voluntary contractions. In this way the new muscle action may be taught, although it will take longer to achieve this than to re-educate an action which the muscle has previously performed.
āĻāĻāĻāĻŋ āĻ¨āĻ¤ā§āĻ¨ āĻĒā§āĻļā§-āĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĒā§āĻ°āĻļāĻŋāĻā§āĻˇāĻŖ āĻā§āĻ¨ā§āĻĄāĻ¨ āĻā§āĻ°āĻžāĻ¨ā§āĻ¸āĻĒā§āĻ˛āĻžāĻ¨ā§āĻā§āĻļāĻ¨ āĻŦāĻž āĻ āĻ¨ā§āĻ¯āĻžāĻ¨ā§āĻ¯ āĻĒā§āĻ¨āĻ°ā§āĻāĻ āĻ¨ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻ˛āĻžāĻĒāĻā§āĻ˛āĻŋāĻ° āĻĒāĻ°ā§ āĻāĻāĻāĻŋ āĻĒā§āĻļā§āĻ° āĻāĻā§ āĻ¯āĻž āĻāĻ°āĻž āĻšāĻ¯āĻŧā§āĻāĻŋāĻ˛ āĻ¤āĻžāĻ° āĻĨā§āĻā§ āĻāĻāĻāĻŋ āĻāĻŋāĻ¨ā§āĻ¨ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¸āĻŽā§āĻĒāĻžāĻĻāĻ¨ā§āĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻāĻāĻŋ āĻ¨āĻ¤ā§āĻ¨ āĻāĻ¨ā§āĻĻā§āĻ˛āĻ¨ā§āĻ° āĻĒā§āĻ¯āĻžāĻāĻžāĻ°ā§āĻ¨ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻ¤ā§ āĻšāĻŦā§āĨ¤ āĻĒā§āĻļā§ āĻĢā§āĻ¯āĻžāĻ°āĻžāĻĄāĻŋāĻ-āĻāĻžāĻāĻĒ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻĻā§āĻŦāĻžāĻ°āĻž āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻ¯āĻžāĻ¤ā§ āĻ¤āĻžāĻ° āĻ¨āĻ¤ā§āĻ¨ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¸āĻā§āĻāĻžāĻ˛āĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻ°ā§āĻā§āĻā§ āĻ āĻŦāĻļā§āĻ¯āĻ āĻ¨āĻĄāĻŧāĻžāĻāĻĄāĻŧāĻžāĻ¯āĻŧ āĻŽāĻ¨ā§āĻ¨āĻŋāĻŦā§āĻļ āĻāĻ°āĻ¤ā§ āĻšāĻŦā§ āĻāĻŦāĻ āĻ¸ā§āĻŦā§āĻā§āĻāĻžāĻ¸ā§āĻŦā§ āĻ¸āĻāĻā§āĻāĻ¨ā§āĻ° āĻ¸āĻžāĻĨā§ āĻ¸āĻšāĻžāĻ¯āĻŧāĻ¤āĻž āĻāĻ°āĻžāĻ° āĻā§āĻˇā§āĻāĻž āĻāĻ°āĻ¤ā§ āĻšāĻŦā§āĨ¤ āĻāĻāĻāĻžāĻŦā§ āĻ¨āĻ¤ā§āĻ¨ āĻĒā§āĻļā§ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻļā§āĻāĻžāĻ¨ā§ āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯āĻĻāĻŋāĻ āĻĒā§āĻļā§ āĻĒā§āĻ°ā§āĻŦā§ āĻ¸āĻā§āĻāĻžāĻ˛āĻŋāĻ¤ āĻāĻāĻāĻŋ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻā§ āĻĒā§āĻ¨āĻ°āĻžāĻ¯āĻŧ āĻļāĻŋāĻā§āĻˇāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻā§āĻ¯āĻŧā§ āĻāĻāĻŋ āĻ āĻ°ā§āĻāĻ¨ āĻāĻ°āĻ¤ā§ āĻāĻ°āĻ āĻŦā§āĻļāĻŋ āĻ¸āĻŽāĻ¯āĻŧ āĻ˛āĻžāĻāĻŦā§āĨ¤
Neurapraxia of a motor nerve
In this case impulses from the brain are unable to pass the site of the lesion to reach the muscles supplied by the affected nerve. Consequently voluntary power is reduced or lost. There is, however, no degeneration of the nerve, so that if it is stimulated with faradism below the site of the lesion, impulses pass to the muscles, çausing them to contract. Electrical stimulation is not usually necessary in neurapraxis, as recovery takes place without any marked changes in the muscle tissue.
āĻāĻāĻāĻŋ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻ¨āĻŋāĻāĻ°āĻžāĻĒā§āĻ°āĻžāĻā§āĻ¸āĻŋāĻ¯āĻŧāĻž āĻāĻ āĻā§āĻˇā§āĻ¤ā§āĻ°ā§ āĻŽāĻ¸ā§āĻ¤āĻŋāĻˇā§āĻ āĻĨā§āĻā§ āĻāĻ¸āĻž āĻāĻŦā§āĻāĻā§āĻ˛āĻŋ āĻĒā§āĻ°āĻāĻžāĻŦāĻŋāĻ¤ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻĻā§āĻŦāĻžāĻ°āĻž āĻ¸āĻ°āĻŦāĻ°āĻžāĻšāĻŋāĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ¤ā§ āĻĒā§āĻāĻāĻžāĻ¨ā§āĻ° āĻāĻ¨ā§āĻ¯ āĻā§āĻˇāĻ¤ āĻ¸ā§āĻĨāĻžāĻ¨āĻāĻŋ āĻ āĻ¤āĻŋāĻā§āĻ°āĻŽ āĻāĻ°āĻ¤ā§ āĻ āĻā§āĻˇāĻŽ āĻšāĻ¯āĻŧāĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ āĻ¸ā§āĻŦā§āĻā§āĻāĻžāĻ¸ā§āĻŦā§ āĻļāĻā§āĻ¤āĻŋ āĻšā§āĻ°āĻžāĻ¸ āĻŦāĻž āĻšāĻžāĻ°āĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻ¤āĻŦā§, āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻā§āĻ¨ āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻ¨ā§āĻ, āĻ¯āĻžāĻ¤ā§ āĻā§āĻˇāĻ¤āĻ¸ā§āĻĨāĻžāĻ¨ā§āĻ° āĻ¨ā§āĻā§ āĻĢā§āĻ¯āĻžāĻ°āĻžāĻĄāĻŋāĻāĻŽ āĻĻāĻŋāĻ¯āĻŧā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ˛ā§, āĻāĻŦā§āĻ āĻĒā§āĻļā§āĻ¤ā§ āĻāĻ˛ā§ āĻ¯āĻžāĻ¯āĻŧ āĻāĻŦāĻ āĻ¤āĻžāĻĻā§āĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻāĻāĻžāĻ¯āĻŧāĨ¤ āĻ¨āĻŋāĻāĻ°āĻžāĻĒā§āĻ°āĻžāĻā§āĻ¸āĻŋāĻ¸ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻž āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧ āĻ¨āĻž, āĻāĻžāĻ°āĻŖ āĻĒā§āĻļā§āĻ° āĻāĻŋāĻ¸ā§āĻ¯ā§āĻ¤ā§ āĻā§āĻ¨ā§ āĻ˛āĻā§āĻˇāĻŖā§āĻ¯āĻŧ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻžāĻĄāĻŧāĻžāĻ āĻĒā§āĻ¨āĻ°ā§āĻĻā§āĻ§āĻžāĻ° āĻāĻā§āĨ¤
Severed motor nerve
When a nerve has been severed, degeneration of the axons takes place and there is no longer a satisfactory response to stimuli of short duration. Degeneration takes several days, and for a few days after the injury a muscle contraction may be obtained with the faradic-type current. If this is so, faradism may be used to exercise the muscle so long as a good response is present, but must be replaced by modified as soon as the response begins to weaken.
āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻšāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻ ā§āĻ¯āĻžāĻā§āĻ¸āĻ¨āĻā§āĻ˛āĻŋāĻ° āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻāĻā§ āĻāĻŦāĻ āĻ¸ā§āĻŦāĻ˛ā§āĻĒ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻāĻ° āĻĨāĻžāĻā§ āĻ¨āĻžāĨ¤ āĻ āĻ§āĻāĻĒāĻ¤āĻ¨ā§ āĻŦā§āĻļ āĻāĻ¯āĻŧā§āĻ āĻĻāĻŋāĻ¨ āĻ¸āĻŽāĻ¯āĻŧ āĻ˛āĻžāĻā§ āĻāĻŦāĻ āĻāĻāĻžāĻ¤ā§āĻ° āĻĒāĻ° āĻāĻ¯āĻŧā§āĻ āĻĻāĻŋāĻ¨ā§āĻ° āĻāĻ¨ā§āĻ¯ āĻĢā§āĻ¯āĻžāĻ°āĻžāĻĄāĻŋāĻ-āĻāĻžāĻāĻĒ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻ¸āĻžāĻšāĻžāĻ¯ā§āĻ¯ā§ āĻĒā§āĻļā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ¯āĻĻāĻŋ āĻ¤āĻžāĻ āĻšāĻ¯āĻŧ, āĻ¤āĻžāĻšāĻ˛ā§ āĻ¯āĻ¤āĻā§āĻˇāĻŖ āĻāĻžāĻ˛ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĨāĻžāĻā§ āĻ¤āĻ¤āĻā§āĻˇāĻŖ āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻĒā§āĻļā§āĻ° āĻŦā§āĻ¯āĻžāĻ¯āĻŧāĻžāĻŽ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻĢā§āĻ¯āĻžāĻ°āĻžāĻĄāĻŋāĻāĻŽ āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¤āĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻŋ āĻĻā§āĻ°ā§āĻŦāĻ˛ āĻšāĻ¤ā§ āĻļā§āĻ°ā§ āĻāĻ°āĻžāĻ° āĻ¸āĻžāĻĨā§ āĻ¸āĻžāĻĨā§ āĻ¸āĻāĻļā§āĻ§āĻ¨ āĻāĻ°ā§ āĻĒā§āĻ°āĻ¤āĻŋāĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻ¤ā§ āĻšāĻŦā§āĨ¤
Improved venous and lymphatic drainage
Increased venous and lymphatic return is brought about by the pumping action of alternate muscle contraction and relaxation and of joint movement on the veins and lymphatics. The treatment is most effective if the current is applied by the method described as ‘faradism under pressure’. It may be used in the treatment of oedema and sometimes for gravitational ulcers.
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Prevention and loosing of adhesions
When there is effusion into the tissues, adhesions are liable to form, but these can be prevented by keeping structures moving with respect to each other. If adequate active exercise is not possible, electrical stimulation may be used for this purpose. Adhesions which have formed may be stretched and loosened by muscle contractions, scar tissue binding muscles or tendons.
āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻāĻŦāĻ adhesions āĻšāĻžāĻ°āĻžāĻ¨ā§ āĻ¯āĻāĻ¨ āĻāĻŋāĻ¸ā§āĻ¯ā§āĻ¤ā§ āĻ¨āĻŋāĻāĻ¸āĻ°āĻŖ āĻšāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻāĻ¨ā§āĻāĻ¤ā§āĻ¯āĻā§āĻ˛āĻŋ āĻ¤ā§āĻ°āĻŋ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¤āĻŦā§ āĻāĻā§ āĻ āĻĒāĻ°ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋ āĻļā§āĻ°āĻĻā§āĻ§āĻž āĻ°ā§āĻā§ āĻāĻžāĻ āĻžāĻŽā§āĻā§āĻ˛āĻŋāĻā§ āĻāĻ˛āĻŽāĻžāĻ¨ āĻ°ā§āĻā§ āĻāĻā§āĻ˛āĻŋ āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ¯āĻĻāĻŋ āĻĒāĻ°ā§āĻ¯āĻžāĻĒā§āĻ¤ āĻ¸āĻā§āĻ°āĻŋāĻ¯āĻŧ āĻŦā§āĻ¯āĻžāĻ¯āĻŧāĻžāĻŽ āĻ¸āĻŽā§āĻāĻŦ āĻ¨āĻž āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻāĻ āĻāĻĻā§āĻĻā§āĻļā§āĻ¯ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻž āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻĒā§āĻļā§ āĻ¸āĻāĻā§āĻāĻ¨, āĻĻāĻžāĻ āĻāĻŋāĻ¸ā§āĻ¯ā§ āĻŦāĻžāĻāĻ§āĻžāĻ āĻĒā§āĻļā§ āĻŦāĻž āĻā§āĻ¨ā§āĻĄāĻ¨ āĻĻā§āĻŦāĻžāĻ°āĻž āĻĒā§āĻ°āĻ¸āĻžāĻ°āĻŋāĻ¤ āĻāĻŦāĻ āĻĸāĻŋāĻ˛āĻž āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŽāĻ¨ āĻāĻ āĻžāĻ˛ā§ āĻ¯āĻž āĻāĻ āĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤
Techniques of treatment with faradic-type currents
Various methods of applying faradic-type current can be used, according to the effects required. The techniques used to obtain group muscle will both be described. All techniques include the following preliminary procedures.
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Preparation of apparatus
A low- frequency electronic stimulator with automatic surgery is commonly used, although some physiotherapists prefer a Smart-Bristow faradic coil. The operator should test the apparatus by attaching leads and electrodes to the terminals, holding the two electrodes in a moistened hand, inserting the core if a Smart-Bristow coil is being used, and turning up the current until a mild prickling sensation is experienced and a muscle contraction produced. Describe to the patient the sensation you feel and make sure the patient can see the muscle contraction produced. If the surging is automatic the duration and frequency of the surge should also be tested.
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The active electrode may be a disc-electrode or a small lint pad with a flat plate-electrode. The latter is preferable for large muscles like the quadriceps and glutei, as it is easier to mould to the surface, so obtaining good contact. A flat plate-electrode and lint pad are used for the indifferent electrode, to complete the circuit. The pads consist of at least eight layers of lint, so that they are thick enough to make good contact with the tissues and with the electrode and to absorb any chemicals which might be formed. They should be folded evenly with no creases, or there will be uneven distribution of current and are soaked in warm 1% saline. Tap-water can be used, but the addition rather lower resistance than the tissue fluids. Electrodes should be I coming in contact with the skin and causing uncomfortable concentration of current and possible damage to the tissues from points may become bent and dig into the pad, again causing concentration of current.
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Preparation of the patient
Clothing is removed from the area to be treated and the patient is supported comfortably in a good light. It is important that the patient is warm, otherwise the muscles do not respond well to the stimulation. It is usually easiest to obtain muscle contractions in response to electrical stimulation if the part is supported so that the muscles are in a shortened position. It may, however, be desirable to modify this position according to the effects required. If the aim of treatment is tore-educate a muscle action, the patient may be arranged so that movement is produced when the muscle contracts; e.g. for training the quadriceps the kriee may be arranged in slight flexion so that extension takes place when the muscles are stimulated. In some cases movement can be obtained by supporting the limb in slings during the treatment; e.g. when training the deltoid muscle, movements of the shoulder joint can often be produced if the arm is supported in slings, though rarely from any other position. The joint movement should, however, be avoided if it causes pain, which will inhibit muscle action.
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The skin has a high electrical resistance as the superficial layers, being dry, contain few ions. The resistance is reduced by washing with soap and water to remove the natural oils and moistening with saline immediately before the pads are applied, in order to provide ions. Breaks in the skin cause a marked reduction in resistance which naturally results in concentration of the current and consequent discomfort to the patient. To avoid this, broken skin is protected by a little petroleum jelly covered with a small piece of non-absorbent cotton wool to protect the pad. The indifferent pad should be large to reduce the current density under it to a minimum. This prevents excessive skin stimulation and also reduces the likelihood of unwanted muscle contractions, as it may not be possible to avoid covering the motor points of some muscles. The indifferent electrode may be bandaged or filled with a rubber strap, or body-weight may be sufficient to hold it in position. If the pad is bandaged in position, or if it is liable to come in contact with the patients clothing, it is covered with jaconet ( a kind of plastic sheeting) to protect the bandage or clothing from moisture.
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Stimulation of motor points
This method has the advantage that each muscle performs its own individual action and that optimum contraction of each can be obtained. It may therefore be selected when training a new muscle action or when isolation of one muscle is indicated, the vastus medial is maybe stimulated to overcome a quadriceps lag, or abductor halluces for muscle weakness.
The apparatus and patient should be prepared as previously described. The indifferent electrode is applied and secured in a suitable area. The active electrode may be a disc electrode, which is in the palm of the hand. It is placed over the motor point of the muscle where possible the whole of the operator’s hand should be in contact with the patient’s tissues so that she can feel the strength of the contractions produced. A suitable duration and frequency of surge must be selected. The intensity of the current is gradually increased until a good muscle contraction is obtained at the maximum point of each surge, then the surging is continued to produce alternate contraction and relaxation of the muscle. To re-educate muscle action, voluntary contractions may be attempted at the same time as those produced by the electrical stimulation, and active exercises may be interspersed with the electrical treatment.
The duration of the treatment session is determined by the length of time for which the patient can concentrate on the movement and assist in its production. Muscle fatigue is indicated by weakening of the contraction, but does not occur rapidly with faradic-type stimulation.
The approximate positions of some motor-points are shown. Motor points are frequently at the junction of the upper and middle one-thirds of the fleshy belly of the muscle, although there are exceptions, e.g. the motor point of the vastus medialis, whose nerve enters the lower part of the muscle, is situated a short distance above the knee joint. Deeply placed muscles may be stimulated most satisfactorily where they emerge from beneath the
more superficial ones, e.g. the extensor hallucis longus in the lower one-third of the lower leg.
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Stimulation of muscle groups
Stimulation which makes all the muscles of a group work together is a satisfactory method of re-educating the action of muscles that normally work as a group, such as the quadriceps, the small muscles of the foot, and the muscles of the pelvic floor.
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Quadriceps
Prepare the apparatus and the patient as previously described. Position the patient on a plinth with the affected knee supported in the desired degree of flexion. One electrode and pad (size approximately.12×9 cm) is placed over the femoral nerve in the femoral triangle and either held in position-with a sandbag or bandaged on firmly. The other electrode pad (size 15×8 cm) is placed across the motor point sand held in position by a strap or bandage. Use jaconet to protect clothes and bandage from moisture.
āĻā§āĻ¯āĻŧāĻžāĻĄā§āĻ°āĻŋāĻ¸ā§āĻĒāĻ¸
āĻĒā§āĻ°ā§āĻŦā§ āĻŦāĻ°ā§āĻŖāĻŋāĻ¤ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻ¯āĻ¨ā§āĻ¤ā§āĻ°āĻĒāĻžāĻ¤āĻŋ āĻāĻŦāĻ āĻ°ā§āĻā§ āĻĒā§āĻ°āĻ¸ā§āĻ¤ā§āĻ¤ āĻāĻ°ā§āĻ¨āĨ¤ āĻ°ā§āĻā§āĻā§ āĻāĻāĻāĻŋ āĻĒā§āĻ˛āĻŋāĻ¨ā§āĻĨā§āĻ° āĻāĻĒāĻ° āĻ°āĻžāĻā§āĻ¨ āĻ¯āĻžāĻ° āĻ¸āĻžāĻšāĻžāĻ¯ā§āĻ¯ā§ āĻāĻā§āĻ°āĻžāĻ¨ā§āĻ¤ āĻšāĻžāĻāĻā§ āĻāĻžāĻā§āĻāĻŋāĻ¤ āĻŽāĻžāĻ¤ā§āĻ°āĻžāĻ¯āĻŧ āĻŦāĻžāĻāĻāĻžāĻ¨ā§ āĻšāĻ¯āĻŧāĨ¤ āĻāĻāĻāĻŋ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻāĻŦāĻ āĻĒā§āĻ¯āĻžāĻĄ (āĻāĻāĻžāĻ° āĻĒā§āĻ°āĻžāĻ¯āĻŧ 12x9 āĻ¸ā§āĻŽāĻŋ) āĻĢā§āĻŽā§āĻ°āĻžāĻ˛ āĻ¤ā§āĻ°āĻŋāĻā§āĻā§āĻ° āĻĢā§āĻŽā§āĻ°āĻžāĻ˛ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻāĻĒāĻ°ā§ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻšāĻ¯āĻŧ āĻāĻāĻāĻŋ āĻŦāĻžāĻ˛āĻŋāĻ° āĻŦā§āĻ¯āĻžāĻ āĻĻāĻŋāĻ¯āĻŧā§ āĻŦāĻž āĻļāĻā§āĻ¤āĻāĻžāĻŦā§ āĻŦā§āĻ¯āĻžāĻ¨ā§āĻĄā§āĻ āĻāĻ°āĻž āĻšāĻ¯āĻŧāĨ¤ āĻ āĻ¨ā§āĻ¯ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻĒā§āĻ¯āĻžāĻĄ (āĻāĻāĻžāĻ° 15x8 āĻ¸ā§āĻŽāĻŋ) āĻāĻāĻāĻŋ āĻāĻžāĻŦā§āĻ āĻŦāĻž āĻŦā§āĻ¯āĻžāĻ¨ā§āĻĄā§āĻ āĻĻā§āĻŦāĻžāĻ°āĻž āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ¨ā§ āĻ°āĻžāĻāĻž āĻŽā§āĻāĻ° āĻĒāĻ¯āĻŧā§āĻ¨ā§āĻ āĻŦāĻžāĻ˛āĻŋ āĻā§āĻĄāĻŧā§ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻž āĻšāĻ¯āĻŧāĨ¤ āĻāĻžāĻŽāĻžāĻāĻžāĻĒāĻĄāĻŧ āĻāĻŦāĻ āĻŦā§āĻ¯āĻžāĻ¨ā§āĻĄā§āĻāĻā§ āĻāĻ°ā§āĻĻā§āĻ°āĻ¤āĻž āĻĨā§āĻā§ āĻ°āĻā§āĻˇāĻž āĻāĻ°āĻ¤ā§ āĻā§āĻ¯āĻžāĻāĻ¨ā§āĻ āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°ā§āĻ¨āĨ¤
Select a suitable duration and frequency of surge to gain a good contraction followed by complete relaxation of the muscle. Give several gentle contractions to allow the patient to become accustomed to the current, then increase the intensity gradually until a strong contraction is achieved.
āĻĒā§āĻļā§ āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻļāĻŋāĻĨāĻŋāĻ˛ āĻĻā§āĻŦāĻžāĻ°āĻž āĻ āĻ¨ā§āĻ¸āĻ°āĻŖ āĻāĻ°ā§ āĻāĻāĻāĻŋ āĻāĻžāĻ˛ āĻ¸āĻāĻā§āĻāĻ¨ āĻĒā§āĻ¤ā§ āĻāĻāĻāĻŋ āĻāĻĒāĻ¯ā§āĻā§āĻ¤ āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ āĻāĻŦāĻ āĻĸā§āĻāĻ¯āĻŧā§āĻ° āĻĢā§āĻ°āĻŋāĻā§āĻ¯āĻŧā§āĻ¨ā§āĻ¸āĻŋ āĻ¨āĻŋāĻ°ā§āĻŦāĻžāĻāĻ¨ āĻāĻ°ā§āĻ¨āĨ¤ āĻ°ā§āĻā§āĻā§ āĻ¸ā§āĻ°ā§āĻ¤ā§āĻ° āĻ¸āĻžāĻĨā§ āĻ āĻā§āĻ¯āĻ¸ā§āĻ¤ āĻšāĻāĻ¯āĻŧāĻžāĻ° āĻ āĻ¨ā§āĻŽāĻ¤āĻŋ āĻĻā§āĻāĻ¯āĻŧāĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻŦā§āĻļ āĻāĻ¯āĻŧā§āĻāĻāĻŋ āĻŽā§āĻĻā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻĻāĻŋāĻ¨, āĻ¤āĻžāĻ°āĻĒāĻ°ā§ āĻāĻāĻāĻŋ āĻļāĻā§āĻ¤āĻŋāĻļāĻžāĻ˛ā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻ¨āĻž āĻšāĻāĻ¯āĻŧāĻž āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻ§ā§āĻ°ā§ āĻ§ā§āĻ°ā§ āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻŦāĻžāĻĄāĻŧāĻžāĻ¨āĨ¤
The patient should be encouraged to contract the muscles voluntarily as the current is applied, and active exercises may be interspersed with the electrotherapy. Once the patient can achieve a voluntary contraction, discontinue the electrical stimulation. This can often be achieved after only one or two treatments.
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Small muscles of the foot
Electrical stimulation by faradic type current may be applied in baths. Water makes perfect contact with the tissues, the encumbrance of pad sand electrodes is avoided, and prolonged soaking reduces the resistance of the skin.
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Prepare the apparatus and patient as previously described Position the patient in sitting on a plinth with the back well supported and the feet on a stool which is covered with a plastic sheet. This position may have to be adapted for older patients or patients with a history of dizziness, but has the advantage that the physiotherapist is able to sit to manipulate the controls of the machine and at the same time observe the muscle contraction achieved. Place the patient’s foot in a bath containing enough warm water to cover the toes.
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Lumbrical muscles and interossei To stimulate the lumbrical muscles, place two electrodes transversely across the bottom of the bath, one under the heel and the other obliquely under the metatarsal heads To stimulate the plantar interossei, place one electrode on each side of the foot at the level of the metatarsal shafts to gain a good contraction followed by complete relaxation of the become accustomed to the current, then increase the intensity to contract the muscles voluntarily with the current. Active exercises may be interspersed with the electrotherapy. Once the patient can achieve a voluntary contraction, discontinue the electrical stimulation.
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Abductor hallucis Place one electrode under the heel and stimulate the muscle through the motor point using a button electrode Follow the procedure outlined for the lumbrical muscles.
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It may be impracticable to stimulate the small muscle groups of the foot in water if, for example, the patient has a foot infection, pes cavus, or an open unhealed wound. In these cases pads and electrodes may be used as an alternative, in exactly the same positions as previously described.
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Muscles of the pelvic floor
Electrical stimulation can be of considerable assistance in the re-education of these muscles in early cases of prolapse of the pelvic organs and in stress incontinence.
There are various methods of applying the current, but a good contraction of the muscles must be obtained and a vaginal electrode is often the most satisfactory method of achieving this. Voluntary contractions must be attempted at the same time as the electrical stimulation; electrotherapy is an accessory to the exercises, which are an essential part of the treatment.
-Male patients suffering from incontinence following prostatectomy may be treated by a corresponding method using a rectal electrode.
Prepare the apparatus and the patient as previously described. Position the patient in the side-lying position with a pillow between the lower legs. Place a plastic sheet under the patient. The in different electrode, with a large pad, is secured on the lumbosacral region. Sterilized lubrication jelly is rubbed onto the vaginal (or rectal) electrode which is then inserted into the vagina (or rectum). If no vaginal (or rectal) electrode is available or if this site is unusable, place a large button electrode over the urogenital region (or the anal region). Select a suitable duration and frequency of surge to gain a good contraction of muscle, then increase the intensity gradually until a strong contraction is achieved.
N.B. The muscles of the pelvic floor are thin, and fatigue rapidly. The duration of treatment should therefore be short.
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Reduction of limb edema
Electrical stimulation of the muscles that generally act as the muscle pump may be combined with compression and elevation of the limb to increase venous and lymphatic return and so relieve edema. This Prepare the apparatus and the patient as previously described lymphatic return. Contractions of many muscles are required, so place the electrodes and pads (large) so that they cover the motor points of the main muscle groups involved in the muscle pump ,e.g. for the on the plantar aspect of the foot. For the upper limb place electrodes over the flexor aspects of the forearm and the arm. Fix the pads in position firmly, with straps if necessary, and test the contraction produced. Adjust the pads as necessary. Then apply an elastic bandage, starting distally. It should be firm but not too tight. Avoid gaps between the turns of the bandage. The bandage increases the pressure on the vessels when the muscles contract, and as the muscles relax its recoil exerts a further pumping effect. The rate of contraction must be slow, to allow maximum contraction of the muscles. The repetition rate is slow also, to give time for relaxation and to allow the vessels time to refill; typical timing would be two to three minutes’ contraction followed by a rest of five minutes. A total per session of up to fifteen minutes contraction of muscles gives an overall treatment time of approximately 35 minutes. With the limb being edema to us the current may spread in the fluid, and so it may be difficult to obtain contractions by stimulation of motor points. In this case the muscles may be stimulated by another method of application, viz. nerve conduction. For this method of stimulation an indifferent electrode is applied to a convenient area and the active electrode to a point at which the nerve trunk is superficial. The method may also be used if the motor points are inaccessible method of stimulating the muscles of facial expression. For this ear, and one just above the angle of the jaw.
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Interrupted direct current
Interruption is the most usual modification of direct current, the flow of current commencing and ceasing at regular intervals. The rise and fall of intensity may be sudden (rectangular impulses) or gradual(trapezoidal, triangular, and saw-tooth impulses). These impulses are illustrated graphically. The impulses in which the current rises gradually are often termed “selective’, because a contraction of the denervated muscle can often be produced with an intensity of current that is insufficient to stimulate the motor nerves because accommodation occurs. The duration and frequency of the impulses can be adjusted, a duration of Ioo ms being commonly used, although it is often an advantage to increase this to 300 or 600 ms. An impulse of Ioo ms duration requires a frequency of about 30 per minute, but if the duration is increased the frequency must be reduced. The interval between the impulses should never be of shorter duration than the impulses themselves and is usually appreciably longer.
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Some equipment allows for a low-intensity reversed current between the impulses giving so-called depolarized impulses. The passage of a direct current (d.c.) through an electrolyte causes chemical changes to take place at the electrodes. Now that constant d.c. is rarely used the chances of chemical burn are much reduced: there is little danger when using pulsed d.c. and the risk is further reduced by the Circuit for the administration of interrupted direct current the slider of the rheostat can be set anywhere between A (zero current) and B (maximum current). use of depolarized impulses. The reverse wave of current between the impulses reduces the chemical formation, and if the quantity electricity passed in the reversed current is equal to that in the forward one any chemicals formed are neutralized and the danger of burn eliminated. There is a consequent reduction in irritation of the skin, smoking the treatment more comfortable for the patient.
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Production of interrupted d.c. is usually accomplished in modern apparatus by circuits which employ transistors and timing devices The length of the pulse of electricity produced can be varied by altering the parts of the circuit through which current flows ,and selector switch provides a choice of several different fixed-interval pulses and frequencies. Current is always applied to the patient via a potentiometer as this allows the intensity of current to be turned up from zero.
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Physiological effects of interrupted d.c–
Provided that the intensity of current and duration of impulses are adequate, a contraction of denervated muscle can be initiated. The contractions are sluggish, the contraction and relaxation being slower than when the mÅtor nerve is stimulated. As denervated muscle tissue has not the same property of accommodation as motor nerves, a current that rises fairly slowly is as effective in producing a contraction as one that rises suddenly. Moreover ,the slowly rising current can often produce a contraction of denervated muscle with a current that is insufficient to stimulate selectively the motor nerve. An impulse with a duration of 1oo ms is the shortest that is generally considered satisfactory for the treatment of denervated muscle, but it is often necessary to lengthen this impulse in order to eliminate contractions of innervated muscles. Both of these factors should be taken into consideration before treating the patient.
āĻ¤āĻŦā§ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻāĻŦāĻ āĻĒā§āĻ°āĻŦāĻžāĻšā§āĻ° āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ āĻĒāĻ°ā§āĻ¯āĻžāĻĒā§āĻ¤ āĻšāĻ˛ā§, āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻļā§āĻ°ā§ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ¸āĻāĻā§āĻāĻ¨ āĻŽāĻ¨ā§āĻĨāĻ° āĻšāĻ¯āĻŧ, āĻ¸āĻāĻā§āĻāĻ¨ āĻāĻŦāĻ āĻļāĻŋāĻĨāĻŋāĻ˛āĻāĻ°āĻŖ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻāĻ¯āĻŧāĻžāĻ° āĻā§āĻ¯āĻŧā§ āĻ§ā§āĻ° āĻšāĻ¯āĻŧāĨ¤ āĻ¯ā§āĻšā§āĻ¤ā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§ āĻāĻŋāĻ¸ā§āĻ¯ā§āĻ¤ā§ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻŽāĻ¤ā§ āĻĨāĻžāĻāĻžāĻ° āĻāĻāĻ āĻ¸āĻŽā§āĻĒāĻ¤ā§āĻ¤āĻŋ āĻ¨ā§āĻ, āĻāĻāĻāĻŋ āĻ¸ā§āĻ°ā§āĻ¤ āĻ¯ā§āĻāĻŋ āĻŽā§āĻāĻžāĻŽā§āĻāĻŋ āĻ§ā§āĻ°ā§ āĻŦā§āĻĻā§āĻ§āĻŋ āĻĒāĻžāĻ¯āĻŧ āĻ¤āĻž āĻšāĻ āĻžā§ āĻāĻ°ā§ āĻŦā§āĻĄāĻŧā§ āĻ¯āĻžāĻāĻ¯āĻŧāĻž āĻāĻāĻāĻŋ āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋāĻ¤ā§ āĻ¤āĻ¤āĻāĻžāĻ āĻāĻžāĻ°ā§āĻ¯āĻāĻ°āĨ¤ āĻ¤āĻĻā§āĻĒāĻ°āĻŋ, āĻ§ā§āĻ°ā§ āĻ§ā§āĻ°ā§ āĻā§āĻ°āĻŽāĻŦāĻ°ā§āĻ§āĻŽāĻžāĻ¨ āĻ¸ā§āĻ°ā§āĻ¤ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻ āĻāĻāĻāĻŋ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻ¸āĻžāĻĨā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¯āĻž āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻā§ āĻŦā§āĻā§ āĻ¨ā§āĻāĻ¯āĻŧāĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ āĻĒāĻ°ā§āĻ¯āĻžāĻĒā§āĻ¤āĨ¤ 1oo ms āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ā§āĻ° āĻāĻāĻāĻŋ āĻāĻŦā§āĻ āĻšāĻ˛ āĻ¸āĻŦāĻā§āĻ¯āĻŧā§ āĻāĻŽ āĻ¯āĻž āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻ° āĻāĻŋāĻāĻŋā§āĻ¸āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻŦāĻ˛ā§ āĻŦāĻŋāĻŦā§āĻāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻāĻŋāĻ¨ā§āĻ¤ā§ āĻ āĻ¨ā§āĻ¤āĻ°ā§āĻ¨āĻŋāĻšāĻŋāĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻĻā§āĻ° āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻāĻ āĻāĻŦā§āĻ āĻĻā§āĻ°ā§āĻāĻžāĻ¯āĻŧāĻŋāĻ¤ āĻāĻ°āĻž āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨āĨ¤ āĻ°ā§āĻā§āĻ° āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻž āĻāĻ°āĻžāĻ° āĻāĻā§ āĻāĻ āĻāĻāĻ¯āĻŧ āĻāĻžāĻ°āĻŖ āĻŦāĻŋāĻŦā§āĻāĻ¨āĻž āĻāĻ°āĻž āĻāĻāĻŋāĻ¤āĨ¤
When interrupted d.c. is applied to the body there is stimulation of sensory nerves. The impulses are of fairly long duration so the effect is rather marked, giving rise to a stabbing or burning sensation. There is reflex dilatation of the superficial blood vessels and consequent erythema of the skin. Stimulation of motor nerves with interrupted d.c. produces contraction of the muscles supplied. The stimuli are frequently repeated, so each one produces a brisk muscle twitch followed by immediate relaxation. There is therefore little beneficial effect on the muscles.
āĻŦāĻžāĻ§āĻžāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ˛ā§ d.c. āĻļāĻ°ā§āĻ°ā§ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻ¸āĻāĻŦā§āĻĻāĻ¨āĻļā§āĻ˛ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĨ¤ āĻāĻŦā§āĻāĻā§āĻ˛āĻŋ āĻŽā§āĻāĻžāĻŽā§āĻāĻŋ āĻĻā§āĻ°ā§āĻ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻšāĻ¯āĻŧ āĻ¤āĻžāĻ āĻĒā§āĻ°āĻāĻžāĻŦāĻāĻŋ āĻŦāĻ°āĻ āĻāĻŋāĻšā§āĻ¨āĻŋāĻ¤ āĻāĻ°āĻž āĻšāĻ¯āĻŧ, āĻ¯āĻž āĻāĻāĻāĻŋ āĻā§āĻ°āĻŋāĻāĻžāĻāĻžāĻ¤ āĻŦāĻž āĻā§āĻŦāĻ˛āĻ¨ā§āĻ¤ āĻ¸āĻāĻŦā§āĻĻāĻ¨ āĻ¸ā§āĻˇā§āĻāĻŋ āĻāĻ°ā§āĨ¤ āĻāĻĒāĻ°āĻŋāĻāĻžāĻā§āĻ° āĻ°āĻā§āĻ¤āĻ¨āĻžāĻ˛ā§āĻā§āĻ˛āĻŋāĻ° āĻ°āĻŋāĻĢā§āĻ˛ā§āĻā§āĻ¸ āĻĒā§āĻ°āĻ¸āĻžāĻ°āĻŖ āĻāĻŦāĻ āĻāĻ° āĻĢāĻ˛ā§ āĻ¤ā§āĻŦāĻā§āĻ° āĻāĻ°āĻŋāĻĨā§āĻŽāĻž āĻ°āĻ¯āĻŧā§āĻā§āĨ¤ āĻŦāĻŋāĻā§āĻ¨āĻŋāĻ¤ āĻĄāĻŋāĻ¸āĻŋ āĻ¸āĻš āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĨ¤ āĻ¸āĻ°āĻŦāĻ°āĻžāĻšāĻā§āĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°ā§āĨ¤ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻā§āĻ˛āĻŋ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻĒā§āĻ¨āĻ°āĻžāĻŦā§āĻ¤ā§āĻ¤āĻŋ āĻšāĻ¯āĻŧ, āĻ¤āĻžāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻāĻāĻāĻŋ āĻĻā§āĻ°ā§āĻ¤ āĻĒā§āĻļā§āĻ° āĻāĻžāĻāĻā§āĻ¨āĻŋ āĻ¤ā§āĻ°āĻŋ āĻāĻ°ā§ āĻ¯āĻžāĻ° āĻĒāĻ°ā§ āĻ¤āĻžāĻ¤ā§āĻā§āĻˇāĻŖāĻŋāĻ āĻļāĻŋāĻĨāĻŋāĻ˛āĻ¤āĻž āĻšāĻ¯āĻŧāĨ¤ āĻ¤āĻžāĻ āĻĒā§āĻļā§āĻ¤ā§ āĻ¸āĻžāĻŽāĻžāĻ¨ā§āĻ¯ āĻāĻĒāĻāĻžāĻ°ā§ āĻĒā§āĻ°āĻāĻžāĻŦ āĻ¨ā§āĻāĨ¤
Indications for use of interrupted d.c.
The main value of interrupted d.c. lies in its ability to produce contractions of denervated muscles. When a muscle is deprived of its nerve supply, changes in its structure and properties tend to occur. There is marked wasting of the muscle fibres and, if degeneration is of long-standing, they tend to become fibrosed and to lose their properties of irritability, contract ability, extensibility and elasticity. Electrical stimulation of the muscle fibres cv may slow down these changes, although no one has ever shown in a controlled experiment that this is so, and it is doubtful whether it is possible to restore the that this is so, and it is doubtful whether it is possible to restore the muscle bulk or properties by these means once they have been lost. Some authorities consider electrical stimulation unnecessary, as it is only after a considerable period of denervation that irreversible changes take place in the muscle fibres, and lost muscle bulk can restored by exercise once re-innervation occurs.
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If electrical stimulation is used, it must be strong enough to produce a muscle contraction, and an adequate number of contractions must be produced. Three hundred contractions of each muscle are desirable a teach treatment. This is not always possible, either because the muscle becomes fatigued or because, if many muscles are affected, the duration of the treatment would be excessive. Ninety is usually regarded as the minimum number of contractions for treatment to be effective, though if fatigue occurs before this number is reached the treatment time should be shortened
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In the early stages of re-innervation, electrical stimulation may be useful as a means of re-education if the patient cannot get the feel of there covering muscle. One should then use a pulse duration which is comfortable for the patient and gives the best contraction. This may well be a long-duration current, even though the patient has shown voluntary movement. It is important not to assume that because the muscle is recovering, a faradic type of current (short-duration current must be used. In fact a long duration current, say 30 ms, may be more comfortable and more effective.
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Selection of type of impulse
If a good muscle contraction is obtained with a rectangular impulse satisfactory. The difference between the various types of impulse lies in the rectangular impulses the rise is sudden, with the trapezoidal it is slower still, provided that the impulses are of the same duration (see advantages that a contraction of the denervated muscle is often obtained current than that required to stimulate motor nerves so that unwanted contractions of normally innervated muscles in the region are eliminated. In long-standing denervation, a muscle contraction may be obtained with a slowly rising current when there is no longer any response to a rectangular impulse.
āĻ¯āĻĻāĻŋ āĻāĻāĻāĻŋ āĻāĻžāĻ˛ āĻĒā§āĻļā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻāĻāĻāĻŋ āĻāĻ¯āĻŧāĻ¤āĻā§āĻˇā§āĻ¤ā§āĻ°āĻžāĻāĻžāĻ° āĻāĻŦā§āĻ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻ¸āĻā§āĻā§ āĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻāĻ°āĻž āĻšāĻ¯āĻŧāĨ¤ āĻŦāĻŋāĻāĻŋāĻ¨ā§āĻ¨ āĻ§āĻ°āĻŖā§āĻ° āĻāĻŦā§āĻā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻĒāĻžāĻ°ā§āĻĨāĻā§āĻ¯ āĻšāĻ˛ āĻāĻ¯āĻŧāĻ¤āĻā§āĻˇā§āĻ¤ā§āĻ°āĻžāĻāĻžāĻ° āĻāĻŦā§āĻā§āĻ° āĻŦā§āĻĻā§āĻ§āĻŋ āĻšāĻ āĻžā§ āĻāĻ°ā§, āĻā§āĻ°ā§āĻ¯āĻžāĻĒāĻŋāĻāĻ¯āĻŧā§āĻĄāĻžāĻ˛ āĻ¸āĻš āĻāĻāĻŋ āĻ§ā§āĻ°āĻāĻ¤āĻŋāĻ° āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻļāĻ°ā§āĻ¤ āĻĨāĻžāĻā§ āĻ¯ā§ āĻāĻŦā§āĻāĻā§āĻ˛āĻŋ āĻāĻāĻ āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ā§āĻ° āĻšāĻ¯āĻŧ (āĻ¸ā§āĻŦāĻŋāĻ§āĻžāĻā§āĻ˛āĻŋ āĻĻā§āĻā§āĻ¨ āĻ¯ā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ā§āĻ° āĻ¤ā§āĻ˛āĻ¨āĻžāĻ¯āĻŧ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯, āĻ¯āĻžāĻ¤ā§ āĻāĻ āĻ āĻā§āĻāĻ˛ā§ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻāĻāĻžāĻŦā§ āĻāĻĻā§āĻāĻžāĻŦāĻŋāĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻ āĻŦāĻžāĻā§āĻāĻŋāĻ¤ āĻ¸āĻāĻā§āĻāĻ¨ āĻĻā§āĻ° āĻšāĻ¯āĻŧ āĻĻā§āĻ°ā§āĻāĻ¸ā§āĻĨāĻžāĻ¯āĻŧā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻļāĻ¨ā§ āĻāĻāĻāĻŋ āĻĒā§āĻļā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻāĻāĻāĻŋ āĻ§ā§āĻ°ā§ āĻ§ā§āĻ°ā§ āĻā§āĻ°āĻŽāĻŦāĻ°ā§āĻ§āĻŽāĻžāĻ¨ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻ¸āĻžāĻĨā§ āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻāĻ¯āĻŧāĻ¤āĻā§āĻˇā§āĻ¤ā§āĻ°āĻžāĻāĻžāĻ° āĻāĻŦā§āĻā§āĻ° āĻāĻ° āĻā§āĻ¨ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¨ā§āĻā§ˇ
When various types of impulse are available it is advisable to attempt stimulation with each in order to ascertain which produces the most satisfactory contraction. It is often found that the more long-standing the denervation the slower the rise in intensity of current that is required.
āĻ¯āĻāĻ¨ āĻŦāĻŋāĻāĻŋāĻ¨ā§āĻ¨ āĻ§āĻ°āĻŖā§āĻ° āĻāĻŦā§āĻ āĻāĻĒāĻ˛āĻŦā§āĻ§ āĻĨāĻžāĻā§ āĻ¤āĻāĻ¨ āĻā§āĻ¨āĻāĻŋ āĻ¸āĻŦāĻā§āĻ¯āĻŧā§ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°ā§ āĻ¤āĻž āĻ¨āĻŋāĻļā§āĻāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋāĻ° āĻ¸āĻžāĻĨā§ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻā§āĻˇā§āĻāĻž āĻāĻ°āĻžāĻ° āĻĒāĻ°āĻžāĻŽāĻ°ā§āĻļ āĻĻā§āĻāĻ¯āĻŧāĻž āĻšāĻ¯āĻŧāĨ¤ āĻāĻāĻŋ āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧ āĻ¯ā§ āĻ¯āĻ¤ āĻŦā§āĻļāĻŋ āĻĻā§āĻ°ā§āĻāĻ¸ā§āĻĨāĻžāĻ¯āĻŧā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻļāĻ¨ āĻ¤āĻ¤ āĻ§ā§āĻ° āĻāĻ¤āĻŋāĻ¤ā§ āĻ¸ā§āĻ°ā§āĻ¤ā§āĻ° āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻŦā§āĻĻā§āĻ§āĻŋāĻ° āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ āĻšāĻ¯āĻŧāĨ¤
Duration of impulse
An impulse of at least 1oo ms is necessary in order to ensure that all the denervated muscle fibres are stimulated: if shorter impulses are used some of the muscle fibres may fail to contract. When attempting to eliminate contractions of normally innervated muscles or to stimulate a muscle which has been denervated for some time, it is usually necessary to increase the duration of the impulses to 300 or 600 ms.
āĻāĻŦā§āĻā§āĻ° āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§ āĻĢāĻžāĻāĻŦāĻžāĻ°āĻā§āĻ˛āĻŋāĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻž āĻ¨āĻŋāĻļā§āĻāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻāĻŽāĻĒāĻā§āĻˇā§ 1oo ms āĻāĻ° āĻāĻāĻāĻŋ āĻāĻŦā§āĻ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨: āĻ¯āĻĻāĻŋ āĻ¸āĻāĻā§āĻˇāĻŋāĻĒā§āĻ¤ āĻāĻŦā§āĻ āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻāĻŋāĻā§ āĻĒā§āĻļā§ āĻ¤āĻ¨ā§āĻ¤ā§ āĻ¸āĻāĻā§āĻāĻŋāĻ¤ āĻšāĻ¤ā§ āĻŦā§āĻ¯āĻ°ā§āĻĨ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ¯āĻāĻ¨ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻāĻāĻžāĻŦā§ āĻāĻĻā§āĻā§āĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻĻā§āĻ° āĻāĻ°āĻžāĻ° āĻā§āĻˇā§āĻāĻž āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻŦāĻž āĻāĻŋāĻā§ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻāĻ¨ā§āĻ¯ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻā§āĻˇā§āĻāĻž āĻāĻ°āĻž āĻšāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻāĻŦā§āĻā§āĻ° āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ 300 āĻŦāĻž 600 ms āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻŦāĻžāĻĄāĻŧāĻžāĻ¤ā§ āĻšāĻ¯āĻŧāĨ¤
Techniques of treatment with interrupted d.c.
Methods of application
When applying modified d.c., the aim of treatment is direct stimulation of the muscle fibres, therefore the treatment must be arranged so that the current passes through all the fibres of the muscle. There are various methods of achieving this.
āĻŦāĻžāĻ§āĻžāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻĄāĻŋāĻ¸āĻŋ āĻ¸āĻš āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻžāĻ° āĻā§āĻļāĻ˛ āĻĒā§āĻ°āĻ¯āĻŧā§āĻā§āĻ° āĻĒāĻĻā§āĻ§āĻ¤āĻŋ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ d.c. āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻžāĻ° āĻ¸āĻŽāĻ¯āĻŧ, āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻžāĻ° āĻ˛āĻā§āĻˇā§āĻ¯ āĻšāĻ˛ āĻĒā§āĻļā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋāĻ° āĻ¸āĻ°āĻžāĻ¸āĻ°āĻŋ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻž, āĻ¤āĻžāĻ āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻžāĻ° āĻŦā§āĻ¯āĻŦāĻ¸ā§āĻĨāĻž āĻāĻ°āĻž āĻāĻŦāĻļā§āĻ¯āĻ āĻ¯āĻžāĻ¤ā§ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻĒā§āĻļā§āĻ° āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻĢāĻžāĻāĻŦāĻžāĻ°āĻā§āĻ˛āĻŋāĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻāĻāĻŋ āĻ āĻ°ā§āĻāĻ¨ā§āĻ° āĻŦāĻŋāĻāĻŋāĻ¨ā§āĻ¨ āĻĒāĻĻā§āĻ§āĻ¤āĻŋ āĻ°āĻ¯āĻŧā§āĻā§āĨ¤
One pad may be fixed over the origin of the muscle group, and each muscle stimulated in turn with the active electrode. The active electrode is a disc or small pad which is either held over the lower end of the fleshy belly of the muscle to be stimulated or stroked slowly down it (labile technique). Moving the electrode over the muscle ensures that the current passes through the maximum number of fibres. There is also less irritation of the skin when the active electrode is held in the same position throughout. Both these method shave the advantages that the current can be regulated to produce the optimum contraction of each muscle, and that each muscle is rested while other muscles of the group are being stimulated. They suffer from the disadvantage that if there are many muscles to be stimulated it is not practicable to produce a large number of contractions of each.
āĻāĻāĻāĻŋ āĻĒā§āĻ¯āĻžāĻĄ āĻĒā§āĻļā§ āĻā§āĻ°ā§āĻĒā§āĻ° āĻā§āĻĒāĻ¤ā§āĻ¤āĻŋāĻ° āĻāĻĒāĻ° āĻ¸ā§āĻĨāĻŋāĻ° āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻāĻŦāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻļā§ āĻ¸āĻā§āĻ°āĻŋāĻ¯āĻŧ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄā§āĻ° āĻ¸āĻžāĻĨā§ āĻĒāĻžāĻ˛āĻžāĻā§āĻ°āĻŽā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻ¸āĻā§āĻ°āĻŋāĻ¯āĻŧ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻšāĻ˛ āĻāĻāĻāĻŋ āĻĄāĻŋāĻ¸ā§āĻ āĻŦāĻž āĻā§āĻ āĻĒā§āĻ¯āĻžāĻĄ āĻ¯āĻž āĻšāĻ¯āĻŧ āĻĒā§āĻļā§āĻ° āĻŽāĻžāĻāĻ¸āĻ˛ āĻĒā§āĻā§āĻ° āĻ¨ā§āĻā§āĻ° āĻĒā§āĻ°āĻžāĻ¨ā§āĻ¤ā§ āĻ§āĻ°ā§ āĻ°āĻžāĻāĻž āĻšāĻ¯āĻŧ āĻ¯āĻžāĻ¤ā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻŦāĻž āĻ§ā§āĻ°ā§ āĻ§ā§āĻ°ā§ āĻ¸ā§āĻā§āĻ°ā§āĻ āĻāĻ°āĻž āĻšāĻ¯āĻŧ (āĻ˛ā§āĻŦāĻžāĻāĻ˛ āĻā§āĻāĻ¨āĻŋāĻ)āĨ¤ āĻĒā§āĻļā§āĻ° āĻāĻĒāĻ° āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻ¸āĻ°āĻžāĻ¨ā§ āĻ¨āĻŋāĻļā§āĻāĻŋāĻ¤ āĻāĻ°ā§ āĻ¯ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻ¸āĻ°ā§āĻŦāĻžāĻ§āĻŋāĻ āĻ¸āĻāĻā§āĻ¯āĻ āĻĢāĻžāĻāĻŦāĻžāĻ°ā§āĻ° āĻŽāĻ§ā§āĻ¯ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻ¸āĻā§āĻ°āĻŋāĻ¯āĻŧ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻā§āĻĄāĻŧā§ āĻāĻāĻ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ¨ā§ āĻĨāĻžāĻāĻž āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ¯āĻŧ āĻ¤ā§āĻŦāĻā§āĻ° āĻā§āĻŦāĻžāĻ˛āĻžāĻ āĻāĻŽ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻāĻāĻ¯āĻŧ āĻĒāĻĻā§āĻ§āĻ¤āĻŋāĻ° āĻ¸ā§āĻŦāĻŋāĻ§āĻžāĻā§āĻ˛āĻŋ āĻļā§āĻ āĻāĻ°ā§ āĻ¯ā§ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻļā§āĻ° āĻ¸āĻ°ā§āĻŦā§āĻ¤ā§āĻ¤āĻŽ āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°āĻ¤ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻāĻā§ āĻ¨āĻŋāĻ¯āĻŧāĻ¨ā§āĻ¤ā§āĻ°āĻŋāĻ¤ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŦāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻļā§ āĻŦāĻŋāĻļā§āĻ°āĻžāĻŽā§ āĻĨāĻžāĻā§ āĻ¯āĻāĻ¨ āĻā§āĻ°ā§āĻĒā§āĻ° āĻ āĻ¨ā§āĻ¯āĻžāĻ¨ā§āĻ¯ āĻĒā§āĻļā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻ¤āĻžāĻ°āĻž āĻāĻ āĻ āĻ¸ā§āĻŦāĻŋāĻ§āĻžāĻ¯āĻŧ āĻā§āĻāĻā§ āĻ¯ā§ āĻ¯āĻĻāĻŋ āĻ āĻ¨ā§āĻāĻā§āĻ˛āĻŋ āĻĒā§āĻļā§āĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻ¤ā§ āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋāĻ° āĻĒā§āĻ°āĻā§āĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻ¤ā§āĻ°āĻŋ āĻāĻ°āĻž āĻŦāĻžāĻ¸ā§āĻ¤āĻŦāĻ¸āĻŽā§āĻŽāĻ¤ āĻ¨āĻ¯āĻŧāĨ¤
As an alternative two disc electrodes may be used, one placed over each end of the muscle to be stimulated. This method is useful for the stimulation of deeply placed muscles which are difficult to isolate such as the extensor pollicis longus ,but it is difficult for the operator to hold both electrodes and at the same time to regulate the current intensity. The two pads may be fixed (stabile technique), one over the origin and the other over the lower end of the muscle group to be stimulated. Provided that all the muscles contract equally, this method as the advantage that it permits a large number of contractions to be elicited. However, great care must be taken that all the muscles contract satisfactorily. There may also be a tendency for current to leak on to surrounding innervated muscles, but their contraction can usually be eliminated by the use of selective impulses of adequate duration. Another technique which may be convenient is to apply an active pad which completely covers the muscle or group of muscles to be stimulated, the circuit being completed with a large directing or indifferent electrode. This method is satisfactory, for example, for the muscles of the shoulder girdle, when an indifferent electrode can be placed on the upper part of the anterior chest wall and a pad with a plate electrode held over each of the muscles in turn.\
āĻāĻāĻāĻŋ āĻŦāĻŋāĻāĻ˛ā§āĻĒ āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻĻā§āĻāĻŋ āĻĄāĻŋāĻ¸ā§āĻ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻāĻāĻāĻŋ āĻĒā§āĻļā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻ°āĻžāĻ¨ā§āĻ¤ā§ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻ¯āĻžāĻ¤ā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻĒāĻĻā§āĻ§āĻ¤āĻŋāĻāĻŋ āĻāĻā§āĻ°āĻāĻžāĻŦā§ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻž āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻĻāĻ°āĻāĻžāĻ°ā§ āĻ¯ā§āĻā§āĻ˛āĻŋāĻā§ āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻāĻ°āĻž āĻāĻ āĻŋāĻ¨ āĻ¯ā§āĻŽāĻ¨ āĻāĻā§āĻ¸āĻā§āĻ¨āĻ¸āĻ° āĻĒāĻ˛āĻŋāĻ¸āĻŋāĻ¸ āĻ˛āĻāĻāĻžāĻ¸, āĻ¤āĻŦā§ āĻ āĻĒāĻžāĻ°ā§āĻāĻ°ā§āĻ° āĻĒāĻā§āĻˇā§ āĻāĻāĻ¯āĻŧ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻ§āĻ°ā§ āĻ°āĻžāĻāĻž āĻāĻŦāĻ āĻāĻāĻ āĻ¸āĻžāĻĨā§ āĻŦāĻ°ā§āĻ¤āĻŽāĻžāĻ¨ āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻ¨āĻŋāĻ¯āĻŧāĻ¨ā§āĻ¤ā§āĻ°āĻŖ āĻāĻ°āĻž āĻāĻ āĻŋāĻ¨āĨ¤ āĻĻā§āĻāĻŋ āĻĒā§āĻ¯āĻžāĻĄ āĻ¸ā§āĻĨāĻŋāĻ° (āĻ¸ā§āĻĨāĻŋāĻ¤āĻŋāĻļā§āĻ˛ āĻā§āĻļāĻ˛) āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻāĻāĻāĻŋ āĻāĻ¤ā§āĻ¸ā§āĻ° āĻāĻĒāĻ°ā§ āĻāĻŦāĻ āĻ āĻ¨ā§āĻ¯āĻāĻŋ āĻĒā§āĻļā§ āĻā§āĻ°ā§āĻĒā§āĻ° āĻ¨ā§āĻā§āĻ° āĻĒā§āĻ°āĻžāĻ¨ā§āĻ¤ā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻļāĻ°ā§āĻ¤ āĻĨāĻžāĻā§ āĻ¯ā§ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻĒā§āĻļā§ āĻ¸āĻŽāĻžāĻ¨āĻāĻžāĻŦā§ āĻ¸āĻāĻā§āĻāĻ¨ āĻāĻ°ā§, āĻāĻ āĻĒāĻĻā§āĻ§āĻ¤āĻŋāĻāĻŋ āĻ¸ā§āĻŦāĻŋāĻ§āĻž āĻšāĻŋāĻ¸āĻžāĻŦā§ āĻ¯ā§ āĻāĻāĻŋ āĻĒā§āĻ°āĻā§āĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖā§ āĻ¸āĻāĻā§āĻāĻ¨ā§āĻ° āĻ āĻ¨ā§āĻŽāĻ¤āĻŋ āĻĻā§āĻ¯āĻŧāĨ¤ āĻ¯āĻžāĻāĻšā§āĻ, āĻŽāĻšāĻžāĻ¨ āĻ¸āĻ¤āĻ°ā§āĻāĻ¤āĻž āĻ āĻŦāĻ˛āĻŽā§āĻŦāĻ¨ āĻāĻ°āĻž āĻāĻŦāĻļā§āĻ¯āĻ āĻ¯ā§ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻĒā§āĻļā§ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻāĻāĻžāĻŦā§ āĻ¸āĻāĻā§āĻāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻāĻļā§āĻĒāĻžāĻļā§āĻ° āĻāĻ¨āĻžāĻ°āĻā§āĻā§āĻĄ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ¤ā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻ˛āĻŋāĻ āĻšāĻāĻ¯āĻŧāĻžāĻ° āĻĒā§āĻ°āĻŦāĻŖāĻ¤āĻžāĻ āĻĨāĻžāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¤āĻŦā§ āĻ¤āĻžāĻĻā§āĻ° āĻ¸āĻāĻā§āĻāĻ¨ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻĒāĻ°ā§āĻ¯āĻžāĻĒā§āĻ¤ āĻ¸āĻŽāĻ¯āĻŧāĻāĻžāĻ˛ā§āĻ° āĻ¨āĻŋāĻ°ā§āĻŦāĻžāĻāĻ¨ā§ āĻāĻŦā§āĻ āĻŦā§āĻ¯āĻŦāĻšāĻžāĻ° āĻāĻ°ā§ āĻ¨āĻŋāĻ°ā§āĻŽā§āĻ˛ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻ°ā§āĻāĻāĻŋ āĻā§āĻļāĻ˛ āĻ¯āĻž āĻ¸ā§āĻŦāĻŋāĻ§āĻžāĻāĻ¨āĻ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¤āĻž āĻšāĻ˛ āĻāĻāĻāĻŋ āĻ¸āĻā§āĻ°āĻŋāĻ¯āĻŧ āĻĒā§āĻ¯āĻžāĻĄ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻ¯āĻž āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖāĻ°ā§āĻĒā§ āĻĒā§āĻļā§ āĻŦāĻž āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻā§āĻ°ā§āĻĒāĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻāĻŦā§āĻ¤ āĻāĻ°ā§, āĻāĻāĻāĻŋ āĻŦāĻĄāĻŧ āĻ¨āĻŋāĻ°ā§āĻĻā§āĻļāĻ āĻŦāĻž āĻāĻĻāĻžāĻ¸ā§āĻ¨ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻĻāĻŋāĻ¯āĻŧā§ āĻ¸āĻžāĻ°ā§āĻāĻŋāĻāĻāĻŋ āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻāĻ°āĻž āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻĒāĻĻā§āĻ§āĻ¤āĻŋāĻāĻŋ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ, āĻāĻĻāĻžāĻšāĻ°āĻŖāĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻāĻžāĻāĻ§ā§āĻ° āĻā§āĻŽāĻ°ā§āĻ° āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻāĻ¨ā§āĻ¯, āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻāĻĻāĻžāĻ¸ā§āĻ¨ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻ¸āĻžāĻŽāĻ¨ā§āĻ° āĻŦā§āĻā§āĻ° āĻĒā§āĻ°āĻžāĻā§āĻ°ā§āĻ° āĻāĻĒāĻ°ā§āĻ° āĻ āĻāĻļā§ āĻ¸ā§āĻĨāĻžāĻĒāĻ¨ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŦāĻ āĻāĻāĻāĻŋ āĻĒā§āĻ˛ā§āĻ āĻāĻ˛ā§āĻā§āĻā§āĻ°ā§āĻĄ āĻ¸āĻš āĻāĻāĻāĻŋ āĻĒā§āĻ¯āĻžāĻĄ āĻĒā§āĻ°āĻ¤āĻŋāĻāĻŋ āĻĒā§āĻļā§āĻ° āĻāĻĒāĻ°ā§ āĻ°āĻžāĻāĻž āĻšāĻ¯āĻŧāĨ¤
Preparation of equipment
The apparatus is tested and the other equipment prepared as for the treatments previously described. Make sure that the coverings of the disc electrodes and the pads consist of at least eight layers of lint. This is because it is possible to get a chemical burn with long-duration pulses if the treatment is given at the same spot for long periods of time, particularly if the current selected is without the reverse wave of current between the impulses (i.e. it is not depolarized). No metal should be allowed to come into contact with the patient’s tissues.
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Preparation of the patient
The skin is prepared by washing and protecting abrasions as for other electrical treatments, It is often an advantage to soak the part in warm water before the treatment to lower the resistance of the skin and to warm the muscles, although if there is extensive loss of sensation care must be taken that the water is not too hot. Contractions are obtained most easily if the part is supported so that the muscles to be stimulated are in a shortened position. Alternatively, the current may be applied with the muscles in a partly lengthened position: this should only be done if the contractions produced are sufficiently strong to cause shortening of the muscle and so joint movement. If this is achieved the load opposing the muscle action should increase the beneficial effects. It is usually possible to produce movement only in the smaller joints, e.g. the wrist.
āĻ°ā§āĻā§āĻ° āĻĒā§āĻ°āĻ¸ā§āĻ¤ā§āĻ¤āĻŋ āĻ āĻ¨ā§āĻ¯āĻžāĻ¨ā§āĻ¯ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻžāĻ° āĻŽāĻ¤ā§ āĻāĻ°ā§āĻˇāĻŖāĻā§āĻ˛āĻŋ āĻ§ā§āĻ¯āĻŧā§ āĻāĻŦāĻ āĻ°āĻā§āĻˇāĻž āĻāĻ°ā§ āĻ¤ā§āĻŦāĻ āĻĒā§āĻ°āĻ¸ā§āĻ¤ā§āĻ¤ āĻāĻ°āĻž āĻšāĻ¯āĻŧ, āĻ¤ā§āĻŦāĻā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻ°ā§āĻ§ āĻā§āĻˇāĻŽāĻ¤āĻž āĻāĻŽāĻžāĻ¤ā§ āĻāĻŦāĻ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻā§ āĻāĻˇā§āĻŖ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻāĻŋāĻāĻŋāĻ¤ā§āĻ¸āĻžāĻ° āĻāĻā§ āĻ āĻāĻļāĻāĻŋ āĻāĻ°āĻŽ āĻāĻ˛ā§ āĻāĻŋāĻāĻŋāĻ¯āĻŧā§ āĻ°āĻžāĻāĻž āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻļāĻ āĻāĻāĻāĻŋ āĻ¸ā§āĻŦāĻŋāĻ§āĻžāĻāĻ¨āĻ, āĻ¯āĻĻāĻŋāĻ āĻ¯āĻĻāĻŋ āĻāĻ° āĻŦā§āĻ¯āĻžāĻĒāĻ āĻā§āĻˇāĻ¤āĻŋ āĻšāĻ¯āĻŧāĨ¤ āĻ¸āĻāĻŦā§āĻĻāĻ¨ āĻ¯āĻ¤ā§āĻ¨ āĻ¨āĻŋāĻ¤ā§ āĻšāĻŦā§ āĻāĻ˛ āĻā§āĻŦ āĻāĻ°āĻŽ āĻ¨āĻž. āĻ¸āĻāĻā§āĻāĻ¨āĻā§āĻ˛āĻŋ āĻā§āĻŦ āĻ¸āĻšāĻā§ āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧ āĻ¯āĻĻāĻŋ āĻ āĻāĻļāĻāĻŋ āĻ¸āĻŽāĻ°ā§āĻĨāĻŋāĻ¤ āĻšāĻ¯āĻŧ āĻ¯āĻžāĻ¤ā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋ āĻāĻāĻāĻŋ āĻ¸āĻāĻā§āĻˇāĻŋāĻĒā§āĻ¤ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ¨ā§ āĻĨāĻžāĻā§āĨ¤ āĻŦāĻŋāĻāĻ˛ā§āĻĒāĻāĻžāĻŦā§, āĻāĻāĻāĻŋ āĻāĻāĻļāĻŋāĻāĻāĻžāĻŦā§ āĻĻā§āĻ°ā§āĻāĻžāĻ¯āĻŧāĻŋāĻ¤ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ¨ā§ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻ¸āĻžāĻĨā§ āĻāĻžāĻ°ā§āĻ¨ā§āĻ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§: āĻāĻāĻŋ āĻļā§āĻ§ā§āĻŽāĻžāĻ¤ā§āĻ° āĻ¤āĻāĻ¨āĻ āĻāĻ°āĻž āĻāĻāĻŋāĻ¤ āĻ¯āĻāĻ¨ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻŋāĻ¤ āĻ¸āĻāĻā§āĻāĻ¨āĻā§āĻ˛āĻŋ āĻĒā§āĻļā§ āĻā§āĻ āĻāĻ°āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ¯āĻĨā§āĻˇā§āĻ āĻļāĻā§āĻ¤āĻŋāĻļāĻžāĻ˛ā§ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻ¤āĻžāĻ āĻāĻ¯āĻŧā§āĻ¨ā§āĻ āĻ¨āĻĄāĻŧāĻžāĻāĻĄāĻŧāĻž āĻāĻ°ā§āĨ¤ āĻāĻāĻŋ āĻ āĻ°ā§āĻāĻ¨ āĻāĻ°āĻž āĻšāĻ˛ā§ āĻĒā§āĻļā§ āĻāĻ°ā§āĻŽā§āĻ° āĻŦāĻŋāĻ°ā§āĻ§ā§ āĻ˛ā§āĻĄ āĻāĻĒāĻāĻžāĻ°ā§ āĻĒā§āĻ°āĻāĻžāĻŦ āĻŦā§āĻĻā§āĻ§āĻŋ āĻāĻ°āĻž āĻāĻāĻŋāĻ¤. āĻāĻāĻŋ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻļā§āĻ§ā§āĻŽāĻžāĻ¤ā§āĻ° āĻā§āĻ āĻāĻ¯āĻŧā§āĻ¨ā§āĻāĻā§āĻ˛ā§āĻ¤ā§ āĻāĻ¨ā§āĻĻā§āĻ˛āĻ¨ āĻāĻ¤ā§āĻĒāĻžāĻĻāĻ¨ āĻāĻ°āĻž āĻ¸āĻŽā§āĻāĻŦ, āĻ¯ā§āĻŽāĻ¨ āĻāĻŦā§āĻāĻŋ
Application of interrupted d.c.
Muscle contractions are often obtained most easily if the active electrode is connected to the anode, but this is not always the case. Each patient should be tested to determine whether the anode or the cathode produces the better response, and the more effective pole used for the active electrode.
When the electrodes have been applied the intensity of current is increased until a good muscle contraction is obtained. A large number of contractions is desirable, but any sign of fatigue, such as weakening of the contraction, is an indication for limiting the length of the treatment. Contractions are usually produced in groups, allowing rest periods between.
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Electro diagnosis
Changes in electrical reactions
When there is disease or injury of motor nerves or muscles, alterations are liable to occur in their response to electrical stimulation. The altered electrical reactions may be of considerable assistance in diagnosing the type and extent of the lesion.
Reduction or loss of voluntary power of a muscle may be due to:
I. A lesion of the upper motor neuron.
2. A lesion of the lower motor neuron.
3. Damage to the muscle itself.
4. A fault at the neuromuscular junction.
5. A functional disorder.
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The parts of the motor pathway that are normally accessible for electrical stimulation are the lower motor neuron below its exit from the vertebral canal and the muscle itself, but not the anterior horn cell or the upper motor neuron.
āĻŽā§āĻāĻ° āĻĒāĻžāĻĨāĻāĻ¯āĻŧā§āĻ° āĻ¯ā§ āĻ āĻāĻļāĻā§āĻ˛āĻŋ āĻ¸āĻžāĻ§āĻžāĻ°āĻŖāĻ¤ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻāĻ¨ā§āĻ¯ āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¸āĻ¯ā§āĻā§āĻ¯ āĻ¤āĻž āĻšāĻ˛ āĻ¨ā§āĻā§āĻ° āĻŽā§āĻāĻ° āĻ¨āĻŋāĻāĻ°āĻ¨ āĻ¯āĻž āĻŽā§āĻ°ā§āĻĻāĻŖā§āĻĄā§āĻ° āĻāĻžāĻ˛ āĻāĻŦāĻ āĻĒā§āĻļā§ āĻĨā§āĻā§ āĻĒā§āĻ°āĻ¸ā§āĻĨāĻžāĻ¨ āĻāĻ°āĻžāĻ° āĻ¨ā§āĻā§ āĻĨāĻžāĻā§, āĻ¤āĻŦā§ āĻ āĻā§āĻ°āĻŦāĻ°ā§āĻ¤ā§ āĻšāĻ°ā§āĻ¨ āĻ¸ā§āĻ˛ āĻŦāĻž āĻāĻĒāĻ°ā§āĻ° āĻŽā§āĻāĻ° āĻ¨āĻŋāĻāĻ°āĻ¨ āĻ¨āĻ¯āĻŧāĨ¤
Upper motor-neuron lesions
When there is a lesion of the upper motor neuron, there are no changes in the lower motor neuron or muscle (i.e. in the accessible part of the more or pathway) which would lead to altered electrical reactions. Consequently, a normal type of response is obtained with electrical stimulation, although sometimes the nerve and muscle are hyper-excitable and react to a lower intensity of current than that normally required.
āĻāĻĒāĻ°ā§āĻ° āĻŽā§āĻāĻ°-āĻ¨āĻŋāĻāĻ°āĻ¨ āĻā§āĻˇāĻ¤ āĻ¯āĻāĻ¨ āĻāĻĒāĻ°ā§āĻ° āĻŽā§āĻāĻ°-āĻ¨āĻŋāĻāĻ°ā§āĻ¨ā§āĻ° āĻāĻāĻāĻŋ āĻā§āĻˇāĻ¤ āĻĨāĻžāĻā§, āĻ¤āĻāĻ¨ āĻ¨ā§āĻā§āĻ° āĻŽā§āĻāĻ°-āĻ¨āĻŋāĻāĻ°āĻ¨ āĻŦāĻž āĻĒā§āĻļā§āĻ¤ā§ (āĻ āĻ°ā§āĻĨāĻžā§ āĻāĻ°āĻ āĻŦāĻž āĻĒāĻĨā§āĻ° āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¸āĻ¯ā§āĻā§āĻ¯ āĻ āĻāĻļā§) āĻā§āĻ¨ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻšāĻ¯āĻŧ āĻ¨āĻž āĻ¯āĻž āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻŋāĻ¤ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ° āĻĻāĻŋāĻā§ āĻ¨āĻŋāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧāĨ¤ āĻĢāĻ˛āĻ¸ā§āĻŦāĻ°ā§āĻĒ, āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻŽāĻžāĻ§ā§āĻ¯āĻŽā§ āĻāĻāĻāĻŋ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧ, āĻ¯āĻĻāĻŋāĻ āĻāĻāĻ¨āĻ āĻāĻāĻ¨āĻ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻāĻŦāĻ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋ āĻ āĻ¤āĻŋ-āĻāĻ¤ā§āĻ¤ā§āĻāĻ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻāĻāĻžāĻŦā§ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ā§āĻ° āĻ¤ā§āĻ˛āĻ¨āĻžāĻ¯āĻŧ āĻāĻŽ āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻžāĻ¯āĻŧ āĻāĻžāĻ°ā§āĻ¨ā§āĻā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĻā§āĻāĻžāĻ¯āĻŧāĨ¤
Lower motor-neurone lesions
Damage to a lower motor-neurone may involve either the anterior horn cell or the fibres of the nerve roots or peripheral nerves. Lesions involving the nerve-fibres can be classified into three groups: neurapraxia, axonotmesis and neurotmesis.
āĻ¨āĻŋāĻŽā§āĻ¨ āĻŽā§āĻāĻ°-āĻ¨āĻŋāĻāĻ°āĻ¨ āĻā§āĻˇāĻ¤ āĻāĻāĻāĻŋ āĻ¨āĻŋāĻŽā§āĻ¨ āĻŽā§āĻāĻ°-āĻ¨āĻŋāĻāĻ°ā§āĻ¨ā§āĻ° āĻā§āĻˇāĻ¤āĻŋ āĻšāĻ¯āĻŧ āĻ āĻā§āĻ°āĻŦāĻ°ā§āĻ¤ā§ āĻļāĻŋāĻ āĻā§āĻˇ āĻŦāĻž āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻļāĻŋāĻāĻĄāĻŧ āĻŦāĻž āĻĒā§āĻ°āĻŋāĻĢā§āĻ°āĻžāĻ˛ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻ¤āĻ¨ā§āĻ¤ā§ āĻāĻĄāĻŧāĻŋāĻ¤ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§-āĻ¤āĻ¨ā§āĻ¤ā§ āĻāĻĄāĻŧāĻŋāĻ¤ āĻā§āĻˇāĻ¤ āĻ¤āĻŋāĻ¨āĻāĻŋ āĻā§āĻ°ā§āĻĒā§ āĻļā§āĻ°ā§āĻŖā§āĻŦāĻĻā§āĻ§ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§: āĻ¨āĻŋāĻāĻ°āĻžāĻĒā§āĻ°āĻžāĻā§āĻ¸āĻŋāĻ¯āĻŧāĻž, āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¨ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸ āĻāĻŦāĻ āĻ¨āĻŋāĻāĻ°ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸āĨ¤
I. Neurapraxia (first-degree injury) is a condition in which bruising or pressure renders the nerve incapable of conducting impulses past the site of the lesion, but the damage is not severe enough to caused generation of the fibres. If the electrical reactions are tested on the affected muscles a normal type of response is obtained, but there is loss of response to a stimulus applied to the nerve trunk above the lesion.
1. āĻ¨āĻŋāĻāĻ°āĻžāĻĒā§āĻ°āĻžāĻā§āĻ¸āĻŋāĻ¯āĻŧāĻž (āĻĒā§āĻ°āĻĨāĻŽ-āĻĄāĻŋāĻā§āĻ°āĻŋ āĻāĻ¨āĻā§āĻ°āĻŋ) āĻšāĻ˛ āĻāĻŽāĻ¨ āĻāĻāĻāĻŋ āĻ āĻŦāĻ¸ā§āĻĨāĻž āĻ¯ā§āĻāĻžāĻ¨ā§ āĻā§āĻˇāĻ¤ āĻŦāĻž āĻāĻžāĻĒ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻā§ āĻā§āĻˇāĻ¤ āĻ¸ā§āĻĨāĻžāĻ¨ā§āĻ° āĻāĻā§ āĻāĻŦā§āĻ āĻ¸āĻā§āĻāĻžāĻ˛āĻ¨ āĻāĻ°āĻ¤ā§ āĻ āĻā§āĻˇāĻŽ āĻāĻ°ā§, āĻāĻŋāĻ¨ā§āĻ¤ā§ āĻā§āĻˇāĻ¯āĻŧāĻā§āĻˇāĻ¤āĻŋ āĻĢāĻžāĻāĻŦāĻžāĻ° āĻ¤ā§āĻ°āĻŋāĻ° āĻāĻ¨ā§āĻ¯ āĻ¯āĻĨā§āĻˇā§āĻ āĻā§āĻ°ā§āĻ¤āĻ° āĻ¨āĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋ āĻĒā§āĻ°āĻāĻžāĻŦāĻŋāĻ¤ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ¤ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĒāĻ°ā§āĻā§āĻˇāĻž āĻāĻ°āĻž āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻāĻāĻāĻŋ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻ§āĻ°āĻŖā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĒāĻžāĻāĻ¯āĻŧāĻž āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻā§āĻˇāĻ¤ā§āĻ° āĻāĻĒāĻ°ā§ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻā§āĻ°āĻžāĻā§āĻā§ āĻĒā§āĻ°āĻ¯āĻŧā§āĻ āĻāĻ°āĻž āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻšā§āĻ°āĻžāĻ¸ āĻĒāĻžāĻ¯āĻŧāĨ¤
2. Axonotmesis (second-degree injury) is liable to occur if the lesion is more severe. Degeneration of the axons takes place, the sheath of the nerve remaining intact. An example of this type of lesion may be observed in a radial nerve palsy associated with fractured shaft of thehumerus. Once the nerve fibres have degenerated, alterations in the electrical reactions occur.
āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¨ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸ (āĻ¸ā§āĻā§āĻ¨ā§āĻĄ-āĻĄāĻŋāĻā§āĻ°āĻŋ āĻāĻ¨āĻā§āĻ°āĻŋ) āĻāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¯āĻĻāĻŋ āĻā§āĻˇāĻ¤ āĻāĻ°āĻ āĻā§āĻ°ā§āĻ¤āĻ° āĻšāĻ¯āĻŧāĨ¤ āĻ ā§āĻ¯āĻžāĻā§āĻ¸āĻ¨āĻā§āĻ˛āĻŋāĻ° āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻāĻā§, āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻāĻŦāĻ°āĻŖ āĻ āĻā§āĻˇāĻ¤ āĻĨāĻžāĻā§āĨ¤ āĻĨāĻŋāĻāĻŽā§āĻ°āĻžāĻ¸ā§āĻ° āĻĢāĻžāĻāĻ˛āĻ¯ā§āĻā§āĻ¤ āĻļā§āĻ¯āĻžāĻĢā§āĻā§āĻ° āĻ¸āĻžāĻĨā§ āĻ¯ā§āĻā§āĻ¤ āĻ°ā§āĻĄāĻŋāĻ¯āĻŧāĻžāĻ˛ āĻ¨āĻžāĻ°ā§āĻ āĻĒāĻ˛āĻ¸āĻŋāĻ¤ā§ āĻāĻ āĻ§āĻ°āĻŖā§āĻ° āĻā§āĻˇāĻ¤ā§āĻ° āĻāĻāĻāĻŋ āĻāĻĻāĻžāĻšāĻ°āĻŖ āĻ˛āĻā§āĻˇā§āĻ¯ āĻāĻ°āĻž āĻ¯ā§āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻāĻāĻŦāĻžāĻ° āĻ¨āĻžāĻ°ā§āĻ āĻĢāĻžāĻāĻŦāĻžāĻ° āĻā§āĻˇāĻ¯āĻŧāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ˛ā§, āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻŦāĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ¯āĻŧ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻā§āĨ¤
3. Neurotmesis (third-degree injury) is severing of the nerve sheath and fibres. The fibres degenerate below the site of the lesion, causing the same alterations in the electrical reactions as axonotmesis. The condition is, however, more serious, as suture of the nerve is necessary before satisfactory regeneration of the nerve can take place. A lesion of this type would be observed if the ulnar nerve were severed by a cut on the front of the wrist.
āĻ¨āĻŋāĻāĻ°ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸ (āĻĨāĻžāĻ°ā§āĻĄ-āĻĄāĻŋāĻā§āĻ°āĻŋ āĻāĻ¨āĻā§āĻ°āĻŋ) āĻšāĻ˛ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻāĻŦāĻ°āĻŖ āĻāĻŦāĻ āĻĢāĻžāĻāĻŦāĻžāĻ° āĻāĻŋāĻ¨ā§āĻ¨ āĻāĻ°āĻžāĨ¤ āĻā§āĻˇāĻ¤āĻ¸ā§āĻĨāĻžāĻ¨ā§āĻ° āĻ¨ā§āĻā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋ āĻā§āĻˇāĻ¯āĻŧāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ¯āĻŧ, āĻ¯āĻž āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¨ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸ā§āĻ° āĻŽāĻ¤ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻŦāĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ¯āĻŧ āĻāĻāĻ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻāĻāĻžāĻ¯āĻŧāĨ¤ āĻ¤āĻŦā§ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻāĻŋ āĻāĻ°āĻ āĻā§āĻ°ā§āĻ¤āĻ°, āĻāĻžāĻ°āĻŖ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻĒā§āĻ¨āĻ°ā§āĻ¤ā§āĻĨāĻžāĻ¨ āĻāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻ¤āĻžāĻ° āĻāĻā§ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻ¸āĻŋāĻāĻ¨āĻāĻŋ āĻĒā§āĻ°āĻ¯āĻŧā§āĻāĻ¨ā§āĻ¯āĻŧāĨ¤ āĻāĻŦā§āĻāĻŋāĻ° āĻ¸āĻžāĻŽāĻ¨ā§āĻ° āĻ āĻāĻļā§ āĻāĻ˛āĻ¨āĻžāĻ° āĻ¨āĻžāĻ°ā§āĻ āĻā§āĻā§ āĻā§āĻ˛ā§ āĻāĻ āĻ§āĻ°āĻ¨ā§āĻ° āĻā§āĻˇāĻ¤ āĻĻā§āĻāĻž āĻ¯āĻžāĻŦā§āĨ¤
-All these types of nerve lesion may be partial or complete, and there may be a combination of two of them, e.g. neurapraxia and axonotmesis. If all the nerve fibres supplying a muscle degenerate, the reactions characterizing complete denervation are observed, while if only some of the fibres degenerate the reaction is that of partial denervation.
āĻāĻ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻ§āĻ°āĻŖā§āĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻā§āĻˇāĻ¤ āĻāĻāĻļāĻŋāĻ āĻŦāĻž āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§ āĻāĻŦāĻ āĻ¤āĻžāĻĻā§āĻ° āĻĻā§āĻāĻŋāĻ° āĻ¸āĻāĻŽāĻŋāĻļā§āĻ°āĻŖ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯ā§āĻŽāĻ¨ āĻ¨āĻŋāĻāĻ°āĻžāĻĒā§āĻ°āĻžāĻā§āĻ¸āĻŋāĻ¯āĻŧāĻž āĻāĻŦāĻ āĻ ā§āĻ¯āĻžāĻā§āĻ¸ā§āĻ¨ā§āĻāĻŽā§āĻ¸āĻŋāĻ¸āĨ¤ āĻ¯āĻĻāĻŋ āĻāĻāĻāĻŋ āĻĒā§āĻļā§ āĻ¸āĻ°āĻŦāĻ°āĻžāĻšāĻāĻžāĻ°ā§ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋ āĻā§āĻˇāĻ¯āĻŧāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻšā§āĻ°āĻžāĻ¸ā§āĻ° āĻŦā§āĻļāĻŋāĻˇā§āĻā§āĻ¯āĻ¯ā§āĻā§āĻ¤ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻā§āĻ˛āĻŋ āĻĒāĻ°āĻŋāĻ˛āĻā§āĻˇāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻ¯āĻāĻ¨ āĻļā§āĻ§ā§āĻŽāĻžāĻ¤ā§āĻ° āĻāĻŋāĻā§ āĻĢāĻžāĻāĻŦāĻžāĻ° āĻā§āĻˇāĻ¯āĻŧāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻŋ āĻāĻāĻļāĻŋāĻ āĻšā§āĻ°āĻžāĻ¸ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĨ¤
The reactions observed in lesions of the anterior horn cells depend on the extent of the damage. If the severity of the lesion is such that there is degeneration of the nerve fibres, the reactions of denervation are observed. If all the nerve cells supplying a muscle are affected, there action is that of complete denervation, while if only a proportion of the cells are involved the reaction is that of partial denervation. In less severe lesions degeneration of the nerve fibres does not occur, and there actions are normal.
āĻĒā§āĻ°ā§āĻŦāĻŦāĻ°ā§āĻ¤ā§ āĻļāĻŋāĻ āĻā§āĻˇā§āĻ° āĻā§āĻˇāĻ¤āĻā§āĻ˛āĻŋāĻ¤ā§ āĻĒāĻ°āĻŋāĻ˛āĻā§āĻˇāĻŋāĻ¤ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻā§āĻ˛āĻŋ āĻā§āĻˇāĻ¤āĻŋāĻ° āĻĒāĻ°āĻŋāĻŽāĻžāĻŖā§āĻ° āĻāĻĒāĻ° āĻ¨āĻŋāĻ°ā§āĻāĻ° āĻāĻ°ā§āĨ¤ āĻ¯āĻĻāĻŋ āĻā§āĻˇāĻ¤ā§āĻ° āĻ¤ā§āĻŦā§āĻ°āĻ¤āĻž āĻāĻŽāĻ¨ āĻšāĻ¯āĻŧ āĻ¯ā§ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋāĻ° āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻšā§āĻ°āĻžāĻ¸ā§āĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻĒāĻ°āĻŋāĻ˛āĻā§āĻˇāĻŋāĻ¤ āĻšāĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋ āĻāĻāĻāĻŋ āĻĒā§āĻļā§ āĻ¸āĻ°āĻŦāĻ°āĻžāĻšāĻāĻžāĻ°ā§ āĻ¸āĻŽāĻ¸ā§āĻ¤ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻā§āĻˇ āĻĒā§āĻ°āĻāĻžāĻŦāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻ¸ā§āĻāĻžāĻ¨ā§ āĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻŋ āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻšā§āĻ°āĻžāĻ¸ā§āĻ° āĻŽāĻ¤ā§ āĻšāĻ¯āĻŧ, āĻ¯āĻāĻ¨ āĻā§āĻˇāĻā§āĻ˛āĻŋāĻ° āĻļā§āĻ§ā§āĻŽāĻžāĻ¤ā§āĻ° āĻāĻāĻāĻŋ āĻ āĻ¨ā§āĻĒāĻžāĻ¤ āĻāĻĄāĻŧāĻŋāĻ¤ āĻĨāĻžāĻā§ āĻ¤āĻŦā§ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻāĻŋ āĻāĻāĻļāĻŋāĻ āĻšā§āĻ°āĻžāĻ¸ā§āĻ° āĻŽāĻ¤ā§āĨ¤ āĻāĻŽ āĻā§āĻ°ā§āĻ¤āĻ° āĻā§āĻˇāĻ¤āĻā§āĻ˛āĻŋāĻ¤ā§ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋāĻ° āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻāĻā§ āĻ¨āĻž āĻāĻŦāĻ āĻ¸ā§āĻāĻžāĻ¨ā§ āĻāĻžāĻāĻā§āĻ˛āĻŋ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻāĨ¤
Defects of the neuromuscular junction
Occasionally, as in the disease myasthenia gravis, reduction of voluntary power is due to faulty conduction at the neuromuscular junction. Methods other than electrical stimulation provide the most satisfactory aids to diagnosis of such conditions.\
āĻ¨āĻŋāĻāĻ°ā§āĻŽāĻžāĻ¸āĻā§āĻ˛āĻžāĻ° āĻ¸āĻāĻ¯ā§āĻā§āĻ° āĻ¤ā§āĻ°ā§āĻāĻŋ āĻŽāĻžāĻā§ āĻŽāĻžāĻā§, āĻŽāĻžāĻ¯āĻŧāĻžāĻ¸ā§āĻĨā§āĻ¨āĻŋāĻ¯āĻŧāĻž āĻā§āĻ°āĻžāĻāĻŋāĻ¸ āĻ°ā§āĻā§āĻ° āĻŽāĻ¤ā§, āĻ¨āĻŋāĻāĻ°ā§āĻŽāĻžāĻ¸āĻā§āĻ˛āĻžāĻ° āĻ¸āĻāĻ¯ā§āĻā§ āĻ¤ā§āĻ°ā§āĻāĻŋāĻĒā§āĻ°ā§āĻŖ āĻ¸āĻā§āĻāĻžāĻ˛āĻ¨ā§āĻ° āĻāĻžāĻ°āĻŖā§ āĻ¸ā§āĻŦā§āĻā§āĻāĻžāĻ¸ā§āĻŦā§ āĻļāĻā§āĻ¤āĻŋ āĻšā§āĻ°āĻžāĻ¸ āĻĒāĻžāĻ¯āĻŧāĨ¤ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻž āĻŦā§āĻ¯āĻ¤ā§āĻ¤ āĻ āĻ¨ā§āĻ¯āĻžāĻ¨ā§āĻ¯ āĻĒāĻĻā§āĻ§āĻ¤āĻŋāĻā§āĻ˛āĻŋ āĻāĻ āĻāĻžāĻ¤ā§āĻ¯āĻŧ āĻ āĻŦāĻ¸ā§āĻĨāĻžāĻ° āĻ¨āĻŋāĻ°ā§āĻŖāĻ¯āĻŧā§āĻ° āĻāĻ¨ā§āĻ¯ āĻ¸āĻŦāĻā§āĻ¯āĻŧā§ āĻ¸āĻ¨ā§āĻ¤ā§āĻˇāĻāĻ¨āĻ āĻ¸āĻšāĻžāĻ¯āĻŧāĻ¤āĻž āĻĒā§āĻ°āĻĻāĻžāĻ¨ āĻāĻ°ā§āĨ¤
Muscle lesions
If reduction of voluntary power is due to weakness or disease of the muscle and there is no degeneration of the motor nerve, the reactions to electrical stimulation are of normal type but are reduced in strength. Should the lesion be so severe that there is complete loss of muscle tissue, there will be no response to electrical stimulation. This absence of response may occur in such conditions as is chemic contracture or in the advanced stages of the myopathies, or may be due to fibrosis of muscles in longstanding denervation.
āĻĒā§āĻļā§āĻ° āĻā§āĻˇāĻ¤ āĻ¯āĻĻāĻŋ āĻ¸ā§āĻŦā§āĻā§āĻāĻžāĻ¸ā§āĻŦā§ āĻļāĻā§āĻ¤āĻŋ āĻšā§āĻ°āĻžāĻ¸ āĻĒā§āĻļā§āĻ° āĻĻā§āĻ°ā§āĻŦāĻ˛āĻ¤āĻž āĻŦāĻž āĻ°ā§āĻā§āĻ° āĻāĻžāĻ°āĻŖā§ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻŽā§āĻāĻ° āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻ° āĻā§āĻ¨ āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻ¨āĻž āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻ§āĻ°āĻŖā§āĻ° āĻšāĻ¯āĻŧ āĻ¤āĻŦā§ āĻļāĻā§āĻ¤āĻŋ āĻšā§āĻ°āĻžāĻ¸ āĻĒāĻžāĻ¯āĻŧāĨ¤ āĻ¯āĻĻāĻŋ āĻā§āĻˇāĻ¤āĻāĻŋ āĻāĻ¤ āĻā§āĻ°ā§āĻ¤āĻ° āĻšāĻ¯āĻŧ āĻ¯ā§ āĻĒā§āĻļā§ āĻāĻŋāĻ¸ā§āĻ¯ā§āĻ° āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻā§āĻˇāĻ¤āĻŋ āĻšāĻ¯āĻŧ, āĻ¤āĻŦā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻāĻĻā§āĻĻā§āĻĒāĻ¨āĻžāĻ° āĻā§āĻ¨āĻ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻšāĻŦā§ āĻ¨āĻžāĨ¤ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ° āĻāĻ āĻ āĻ¨ā§āĻĒāĻ¸ā§āĻĨāĻŋāĻ¤āĻŋ āĻ°āĻžāĻ¸āĻžāĻ¯āĻŧāĻ¨āĻŋāĻ āĻ¸āĻāĻā§āĻāĻ¨ āĻŦāĻž āĻŽāĻžāĻ¯āĻŧā§āĻĒā§āĻ¯āĻžāĻĨāĻŋāĻ° āĻāĻ¨ā§āĻ¨āĻ¤ āĻĒāĻ°ā§āĻ¯āĻžāĻ¯āĻŧā§āĻ° āĻā§āĻˇā§āĻ¤ā§āĻ°ā§ āĻāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ āĻĨāĻŦāĻž āĻĻā§āĻ°ā§āĻāĻ¸ā§āĻĨāĻžāĻ¯āĻŧā§ āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻļāĻ¨ā§ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻĢāĻžāĻāĻŦā§āĻ°ā§āĻ¸āĻŋāĻ¸ā§āĻ° āĻāĻžāĻ°āĻŖā§ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§
Functional disorders
Loss of voluntary power may be due to hysterical paralysis, in which case there is no alteration in the electrical reactions.
āĻāĻžāĻ°ā§āĻ¯āĻāĻ°ā§ āĻŦā§āĻ¯āĻžāĻ§āĻŋ āĻ¸ā§āĻŦā§āĻā§āĻāĻžāĻ¸ā§āĻŦā§ āĻļāĻā§āĻ¤āĻŋāĻ° āĻā§āĻˇāĻ¤āĻŋ āĻšāĻŋāĻ¸ā§āĻā§āĻ°āĻŋāĻā§āĻ¯āĻžāĻ˛ āĻĒā§āĻ¯āĻžāĻ°āĻžāĻ˛āĻžāĻāĻ¸āĻŋāĻ¸ā§āĻ° āĻāĻžāĻ°āĻŖā§ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻāĻ āĻā§āĻˇā§āĻ¤ā§āĻ°ā§ āĻŦā§āĻĻā§āĻ¯ā§āĻ¤āĻŋāĻ āĻŦāĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ¯āĻŧ āĻā§āĻ¨ā§ āĻĒāĻ°āĻŋāĻŦāĻ°ā§āĻ¤āĻ¨ āĻšāĻ¯āĻŧ āĻ¨āĻžāĨ¤
Stages of denervation
When a nerve fibre is severed, Wallerian degeneration takes place below the site of the lesion, and above it as far as the first node of Ranvier. This degeneration may take as long as fourteen days to become complete. If the nerve is stimulated below the site of the lesion before degeneration has taken place, an impulse is initiated and a normal response of the muscle produced. Because of this, it may not be possible to make a full assessment of the lesion until three weeks often a suspected nerve injury, by which time any nerve fibres that have been severed will have degenerated. Tests carried out before this date can, however, provide useful information.
āĻĄāĻŋāĻ¨āĻžāĻ°āĻā§āĻļāĻ¨ā§āĻ° āĻĒāĻ°ā§āĻ¯āĻžāĻ¯āĻŧ āĻ¯āĻāĻ¨ āĻāĻāĻāĻŋ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻĢāĻžāĻāĻŦāĻžāĻ° āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻšāĻ¯āĻŧā§ āĻ¯āĻžāĻ¯āĻŧ, āĻ¤āĻāĻ¨ āĻā§āĻˇāĻ¤ āĻ¸ā§āĻĨāĻžāĻ¨ā§āĻ° āĻ¨ā§āĻā§ āĻāĻ¯āĻŧāĻžāĻ˛ā§āĻ°āĻŋāĻ¯āĻŧāĻžāĻ¨ āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻāĻā§ āĻāĻŦāĻ āĻāĻ° āĻāĻĒāĻ°ā§ āĻ°ā§āĻ¯āĻžāĻ¨āĻāĻŋāĻ¯āĻŧāĻžāĻ°ā§āĻ° āĻĒā§āĻ°āĻĨāĻŽ āĻ¨ā§āĻĄ āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤āĨ¤ āĻāĻ āĻ āĻŦāĻā§āĻˇāĻ¯āĻŧ āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻšāĻ¤ā§ āĻā§āĻĻā§āĻĻ āĻĻāĻŋāĻ¨ā§āĻ° āĻŽāĻ¤ā§ āĻ¸āĻŽāĻ¯āĻŧ āĻ˛āĻžāĻāĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤ āĻ āĻ§āĻāĻĒāĻ¤āĻ¨ā§āĻ° āĻāĻā§ āĻ¯āĻĻāĻŋ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§āĻāĻŋ āĻā§āĻˇāĻ¤ āĻ¸ā§āĻĨāĻžāĻ¨ā§āĻ° āĻ¨ā§āĻā§ āĻāĻĻā§āĻĻā§āĻĒāĻŋāĻ¤ āĻšāĻ¯āĻŧ, āĻ¤āĻžāĻšāĻ˛ā§ āĻāĻāĻāĻŋ āĻāĻŦā§āĻ āĻļā§āĻ°ā§ āĻšāĻ¯āĻŧ āĻāĻŦāĻ āĻĒā§āĻļā§āĻā§āĻ˛āĻŋāĻ° āĻāĻāĻāĻŋ āĻ¸ā§āĻŦāĻžāĻāĻžāĻŦāĻŋāĻ āĻĒā§āĻ°āĻ¤āĻŋāĻā§āĻ°āĻŋāĻ¯āĻŧāĻž āĻ¤ā§āĻ°āĻŋ āĻšāĻ¯āĻŧāĨ¤ āĻāĻ āĻāĻžāĻ°āĻŖā§, āĻ¤āĻŋāĻ¨ āĻ¸āĻĒā§āĻ¤āĻžāĻš āĻĒā§āĻ°āĻžāĻ¯āĻŧāĻ āĻ¸āĻ¨ā§āĻĻā§āĻšāĻāĻ¨āĻ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻāĻāĻžāĻ¤ āĻ¨āĻž āĻšāĻāĻ¯āĻŧāĻž āĻĒāĻ°ā§āĻ¯āĻ¨ā§āĻ¤ āĻā§āĻˇāĻ¤āĻāĻŋāĻ° āĻ¸āĻŽā§āĻĒā§āĻ°ā§āĻŖ āĻŽā§āĻ˛ā§āĻ¯āĻžāĻ¯āĻŧāĻ¨ āĻāĻ°āĻž āĻ¸āĻŽā§āĻāĻŦ āĻ¨āĻžāĻ āĻšāĻ¤ā§ āĻĒāĻžāĻ°ā§, āĻ¯ā§ āĻ¸āĻŽāĻ¯āĻŧā§āĻ° āĻŽāĻ§ā§āĻ¯ā§ āĻŦāĻŋāĻā§āĻāĻŋāĻ¨ā§āĻ¨ āĻ¸ā§āĻ¨āĻžāĻ¯āĻŧā§ āĻ¤āĻ¨ā§āĻ¤ā§āĻā§āĻ˛āĻŋ āĻā§āĻˇāĻ¯āĻŧāĻĒā§āĻ°āĻžāĻĒā§āĻ¤ āĻšāĻŦā§āĨ¤ āĻ¤āĻŦā§ āĻāĻ āĻ¤āĻžāĻ°āĻŋāĻā§āĻ° āĻāĻā§ āĻāĻ°āĻž āĻĒāĻ°ā§āĻā§āĻˇāĻžāĻā§āĻ˛āĻŋ āĻĻāĻ°āĻāĻžāĻ°ā§ āĻ¤āĻĨā§āĻ¯ āĻĒā§āĻ°āĻĻāĻžāĻ¨ āĻāĻ°āĻ¤ā§ āĻĒāĻžāĻ°ā§āĨ¤
If a normal motor nerve trunk is stimulated with a current of adequate intensity, there is contraction of all the muscles it supplies beyond the point of stimulation. If, however, there is degeneration of the nerve fibres this response is reduced or lost, and the changes become evident three or four days after the injury. Changes in the reactions obtained on stimulation over the muscles may be observed before the end of the first week, and indicate that the nerve is degenerating, although the ultimate extent of the degeneration cannot be assessed at this stage. A reaction indicating partial denervation shows that some of the nerve fibres have degenerated but does not indicate how many more are still in the process of degeneration, or whether the denervation will ultimately become complete. If, however, the reaction of complete denervation is obtained, the severity of the lesion is immediately apparent.
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Strength-duration curves
The plotting of strength-duration curves, which indicate the strength of impulses of various durations required to produce contraction in a muscle, is the most satisfactory method at present available for the routine testing of electrical reactions in peripheral nerve lesions. The advantages of this method of testing electrical reactions are that it is simple and reliable and indicates the proportion of denervation, while a series of tests shows changes in the condition. Its disadvantages are that in large muscles only a proportion of the fibres may respond so that the full picture is not clearly shown, and that it does not indicate the site of the nerve lesion. However, the site may be determined by testing nerve conduction.
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Apparatus
The apparatus used for obtaining strength-duration curves supplies rectangular impulses of different durations. Both the form and the duration of the impulses must be accurate, so it is necessary to use a stimulator specially designed for muscle testing, and the apparatus should be checked at regular intervals to ensure satisfactory working. Impulses with durations of 0.01,0.03, 0.I, 0.3, I, 3, I0, 30 and 1oo ms are required.
The stimulator may be of either the constant-current or the constant-voltage type. The differences between these two types of stimulators are beyond the scope of this book, but the former records the intensity of current used, the latter the voltage. Recent work indicates that the differences in the results obtained with the two types of stimulator have in the past been overestimated. The constant current stimulator was thought to produce more accurate results, but the constant-voltage stimulator is rather more comfortable for the patient. The discomfort of both types of stimulators can be minimized by ensuring that the skin resistance is as low as possible.
Before the current is applied, the skin resistance is reduced by washing and soaking in warm water and any abrasions are protected. The patient must be warm, fully supported and in a good light. An indifferent electrode may be applied to some convenient area, usually on the midline of the body or over the origin of the muscle group, and the active electrode over the fleshy part of the muscle. Alternatively two small electrodes may be used, one over each end of the muscle belly. In either case the active electrodes should be fairly small in order that the muscles may be isolated from each other.
Current is applied, using the longest stimulus first, and increased until the minimum observable contraction is obtained.
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