An international system of electro-therapeutics : for students, general practitioners, and specialists . TROPHTSICS. A-31 The resistance of a conductor depends on the dimensions of theconductor, as follows: If the length of the conductor be 1, the area ofits cross-section a, then its resistance is R = k-, a where k is a constant independent of the dimensions of the wire. If wetake a piece of the conductor of unit length and unit cross-section, 1^ iand a = 1, and then R = k; or, k is the resistance of a piece of the con-ductor of unit length and unit cross-sectional area, and is called thespeci
An international system of electro-therapeutics : for students, general practitioners, and specialists . TROPHTSICS. A-31 The resistance of a conductor depends on the dimensions of theconductor, as follows: If the length of the conductor be 1, the area ofits cross-section a, then its resistance is R = k-, a where k is a constant independent of the dimensions of the wire. If wetake a piece of the conductor of unit length and unit cross-section, 1^ iand a = 1, and then R = k; or, k is the resistance of a piece of the con-ductor of unit length and unit cross-sectional area, and is called thespecific resistance of the substance of which the conductor consists. Conductivity.—The conductivity, or conducting power of a conductor,will be greater, of course, the less its resistance. We may, then, take it asthe reciprocal of the resistance, or = ~, and then from Ohms law it willmean the ratio of the current in the conductor to the electro-motiveforce that produces it. 46. Conditions on which Resistance Depends.— The Material ofthe Conductor.—Some substances are naturally good conductors, and. others naturally poor. Among metals, copper is one of the best conduc-tors, and hence it is the most commonly employed. Tlie conductivityof silver stands higher than that of copper, but its cost prevents its copper come platinum, iron, lead, mercury, German silver, in theorder named. It is interesting to note that this is also the order of themetals as regards conducting power for heat. In fact, the ratio of theconductivities for heat and electricity is nearly constant not only formetals, but also for alloys. Physical State.—In all ordinary metal conductors the conductivitydecreases as the temperature rises. The increase of specific resistance perunit rise of temperature is called the temperature coefficient of the sub-stance. It is nearly the same for all pure metals except thallium and iron,being .0037647 per degree centigrade. Again, metals become worse con-ductors o
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectuterus, bookyear1894
