ON STANDARDS OF ELECTRICAL RESISTANCE. 157 
It is somewhat more convenient to refer to the unit of mass with long 
uniform conductors, such as metal wires, of which the size is frequently and 
easily measured by the weight per foot or metre ; and it is, on the other hand, 
more convenient to refer to the unit of volume bodies, such as gutta percha, 
glass, &c., which do not generally occur as conducting-rods of uniform section, 
while their dimensions can always be measured with at least as much accu- 
racy as their weights. 
51. Specific Inductive Capacity*.—Faraday discovered that the capacity 
of a conductor does not depend simply on its dimensions or on its position 
relatively to other conductors, but is influenced if amount by the nature of 
the insulator or dielectric separating it from them. The laws of induction 
are assumed to be the same in all insulating materials, although the amount 
be different. The name “inductive capacity’’ is given to that quality of an 
insulator in virtue of which it affects the capacity of the conductor it sur- 
rounds, and this quality is measured by reference to air, which is assumed 
to possess the unit inductive capacity. The specific inductive capacity of a 
,material is therefore equal to the quotient of the capacity of any conductor 
insulated by that material from the surrounded conductors, divided by the 
capacity of the same conductor in the same position separated from them by 
air only. It is not improbable that this view of induction may be here- 
after modified. 
52. Heat produced in a Conductor by a Current.—The work done in driving 
a current, C, for a unit of time through a conductor whose resistance is R, byan 
electromotive force E, is EC=RC* ($17). This work is lost as electrical 
energy, and is transformed into heat. As Dr. Joule has ascertained the 
quantity of mechanical work equivalent to one unit of heat, we can calculate 
the quantity of heat produced in a conductor in a given time, if we know 
C and R in absolute measure. In the metrical series of units founded on 
the metre gramme and second, if we call the total heat ©, taking as unit the 
quantity required to raise one gramme of water one degree Centigrade, we 
have | RC% 
8 => *4157° . . * ° . . > : . . 
In the British system, founded on feet, grains, seconds, with a unit of heat 
equal to the quantity required to raise one grain of water one degree Fahr., 
we must substitute the number 24°861 for 4157 in the above equation. 
53. Electrochemical Equivalents—Dr. Faraday has shown + that when an 
electric current passes through certain substances and decomposes them, the 
quantity of each substance decomposed is proportional to the quantity of 
electricity which passes. Hence we may call that quantity of a substance 
which is decomposed by unit current in unit time the electrochemical equi- 
valent of that substance. 
This equivalent is a certain number of grammes of the substance. The 
equivalents of different substances are in the proportion of their combining 
numbers ; andif all chemical compounds were electrolytes, we should be able 
to construct experimentally a table of equivalents in which the weight of 
each substance decomposed by a unit of electricity would be given. The 
electrochemical setae of water, in electromagnetic measure, is about 
0-02 in British, 0-0092 + in the metrical system. The electrochemical equi- 
valents of all other electrolytes can be deduced from this measurement with 
the aid of their combining numbers. 
(28) 
* Experimental Researches, series xi. + Experimental Researches, series vii. 
} ‘009375 by Weber and Kohlrausch. 
