SCIENCE AND PRACTICE. 265 



In this way we have to compare the conducting power 

 c' of any given body with the conducting power c of another, 

 which we take as unit, and we may do this by a simple 

 proportion, 



1' I' 1 



= 7V : 77" 



when we know the lengths I' and /", the sections s' and s", and 

 the resistances r' and r", of the two bodies in common 

 measure. We can prevent errors arising from the measure- 

 ment of these dimensions if we take all the bodies to be 

 compared with each other of a common length and section, 

 by which the proportion becomes simply, 



or, the dimensions being the same, the conducting powers 

 are to each other inversely as the resistances. The section 

 of the wires, when the materials under inquiry are metals, 

 is obtained, the same for all, by drawing them all through 

 the same die, usually an agate plate, and they are easily cut 

 to the same length. 



When this is not possible, the section is arrived at by 

 measuring the length, weight, and specific gravity. If W is 

 the weight, I the length, or the specific gravity, and s the 

 section, we know that the volume, Is, multiplied by the 

 specific gravity, is the weight of the body ; or, 



W = Is <r 



Setting this value of s in (I., we have, 



The resistances of the wires of metals are measured by one 

 of the methods which will be given further on, at an uniform 

 temperature, generally the freezing point of water, and the 

 conducting power found by means of one of the above 

 formulae. 



