SCIENCE AND PRACTICE. 81 



concentric cylinders. It is evident then, from what has gone 

 before, that the resistance through such a cylinder concentric 

 with the conductor will be directly proportional to its thick- 

 ness and inversely proportional to its surface, that is to say, 

 of its length and circumference, and to the conducting power 

 of the material. If we have a metallic conductor insulated 

 with a material whose conducting power is c, the diameter of 

 the conductor, 2 r, and the outer diameter of the insulating 

 covering, 2 R, the resistance, d W, of a differential cylinder, 

 whose thickness is dx, diameter 2#, and length /, will be 



dx 



and by integration between the limits of x = r and x = R, 

 the sum of all the differential cylinders which make up the 

 space occupied by the insulator, or, in other words, the 

 resistance of insulation will be 



../ 



dx ' r (I. 



W=l Zxirlc ~2 



r 



whence the conducting power, c, is 



R 



: 2*1 W 



I3y measuring the value of W by any of the known methods 

 of determinating great resistances (which will be treated of 

 further on), and being in possession of the dimensions of the 

 cable, we can calculate the conducting power. 



Having another cable, insulated with a different material 

 whose conducting power is c', length /', resistance W, and 



R' 

 ratio of diameters , the conducting power of these two 



cables will obviously stand in the relation 

 log-. log. e ;r 



