CHAP, vi.] ELECTRICITY AND MAGNETISM. 309 



347. Laws of Resistance. The following are the 

 laws of the resistance of conductors : 



fc- i. The resistance of a conducting wire is proportional 

 to its length. If the resistance of a mile of 

 telegraph wire be 13 ohms, that of fifty miles 

 will be 50 x 13 = 650 ohms. 



ii. The resistance of a conducting wire is inversely 

 proportional to the area of its cross section, and 

 therefore in the usual round wires is inversely 

 proportional to the square of its diameter. Ordi- 

 nary telegraph wire is about |th of an inch thick ; 

 a wire twice as thick would conduct four times as 

 well, having four times the area of cross section : 

 hence an equal length of it would have only ^th 

 the resistance. 



iii. The resistance of a conducting wire of given length 

 and thickness depends upon the material of which 

 it is made, that is to say, upon the specific 

 resistance of the material. 



348. Specific Resistance. The specific resistance 

 of a substance is best stated as the resistance in 

 "absolute" C.G.S. units (i.e. in thousand millionths of 

 an ohm) of a centimetre cube of the substance. The 

 following Table also gives the relative conductivity when 

 that of^ilver is taken as 100. 



