ELECTRICITY. 



47 



(64) THE SPECIFIC RESISTANCE OF SUBSTANCES. 



The specific resistance of a substance is the resistance between the 

 opposite faces of a cube of it at C. which measures 1 cm. each 

 way. 



The conductivity of a substance is the reciprocal of its resistance. 



The resistance of a metal at any temperature t C. may be cal- 

 culated from its resistance at C. by the formula 



+ at db bt 2 ) where 



a b 



003 824 + -000 001 26 



000 748 5 - -000 000 398 



000 443 3 + '000 000 152 

 000 31 



000 7 - '000 000 062 



For most pure metals 

 ,, mercury 

 ,, German-silver 



,, platinum-silver 

 ,, gold-silver 



The resistance of a platinum wire at T on the absolute scale is 

 given by the formula 



Ji = AVjo-039369 7'* + 0-00216407.2'.+ D'2413}. 



The resistances of commercial metals are usually much higher 

 than those of pure metals. 



If the resistance of pure copper at C. be taken as 1, the resist- 

 ance at any temperature t C. is 



