THE MEASUREMENT OF RESISTANCE 231 



If the effects of expansion had been neglected, the result would 

 have been 0.000602. 



Using the volume resistivity, in general, 



RA 

 S A =- --- 



At t, 



, , fl2oA 2u (l + a 2 o[< - 20]) (1 + 2y[t - 20]) 



Lo(l + y[t - 20]) 



= [&V|2o{l 4- (#20 + y)[t 20]} approximately. 

 Using microhms, 



[N,t] t = 1.7241 { 1 + (0.00393 + 0.000017)[* - 20]} ; 

 = 1.7241 + 0.00681[* - 20]. 



In this case the " resistivity-temperature constant" is 0.00681. 

 Again, using the ohm (mil, foot) resistivity, 



[&>] = 10.371 + 0.0409[* - 20]. 



Here the " resistivity-temperature constant" is 0.0409. 



Relation Between Resistivity and the Temperature Coefficient of 

 Resistance. As shown above, the change in the ohm (meter, gram) 

 resistivity per degree C. is 0.000597 ; consequently, the temperature 



... 0.000597 

 coefficient of the ohm (meter, gram) resistivity = fc . 1 



L^AfJtj 



The resistance of n wire at t, if t\ is the temperature of 

 reference, is given by 



= R, 1 1 + ("-'""'f 97 + 0.000034) (t - ,)} approx. 

 V IOJTJI, 



For the copper met with in practice this is approximately 



, 0.000602 



R t = R t \l + 



.'. the temperature coefficient of resistance at the reference 



, 0.000602 

 temperature ^ i is -TO r 



