230 ELECTRICAL MEASUREMENTS 



engineers had already in use a standard of conductivity, 58 



(meter, mm. 2 ) at 20, which is slightly different from the above. 

 The corresponding figure for the resistivity was the one finally 

 recommended for adoption in America and Germany, with a 

 likelihood of its immediate adoption in other countries. It is: 



INTERNATIONAL ANNEALED COPPER STANDARD 



Mass resistivity 0. 15328 ohm (meter, gram) at 20C. 



875.20 ohms (mil, pound) at 20C. 

 Volume resistivity. . . 1.7241 microhm (cm.) at 20C. 



0.017241 ohm (meter, mm. 2 ) at 20C. 

 0.67879 michrom (inch) at 20C. 

 10.371 ohms (mil, foot) at 20C. 

 Density (grams per 

 cubic cm.) 8 . 89 at 20C. 



Resistivity-temperature Constant. The changes in the re- 

 sistivity of copper due to alterations of temperature are com- 

 plicated by the expansion of the material. This effect is very 

 small and is readily allowed for. 



Assuming that the resistance is measured between terminals 

 rigidly attached to the specimen, in general for the ohm (meter, 

 gram) resistivity, 



MR 

 '' L 2 * 



Introducing the temperatures and denoting the coefficient of 

 linear expansion by 7, 



rq 1 _ MR 20 (l + a z o[t 20]) 



L^A/J "" TO /-i *"fcj-kl"\~o 



2 2o(l + y[t 20]) 2 

 For copper, 7 is a very small quantity, 0.000017, and so 



[S M ]t = [S M ]zo{l + (a 20 - 2y)[t - 20]} approximately. 

 For standard copper, 100 per cent, conductivity, 



[S M ]t = 0.15328(1 + (0.00393 - 0.000034)[* - 20]}; 

 = 0.15328 + 0.000597[* - 20]. 



The change per degree in the resistivity is seen to be 0.000597 ohm; 

 this figure is independent of the temperature of reference, and 

 in consequence of (25) applies to coppers of all conductivities. It 

 is called the " resistivity-temperature constant." 



