232 ELECTRICAL MEASUREMENTS 



Similarly for S A , 



_ 0.00678 



Q>1, ~fcr 1 



[^A\t l 



For S D it is 



Per Cent. Conductivity. The per cent, conductivity is ob- 

 tained by dividing the resistivity of the annealed copper stand- 

 ard at 20 by that of the sample at 20. 



It is to be noticed that on account of the relation of the tem- 

 perature coefficient to the conductivity, the per cent, conductivity 

 of a sample when referred to the standard copper will depend 

 somewhat on the temperature at which the conductivity is 

 computed. For instance, if copper of resistivity 0.15328 ohm 

 (meter, gram) at 20 be taken as a standard, the resistivity at 

 will be 0.15328 - 0.000597 X 20 = 0.14134; a copper which 

 has 95 per cent, conductivity at 20 will have a resistivity of 

 0.16134, and at zero a resistivity of 0.16134 -- 0.000597 X 

 20 = 0.14940. Therefore the per cent, conductivity at is 



100 ~ = 94.6 per cent. 



To avoid possible confusion, per cent, conductivities are to be 

 computed at 20C. 



Resistivity of Aluminum. The Aluminum Co. of America 

 furnishes the following data concerning the resistivity of their 

 commercial product of hard-drawn aluminum. 



Mass resistivity ..... 0.0764 ohm (meter, gram) at 20C. 

 436.0 ohm (mils, pound) at 20C. 



Volume resistivity. . . 2.828 microhm (centimeter) at 20C. 

 1.113 microhm (inch.) at 20C. 



Density, grams per cubic cm. 2.70. 

 Mass per cent, conductivity 200.7%. 

 Volume per cent, conductivity 61.0%. 



Conductivity Bridges. In wire works and in the testing 

 laboratories of large consumers of wire, it is necessary to have 

 special apparatus for the determination of conductivity, the 

 requirements being: 



