CONDUCTORS 55 



The resistances for direct current tabulated in Part III 

 have been calculated in accordance with the recommenda- 

 tions of the Bureau of Standards for the preparation of wire 

 tables.* The Standardization Rules of the American In- 

 stitute of Electrical Engineers are in agreement with these 

 recommendations. According to the Bureau of Standards 

 and the A. I. E. E., the "Annealed Copper Standard," 

 which is of 100% conductivity, is represented by a resis- 

 tivity of 0.153022 ohm per meter-gram at 20 C. This is 

 equivalent to 1.72128 micro-ohms per centimeter cube at 

 20 C., assuming a density of 8.89. This is the same as 

 the resistivity of Matthiessen's Standard at 20 C., for- 

 merly used by the A. I. E. E. The conductivity of hard 

 drawn copper recommended for wire tables by the Bureau 

 of Standards is 97.3%, this value representing an average 

 for good commercial copper. The "average conductivity 

 given by the Bureau of Standards for hard drawn alu- 

 minum on the centimeter cube basis, assuming a density of 

 2.699, * s 60.86%. The above values have been used in 

 preparing the tables in Part III, i% being added to the 

 resistance for the effect of spiralling, as already noted. 



If it is desired to calculate the resistance of copper con- 

 ductors for other temperatures than 20 C., the tem- 

 perature coefficient, 0:20, for hard drawn copper of 97.3% 

 conductivity should be used in connection with the formula 



R t = #20 { I + <* 20 (/ - 20) } 



where / is the temperature in degrees Centigrade ior which 

 the resistance R t is desired and where 



0:20 = 0.00383. 



* Bulletin of the Bureau of Standards, Vol. VII, pp. 71-126, Washington, 

 1911; Proc. A. I. E. E., Dec., 1910. 



