THE MEASUREMENT OF RE $1 STANCE 229 



the least experimental difficulty, as the mechanical measurements 

 are simply the determination of a length and a weight, both of 

 which can be very accurately made; consequently the ohm 

 (meter, gram) resistivity is very commonly employed, and is 

 recommended in preference to the " length-section " or "volume" 

 resistivities denoted above by S A and S D . 



The standard temperature at which results are expressed should 

 not differ greatly from the ordinary average atmospheric tem- 

 perature; therefore 20C. is to be taken. 



The electrical qualities of copper are frequently stated in terms 

 of the conductivity, which is the reciprocal of the resistivity, 

 and in business transactions guarantees are given as to per cent 

 conductivity. In order that such guarantees may possess defi- 

 niteness, some standard must be adopted. Obviously, the 

 conductivity of pure copper would be the most proper standard, 

 but this is unknown. So for the greatest convenience some 

 reasonable figure must be agivcd upon. What this figure may 

 be is not of consequence, but that it should be universally 

 recognized is of the greatest moment. 



Many different standard values of the resistivity of annealed 

 copper have been in use and sanctioned by various electrotech- 

 nical societies. Generally these values have been based on the 

 work of Matthiessen on supposedly pure copper (1862), but the 

 results on annealed copper involve an assumption as to the ratio 

 of the resistivity of the hard-drawn to the annealed wire. There 

 are also uncertainties as to the temperature coefficients, the many 

 digits usually given in the constants bc'ng without significance; 

 consequently the values derived by various persons from Mat- 

 thiessen 'a work do not agree, and the so-called Matthiessen 

 standard has had no universal significance. 



To obviate this difficulty the Bureau of Standards, at the 

 request of the American Institute of Electrical Engineers, has 

 investigated the subject, and as the result of measurements 

 upon 89 samples of commercial copper, procured from 14 differ- 

 ent refiners, an average result of 0.15292 ohm (meter, gram) 

 at 20C. was obtained. This value is seen to be in close agree- 

 ment with the figure 0.15302 ohm (meter, gram) at 20, which 

 had previously been adopted by the Bureau. This latter figure 

 was suggested for international adoption, but the German 



