132 



REPORT — 1894. 



Only in cases where considerable masses of the material are used 

 can the specitic gravity, and from this the cross-section of the wires, be 

 accurately determined. There is, therefore, an evident advantage in 

 expressing results in terms of weight, as then the determination of the 

 cross-section of the wires becomes unnecessary, and there is no reason 

 why an accuracy of one in two or three thousand should not be attained. 



Again, it is found that different samples of copper have different 

 densities, according to the method by which they have been prepared ; in 

 a table • which I published on a previous occasion the variation is from 

 3-86 to 8-95. Mr. Swan - gives a value as high as 8-9587. 



From samples of copper of the same quality I have had wires drawn 

 which differed in density ; it was always found that the denser the copper 

 the less is its resistance, and the difference affects much more the results 

 expressed as specific resistances than when expressed as the resistance for 

 a gramme per metre. 



This is another reason for expressing the results in these terms, at least 

 as well as specific resistances, and for actual practical purposes it is a 

 question of weight rather than volume. 



In the following tables the results are given for a temperature of 

 18° C. in CG.S. units :— 



Table A. — Hard-drawn Copper Wires. 



Table B. — Annealed Wires. 



From Table A it will be seen that Messrs. Swan and Rhodin obtained a 

 value rather lower than that which I got for copper, prepared by myself, 

 and which, expressed as the resistance of a wire one metre long weigh- 

 ing one gramme, is identical with the value that Matthiessen obtained ; but 

 the resistance of all these specimens is distinctly greater than that of the 

 copper kindly sent me by Messrs. Bolton — which seems to bear out the 



' B.A. Jlejwrt, 1890, p. 125. 



' B.A. Report, 1864. 



' B.A. Rej)0Tt, 1890, p. 125 



* Nature, vol. 1. p. 165. 

 ■* J\ature, vol. 1. p. 165. 



* Ffiil. Mag , vol. xxxvi. p. 287. 



