ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



133 



statement, which I liave previously made, that it is impossible to prepare 

 wires on the small scale which are of the same quality, i.e., probably due 

 to density, as the best specimens specially prepared by large manufacturers. 



In Table B are given the results of measurements on three specimens 

 of copper prepared by Mr. Swan : one was given to Profs. Dewar and 

 Fleming ; a second was examined by Mr. Swan liimself, and a third 

 specimen he kindly sent to me ; the quality of the copper in the three 

 cases may therefore be expected to be the same. 



The results of Profs. Dewar and Fleming and Messrs. Swan and 

 Rhodin are expressed only as specific resistances, whilst the result of my 

 measurement is only given for a wire one metre long weighing one gramme. 

 The weight of the copper wire, as measured, was only three grammes, and 

 that does not allow the accurate determination of the specific gravity of the 

 sample. The value I obtain for its resistance is identical with that for the 

 sample of annealed copper wire sent me by Messrs. Bolton. 



If it be considered to have the same specific gravity as that sample 

 (8-94) its specific resistance in C.G.S. units is 1665 ; a value distinctly 

 smaller than that obtained by Messrs. Swan and Rhodin, whose result is 

 practically identical with that of Profs. Dewar and Fleming. 



Not only may wires drawn from the same specimen of copper have 

 different densities and different resistances, but the variation of that 

 resistance with change of temperature may be also diflferent. 



In the following table are given the temperature coefficients of various 

 specimens of copper : — 



* Hard-drawn wires. 



Influence of Annealing. — As is well known, annealed wires have a 

 less resistance than hard-drawn wires, but the variation of resistance 

 according as the wires are annealed or hard-drawn differs considerably 

 for different materials. For silver it is as much as 10 per cent., whereas 

 for copper it is less than 3 per cent. 



I have made observations from time to time on the resistance value 

 of specimens of hard-drawn copper wire, all pieces of the same coil, which 

 were sent me in 1889 by Messrs. Bolton and Son. Fi'om the results of 

 these measurements it will be seen that a hard-drawn wire seems to fall 

 in resistance with lapse of time. The coil of wire has been left hanging 

 in the laboratory, and has not been treated with any special care. 



' B.A. Report, 1864. - Phil Mag., vol. xxxvi. p. 287. 



' Nature, vol. 1. p. 165. ♦ Electricity , vol. v. p. 165. 



' Comptes Jiendus, Ixxvi. p. 345. 



