1865.] of Electrical Resistance. 157 



of 4 millimetres diameter. Several other units were from time to time 

 proposed by Langsdorf *, Jacobif, Marie-Davy J, Weber , W. Thomson ||, 

 and others, with a gradually increasing perception of the points of chief 

 importance in a standard ; but none of these were generally accepted as the 

 one recognized measure in any country. To remedy the continually in- 

 creasing evils arising from the discrepancies invariably found between dif- 

 ferent sets of coils, Dr. Werner Siemens (in 18601F) constructed standards, 

 taking as unit the resistance of a column of chemically pure mercury 

 1 metre long, having a section equal to 1 millimetre square, and maintained 

 at the temperature of Centigrade**. Dr. Siemens supposed that this 

 standard could be reproduced without much difficulty where copies could 

 not be directly obtained. Mercury had been proposed before as a fitting 

 material for a standard by Marie-Davy and De la Rive ; but Dr. Siemens 

 merits especial recognition, as the coils and apparatus he issued have been 

 made with great care, and have materially helped in introducing strict 

 accuracy ff. 



The question had reached this point when (in 1861) the British Asso- 

 ciation, at the suggestion of Professor W. Thomson, appointed a Committee 

 to determine the best standard of electrical resistance. This Committee, 

 aided by a grant from the Royal Society, has now issued a new standard, 

 the subject of the present paper. 



The writer has hitherto described those units only which are founded 

 on a more or less arbitrary size and weight of some more or less suitable 

 material ; but measurements of resistance can be conceived and carried out 

 entirely without reference to the special qualities of any material whatever. 

 In 1849 Kirchhoff had already effected a measurement of this kind ; but 

 it is to W. Weber that we owe the first distinct proposal (in 1851) of a 

 definite system of electrical measurements, according to which resistance 

 would be measured in terms of an absolute velocity. This system of mea- 

 sures he called absolute electromagnetic measure, in analogy with Gauss's 

 nomenclature of absolute magnetic measure. The Committee have decided 

 that Weber's proposal is far preferable to the use of any unit of the kind 

 previously described. Setting aside the difficulties in the way of their 



* Liebig's Ann. vol. Ixxxv. p. 155. t Pogg. Ann. vol. Ixxviii. p. 173. 



J Ann. Chim. et Phys. 3rd series, vol. ix. p. 410. Pogg. Ann. vol. Irani, p. 337. 



|| Phil. Mag. Dec. 1851, 4th ser. vol. ii. p. 551. ^ Pogg. Ann. vol. ex. p. 1. 



** Dr. Siemens, while retaining his definition, has altered the value of his standard 

 about 2 per cent, since the first issue ; and it is doubtful^whether even the present stand- 

 ard represents the definition truly : his experiments were made by weight ; and in 

 reducing the results to simple measurements of length he has used a specific gravity for 

 mercury of 13-557 instead of 13'596 as given by Eegnault, 13'595 by H. Kopp, and 

 13-594 by Balfour Stewart, 



tt Many of the different units described above were represented by resistance-coils 

 in the International Exhibition of 18G2 : vide Jury Eeport, Class XIII. p. 83, where 

 their relative values are given : vide also Appendix A. to present paper. 



JJ Pogg. Ann. vol. Ixxvi. p. 412. Ibid. vol. Ixxiii. p. 337. 



