480 Royal Society : — 



Dr. Siemens supposed that this standard could be reproduced without 

 much difficulty where copies could not be directly obtained. Mer- 

 cury 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*. 



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

 Association, 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 con- 

 ceived and carried out entirely without reference to the special 

 qualities of any material whatever. In 1849 Kirchhofff had 

 already effected a measurement of this kind ; but it is to W. Weber J 

 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 measures 

 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 reproduction, which are by no means contemptible, 

 arbitrary material standards, whether of mercury, gold, silver, plati- 

 num, or any other material, would be heterogeneous isolated units 

 without any natural connexion with any other physical units. The 

 unit proposed by Weber, on the other hand, forms part of a sym- 

 metrical natural system, including both the fundamental units of 

 length, time, and mass, and the derived electrical units of current 

 quantity and electromotive force. Moreover it has been shown by 

 Professor W. Thomson §, who accepted and extended Weber's pro- 

 posal immediately on its appearance, that the unit of absolute work, 

 the connecting link between all physical forces, forms part of the 

 same system, and may be used as the basis of the definition of the 

 absolute electromagnetic units. 



The full grounds of the choice of the Committee could only be ex- 

 plained by a needless repetition of the arguments given in the reports 

 already made to the British Association. It will be sufficient here 

 to state that, in the absolute electromagnetic system, the following 

 equations exist between the mechanical and electrical units : — 



W=C 2 R£, (1) 



where W is the work done in the time t by the current C conveved ' 



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

 coils in the International Exhibition of 1862: vide Jury Eeport, Class XIII. 

 p. 83, where their relative values are given : vide also Appendix A. to present paper. 



t Pogg. Ann. vol. lxxvi. p. 412. \ Ibid. vol. lxxxii. p. 337. 



§ Phil. Mag. Dec. 1851, 4th series, vol. ii. p. 551. 





