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THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES 



AUGUST 190( 



XVI. On the Experimental Determinatioixyfitlie Batio of the 

 Electrical Units. By Lord Rayleigh, OJ^JPres. R.S* 



AN able discussion of the principal determinations of the 

 above quantity, usually denoted by v, has been given 

 in the Reports of the Paris Physical Congress (1900) by 

 H. Abraham — himself a contributor to the series. This 

 ground it is unnecessary to retraverse, but I desire to place 

 on record one or two suggestions which have occurred to me 

 but which I may probably have no opportunity of myself 

 putting into practice. 



The most approved methods involve the construction either 

 of a condenser or of an electrometer, of which in the first case 

 the capacity, and in the second the potential, can be calculated 

 in electrostatic measure. The first method, on the whole, 

 offers the greatest advantages, and I preferred it when (about 

 1882, and with the advice of Prof. Stuart) the Cambridge 

 condenser was designed f. In this method two currents are 

 compared by a galvanometer. The first is that due to a given 

 electromotive force in a resistance whose value is known in 

 electromagnetic measure. The second is the intermittent 

 current due to the same electromotive force charging n times 

 per second a condenser whose capacity is known from the 

 data of construction in electrostatic measure. The comparison 

 may be conducted by the aid of Wheatstone's bridge. 



* Communicated by the Author. 



t For description see J. J. Thomson, Phil. Trans. 1883, p. 711 ; 

 Thomson and Searle, Phil. Trans. 1890, p. 586. 



Phil. Mag. S. 6. Vol, 12. No. 68. Aug. 1906. H 



