THE 



LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



NOVEMBER 1882. 



XXXVIII. Comparison of Methods for the Determination of 

 Resistances in Absolute Measure. By Lord Rayleigh, 

 F.R.S., Professor of Experimental Physics in the University 

 of Cambridge* '. 



AT the present time, and in view of the projected confer- 

 ence at Paris, the subject of the present paper is enga- 

 ging a large share of attention ; and Prof. Gr. Wiedemann t 

 has published an interesting discussion of some of the methods 

 that have been employed. I have thought it might be of ser- 

 vice if I also were to place upon record the views that I have 

 been led to entertain, and which are the result of a good deal 

 of experience. 



Resistance being of the dimensions of velocity, its absolute 

 measurement involves the absolute measurement of a length 

 and of a time. The latter is usually the time of a vibration of 

 a suspended magnet, and can be determined without much 

 difficulty. In the B.A. method it is the time of rotation of 

 the revolving coil, and can be obtained with all desirable accu- 

 racy. In this respect there is not much to choose between one 

 method and another; but when we come to consider the man- 

 ner in which the linear measurement enters, important differ- 

 ences reveal themselves. These will be discussed in detail 

 presently; but for the moment it will be sufficient to say that 



* Communicated by the Author. 



t " Ueber die bisherigen Methoden zur Feststellung des Ohm." Se- 

 paratabdruck ans der Electrotechnischen Zeitschrift, July 1882. [Phil. 

 Mag. for October, p. 258.] 



Phil. Mag. S. 5. Vol. 14. No. 89. Nov. 1882. Z 



