CONSTANT ELECTROMOTIVE FORCES. 527 



The electromotive force e is measured by compensating it with 

 the difference of potential RI' which exists between the two ends 

 A and B of a resistance R traversed by a constant current, so that 

 there is no current in the disc. The result is also independent 

 of the necessarily variable resistance of the sliding contacts, and 

 we have 



Any measurement of intensity is avoided if the current which 

 traverses the resistance R is that which traverses the coil. The 

 formula then becomes 



(")' *-. 



It is sufficient then to calculate the coefficient M and to deter- 

 mine the period of rotation T which establishes equilibrium. 



1112. The calorimetric method (i) was employed by Joule* 

 in 1866. 



The first absolute determination of a resistance by induction 

 is due to Kirchhoff,f who measured the discharge produced by the 

 action of an auxiliary current. 



W. WeberJ suggested methods (2), (4), and (9), and applied 

 the first two. 



To the British Association, on the proposition of Sir W. Thom- 

 son, is due the credit of having undertaken a series of experiments 

 with a view of establishing a standard of resistance ; and the perma- 

 nent deflection (9) produced by a rotating frame was used. 



The determination of an instantaneous electromotive force (10) 

 was proposed by Professor G. C. Foster. || 



The last method (12), based on the production of a continuous 

 current by induction, is due to Lorenz.1T 



* J. P. JOULE. British Association Report. Dundee, 1867. Reprint of 

 Reports of the Committee on Electrical Standards, p.* 175. JOULE. Scientific 

 Papers, Vol. I., p. 542. 



t .KIRCHHOFF. Pogg. Ann., Vol. LXXVII. 1849. Gesamm. Abhand., p. 118. 

 W. WEBER. Ekktrodyn. Maasbestimmungen. Abh. d. Konig. Sachs. 

 Gesellschaft, Vol. I., p. 199. 1851. 



British Association Report. Newcastle, 1863. Reprint, p. 39. 

 II G. C. FOSTER. Electrician, Vol. vn., p. 266. 1881. 

 IT LORENZ. Pogg. Ann., Vol. CXLIX., p. 251.- 1870. 



