MEASUREMENT OF AN ELECTROMOTIVE FORCE. 559 



The fixed coil of the electrodynamometer (865) is formed of two 

 distinct coils. The current arrives by the wire by which the movable 

 coil is suspended, and emerges by a very light wire wound in a spiral 

 suspended to the coil, and then plunging in a mercury cup. 



Magnets conveniently arranged nullify almost completely the 

 action of the terrestrial field on the coil. As the compensation 

 thus obtained is never complete, the current was passed alternately 

 in opposite directions, and the mean of two readings which dif- 

 fered very little from each other was taken as final deflection. 



The movable coil was made of 3000 turns of a very fine wire ; 

 its radius being too small for the surface S to be calculated, this 

 being measured by a standard surface. 



The resistance of the electrodynamometer was about 15,600 

 ohms. In order to have greater differences of potential on the 

 electrometer, a supplementary resistance of 10,000 ohms was intro- 

 duced between the points A and B, and in consequence the number 

 of the couples was varied. This was sometimes 90 and sometimes 

 1 80 Daniell in series. 



The first experiments were made by Mr. King* in 1869 under 

 the direction of Sir W. Thomson. They were then resumed in 1870 

 to 1872 by Mr. Dugald M'Kichan.f 



1132. The same method was applied by M. Shidaj: with some 

 modifications. 



It was not attempted to bring the charge of the electrometer to 

 a fixed value by using a replenisher and a gauge; but putting alter- 

 nately the plate and the cage in connection with the two poles 

 A and B of a battery closed by a resistance R, and working at 

 equal intervals, readings D 15 D' x ; D 2 , D 2 ; D 3 , D' 3 were ob- 

 tained, and, as a mean of the differences, the values 



were taken, which should be equal to within errors of reading. 



The intensity of the current was determined by a tangent gal- 

 vanometer arranged like that of Joule (837). If the deflection A 



* KING. Report of the Committee on Electrical Standards. 1869. Reprint, p. 1 86. 

 t DUGALD M'KICHAN. Phil. Trans, for 1879, pp. 409, 427. 

 % SHIDA. Phil. Mag. [5], Vol. x., p. 401. 1880. 



