MEASUREMENT OF AN ELECTROMOTIVE FORCE. 561 



is always a position of the two systems for which there is equilibrium. 

 It must be observed that this is a position of unstable equilibrium, 

 which makes the experiments somewhat difficult. 



Fig. 237 shows the arrangement of the two systems: C and C' 

 are the two plates, B and B' the two coils. The plate C' is insu- 

 lated, and with the corresponding coil B' may be moved parallel to 

 itself by means of a micrometric screw V. The plate C, which is in 

 connection with the earth, is placed at the end of a beam movable 

 about the point O. It passes exactly through the aperture of a 

 box S, also in connection with the ground, which contains the plate 

 C', and serves as guard-ring for the plate C. 



In observing, the plate C should be kept in the plane of the 

 guard-ring, the distance of the two systems being regulated by the 



Fig. 237. 



play of the micrometric screw V. This position is defined by means 

 of two small silvered mirrors, one of which is on the posterior part 

 of the plate C, and the other on the ring itself; the disc and the 

 ring are in the same plane when the positions of the images of the 

 same straight line given by two mirrors are exactly in the pro- 

 longation of each other ; this position is marked on a divided plate 

 supported by the movable system, and which is observed with a 

 microscope. The distance of the two plates C and C' is deduced 

 from comparative measures with the micrometer and divided scale ; 

 while the two discs C and C', put in contact with each other, are 

 simultaneously displaced. 



In order to compensate the action of the earth on the coil B, 

 when it is traversed by the current, there is an identical coil B 15 

 through which the current circulates in the same direction. The 

 system constituted of these two coils is perfectly astatic. 



VOL. II. O O 



