562 RATIO OF UNITS. 



The beam is suspended to a copper wire, by which the current 

 reaches the coils ; this emerges thence by a wire dipping in a mer- 

 cury cup. The time of oscillation of the system was 7 seconds ; 

 these oscillations are rapidly extinguished, owing to the changes in 

 pressure which they determine in the box S. The torsion of the 

 wire need not be allowed for, as the position of equilibrium corre- 

 sponds to zero torsion. 



The difference of potential established between the two plates 

 C and C, is that of two points A and A', of a circuit traversed by 

 the current of a battery P of 2600 bichloride of mercury elements. 

 The resistance R, which separates the two points A and A', is 

 formed of a standard coil with about a million of ohms, and the 

 current is determined by a galvanometer G. 



A second battery P' furnishes a current which traverses the three 

 coils B, B', and B x . This current traverses, in the galvanometer G, 

 a second frame, formed of a small number of turns, and superposed 

 on the first, so as to form a differential galvanometer. A portion of 

 the current is diverted through a shunt, the resistance of which is 

 modified so that the needle remains at zero. 



The two currents are closed for a very short time, the moment 

 the oscillation of the beam brings it into the position of equilibrium ; 

 the distance of the two plates is then varied, so that the movable 

 plate is neither attracted nor repelled ; and, on the other hand, the 

 shunt is modified until the needle remains at zero. 



Two circumstances make the experiment difficult the instability 

 of the equilibrium of the beam, and the rapid variation of the 

 electromotive force of the battery P, from the moment the circuit 

 is closed. 



The attraction of two discs for the distance x, and with an 



A e z 



electrostatic difference of potential equal to e, is - -- - ; the repul- 



"MVT ^ 



sion of the two coils is I' 2 , where M is the coefficient of 



ox 



mutual induction, and I' the intensity of the current. As these 

 two forces are equal, 



The intensity I of the current of the battery P gives the difference 

 of electromagnetic potential E = IR of the two plates. As, more- 

 over, the needle of the galvanometer is at zero, when a shunt s is 



