268 



MEASUREMENT OF CURRENTS. 



its first positive elongation A (Fig. 169), then the following negative 

 elongation B'. The instant it is at zero, having moved in the 

 positive direction, a negative charge is sent; the needle stops sud- 

 denly, and returns in the negative direction. It is allowed to attain 

 its first negative elongation A', then the following positive one B 



Fig. 169. 



and when it again passes zero a positive discharge is sent, which 

 stops the needle and restores it to a positive elongation A 1? etc. 



If a, b', a' 



are the successive elongations 



From which we deduce 



b' b b + b' 



? = - = = ; = e S 



a a a + a 



2a Q = a' + a 1 + (b' + b) q = a l + a' + b + b 1 - (b + b') ( i - q] . 



The decrement A and the elongation for a single throw are thus 

 determined by the sum of the swings to the right and left that is to 

 say, by the difference of the readings of the scale without its being 

 necessary to know the position of zero. This position, moreover, 

 would be determined by two successive elongations A and B', A' and 

 B of a free oscillation, taking into account the damping. 



