THE MEASUREMENT OF CURRENT 27 



At t = both the deflection and the angular velocity are zero, or 

 t = = 



Imposing the first of these conditions on (10) gives 



o - cl + c + " 



CI 



but - = 6 F , the final value of the deflection. 



d0 



= CI?HI + Corn* = 

 at J <_o 



,andC 2 



?/i -2 /// 1 m* in 1 



The value of 6 which fulfils the conditions is therefore 



(13) 



If, when the needle is at rest in its deflected position, the cir- 

 cuit is broken, the deflecting moment due to the current becomes 

 zero, so 



rg + *+*-o 



.-. H - CV* + <'***'. 



When 



t = e = 8 f 

 and 



t = Q , = 



and 



miC i -{- m 2 C2 == 0. 

 Therefore 



r AM . AM -I 



(15) 



Non-osciUatory Deflection. If fc 2 > 4rP, both mi and m 2 are 

 real and negative and (13) is the equation of a curve which 

 gradually rises toward the value F ; in this case, the galva- 



