THE MEASUREMENT OF CURRENT 41 



Current and Voltage Sensitivity. Using in radians and I F 

 in absolute units 



(31) 



R is the resistance of the circuit necessary for critical damping. 

 Using the microampere sensitivity (page 20), if R is expressed 

 in ohms and the reading in millimeters, on a scale at a meter's 

 distance, 



X 



= 80--J- (32) 



-S, = 8 X 



\j 



The voltage sensitivity or the deflection per microvolt applied 

 to the circuit is 



s, 



S * " IK 



SO 



(33) 



In designing, the formulae may of course be worked backward to 

 find the values of P, T, etc., corresponding to the conditions of the 

 problem. 



To illustrate the utility of (32) and (33), suppose that an 

 instrument, already constructed, is to be used in a circuit having 

 a fixed resistance and that the conditions are such that it is 

 critically damped or dead beat but not sufficiently sensitive for 

 the work in hand. There are two factors which may be altered 

 to increase the sensitivity without using a new coil; the torsional 

 control T and the field strength H. If the field strength is 

 increased, the ultimate deflection due to any current will be 



