THE MEASUREMENT OF CURRENT 39 



v total length of vertical wire. 



h = total length of horizontal wire, across ends of coil. 

 m = mass per unit length of bare wire. 

 m' = mass per unit length of covered wire. 



m' 



n = 



m 



a = area of bare wire. 



p = resistivity of copper, 1. 724 microhms centimeter at 20C. 



5 = density of copper, 8.89. 



P = moment of inertia of entire moving system. 

 P' = moment of inertia of coil fittings. 



T = torsion constant of suspension, restoring moment for unit angular 

 deflection. 



6 = deflection at any instant. 



dp = final value of 6 when a constant current IF flows in coil, or initial 



value of 6 when circuit is broken. 

 i = current in coil at any instant. 

 IF = final value of current after coil has come to rest in its deflected 



position. 



To = time of an undamped vibration of movable system. 

 R = total resistance of circuit, including galvanometer. 

 L = total inductance of circuit. 

 RG = resistance of coil of galvanometer. 

 E = e.m.f. of battery. 



EB = back e.m.f. generated by movement of coil. 



C = Hlb, turning moment which acts on coil when it carries unit current. 

 Si = current sensitivity. 



o 



SK = ^ = voltage sensitivity. 



K 



The turning moment acting on the coil at any instant is iHlb 

 = iC, while the restoring moment is rB. After the coil has come 

 to rest 



r0, = I f C 

 or 



/, = f (28) 



An important use of the moving-coil instrument is in a closed 

 circuit of moderate resistance as occurs with the Dieselhorst or 

 the Brooks potentiometer or when the instrument is used in 

 connection with thermo-electric junctions. In these cases the 

 resistance of the circuit is constant or nearly so. 



Consider the instrument to form a part of a galvanic circuit 

 which includes a source of e.m.f. and a resistance external to the 



