310 ELECTRICAL MEASUREMENTS 



in the potential coil and shifts its phase by an amount dependent 

 on the frequency. Thus the mean product of the currents in 

 the fixed and movable coils is altered from its proper value. 

 If sinusoidal currents are assumed, the magnitude of the error 

 thus introduced may readily be computed. 



Symbols used in the Discussion of the Theory of the Wattmeter 

 Maximum values of currents and voltages are denoted by large letters, 

 instantaneous values by small letters. 



Referring to Fig. 180 



ab = V = voltage at terminals of potential-coil circuit. 

 IP = current in potential coil. 

 RP = resistance of potential-coil circuit. 

 ac = IpRp = ohmic drop in potential-coil circuit. 

 IL = current in load. 

 R L = resistance of load. 

 RC = resistance of current coils. 

 I c = current in current coils. 



ad = IL(RL + RC} ohmic drop in load circuit between potential terminals. 

 6 P = phase displacement in potential-coil circuit. 

 &L = phase displacement in load circuit between potential terminals 

 L P = inductance of potential-coil circuit. 

 LL = indutance of load. 

 L c = inductance of current coils. 

 ZL = impedance of load. 

 Zp = impedance of potential-coil circuit. 

 Z c = impedance of current coils. 



= 2 TT times frequency. 



m = mutual inductance of fixed and movable-coil circuits. 

 PL = power in load. 



P w = power as read from the wattmeter. 

 HP = heating loss in potential circuit. 

 H c = heating loss in current coils. 



K = constant of dynamometer. 



D = deflection of instrument. 



Consider case / on page 307 where the current coils are 

 traversed by the load current only and the power measured 

 includes the heating loss in the current coils. 



On account of the power factor of the reactive load circuit, the 

 current, I L , will lag behind V by an angle 6 L , and in consequence 

 of the reactance of the potential-coil circuit, the current in it will 

 be out of phase with V by an angle 9 P and will be altered from 



the value to Fc s ^, see Fig. 180. 



ftp KP 



