56 ALTERNATING CURRENTS 



mometer voltmeter, Fig. 50. It is connected across the line in 

 series with a high resistance R. The current is led into this coil 

 through springs. The two fixed coils FF are wound with a few turns 

 of heavy wire, capable of carrying the load current. As there is no 

 iron present, the field due to the current coils FF is proportional 

 to the load current at every instant. The current in the moving 

 coil M is proportional to the voltage at every instant. There- 

 fore, for any given position of the moving coil, the torque is 

 proportional at every instant to the product of the current and 

 voltage or to the instantaneous power of the circuit. If the 

 power-factor is other than unity, there is negative torque for part 

 of the cycle. That is, during the periods when there are negative 

 loops in the power curve, Fig. 21, page 24, the current in the 

 fixed coil and the current in the moving coil reverse their direc- 

 tions with respect to each other, and so produce a negative torque. 

 The moving coil takes a position corresponding to the average 

 torque. The torque is also a function of the angle between the 

 fixed and moving coil axes, but this factor is taken into account 

 by the scale calibration. 



As the torque acting on the moving coil varies from instant to 

 instant, having a frequency twice that of either the current or 

 the voltage, the coil tends to change its position to correspond 

 with these variations of torque. If the moving system had little 

 inertia, the needle would vibrate so that it would be impossible 

 to obtain a reading. Because of the relatively large moment of 

 inertia of the moving system, the needle assumes a steady 

 deflection for constant values of average power. The position 

 taken by the coil corresponds to the average value of the power, 

 which is the result desired. 



It should be noted in Fig. 52 that the voltage terminal marked 

 "0" is connected directly to one end of the moving coil. This 

 terminal always should be connected directly to that side of the 

 line to which the current-coil is connected. The fixed and moving 

 coils are then at the same potential. If the moving coil is 

 connected to the other side of the line, the potential difference 

 between the fixed and moving coils is equal to the full line poten- 

 tial, as shown in Fig. 53. In this diagram, the fixed coils are con- 

 sidered as being at zero or ground potential. The moving coil is 

 then at the potential of the other side of the line, or 550 volts, 



