PRINCIPLES OF ALTERNATING CURRENT 207 



49. The Wattmeter Power Measurement. From what 

 has been said in the previous paragraph it is evident that 

 the power used in an a-c. circuit cannot be determined by 

 employing a voltmeter and ammeter as would be done in a 

 continuous current circuit. If it were possible to deter- 

 mine cj>, then the power could be calculated from the readings 

 of these instruments, but to determine cj> directly it is 

 necessary to use either a laborious method of curve plotting 

 or else the use of special instruments as the oscillograph 

 or power factor meter; these instruments are not generally 

 available. 



The Wattmeter. An indicating instrument has been 

 designed, however, and is constantly used on a-c. circuits, 

 which will actually indicate the El cos < value for any 

 circuit; it is called the wattmeter. There are two coils 

 in this instrument, one fixed and made of a few turns of 

 comparatively large conductor and the other, movable 

 and made of many turns of fine wire. This movable coil 

 is mounted on a shaft, fitted with jewelled bearings, so 

 that it turns inside the stationary coil. It carries an 

 indicating finger which moves over a suitably graduated 

 scale. 



It may be proved theoretically and experimentally 

 that such an instrument, when properly calibrated, actually 

 does read El cos cj>, E and / being the virtual values of 

 voltage and current in the circuit to which the wattmeter 

 is connected. Fig. 125 shows a well known type of portable 

 wattmeter. The current coil terminates in the heavy 

 copper lugs on the left of the instrument and the potential 

 coil is connected to the two smaller posts seen at the upper 

 end of the instrument case. 



The rating of a wattmeter is always given in terms of volts 

 and amperes; the current coil can stand only a certain 

 number of amperes and the potential coil only a certain 

 pressure in volts; and if the power factor of a circuit is low 

 it is readily seen that the capacity of the meter in watts on 



