ALTERNA TIXd-CCItRENT INSTRUMENTS 



55 



take such a position that its axis coincides with the axis of the 

 fixed coil, so that their magnetic fields act in conjunction. In 

 turning, the moving coil is opposed by flat spiral springs. The 

 scale is calibrated in volts. 



As this instrument is of the dynamometer type, its scale read- 

 ings hold for both direct and alternating currents. 



31. Dynamometer Ammeters. Owing to the difficulty of 

 leading a heavy current into the moving coil, dynamometer 

 ammeters of the portable type and of the switchboard type are 



ommon. It might appear that this type of instrument 

 could be devised for use with a shunt, which would allow but a 

 small portion of the total current to pass through the moving 

 coil. This involves two difficulties. Alternating currents divide 

 inversely as the circuit impedances. Impedances are determined 

 by the frequency. Unless the ratio of inductance to resistance 

 were the same in the shunt as in the moving coil, the instrument 

 would be correct at only one frequency and might be in consider- 

 able error with an irregular wave shape owing to the presence of 

 higher-frequency harmonics. There would also be a considerable 

 voltage drop across such a shunt. 



Instruments of the dynamometer type, in which the above 

 difficulties are in part overcome, are available, but the iron-vane 

 type of in<trument described in 



; i is so much simpler and 

 less expensive that the shunted 



type is little used. 



32. The Wattmeter. Alter- 



nating-current power is equal 

 to the product of the effective 

 (in-rent and the effective volt- 



.,,_.,.,,. ; . ; 



i> unity. Therefore, the am- 



mete! and voltmeter method. 



as used with direct CUITentS, can seldom be u-ed to measure 



alternatiim-current power. Consequently, a inittmeter is ii- 



. for measuring alternating-current power. 

 The wattmeter shown in I-'ii;. 52 operates on the eta 

 dynamometer principle. M i- a moving coil wound with tine 

 UK! i- practically identical with the moving coil of the dyna- 



Load 



K I() . .vj, r ()MIUM -t inns for a wattmeter. 



