ALTBRNAT1NG-CVRRBN T /.v> Th'i 'Ml-XTS 63 



35. Ammeters. Owing to the difficulty of carrying ai 



the smallest currents into the moving system of dynamometer 

 instruments, iron-vane ammeters are practically the only type 

 u.-od for commercial instruments. The Weston iron-vane 

 ammeter operates on the same principle as the iron-vane volt- 

 meter, Par. 34. The magnetizing coil in the ammeter is wound 

 with u few turns of heavy wire instead of with the large number 

 of turns of fine wire used witli the voltmeter. 



The General Electric Co.'s inclined-coil ammeter is of the 

 same construction as the voltmeter, except that the coil is wound 

 with coarse, instead of with fine, wire. (See Fig. 60.) 



36. Hot-wire Instruments. This type of instrument, described 

 in Vol. I, Chap. VII, page 136, reads equally well on both direct- 

 and alternating-current circuits. As its deflection depends upon 



luare of the current (I 2 R loss) the hot-wire instrument can 

 be used as a transfer from alternating to direct current and vice 

 versa. This type of instrument lacks high precision. 



37. Alternating-current Watthour Meter. The direct -current 

 watt hour meter can be used with alternating current. a> the 

 reversal of line voltage reverses both its armature and its field 

 current simultaneously and the direction of the torque remains 

 unchanged. At low power-factors, however, considerable error 

 may be introduced by the inductance of the armature circuit. 

 This oauaee the armature current to lag the line voltage by a 

 small anirle and although this has nogliidble etYect at or near 

 unity power-factor, the error at low power-factor is quite pro- 

 nounced. This error may bo compensated by shunting the cur- 

 rent coils of the meter with a low non-inductive 



The induction watthour meter is SO much cheaper and <o -upe- 

 rior to the direct-current type that there i- little neceor 

 u.-iim the direct-current type on alternating-current OUTOU 



A rear view of one type of induction meter is shown r 



potential coil which i.- highly inductive and i- placed on one 

 lu^ of the laminate. I magnetic circuit, thi- hitf beinu over the di>c 

 h. CC :ire two series Or current coiU placed on two project inn 



lim< beneath the disc. Tlie-e mil- are >o Wolllid th tend- 



:id flux upward the other tend- to send it dowir, 

 -mall auxiliary or cmn M the 



:ial hm and it- end- the resistance l\. In 



order that the meter m i!y. the pot cut nil-coil 



