POWER MEASUREMENT 300 



stray fields, to have any effect, must be of the frequency of the 

 current under measurement. 



As high-capacity instruments have comparatively few turns 

 on the fixed coil, special care must be taken that there are no 

 loops in the leads near the instrument and that the current 

 leads occupy the same position with respect to the instrument 

 during its calibration and subsequent use. 



To obviate stray field errors, the working parts of the instru- 

 ment are frequently surrounded by a soft iron shield built up 

 of stampings (see Fig. 177). 



Voltage between Current and Potential Coils. The movable 

 coil of a wattmeter, being in series with a large resistance, may 

 inadvertantly be connected to the circuit so that practically 

 the full line potential exists between the current and the potential 

 coils. This may give rise to errors due to the electrostatic 

 attraction between these two coils ; also there is danger that the 

 insulation between the coils may be punctured. The connec- 

 tions should be so made that the current and potential coils 

 are, as nearly as possible, at the same potential. If when so 

 connected, the deflection is in the wrong direction, the current 

 coil should be reversed The proper position of a multiplier, 

 when one is used, is governed by the same consideration. 



It is well to mark the terminals of a wattmeter once for all 

 so that there can be no mistake in making the connections. 

 Such a marking also obviates any question as to the algebraic 

 sign of the readings when measuring polyphase power. 



When instrument transformers are used, electrostatic troubles 

 may be avoided if the two coils of the v wattmeter be connected 

 by a piece of very fine fuse wire. 



The Effect of Reactance in the Potential Circuit. -It has been 

 assumed that the potential circuit is non-reactive; this can 

 never be strictly true since it must contain the movable coil. 

 In commercial instruments, when used at ordinary frequencies 

 and power factors, the resistance of the potential circuit is so 

 high in comparison with its reactance that the effect of the 

 inductance is entirely negligible. In special investigations, how- 

 ever, cases arise where the utmost care must be exercised if 

 reliable results are to be obtained. The presence of reactance 

 has two effects: it cuts down to a certain extent the current 



