308 ELECTRICAL MEASUREMENTS 



in a voltmeter connected directly across the load may be allowed 



for, a second compensating winding being added and connected in 



series with the voltmeter; allowance must be made for the 



changed resistance of the voltmeter circuit. 



Grouping of Instruments. The heating losses have a bearing 



on the grouping of the instruments when the voltage, current, 



power and power factor of a small reactive load are to be 



determined. 



Electrical apparatus is generally sold to be operated at some 



definite voltage, so the voltmeter is placed across the load as 



in Fig. 179, 



As shown, the wattmeter measures in addition to the load, 



the power in its own current coils, in the ammeter, and in the 



voltmeter. The ammeter gives 

 the vector sum of the currents in 

 the load and in the voltmeter. 

 Load Errors due to Local Fields. In 

 industrial testing, it is not safe 

 to assume that a wattmeter will 

 be uninfluenced by local mag- 



F , IG ' 17 ?'~7 Showing g rou P in g of netic fields. In the ordinary port- 



instruments for measuring, power, 



current and voltage. able instruments this error, if 



present, will depend on the de- 

 flection. Its presence or absence may be made obvious by 

 connecting the potential coil alone to the circuit and observ- 

 ing the deflection when the instrument is turned to several 

 different azimuths. If the error is present and it is not feasible 

 to change the location of the observing station, the instrument 

 may be turned to the position where the pointer is undeflected, 

 that is, where the movable coil is threaded by the maximum 

 number of lines of force, due to the stray field. When the 

 proper position has been found, the direction of the pointer 

 may be noted. If in the subsequent use of the wattmeter the 

 pointer is always kept in approximately this direction, the body 

 of the instrument being turned as the load is varied, the error 

 will be rendered negligible, provided the direction of the local 

 field does not change. 



Direct-current stray fields have no influence on the readings 

 when alternating currents are being dealt with, and alternating 



