602 ELECTRICAL MEASUREMENTS 



To detect the presence of a stray field, the instrument should 

 be read and then immediately turned through 180 and read 

 again, the circuit conditions being maintained as constant as 

 possible. If no stray field is present, the readings will check. 

 If a wattmeter is being used, current may be sent through the 

 potential circuit alone and the instrument slowly turned in 

 azimuth. Any deflection observed will be due to the stray field. 



The stray fields due to heavy currents in the leads to the in- 

 struments themselves must not be neglected. The leads must be 

 free from loops and coils, should run straight away and should be 

 twisted. Especial care must be exercised when testing direct- 

 current watt-hour meters in situ, that the stray fields from the tem- 

 porary connections, such as the necessary jumpers, do not 

 vitiate the results, especially at light loads. Do not set moving- 

 coil instruments on sheets of "tin," which are tinned iron, and 

 do not place instruments too near together. As the field strength 

 in alternating-current instruments is small, they are more sus- 

 ceptible to these errors than direct-current instruments of the 

 moving-coil type. In general, one cannot assume that stray 

 fields are constant in either magnitude or direction. 



Careful attention must be given to these points when deciding 

 on the location and arrangement of apparatus for a test; for in 

 any case where results are called in question, unless one can prove 

 that there were no stray-field errors, the measurements have no 

 standing. Shielded instruments obviate these troubles. 



Electrostatic Attraction. Electrostatic attraction between the 

 fixed and movable members may cause erroneous deflections; 

 for instance, in wattmeters which are operated from instrument 

 transformers. In this case the remedy is to connect the current 

 and potential circuits by a bit of the finest fuse wire. Again, 

 glass and hard-rubber covers sometimes give trouble. They 

 should not be rubbed immediately before a reading is taken. 

 The surface charges may be dissipated by breathing on the 

 instrument. High range instruments having metal covers which 

 are supported by insulating bases are likely to give trouble. 

 The secondary circuits of instrument transformers should be 

 grounded. 



Eddy Currents. Eddy currents induced in massive coils, in 

 metal frames supporting the coils or in metal covers, may be a 



