THE MEASUREMENT OF CURRENT 



83 



When alternating currents are used, the term due to the local 

 field is absent, consequently the calibration curve will be obtained 



by plotting a new curve, the abscissae being D = - - For 



this case D = 25. 8/ 2 . The new curve is shown by the dotted 

 line in Fig. 42. 



It is to be noted that a high sensitivity is obtained by having 

 many turns on the coils. At high periodicities, therefore, both 

 the resistance and reactance of the coils conspire to alter the 

 circuit conditions when the instrument is introduced. 



4 2 



4 6 8 10 12 



CALIBRATION CURVES FOR SENSITIVE 



ELECTRODYNAMOMETER 

 * /-With Direct Current 

 *//- With Alternating: Current 

 Currents are in Milliamperes 



14 R 16 ad .18 20 J 



FIG. 42. Calibration curves for sensitive electrodynamometer. 



Astatic Electrodynamometers. All troubles due to uniform 

 local and stray fields may be obviated by making the instrument 

 astatic. Fig. 43 shows a simple arrangement of this sort. 



The current circulates in opposite directions in the two rigidly 

 connected movable coils which are identical in dimensions and 

 numbers of turns and are connected in series; hence, the movable 

 system when traversed by direct currents will experience no 

 turning moment due to the earth's field. Again, stray fields due 

 to either direct or alternating currents have no effect provided 

 they are sufficiently uniform so that the strength is the same 

 at both the upper and lower coils. The two sets of fixed coils 

 are so connected that they both tend to turn the movable member 

 in the same direction. 



