THE MEASUREMENT OF CURRENT 



85 



Rewinding an Electrodynamometer to Obtain a Given Per- 

 formance. As the deflection of an electrodynamometer when 

 used with alternating currents is proportional to the square of 

 the current, the range of the instrument is limited. It is desirable, 

 therefore, to have the fixed coil subdivided by taps so that the 

 number of turns may be varied. 



It is sometimes necessary to alter the sensitivity of secondary 

 dynamometers so that definite deflections may be obtained with 

 stated currents. If the corresponding dimensions of the coils of 

 the original and the rewound instruments are the same, the calcu- 



FIG. 44. Irwin astatic electrodynamometer. 



lation of the proper number of turns is simply a matter of pro- 

 portion, provided one has data concerning the number of turns 

 on the coils and the performance of the original instrument. If 

 the instrument is properly set up, 



r0 = KFMI* (65) 



F and M are the numbers of turns on the fixed and movable coils 

 respectively, and K is a constant depending upon the geometry 

 of the coil system. This may be determined from a knowledge 

 of the torsion constant of the spring, the number of fixed and 

 moving turns on the original instrument and the deflection corre- 

 sponding to a given current. After K has been found, the value 



