82 



ELECTRICAL MEASUREMENTS 



deflection is small and cos is practically unity, 

 the scale reading then, nearly enough, 



D l = K'J* + K' 2 I 

 and on reversing the current 



FIG. 41. Sensitive electrodyna- 

 mometer. 



If D represents 



- K'J 



where K\ and K' 2 are con- 

 stants. I is the numerical 

 value of the current without 

 regard to sign. The law of a 

 sensitive electrodynamometer 

 when used with direct currents 

 is shown in Fig. 42. The in- 

 strument was adjusted so that 

 initially the axes of the coils 

 were perpendicular, with the 

 axis of the fixed coil coincid- 

 ing in direction with the local 

 field. 



To make this adjustment 

 an alternating current may 

 be sent through the fixed coil, 

 the circuit of the movable coil 

 being closed through a tele- 

 phone. The desired position 

 is attained when the mutual 

 inductance is zero, that is, 

 when the telephone is silent. 

 This adjustment having been 

 made, the axis of the fixed 

 coil may be made to coincide 



with the local field if a direct current be sent through the mov- 

 able coil alone and the instrument turned in azimuth until the 

 deflections with reversed currents are equal. 



Referring to Fig. 42 and employing 10~ 3 amp. as the unit of 

 current, for this particular instrument, 



D 1 = 25.8/ 2 + 18.37 

 Z> 2 = 25.8J 2 - 18.3/ 



