ALTER\.\ T1\C,-('rRRE\T I\*TRVMENTS 



53 



direct-current instruments (see Vol. I, page 129, Fig. 117) oppose 

 the turning of coil M and at the same time carry the current into 

 the coil. As the springs can carry but a very small current, the 

 movable coil is wound with fine wire. 



Assume that at some instant the direction of the magnetic 

 field 0i, which is due to the fixed coils, is from left to right. At 

 the same instant the current in coil M produces a field < 2 whose 

 direction is along the axis of M. Coils tend to so align themselves 

 that the number of magnetic linkages in the system is a maximum. 

 Therefore, the moving coil M tends to turn in a clockwise direc- 

 tion so that its field will act in conjunction with <i. The turn- 

 ing of .!/ is opposed by the control springs. 



Obviously the torque developed is 

 promotional to #1, < 2 and sin 0, 

 where /3 is the angle between the axis 

 of coil M and the axis of coils FF'. 

 v>i and 02 are proportional to the 

 currents in the coils FF' and 717 re- 

 spectively, the torque is proportional 

 to the product of the two currents 

 and sin 0. 



29. The Electro-dynamometer Volt- 

 meter. Some types of alternating- 

 current voltmeter operate on the 

 electro-dynamometer principle. The 

 fixed coils FF', Fig. 50, are wound 

 with fine wire and are connected in 

 I with the moving coil M. A 

 high resistance /' is connected in 

 with the dynamometer to 



limit the current when the instrument is connected across the 

 line. The current pa irm through the dynamometer is therefore 

 proportional to the line voltage. The current passing through 

 the muniment causes coil M to turn and the pointer attached to 

 it m. i a scale graduated in volts. The scale will not In- 



divided uniformly like that of the direct-current voltmeter, for 

 the d - are very nearly proportional to t he square of the 



The di\ urionfl at the lower part of the scale are SO small 

 that poor precision i> obtained. The divisions at the middle 



.">(). I)i:inr;iin of H dyna- 

 mometer- voltmeter. 



