THE MEASUREMENT OF CURRENT 



33 



coil the current is taken out by a loosely coiled metallic spiral. A 

 proper torsion head for adjusting the coil vertically and setting 

 the zero reading is provided. The entire coil system is mounted 

 in a removable frame so that the coils may be readily changed. 

 Suppose the coil is rectangular, the total length of active wire 

 is I and the half breadth of the coil is b, and that the field, H , in 

 which the coil is placed is uniform. The turning moment due to 

 the current, 7, is 



M = IHlb cos 6 



Fir,. 15. Moving-coil galvanometer. 



where 6 is the angle between the lines of force and the plane of 

 the coil. If the motion of the coil be resisted by a spring, the 

 latter will be twisted until the restoring moment due to it is equal 

 to the deflecting moment due to the current. If the zero posi- 

 tion of the plane of the coil is in the direction of the lines of force, 

 the restoring moment will be rO where r is the restoring moment 

 for unit angular deflection (a constant). Thus, at equilibrium 



rB = IHlb cos 6 



If there are n turns, 



nlllb \cos0 



The D'Arsonval galvanometer is a secondary instrument; that 

 is, the relation between the deflection and the current is always 



