Ans: The length of coil be l meter and width of coil be d meter. Assume I is the current flowing in the ciol having N turns. Assume B as the flux density in the air gap then F=BIL(N)
The equation for the developed torque, derived from the basic
law for electromagnetic torque is
Ʈ= B*A*I*N
Where r= torque Newton. Meter
B = flux density in the air gap. Wb/m2
A = effective coil area (m)
N= number of turns of wire of the coil
I = current in the movable coil (amperes)
The equation shows that the developed torque is proportional to the flux density of the field in which the coil rotates, the current coil constants (area and number of turns). Since both flux density and coil constants are fixed for a given instrument, the developed torque is a direct indication of the current in the coil. The pointer deflection can therefore be used to measure current.