# Measurements and Instruments of Milliammeter

A rectangular moving coil of a milliammeter is wound with 30.5 turns. The effective axial length of the magnetic field is 20 mm and the effective radius of the coil is 8 mm. The flux density in the gap is 0.2 T and the controlling torque of the hairsprings is 0.5 x 10^-6 N m/degree of deflection. Calculate the current to give a deflection of 60 degrees.

Many thanks.

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#### Solution Preview

** Please see the attached file for the diagram **

A magnetic field B is radial to the pole, which faces at each point (B = 0.2 T). Please see the attachment for the diagram of this.

Magnetic moment of the coil = M = NIA (Equation (1)), where N = total number of turns = 30.5, I = current in the coil, and A = area of the coil = 16 x 20 mm2 = 320 mm2 or 320 x 10-6 m2. ...

#### Solution Summary

In this solution we calculate the current in a given physics problem.