Question

In ballistic galvanometer, the frame on which the coil is wound is non-metallic to:
A. Avoid the production of induced emf.
B. Avoid the production of eddy currents.
C. Increase the production of eddy currents.
D. Increase the production of induced emf.

Answer 1

Hint: To solve this question we must know what is a ballistic galvanometer and why is it important for the coil of ballistic galvanometer to be non-metallic. So, if we were to find out what would happen to a ballistic galvanometer if the coil was metallic we can answer this particular question.

Complete answer:
A ballistic galvanometer is a sensing galvanometer. Ballistic galvanometer measures the amount of charge which flows across the magnetic coil. This initiates the coil to move and show the deflection. So, the working principle of the galvanometer depends on deflection of the coil which is directly proportional to the charge passing through it. The coil of ballistic galvanometer consists of a copper wire which is wound on the non metallic or non-conducting frame. If the frame of the coil was to be metallic or conducting eddy currents would be generated. Consider that in a galvanometer current to be measured is passed which will generate some magnetic field. This magnetic field in turn will induce current in the metallic frame. This induced current will be in the form of eddies. This will affect the reading of the galvanometer as the eddy current is also present. But, if the frame is non-metallic there would be no induced current as the magnetic field generated would not affect the non metallic frame. So, in order to avoid the eddy current the coil is wound in a non metallic frame.

Therefore, the correct answer is option B.

Note: In a moving coil galvanometer the coil is wound on a metallic frame so that the opposing eddy currents provide electromagnetic damping which opposes the motion of the coil.