Question

The pointer in a dead beat galvanometer gives a steady deflection because
(A) Eddy currents are produced in the conducting frame over which the coil is wound
(B) Its magnet is strong
(C) Its pointer is very light
(D) Its frame is made of abonite

Answer 1

Hint : Eddy current can be induced as a result of the movement of a conductor through a changing magnetic field. It generates a force that opposes the motion of the conductor.

Complete step by step answer
In general, when currents flow through the coil of a galvanometer, this generates a force that is proportional to the current and moves the pointer in a particular direction until the force is balanced by the torsional forces in the string. However, due to Newton’s first law, when the forces balance, the pointer tends to continue at constant velocity. However, the extra movement allows the torsional forces to become greater than the force generated by the current, hence, the pointer begins to accelerate backward, until again, the force due to the current is once again greater than the torsional forces and thus accelerates forward again. This process repeats. This causes a galvanometer to oscillate about its mean (true) position before coming to rest due to damping. This makes it hard and time consuming to take readings.
However, in a dead beat galvanometer, the coil is wound on a metallic frame such that when the current flows through the conductor, as the pointer moves through a changing magnetic field, an eddy current in induced in the metallic frame which opposes the motion of the pointer beyond the mean value, hence dampens the motion very quickly.
Hence, the correct answer is A.

Note
For clarity, the eddy current acts similarly to friction. Note that when forces balance, only acceleration is zero, the body usually continues to move at the velocity at which acceleration became balanced. Like a body moving in outer space will keep moving continuously. However, on the ground kinetic friction acts on the body while it is moving and stops acting once it stops. The eddy current set up works similarly in the galvanometer. When the pointer is moving towards the reading, the eddy current gradually acts in the opposite direction but stops acting once the pointer comes to rest.