Question

1. State the working principle of a moving coil galvanometer? What modification is required in the galvanometer to make its scale linear?  (2)
2. If a galvanometer of resistance 49.5 Ω has a range of 0.05 A. What will be the value of resistance needed to convert it into an ammeter of range 5 A?  (2)
3. How should these two resistors be connected to the galvanometer in both cases?  (1)

Answer 1

A. A moving-coil galvanometer works on the principle that a current-carrying coil placed in a magnetic field experiences a torque.

The deflecting torque produced is directly proportional to the current flowing through the coil.

To make the galvanometer scale linear, the magnetic field is made radial by:

1. Make the pole pieces of the magnet spherical.
2. This is achieved by inserting a soft-iron core into the coil.

This produces a uniform radial magnetic field, so the torque becomes proportional to the current over the entire scale.

B. Given: Galvanometer resistance (R_g) = 49.5 Ω

Galvanometer full-scale deflection current (I_g) = 0.05 A

Required ammeter range (I) = 5 A

To convert a galvanometer into an ammeter, a shunt resistance (S) is connected in parallel.

Formula: S = `(I_g R_g)/(I - I_g)`

= `(0.05 xx 49.5)/(5 - 0.05)`

= `2.475/4.95`

= 0.5 Ω

C. Shunt resistance is connected in parallel with the galvanometer (for ammeter), and a high resistance is connected in series with the galvanometer (for voltmeter).