Question

(i) Why is the magnetic field radial in a moving coil galvanometer ? Explain how it is achieved ?

(ii) A galvanometer of resistance 'G' can be converted into a voltmeter of range (0 - V) volts by connecting a resistance 'R' in series with it. How much resistance will be required to change its range from 0 to V/2 ?

Answer 1

 (i) Need for a radial magnetic field :

The relation between the current (i) flowing through the galvanometer coil, and the angular deflection (ϕ) of the coil (from its equilibrium position), is

where θ is the angle between the magnetic field vector B and the equivalent magnetic moment vector μ_m of the current carrying coil.

Thus I is not directly proportional to ϕ . We can ensure this proportionality by having θ = 90°. This is possible only when the magnetic field vector B, is a radial magnetic field. In such a field, the plane of the rotating coil is always parallel to vector B .

To get a radial magnetic field, the pole pieces of the magnet, are made concave in shape. Also a soft iron cylinder is used as the core.

where R' = Resistance required to change the range from 0 to V/2