Question

Describe Kelvins method to determine the resistance of galvanometer by using meter bridge.

Answer 1

Kelvins method to determine the resistance of the galvanometer by using a meter bridge:

A galvanometer whose resistance 'G is to be determined is connected in one gap (left gap) of a Wheatstones meter bridge and a resistance box is connected in the other gap (right gap).

G : Galvanometer

R : Resistance from the resistance box

AC : Metal wire one meter long

Rh : Rheostat

E : Cell.

K :Plug key

K : Jockey

A cell of emf 'E', key K and rheostat Rh are connected in series with the bridge wire AC. The junction B of the galvanometer and the resistance box is connected to the jockey which can slide along wire AC.

A suitable resistance K is taken in the resistance box and a current J is sent round the circuit. Without touching the jockey to any point of AC, note the deflection in the galvanometer.

A rheostat is adjusted to get a suitable deflection (e.g. 0.15, 20 divisions) in the galvanometer.

Place the jockey at points A and C, and see the deflection on the galvanometer. It should be on opposite sides.

By touching the jockey to different points of wire AC, find (obtain) the point of contact 'D' for which the galvanometer shows, the same deflection as before I.e., points B and D are equipotential (i.e. the point gives the same deflection in the galvanometer with or without the contact of the jockey.)

In this method, the null point is not obtained. Thus, Kelvins method is a deflection method. The point D is called the balanced point.

Let lglglglglglglglglg, in the above equation.