Describe Kelvin’s method to determine the resistance of galvanometer by using metre bridge.
Kelvin’s 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 Wheatstone's meter bridge and a resistance box is connected in the other gap (right gap).
R: Resistance from the resistance box
AC: Metal wire one metre long
K: Plug key
- 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 'R' 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, Kelvin's method is a deflection method. The point 'D' is called the balanced point.
- Let lg and lr be the distances of point 'D' from ends 'A' and 'C' of wire AC, respectively. The resistance per unit length of wire AC is ''. Here also G, R and resistances of wire of lengths lg and lr form four arms of a balanced Wheatstone’s network.
- Thus, the resistance of galvanometer 'G' can be calculated by knowing the values of R and lg in the above equation.