Class 12 Physics practical reading To determine the resistance of a galvanometer by half-deflection method and to find its figure of merit. Class 12, Physics

Aim

To determine the resistance of a galvanometer by half-deflection

method and to find its figure of merit

Apparatus

A moving coil galvanometer, a battery or a battery eliminator (0 - 6 V),

one high resistance box , one small resistance box , two one way keys, voltmeter, connecting 

wires and a piece of sand paper.

Theory

Circuit diagram

Procedure

1. Clean the connecting wires with sand paper and make neat and

tight connections as per the circuit diagram (Fig.) 

2. From the high resistance box , remove a key

and then close the key K1 . Adjust the resistance R from this 

resistance box to get full scale deflection on the galvanometer dial.

Record the values of resistance, R and deflection θ.

3. Insert the key K2

and keep R fixed. Adjust the value of shunt

resistance S to get the deflection in the galvanometer which is

exactly half of θ. Note down S. Remove plug K2

 after noting down

the value of shunt resistance, S.

4. Take five sets of observations by repeating steps 2 and 3 so that

θ is even number of divisions and record the observations for R, S, θ and  2θ in tabular form. 

5. Calculate the galvanometer resistance G and figure of merit k of

galvanometer. 

Observations & Calculations

     

Precautions

1. Key K1

 should be inserted only after high value of R has been

taken out from resistance box otherwise galvanometer coil

may burn.

2. Adjust R such that deflection in galvanometer is of even division

so that θ/2 is more conveniently obtained.

3. Emf of the battery should be constant.

4. Use as high values of R as practically possible. This ensures correct

value of G.

5. All the connections and plugs in the resistance box should

be tight.

Sources of Error

1. Plugs in the resistance boxes may be loose or they may not

be clean.

2. The emf of the battery may not be constant.