Determine the Resistance of a Galvanometer by Half-Deflection Method

Aim

To determine the resistance of a galvanometer by the half-deflection method and to find its figure of merit.

Apparatus

A moving-coil galvanometer, battery or battery eliminator (0–6 V), high-resistance box, small resistance box, two one-way keys, voltmeter, connecting wires, and sandpaper.

Theory

Circuit Diagram

Procedure

1. Clean all connecting wires using sandpaper and make tight connections as per the circuit diagram.
2. From the high resistance box, remove a plug and close key K1.
   Adjust resistance R to obtain full-scale deflection on the galvanometer.
   Record values of R and deflection θ.
3. Insert key K2 and keep R fixed.
   Adjust shunt resistance S to obtain a deflection exactly half of θ.
   Note the value of S, then remove plug K2.
4. Repeat steps 2 and 3 for five sets of observations such that θ is even (for easier halving).
   Record R, S, θ, and 2θ in a table.
5. Calculate:
   • Galvanometer resistance ( G )
   • Figure of merit ( k )

Observations & Calculations

Precautions

1. Insert key K1 only after taking out a high value of R from the resistance box; otherwise the galvanometer coil may burn.
2. Choose R so that the galvanometer shows an even deflection, making θ/2 easier to obtain.
3. Ensure the EMF of the battery remains constant.
4. Use the highest possible value of R for better accuracy of G.
5. Ensure all plugs and connections are tight.

Sources of Error

1. Plugs in the resistance boxes may be loose or unclean.
2. The EMF of the battery may fluctuate during the experiment.