Verify Series and Parallel Laws of Resistance Using a Metre Bridge

Aim

To verify the laws of combination of resistances (series and parallel) using a metre bridge.

Material Required

• Metre bridge
• Sensitive galvanometer
• Two different resistances (carbon or wire-wound)
• Resistance box
• Jockey
• Rheostat
• Plug key
• Cell or battery eliminator
• Thick connecting wires
• Sand-paper

Theory

• Series Combination: If resistances ( R_1 ) and ( R_2 ) are in series, the total ( R_s = R_1 + R_2 )
• Parallel Combination: If in parallel,
  [ \frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} ]

Circuit Diagrams

Series Circuit:

Parallel Circuit:

Procedure

1. Setup: Make all connections neatly. Tighten all plugs of resistance box. Clean wire ends with sandpaper.
2. Series Method:
   • Insert plugs to get a suitable resistance ( R ) from the box.
   • Slide jockey along the metre bridge wire to find the null point D (zero deflection of galvanometer).
   • Measure lengths ( AD ) and ( DC ) in the metre bridge wire.
   • Record ( R, AD, DC ).
   • Repeat for at least 5 different values of ( R ), compute the series-equivalent resistance ( X ).
3. Parallel Method:
   • Reconfigure the resistors ( R_1, R_2 ) in parallel, rewire the bridge.
   • Repeat steps to find null point and corresponding lengths.
   • Calculate parallel-equivalent resistance ( X ) for parallel combination.

Observations & Calculations

Precautions

1. Ensure all connections and resistance box plugs are tight.
2. Move the jockey gently on the metre-bridge wire.
3. Tighten plug keys in the resistance box properly (clockwise).
4. Try to get null point in the central region of the bridge wire (30 cm to 70 cm).

Sources of Error

1. Wire thickness may not be uniform.
2. Plugs in the resistance box may not be fully tight.
3. Wire may heat up due to continuous current, changing resistance.
4. Contact resistance from bad connections can affect readings.