To Find Resistance Using a Metre Bridge (Class 12 Physics Practical)
To determine the unknown resistance of a wire using the metre bridge and the principle of balanced Wheatstone bridge.
TL;DR: To determine the unknown resistance of a wire using the metre bridge and the principle of balanced Wheatstone bridge.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Aim: To determine the unknown resistance of a wire using the metre bridge and the principle of balanced Wheatstone bridge.
Materials Required
- Metre bridge
- One known resistance (standard box)
- Unknown resistance wire
- Battery/cell
- Key
- Galvanometer
- Connecting wires
- Jockey
Theory
A metre bridge is a practical application of the Wheatstone bridge. When the bridge is balanced, the condition is P/Q = R/S, where P is unknown resistance, Q is standard resistance, R is the length on the unknown side, and S is the length on the standard side. The balancing length is found where the galvanometer shows no deflection.
Procedure
- Connect the unknown resistance and standard resistance box to the metre bridge wire
- Attach the galvanometer and key in the circuit as shown in the apparatus diagram
- Adjust the standard resistance box to a convenient value
- Close the key and touch the jockey along the metre bridge wire to find the balancing point
- The balancing point is where the galvanometer shows zero deflection
- Record the lengths from both ends at the balancing point
- Repeat the experiment by changing the value in the standard resistance box (at least 4 times)
- Calculate the unknown resistance using P = Q x R/S for each set
Observation
When the jockey is touched at different points on the metre wire, the galvanometer shows deflections. At one particular point, the galvanometer shows zero deflection - this is the balancing point. The position of the balancing point changes when the standard resistance is changed.
Result
The unknown resistance is found to be approximately [value] Ohm. The average value from multiple observations confirms the accuracy of the metre bridge method.
Precautions
- Ensure the metre bridge wire is clean and uniform in cross-section
- Use a sensitive galvanometer for accurate zero deflection
- Do not apply excessive current to avoid heating and change in resistance
- Take the balancing lengths accurately from the metre scale
- Repeat the observation with different standard resistances to reduce error
Viva Questions
What is the principle of the metre bridge?
The metre bridge works on the principle of the Wheatstone bridge where P/Q = R/S at the balancing point.
Why must the galvanometer show zero deflection at the balancing point?
Zero deflection means no current flows through the galvanometer, indicating that the potential at the junction is the same on both sides.
Why is the unknown resistance placed on one side of the bridge?
The unknown resistance is compared with a known standard resistance to determine its value using the balance condition.
What happens if the metre bridge wire is not uniform?
Non-uniform wire will have varying resistance per unit length, leading to incorrect balancing point and erroneous results.
More Class 12 Physics Practicals
- To Find the Focal Length of a Concave Mirror
- To Verify Ohm's Law
- To Study the Laws of Reflection
- To Determine the Density of a Solid
- To Verify the Laws of Refraction (Glass Slab)
- To Study a Simple Pendulum and Find the Acceleration Due to Gravity (g)
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