PHY043 · Electricity & Circuits · Class 12 + College
Wheatstone bridge
Objective: To find an unknown resistance using a balanced Wheatstone bridge.
🧰 Apparatus
- Metre bridge or four resistors
- Galvanometer
- Resistance box
- Cell and key
- Jockey
📐 Formulas and SI units
Balanced when R₁/R₂ = R₃/R₄
R₄ = R₂R₃ / R₁
💡 Theory
Four resistors form a diamond with a galvanometer across the middle. When R₁/R₂ = R₃/R₄ the two middle points are at the same potential and no current flows through the galvanometer: the bridge is balanced. Then the unknown R₄ = R₂R₃/R₁.
🪜 Procedure
- Connect the bridge with the unknown resistor as R₄.
- Choose R₁, R₂ and R₃ from the resistance box.
- Close the key and watch the galvanometer.
- Adjust R₃ (or slide the jockey on a metre bridge) until there is no deflection.
- Calculate R₄ = R₂R₃/R₁.
📋 Observations Simulated values
✅ Result and conclusion
With R₁ = 100 Ω, R₂ = 200 Ω and R₃ = 50 Ω the bridge balances at R₄ = 100 Ω; the galvanometer reads zero.
🔬 In a real lab
Contact resistance at the jockey and end resistance of the metre bridge wire give small errors; swapping R and the unknown and averaging reduces them.
⚠️ Safety
Press the jockey briefly so the galvanometer is not overloaded; use a low-voltage cell.
❓ Quick check
1. At balance the galvanometer current is…
2. Why is the Wheatstone bridge accurate?
3. R₁ = 10 Ω, R₂ = 20 Ω, R₃ = 15 Ω. Balance R₄?
🏋️ Practice
EasyR₁ = R₂. R₄ at balance?
Answer: Equal to R₃
MediumR₁ = 50, R₂ = 100, R₃ = 30. R₄?
Answer: 60 Ω
ChallengeName a device that uses a bridge.
Answer: Strain gauges, metre bridge, sensors