Simulic
English

Wheatstone bridge and metre bridge – resistance and resistivity

PhysicsElectricity & CircuitsAges 16–17Virtual lab

Loading…

Use with my class

A metre bridge: a uniform 100 cm wire, a resistance box R in one gap and the specimen X in the other. Slide the jockey along the wire until the galvanometer reads zero: at the null point R/X = l/(100 − l). Interchange R and X and average to cancel the end errors, work out the resistivity of a wire from its length and its screw-gauge diameter, and verify the laws of series and parallel combination. The Wheatstone bridge screen shows the balance condition P/Q = R/S.

Lesson: Balanced Wheatstone bridge; measuring resistance with a metre bridge; resistivity; series and parallel combination of resistors

What it shows

In a Wheatstone bridge no current flows through the galvanometer when P/Q = R/S, whatever the cell. A metre bridge uses a uniform wire as the two lower arms, so their ratio is the length ratio l/(100 − l). The network here is solved in full, so the deflection away from the null point is real, and the end strips add a small resistance at each end of the wire. That is why one arrangement is slightly off and interchanging R and X helps. The specimens are two wires and two coils; their values are hidden.

How to use

Choose a Specimen and a Resistance box R, then drag the jockey along the wire or use Jockey position l until the galvanometer reads zero, and press Record. Press Interchange R and X, find the new null point and record again. Answer the tasks with your mean value. The Wheatstone bridge tab lets you change P, Q, R and S and watch the galvanometer.

Parameters you can change

  • Screen Metre bridge, Wheatstone bridge
  • Specimen Wire A, Wire B, Coil X₁, Coil X₂, X₁ and X₂ in series, X₁ and X₂ in parallel
  • Resistance box R 0.5–20 Ω

Questions to explore

  1. Why is the null point independent of the cell voltage and of the rheostat setting?
  2. Why is the metre bridge most sensitive when the null point is near the middle of the wire?
  3. How does interchanging R and X remove the error caused by the end resistances?

Related simulations