Potentiometer: Comparing EMFs
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flagWhat you'll discover
- arrow_forwardExplain the principle of the potentiometer (potential drop ∝ length)
- arrow_forwardFind balancing lengths for two cells with a jockey
- arrow_forwardCompute E₁/E₂ = l₁/l₂ and explain why it measures true EMF
- arrow_forwardState why the driver-cell voltage must exceed both EMFs
Why a potentiometer beats a voltmeter
A voltmeter must draw a little current to deflect, and that current causes a drop across the cell’s internal resistance — so a voltmeter always reads slightly LESS than the true EMF. The potentiometer dodges this completely: at balance, the cell drives zero current, so nothing is lost internally and the balancing length measures the genuine EMF.
The principle: a steady current from a driver cell flows down a long uniform wire, so potential falls linearly with distance — a constant gradient k volts per centimetre. A point l cm from the start sits at potential kl below the start.
Procedure: two balances, one ratio
Connect the driver circuit (accumulator, key, rheostat) across the full wire. Connect cell 1 between the wire’s start and the galvanometer-jockey, with like poles (both positives) at the same end — otherwise no balance exists anywhere. Slide the jockey until the galvanometer is null: that is l₁, where kl₁ = E₁. Switch the two-way key to cell 2 and find l₂.
Dividing kills the unknown gradient: E₁/E₂ = l₁/l₂. Everything depends on the driver current staying constant, so do NOT touch the rheostat between the two balances, and check l₁ again at the end — if it has drifted, the driver cell is sagging and the run must be repeated.
Sensitivity, errors, precautions
A longer balancing length means a smaller percentage error, so adjust the rheostat to push balances toward the far end of the wire. The wire must be uniform — its non-uniformity is the chief systematic error. Contact must be gentle: dragging the jockey scrapes the wire thin.
If the galvanometer deflects the same way at both ends of the wire, either the driver voltage is below the cell’s EMF (gradient too small — reduce the series rheostat) or a polarity is reversed. Standard viva: "Why is the potentiometer called an ideal voltmeter?" — because it draws no current at balance; effectively infinite resistance.