What it does
A potentiometer is a long, uniform wire with a steady current through it from a driver cell. The potential falls evenly along the wire, so the voltage between end A and any point is proportional to the length. A test cell and a galvanometer are joined between A and a sliding contact (the jockey). Where the fall of potential along the wire exactly matches the cell’s emf, no current flows through the galvanometer: that is the balance point. Because no current is drawn from the cell at balance, a potentiometer measures its true emf, which an ordinary voltmeter cannot.
This page is a virtual version of the two classic experiments:
- Compare two emfs. Balance cell 1 (a known 1.50 V) at l₁ and cell 2 (unknown) at l₂. Then E₂ / E₁ = l₂ / l₁.
- Internal resistance. Balance the cell on open circuit (l₁), then with a resistance box R across it (key K₂ closed, l₂). Then r = R (l₁ − l₂) / l₂.
The galvanometer needle swings left or right as you move the jockey, worked out exactly from the circuit, so you find the balance point just as in the lab.
It follows the Class 12 Physics practicals on the potentiometer (comparing emfs, internal resistance of a cell) and the current electricity chapter. For the theory of internal resistance see internal resistance; the wheatstone bridge uses the same null method to find a resistance.
How to use it
- Pick Compare two emfs or Internal resistance.
- Set the rheostat in the driver circuit. If it is too big, there is no balance point; the page says so.
- Slide the jockey, then use the −10 … +10 cm buttons to bring the needle to zero. The needle turns green at balance.
- Press Record this balance length. It only records when the galvanometer reads zero.
- For emfs: switch to Cell 2 and repeat. For internal resistance: Close key K₂ and repeat. The result appears under the table.
- Show the true values to check your answer; New cells gives you new mystery cells.
Worked example
The driver cell is 2 V with 0.1 Ω inside, the wire is 4 m of 4 Ω and the rheostat is set to 0.5 Ω. The current in the wire is 2 / 4.6 = 0.435 A, so the potential gradient is 0.435 V/m, or 4.35 mV per cm. Cell 1 (1.50 V) balances at 1.50 / 0.435 = 345 cm. If cell 2 balances at 276 cm, its emf is 1.50 × 276 / 345 = 1.20 V.
For internal resistance: a cell balancing at 340 cm on open circuit and at 283.3 cm with R = 5 Ω across it has r = 5 × (340 − 283.3) / 283.3 = 1.00 Ω.
Tips
- A longer balance length gives a more precise answer; raise the rheostat as long as a balance point remains.
- Both balance lengths must be measured with the same current in the wire, so do not touch the rheostat between readings.
- The driver cell’s emf must be bigger than the cells you test.
FAQ
Why not just use a voltmeter? A voltmeter draws a little current, so it reads the terminal voltage, a bit less than the emf. At balance the potentiometer draws none.
Why does the needle go off the scale far from balance? The galvanometer is very sensitive; real setups add a protective resistance first and remove it near balance.
Is this an official NCERT resource? No. fibub is independent; the page follows the topics in the syllabus in its own words.
Where the numbers come from
- Potentiometer principle: with a steady current in a uniform wire the fall of potential is proportional to length (k = V/L); at balance no current flows in the cell's branch, so E = k l. E₁/E₂ = l₁/l₂ and r = R (l₁ − l₂)/l₂: standard results.
- The galvanometer current away from balance is found exactly from the Thévenin equivalent of the wire between end A and the jockey.
- Wire, driver cell, galvanometer and test-cell values are made-up teaching values.
- Checked against a separate nodal analysis of the whole circuit in the tests (galvanometer current at several jockey positions) and by hand (2 V driver, 0.5 Ω rheostat, 4 Ω wire: k = 0.435 V/m, a 1.50 V cell balances at 345 cm).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.