A potentiometer works on one simple principle: along a uniform wire carrying a steady current, the potential drop is directly proportional to the length of wire, so V = k * L (k is the potential gradient, in volt per metre). We move a jockey until the galvanometer shows zero (the null point); at that instant the wire's potential drop exactly equals the cell's EMF and NO current flows through the cell. Memory hook: "No current, true EMF" — a potentiometer measures the real EMF because it draws zero current at balance, unlike a voltmeter.
A steady driver current makes the potential drop grow uniformly from A to B, so V = k*L. The jockey J is moved until the galvanometer G reads zero (null point); there the test cell's EMF equals k*(balance length) and no current flows through the cell.
Your doubts, answered
Why does no current flow through the cell at the null point?
At the balance (null) point the potential drop across the length of potentiometer wire is exactly equal and opposite to the EMF of the test cell. Two equal opposing voltages leave zero net voltage in that branch, so the galvanometer reads zero and no current is drawn from the test cell. This is the whole reason a potentiometer measures true EMF: since I = 0 through the cell, there is no drop across the cell's internal resistance (I*r = 0), so terminal voltage = EMF.
What exactly is the potential gradient k?
The potential gradient k is the fall of potential per unit length of the wire, k = V/L (unit: volt per metre). If a driver cell keeps a steady voltage V across the full wire of length L, then k is constant everywhere because the wire is uniform. The balance condition is simply EMF = k * (balance length). A LONGER wire or SMALLER driver voltage gives a smaller k, which makes the instrument more sensitive.
How is a potentiometer different from a rheostat?
They look similar but do different jobs. A rheostat is a two-terminal variable resistor used to control CURRENT in a circuit. A potentiometer is a three-terminal device used to tap off a variable POTENTIAL (voltage) by comparing potential drops along a wire. For NEET, remember: rheostat = adjust current; potentiometer = measure/compare EMF or voltage with no current drawn at balance.
Why must the driver (auxiliary) cell's EMF be larger than the cell being measured?
The potential drop across the whole wire comes from the driver cell. To find a balance point somewhere on the wire, the driver's potential drop over the FULL wire must be able to reach the value of the test EMF. If the driver EMF is smaller, the wire's total drop is less than the test EMF and no null point exists anywhere — the galvanometer never reads zero. So the driver EMF must exceed every EMF you want to measure.
⚠️ The NEET trap ✗ Thinking the balance length depends on the test cell's internal resistance. ✓ At the null point the current through the test cell is zero, so the I*r drop is zero and the balance length depends only on the EMF (not on internal resistance). Internal resistance only matters when you deliberately draw current, e.g. by connecting an external resistor across the cell. 🧠 Zero current at balance means internal resistance drops out — balance length tracks EMF, not r.
Real NEET questions
NEET 2017
A potentiometer is an accurate and versatile device to measure EMF because the method involves:
A · cells
B · potential gradients
C · a condition of no current flow through the galvanometer ✓
D · a combination of cells, galvanometer and resistances
Solution: The key idea is the balance (null) condition. Step 1: The jockey is moved until the galvanometer shows zero deflection. Step 2: Zero deflection means no current flows through the galvanometer branch, so no current is drawn from the test cell. Step 3: With I = 0 through the cell, the internal-resistance drop I*r = 0, so terminal voltage = true EMF. Therefore the accuracy comes from the no-current condition. Correct option: (C).
NEET 2021
In a potentiometer, a cell of EMF 1.5 V balances at 36 cm. If a 2.5 V cell replaces it, the balance length is:
A · 64 cm
B · 62 cm
C · 60 cm ✓
D · 21.6 cm
Solution: Use the principle EMF is proportional to balance length: E = k * L, with the same potential gradient k for both cells. Step 1: Write E1/E2 = L1/L2. Step 2: Substitute 1.5/2.5 = 36/L2. Step 3: Solve L2 = 36 * (2.5/1.5) = 36 * (5/3) = 60 cm. Correct option: (C) 60 cm.
Solved Current Electricity NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the principle of a potentiometer in one line?
For a uniform wire carrying a steady current, the potential drop across any section is directly proportional to its length: V = k * L, where k is the constant potential gradient (volt per metre).
What is a null point or balance point?
It is the position of the jockey on the wire where the galvanometer shows zero deflection. At this point the wire's potential drop equals the test cell's EMF and no current flows through the cell.
Why is a potentiometer called an ideal voltmeter?
Because at balance it draws zero current from the cell, exactly like an ideal voltmeter of infinite resistance. So it measures the true EMF without any loading error.
How do you increase the sensitivity of a potentiometer?
Reduce the potential gradient k. Use a longer wire or lower the driver current/voltage. A smaller k means a small EMF spreads over a longer balance length, so you can read it more precisely.
Is the potentiometer still in the NCERT text?
The rationalised NCERT Current Electricity chapter trimmed the potentiometer, but NEET has repeatedly tested it (2016, 2017, 2021, 2023). So you must still study its principle, working and its two uses (comparing EMF and finding internal resistance).
