Q.State the principle of the potentiometer.

The Intuition: Why "No Current" Matters

Imagine you want to measure the voltage of a battery. You grab a voltmeter and connect it across the terminals. But here's the problem: a voltmeter draws a tiny current to work. That current flows through the battery's internal resistance, causing a small voltage drop inside the battery itself. So the voltmeter shows a value slightly less than the battery's true EMF (electromotive force). For a fresh battery this error is tiny, but for a weak cell or a sensitive measurement it becomes significant.

What if you could measure voltage without any current flowing out of the source? That would give you the true, undisturbed EMF. That's exactly what a potentiometer does.

The Setup: A Uniform Wire and a Known Reference

A potentiometer uses a long, uniform wire (often 10 m) stretched along a scale. A known voltage, say from a standard cell or a driver circuit, is applied across the entire wire. Because the wire is uniform, the potential drop per unit length is constant. If the total voltage across the wire is $V$ and its length is $L$, then the potential drop per unit length (the potential gradient) is:

This $k$ is a known constant, typically a few millivolts per centimetre.

Now, the unknown EMF $E$ is connected in series with a galvanometer and a jockey (a sliding contact). One end of the unknown cell connects to one end of the wire; the other end goes through the galvanometer to the jockey.

The Principle: Balancing to Zero Current

You slide the jockey along the wire until the galvanometer shows exactly zero deflection. At that point, no current flows through the unknown cell. This is the balance point or null point, at some length $l$ from the start of the wire.

Why does the galvanometer read zero? Because the potential difference across the length $l$ of the wire exactly opposes the unknown EMF. They are equal and opposite, so no net voltage drives current through the galvanometer branch.

At balance:

Since $k = V/L$, we can also write:

The Critical Point: "No Current" Is the Whole Point

At balance, the unknown cell supplies zero current. This means there is no voltage drop across its internal resistance. The potentiometer measures the cell's true EMF, not its terminal voltage under load. This is the fundamental advantage over a voltmeter.

If you accidentally slide past the balance point, the galvanometer deflects in the opposite direction — that tells you which way to go back.

A Common Comparison

Think of a seesaw. You want to find the weight of an unknown object. You put it on one side, then add known weights on the other side until the seesaw balances perfectly level. At balance, the torque from the unknown weight equals the torque from the known weights. No motion, no acceleration — just a static equilibrium. …