Potentiometer — Definition

Definition

Imagine you want to measure how much 'push' an electrical battery has, which we call its electromotive force (EMF). A regular voltmeter does this, but it always draws a tiny bit of current from the battery, which can slightly affect the reading, especially for batteries with internal resistance. This is where a potentiometer comes in – it's like a super-accurate voltmeter that measures the battery's 'push' without taking any current from it at the moment of measurement.

Think of a long, uniform wire stretched across a board. We connect a main battery (called the driver cell) to this wire, creating a steady flow of current through it. Because the wire is uniform, the electrical potential gradually drops along its length. This gradual drop in potential per unit length is called the 'potential gradient'.

Now, to measure the EMF of an unknown battery, we connect its positive terminal to the same end of the long wire as the driver cell's positive terminal. Its negative terminal is connected through a galvanometer (a device that detects tiny currents) to a sliding contact, called a jockey. We then slide this jockey along the long wire until the galvanometer shows zero deflection. This 'zero deflection' point is called the null point.

What does a null point mean? It means that at this specific length of the wire, the potential drop across that length due to the driver cell is exactly equal to the EMF of the unknown battery. Since no current flows through the galvanometer at this point, the unknown battery isn't supplying any current, and thus its true EMF is measured without any error caused by its internal resistance.

So, in essence, a potentiometer works by balancing an unknown EMF against a known, adjustable potential difference created across a uniform wire. It's a highly precise instrument used in laboratories for various measurements, including comparing EMFs of different cells and determining the internal resistance of a cell. Its key advantage is its ability to measure EMF without drawing current, making it superior to a voltmeter for precise EMF measurements.