Principle of A Potentiometer
Principle of Potentiometer :
“The potential difference across any length of a wire of uniform cross-section and uniform composition is proportional to its length when a constant current flows through it.”
The battery in primary circuit creates a potential difference across the length of the potentiometer wire. A potential gradient K is also developed across the wire. (K=Potential supplied by main batterry divided by length of wire). The potentiometer wire is thus a source of variable potential difference. That is, potential depends on length of wire selected. (Potential = length * potential gradient).
The cell in secondary is connected opposite to that in primary. This current opposes the current flowing across the potentiometer. When the both potential, i.e, potential difference across the chosen length of potentiometer wire and potential difference caused by the cell in secondary circuit, becomes equal and opposite, no current flows through the secondary circuit. Thus the galvanometer shows null deflection.
Sensitivity of a Potentiometer :
The sensitivity of the potentiometer depends upon the value of potential gradient K. (K=Potential supplied by main battery divided by length of wire) Smaller the value of K, smaller the potential difference that a potentiometer can measure and more is the sensitivity of the potentiometer. Thus, for a given potential difference, the sensitivity of the potentiometer increases with the increase in length of potentiometer wire.
? Why Potentiometer is chosen over Voltmeter to measure the potential of a cell?
Ans : When Voltmeter is used, current flows through the circuit, and because of the internal resistance of the cell, always terminal potential will be less than the actual cell potential. But in a potentiometer circuit, when the potential difference is balanced, no current flows in the circuit, so terminal potential will be equal to actual cell potential. i.e, Voltmeter measures the terminal potential of a cell, but Potentiometer measures actual cell potential.