A Direct Current (D.C.) potentiometer is a precision instrument used for measuring voltage, electromotive force (EMF), and potential difference. It works on the principle of the potentiometer, which is a type of resistive network used to measure and compare electrical potentials.
Here's how a D.C. potentiometer typically works:
Basic Setup: The basic setup of a D.C. potentiometer consists of a primary cell or a standard voltage source, a potentiometer wire (a long uniform wire with a known resistance), a galvanometer (a sensitive current measuring instrument), a battery, and a jockey (a sliding contact).
Principle: The principle of operation is based on the fact that when a steady current flows through a uniform wire, the potential drop across any segment of the wire is directly proportional to its length.
Procedure:
The potentiometer wire is stretched out and connected in series with the primary cell (whose EMF is to be measured) and a battery that provides a constant current.
The galvanometer is connected in parallel with the primary cell, and the jockey is used to make contact with the potentiometer wire.
Initially, the jockey is slid to one end of the potentiometer wire, and its position is adjusted to balance the galvanometer by nullifying the current passing through it. This is achieved by adjusting the length of the potentiometer wire segment that's connected in series with the primary cell.
Once balance is achieved, the length of the potentiometer wire segment (L1) is noted.
Next, the primary cell is replaced with a standard cell (whose EMF is known) without disturbing the rest of the setup.
The jockey is moved along the potentiometer wire to find the new balance point, and the length of the potentiometer wire segment (L2) is noted.
Calculations:
The EMF of the primary cell can be calculated using the formula:
EMF of primary cell = EMF of standard cell * (L1 / L2)
Here, L1 and L2 represent the lengths of the potentiometer wire segments when balancing the primary and standard cells, respectively.
D.C. potentiometers are highly accurate instruments and are often used in laboratory settings for calibrating voltmeters and determining the EMF of cells. They offer a way to measure voltages with great precision by directly comparing the unknown voltage with a known standard voltage.