Potentiometer

The potentiometer is a device used to compare e.m.f of cells. It always gives accurate measurements of the e.m.f of a cell.

It consists of a length AB of wire of cross-sectional area about 100 cm long. A sensitive galvanometer is attached to a sliding jockey (J). The balance point is read from A on the meter rule when the galvanometer shows null deflection (read zero).

The potential difference across any length of the potentiometer wire is directly proportional to the length provided the wire is of uniform cross-sectional area. i.e. V ∝ L 

Use of Potentiometer to Measure E.M.F:

The circuit is connected as shown; the cell C₁ whose e.m.f is required is introduced into the circuit, the positive C₁ is joined to point A and the negative terminal of C₁ to the jockey through the centre-zero galvanometer.

With the key closed, a balanced point is obtained at P. At this point; the p.d (V) across (AP) is equal to the e.m.f (E₁) of C₁, while the p.d between AP is directly proportional to the length (l₁) of AP.

i.e. E₁ ∝ l₁

If cell C₁ is replaced by another cell, C₂ and the procedure is repeated, a new balance point is obtained, such that E₂ ∝ l₂ then \( \frac{E_1}{E_2} = \frac{l_1}{l_2} \)  

With this, the e.m.f of a cell can easily be obtained and it gives the actual value since it is not driving any current through the circuit.