Question

State the working principle of a potentiometer. With the help of the circuit diagram, explain how a potentiometer is used to compare the emf's of two primary cells. Obtain the required expression used for comparing the emfs.

Answer 1

Working principle of Potentiometer:

When a constant current is passed through a wire of uniform area of cross-section, the potential drop across any portion of the wire is directly proportional to the length of that portion.

Applications of Potentiometer for comparing emf of two cells:

The following Figure shows an application of the potentiometer to compare the emf of two cells of emf E₁ and E₂

E₁, E₂ is the emf of the two cells.

1, 2, 3 forms a two-way key.

When 1 and 3 are connected, E₁ is connected to the galvanometer (G).

Jokey is moved to N₁, which is at a distance l₁ from A, to find the balancing length.

Applying loop rule to AN₁G31A,

Φ l₁ + 0 − E₁ = 0 ..(1)

Where Φ is the potential drop per unit length

Similarly, for E₂ balanced against l₂ (AN₂),

Φ l₂ + 0 −E₂ = 0 ..(2)

From equations (1) and (2),

`E_1/E_2 = I_1/I_2` .....(3)

Thus we can compare the emfs of any two sources. Generally, one of the cells is chosen as a standard cell whose emf is known to a high degree of accuracy.