What is potentiomter ? What is its principle ? How will you find the internal resistance of a cell by a potentiometer?

Explanation

Potentiometer. It is an instrument used to compare to e.m.f. of two cells nd to find the internal resistance of a cell.
Principle. It is based on the principle that when a constant currents is passed through a wire of uniform area of cross-section, the potential drop across any portion of the wire is directly proportional tot he length of that portion.

Internal resistance of a cell.
The resistance offered by the electrolyte of the cell to the flow of the ions through the electrolyte is called internal resistance of the cell.
To determine internal resistance of a cell by a potentiometer.
The circuit used for finding the internal resistance of a cell is as shown in the figure.
A constant current I is maintained through the potentiometer wire with the help of the rheostat.
potentiometer wire with the help of the rheostat. Plug in the key `K_(2)` is kept out and the jockey is moved over the potentiometer wire so as to balance the e.m.f. E of the cell, whose intenral resistance is to be found. let `l_(1)` be the balancing length of the potentiometer wire between point A and jockey J. then
`Epropl_(1)` . . (1)
With the help of resistance box S, introduce resistance, say S and then put in key `K_(2)`. now find the balance point for the terminal potential difference V between the two poles of the cell. if `l_(2)` is the balancing length, then
`Vpropl_(2)` . . (2)
From equation (1) and (2), we have
`(E)/(V)=(l_(1))/(l_(2))` . . (3)
Since internal resistance (r) of cell is given by
`r=[(E)/(V)-1]R`
Using equation (3), we get
`r=[(l_(1))/(l_(2))-1]R=[(l_(1)-l_(2))/(l_(2))]R`.