Write the principle of working of a potentiometer. Describe briefly, with the help of circuit diagram, how a potentiometer is used to determine the internal resistance of a given cell.

Explanation

Working principle: When constant current flows through a wire of uniform cross section then potential difference across the wire is directly proporational to the length.
`V alpha l`

With key `K _(2)` balance is obtained at length `l _(1) (AN _(1)).` Then,
`epsi = phi l _(1)` (`phi=` potential gradient)
When key `K _(2)` is closed, the cell sends a current (I) through the resistance box (RB). If V is the terminal potential difference of the cell the balance is obtained at length `l _(2) (AN _(2)).`
`V = phi l _(2)`
implies `(epsi )/(V) = (l _(1))/( l _(2))`
But `(epsi )/( V) = (I ( R+r ))/( IR) = (1 + (r )/( R ))`
` therefore (1 + (r )/( R)) = (l _(1))/( l _(2))`
`implies r = (( l _(1) - l _(2)))/( l _(2)) R`