Consider the potentiometer circuit for determining the internal resistance of a cell. When switch `S` is open, the balance point is found to be at `75 cm` os the wire. When switch `S` is closed and value of `R` is `4 Omega`, the balance points shifts to `60 cm`. find the internal resistance of cell.
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Explanation

When `S` is open, balance length `l = 75 cm`
When `S` is closed, balance length `l' = 60 cm`
`r = R((l)/(l') -1)`
=`4((75)/(60) - 1)`
=`1 Omega`.