When the switch is open in the lowermost loop of a potentiometer

PHYSICS · Subjective · CBSE · Physics · Current Electricity · Current Electricity

Problem

When the switch is open in the lowermost loop of a potentiometer, the balance-point length is

60\,\text{cm}

. When the switch is closed with a known resistance

R=4\,\Omega

, the balance-point length decreases to

40\,\text{cm}

. Find the internal resistance of the unknown battery.