Current Electricity Part 1

The length of a wire of a potentiometer is 100‌cm, and the e.m.f. of its standard cell is E volt. It is employed to measure the e.m.f. of a battery whose internal resistance in 0.5Ω. If the balance point is obtained at 1=30‌cm from the positive end, the e.m.f. of the battery is
where i is the current in the potentiometer wire.