The length of a wire of a potentiometer is 100 cm and the emf of its stand and cell is E volt. It is employed to measure the emf of a battery whose internal resistance is 0.5Ω . If the balance point is obtained at l=30 cm from the positive end, the emf of the battery is
(a) 30E/100.5
(b) 30E/(100-0.5)
(c) (30(E-0.5I))/100, where I is the current in the potentiometer wire
(d) 30E/100
