A potentiometer wire has a length 10 m and a resistance 2012 . Its terminals are connected to a battery of e.m . f 4v internal resistance 52 . What are the distance at which null points are obtained when two cells of e.m.f.s and 13 v are connected so as to (a) assist (b) oppose each other ? ( Ans L 1 = 8.75 m , l 2 = 0.625m )