In the given circuit of potentiometer, the potential difference E across AB (10 m length) is larger than  E1 and E2  as well. For key K1 (closed), the jockey is adjusted to touch the wire at point  J1 so that there is no reflection in the galvanometer. Now the first battery E1  is replaced by second battery E2  for working by making K1  open and  K2 closed. The galvanometer gives then null deflection at J2 . The value of E1E2  is ab , where  a=______.