Potential difference across AB and CD is zero.

Potential difference across BOC is same as if we place a single rod between B and C , hence length of the rod should be $ L = \sqrt { BO^2 + OC^2)} = \sqrt 2 m$

$ \Rightarrow \xi = BLV = 1\times \sqrt 2 \times 1 = \sqrt 2 = 1.41 V$