Two sources of equal emf are connected to an external resistance R the internal resistance of the two soureces are $R_1 and R_2 ( R_ 2 > R_1 )$ if the potential difference across the source having internalresistance R 2 is Zero, then
$ I = { E+E \over R_1 + R_2 +R } $ $ R_2 = { 2E \times R_2 \over (R_1 + R_2 + R)}$ $ \Rightarrow E - { 2ER_2 \over R_1 + R _2 +R } = 0 or E = { 2ER_2 \over R_1 + R_2 +R} $ $ R = R_2 - R_1 $
