1. The problem statement, all variables and given/known data In the potentiometer circuit shown,the balanced length l is reduced to l/2,when a resistor of resistance R is connected across the terminals of the cell E.The internal resistance of the cell E is (1) R/2 (2)R (3) 2R (4) 3R/2 (5) 3R http://img268.imageshack.us/img268/4080/potentiometer.png [Broken] 2. Relevant equations [tex]\Sigma[/tex]E = [tex]\Sigma[/tex]IR 3. The attempt at a solution [tex]\rho[/tex] = potential gadient R1 is the resistance of the potentiometer wire In the 1st case, E = I*(r+R1) = [tex]\rho[/tex] * l ? Now when the resistor R is connected parallel to E, V = I* R = [tex]\rho[/tex]*l/2 ? I'm not quite sure how I'm supposed to find r from this,interms of R only. Hope someone can help. Thank you.