1. The problem statement, all variables and given/known data http://img408.imageshack.us/img408/6537/screenshot20100808at134.png [Broken] 2. Relevant equations 3. The attempt at a solution I redrew the circuit as follow: http://img293.imageshack.us/img293/6247/screenshot20100808at135.png [Broken] Is this correct (or even useful in the first place)? Anyway when switch S1 is closed, the total resistance = rx + ry, the internal resistances of the batteries. But when both switches are closed, I figured that the voltage increased by 0.02V, to 0.07V. However I am not sure this is the voltage across which components because the millimeter is connected in a rather strange way. I'm lost from here onwards. I'm mostly troubled by the placement of the millivoltmeter I guess. Am I on the right track? Any alternative method? The answer is (A) by the way.