How do I find the emf and internal resistor of this cell of this circuit?

1. Jul 2, 2012

kyna_KUzfairy

This is a potentiometer circuit. I don't have the diagram so I hope you can follow me. This is the main circuit with the unknown emf E with internal resistance r of the main cell and with two resistors P and Q connected in series at the bottom of this rectangular circuit, p being connected near end a and Q near end b of the bottom of the main circuit. Another circuit is connected to the main circuit at end a and to c in between the two resistors P and Q. This circuit has a cell E1 of emf 1V, a resistor S and a galvanometer G.

This is the question.
When P is 20Ω it is found that the deflection of g is zero for Q when it is 17Ω. When P is 40Ω, again it was found that the deflection of G is zero for Q when it is 35Ω. Find the emf E and the internal resistance r of the cell.

I tried finding the current through P and then finding the voltage through Q in both instances. I assumed the difference in the total voltages in both instances to be the voltage through internal resistance r. Am i correct? If not, how do I answer this question?