A potentiometer is a circuit to measure emfs. In the diagram with switch S1 closed and S2...

A potentiometer is a circuit to measure emfs. In the diagram with switch S1 closed and S2 open, there is no current through the galvanometer G for R1, = 20.0 Ω with a standard cell ℰs of 2.00 V. With switch S2 closed and S1 open, there is no current through the galvanometer G for R2, = 80.0 Ω. (a) What is the unknown emf ℰ x (b) Explain why the potentiometer accurately measures the emf even for a source with substantial internal resistance.