A potentiometer is a resistor with a sliding contact. It can be used to measure emfs accurately

Question:

A potentiometer is a resistor with a sliding contact. It can be used to measure emfs accurately (Problem 131) or to supply a variable voltage to a circuit (Problem 132). In the diagram with switch S₁ closed and S₂ open, there is no current through the galvanometer G (a sensitive detector of current whose operation is based on magnetic forces) for Ra₁ = 20.0 Ω with a standard cell of 2.00 V. With switch S₂ closed and S₁ open, there is no current through the galvanometer G for R₂= 80.0 Ω.