Balance currents on both sides of the rheostat(using the rheostat pointer) such that they are almost equal. The separate circuit on the right is to produce an arbitrary voltage. The left voltage supply is known and constant. Once the currents are balanced using the central rheostat the ratio of the voltages supplied is equal to the ratio of the resistances of the rheostat respectively. ie (V right/V left)=(R right/R left).
