Fig. 18.43 shows a schematic circuit of an instrument called a potentiometer, which is a device for determining very accurate emf values of power supplies. It consists of three batteries, an ammeter, several resistors, and a uniform wire that can be €œtapped€ for a specific fraction of its total resistance. is the emf of a working battery, designates a battery with a precisely known emf, and designates a battery whose emf is unknown. The switch S is thrown toward battery 1, and the point T (for €œtapped€) is moved along the resistor until the ammeter reads zero. Let€™s call the resistance of this arrangement R1. This procedure is repeated with the switch thrown toward battery 2, and the point T is moved to T until the ammeter again reads zero. Let€™s designate the resistance of this arrangement as R2. Show that the unknown emf can be determined from the following relationship:

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