Theory: Let the standard cell SC be connected with its negative terminal through selector switch S, galvanometer G and key K to the sliding contact J. Since slide wire AB is of uniform cross section, the resistance per unit length r will be constant and equal to R/L. Where R is resistance of whole wire and L is the length of wire. If current flowing through the potentiometer wire is I amperes then potential drop per unit (cm) length will be equal to i r volts (let it be equal to v0 volts) and voltage drop between points A and J in cm.

The potential difference between A and J will try to send a current in mesh ASGJA in counter-clockwise direction whereas emf of standard cell SC will try to oppose it. If the standard cell SC is equal to the potential difference across A and J then no current will flow through the branch and galvanometer will, therefore, show zero deflection.

Operation with Alternating Current: Dry dale Tinsley ac potentiometer is shown in Fig. 15.22. In the circuit VG is a vibration galvanometer, R is a variable resistance for controlling the slide wire circuit current, r is a shunting resistance for changing the range of the potentiometer. R´ is the resistance comes into the slide wire circuit as soon as the shunt resistance r is put in the circuit by switch S1 so that the resistance of the working portion of the potentiometer remains the same and A is a precision ammeter.

The phase shifting transformer, whose connections are shown in Fig. 15.21, is omitted for clearance.

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