Draw the circuit diagram of DC potentiometer & explain how it works.
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.
The instrument is first standardised, as discussed earlier, and the potentiometer is made direct reading. Then the currents in the stator windings of the phase shifter are adjusted to exact quadrature by means of the variable resistor and capacitor, this being adjusted until the alternating current in the slide wire becomes constant for all position of the rotor.
The unknown voltage is then impressed to the potentiometer test terminals. Balance is obtained by adjusting the position of slide wire contact and position of phase rotor till zero deflection is obtained in the vibration galvanometer. Now the magnitude and phase angle of unknown voltage with reference and to an arbitrary reference phasor are read from the slide wire and phase shifter scales respectively. For accurate results it is necessary that the voltage and frequency of the supply are steady and that the waveform of the voltage are reasonably sinusoidal.The accuracy obtainable by this method largely depends on the accuracy of the potentiometer.
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Fig.22(a)