#### The various applications of ac potentiometer are,- Calibration of voltmeter
- Calibration of ammeter
- Calibration of wattmeter
- Measurement of self reactance or inductance of a coil.

## Calibration of Voltmeter :

#### The circuit diagram for the calibration of the voltmeter is shown below. It consists of a stabilized ac supply, rheostats, voltmeter, voltage ratio box, and potentiometer. The basic and important requirement of the circuit is that the input ac supply must not have any fluctuations. Any fluctuation in the supply will have a corresponding change in the calibration of the voltmeter. Hence, to avoid this a stabilized ac supply must be used. Two rheostats Rh_{1} and Rh_{2} are used to have a very precise control so that the voltmeter accurately coincides with the major divisions. A voltage-ratio box is used to reduce the voltage across the voltmeter and applied to the potentiometer.

#### The potentiometer reads the true value of the voltage. If this value is matched with the voltmeter readings, then the error is zero. However, if these two readings do not match, an error is encountered. This error may be positive or negative depending on the relative value of the voltmeter and the potentiometer. In order to have greater accuracy, the voltage should be measured near the maximum range of the potentiometer.

A voltage ratio box or two capacitances in series are used along with an ac potentiometer if the voltage to be measured is of medium or high magnitude. Otherwise, an ac potentiometer can be used directly for voltages under 1.5 V.

## Calibration of Ammeter :

#### The connection diagram for the calibration of the ammeter is shown in the figure below. The circuit consists of a stabilized ac supply, variable resistor, ammeter, a standard resistor, and a potentiometer. The ammeter is connected in series with the standard, non-inductive resistance of known value R. The potentiometer is connected across the standard resistor.

#### Let V_{R} be the potentiometer reading which corresponds to the voltage across R. Then the current through R is given by, I_{R} = V_{R}/R. Let I be the ammeter reading. The two values i.e., I_{R}, I will be the same since the ammeter and standard resistor are in series. Since the value of standard resistor is accurately known, accurate calibration of ammeter can be obtained by this method.

## Measurement of Unknown Inductance :

#### The circuit diagram for the measurement of unknown inductance of an inductor or coil is shown in the below figure.

#### In the figure,- R
_{S} = Known standard non-inductive resistance - R
_{C} = Internal resistance of the coil - L = Inductance of the coil

In this method, two voltage measurements are carried out by switching the DPDT switch to both positions. Let the readings be,- The voltage across R
_{S}, v̅_{R} = V_{R}∠θ_{R} - The voltage across the coil, v̅
_{C} = V_{C}∠θ_{C}

Therefore, the current, impedance, and resistance of the circuit is given by,

The reactance of the coil is,

Hence, the value of unknown inductance can be obtained.

_{S}= Known standard non-inductive resistance_{C}= Internal resistance of the coil_{S}, v̅_{R}= V_{R}∠θ_{R}_{C}= V_{C}∠θ_{C}## Measurement of Unknown Capacitance :

#### The circuit diagram for the measurement of unknown capacitance supplied from a single-phase ac supply of known frequency is shown below.

#### In the figure,- R
_{S} = Known standard non-inductive resistance - R
_{C} = Series resistor representing the loss in the capacitor - C = Capacitance of the capacitor

In this method, two voltage measurements are carried out by switching the DPDT switch to both positions. Let the readings be,- The voltage across R
_{S}, v̅_{R} = V_{R}∠θ_{R} - The voltage across the capacitance, v̅
_{C} = V_{C}∠θ_{C}

Therefore, the current, impedance, and resistance of the circuit is given by,

The reactance of the coil is,

Hence, the value of unknown capacitance can be obtained.

_{S}= Known standard non-inductive resistance_{C}= Series resistor representing the loss in the capacitor_{S}, v̅_{R}= V_{R}∠θ_{R}_{C}= V_{C}∠θ_{C}