AC potentiometer

The potentiometer is an instrument that is used for the measurement of potential differences across a known resistance between two terminals of a circuit. Potentiometers are of two types DC potentiometer and AC potentiometer. The working principle of both the potentiometer is the same except for one difference that is discussed later in this article.

What is ac potentiometer?

Alternating current (AC) potentiometer is the potentiometer in which the magnitude and the phase angle of unknown emf are to be compared with the known emf to obtain balance.

The working principle of the AC potentiometer is the same as the DC potentiometer. But there is a difference between both the potentiometer, that is in the DC potentiometer only the magnitude of unknown voltage is compared with the known. On the other side, in the AC potentiometer, the magnitude and the phase angle of unknown emf are to be compared.

Thus a DC potentiometer can’t be used for AC measurements. So, some modifications and additions have to be made for ac measurements.

The following points must be considered

  1. The slide wire and the resistance coil of an ac potentiometer should be non-inductive, this is to be done to avoid errors in reading.
  2. The AC supply source should be free from harmonics, because balance may not be achieved in presence of harmonics.
  3. The reading is affected by the external magnetic field, so they must be eliminated in the time of measurements.
  4. The AC supply source should be sinusoidal.

Types of ac potentiometer

Polar potentiometer

In this type of potentiometer, the unknown emf is measured in polar form. This means that the unknown emf is measured in terms of its magnitude and its relative phase. The magnitude is measured by one scale and the phase is indicated by another scale.

There is provision for reading phase angle up to 360 degrees.

The voltage is read in the form of V-theta

Read More: How polar potentiometer works?

Coordinate potentiometer

In this type, the unknown emf is measured in cartesian form. It has two different scales to read the in-phase V1 and the other is quadrature V2. There is provision is made in this potentiometer to read both positive and negative values of voltages and cover all angles up to 360degree.  


  • The versatility of the potentiometer is not hidden from anyone. It can measure a wide range of current, voltage, and resistances by using a shunt and volt-ratio box.
  • As it can measure both magnitudes as well as phase of two signals, it is used to measure power, inductance, and phase angle of a coil, etc.


  • Inaccuracy in the measured value of frequency will also result in I the quadrature potentiometer wire current differing from that of the in-phase potentiometer.
  • The slightest difference in the reading of the dynamometer instrument either ac or dc calibration brings an error in the alternating current to be set at standard value.
  • The potential gradient of the wires is affected because of the presence of mutual inductances in various parts of the potentiometer.
  • When mutual inductances of various parts of the potentiometer affect the normal value of the mutual inductance.


Practical applications of ac potentiometer are listed below:

  • Calibration of voltmeter
  • Calibration of ammeter
  • Calibration of wattmeter
  • Measurement of self-inductance

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