The most important difference between a de. And a.c. potentiometer is that, in a d.c, potentiometer, only the magnitude of the unknown emf and potentiometer voltage drop have to be made equal to obtain balance. But in a.c. potentiometer, both magnitude and phases of unknown emf and potentiometer voltage drop have to be same to balance.
The a.c. potentiometer is a complicated instrument and therefore certain important factors are considered for its operation. They are:
- It is necessary that the frequency and wave front of current in the potentiometer circuit must be same as that of voltage being measured.
- A vibration galvanometer, which is a tuned device, is usually used as a detector in a.c potentiometer. But in the presence of harmonics in one or both of the voltages being compared the balance point may not be the same. Only the balance is only minimum. Therefore, the source of a.c. supply is made as sinusoidal as possible.
- The absolute accuracy with which a voltage can be measured in an ac potentiometer cannot be comparable with corresponding type of d.c. measurement.
- The stray field effects must be compensated before measurement.
Standardizing of a.c. Potentiometer:
In a.c. potentiometer, the readings are read off directly from the dial settings. In order to do that. the a.c. be standardized. The standardization is done with the help of a standard d.c. source i.e. a standard cell or a Zener source and a transfer instrument. The transfer instrument is usually an electrodynamometer milli-ammeter. It is so constructed that its response to a.c. is same as it is to d.c, such an instrument can be calibrated potentiometer must on d.c. and then brought to the same setting on a.c.
Types of A.C. Potentiometer:
The A.C. potentiometers are mainly divided into two types depending on the manner in which the unknown voltage can be measured. They are: