What is the main difference between and potentiometer, rheostat and a varistor? And how do the application of the three in a circuit differ? Or to be more concise, in a variable power supply why would you want to use each one rather than the others, or what benefits would one give over another in a variable power supply circuit.
A Potentiometer is a fixed resistance with a mechanically adjustable wiper which can be moved from one end of the fixed resistance to the other, forming an adjustable voltage divider. It is a 3-terminal device.
A Rhoestat is not the same thing as a Potentiometer, since a Rheostat is typically considered to be mechanically variable resistor and is a 2-terminal device.
You can use a Potentiometer as a Rheostat by simply ignoring one of its fixed-resistor terminals.
The application in-circuit of a Varistor vs a Potentiometer is not really comparable as they are completely different devices.