Variable resistor //
A variable resistor can be made from a wirewound element, rather than a solid strip of material.
This is called a rheostat.
A rheostat can have either a rotary control or a sliding control. This depends on whether the nichrome wire is wound around a dough-nut-shaped form (toroid) or a cylindrical form (solenoid).
A rheostat is not continuously adjustable, as a potentiometer is. This is because the movable contact slides along from turn to turn of the wire coil. The smallest possible increment is the resistance in one turn of the coil. The rheostat resistance therefore ad-justs in a series of little jumps.
Rheostats are used in high-voltage, high-power applications. A good example is in a variable-voltage power supply.
This kind of supply uses a transformer that steps up the voltage from the 117-V utility mains, and diodes to change the ac to dc. The rheostat can be placed between the utility outlet and the transformer.
This results in a variable voltage at the power-supply output. A potentiometer would be destroyed instantly in this application.
Rheostat Working Principle (VIDEO)
Resource: Teach Yourself Electricity and Electronics – Stan Gibilisco