What is a rheostat

Reostat is an adjustable or switchable resistor. It is most often done as a set of resistors with many position switches or as a sliding resistor that is wound on the insulating body. The sliding rheostats have a spring lock. The rheostat is used to set greater power from 1 W higher. Today it is used only from old machines and laboratories. The semiconductor elements have been replaced. The disadvantage of the rheostats is a greatly impaired performance. 

The reostat is connected to the appliance in series, where the change of resistance of the rheostat can regulate the current flowing through the circuit. With full rheostat resistance, you reach the lowest values.