A variable autotransformer, known generically as a variac, is a type of transformer with just one coil that is shared by the primary and secondary side of the circuit. The ratio of the primary to the secondary windings is variable as is the ratio of the secondary to the primary voltage.
Inside the variac looks much like an oversized rheostat and there is a single partially exposed winding, which allows the movable wiper to make an electrical connection. The transformer’s primary connection is made on both ends of the winding, and the secondary connection is made to one end of the winding, known as the common connection, as well as to the movable wiper. When the wiper moves the transformer’s ratio changes.
One use for a variac is slowly bringing electronic equipment back into operation after long periods of disuse. Variacs are also used in experiments and electronics testing for simulating different line conditions and voltages. In addition you can use a variable autotransformer for powering electrical equipment .