A variac is basically a variable autotransformer that allows you to adjust the voltage between zero and the local mains supply voltage, or more if tapped midwinding.
They find use in specialist applications where it's useful to ramp the mains voltage up slowly for regulation or testing. Some applications would be inverter, motor drive repair, soft start of vintage electronics and some specialist lighting applications where lights need to be dimmed without phase angle noise.

These units do not provide isolation from the mains supply, as they are just a single winding with an adjustable tap.

I've sometimes wondered how they can seamlessly jump from one winding to another without bridging loops and crating shorted transformer turns. Since I had my unit open for fitting a new digital meter, I thought I'd test that with a thermal imaging camera.

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