Variac

A Variac is a sturdy variable (auto) transformer of which the secondary voltage is continuously adjustable by means of a sliding contact. A Variac is frequently used in experiments, testing, measurement set-ups, and the like. Searching for error or when in development of a device that is measured from the power line can be tested or measured under extreme or even safe conditions from the mains with a Variac. Something you do not use every day, but it is indispensable in any workshop.