STACO variable transformer 

Suitable for controlling electric heating elements, lights, electric motor speed, or power. Output voltage controlled by a large slip-resistant knob with the output increasing linearly as the knob is turned clockwise. All models include a three-conductor grounded line cord, matching receptacle, switch, pilot lamp, and fuse.

Transformers include a dual-range output switch for either overvoltage or line voltage from the off position.