The variable autotransformer (Variac is the trade name of a popular brand) is one of the most useful and efficient devices ever invented for the control of AC votage and of voltage dependent parameters such as current, power, temperature, light intensity, motor speed etc. It enables the AC input to a mains powered appliance to be easily varied from 0 to full line voltage (or greater) max 3 amps. Encased in heavy-duty steel housing it also features on/off switch, output-voltage meter, mains-supply lead and standard 3-pin power output. A must for testing appliance performance under real (simulated) mains fluctuations.
Now supplied with slow blow fuses for extra reliability.