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). 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.