A variable AC transformer (Variac) is a very handy device and should be part of any electrical experimenters lab.
The Variac allows adjusting the line voltage anywhere from 0 to 130 VAC. The voltage output is usually provided through a single or dual 120 VAC outlet located on the Variac housing.
A Variac is available in many different VA output ratings.
Some uses for a Variac:
1.) Adjusting the speed of small AC power tools
2.) Creating a variable DC power supply by adjusting the primary voltage of a power transformer with the secondary output connected to a bridge rectifier and capacitor filter.
3.) Controlling the temperature of a heater or the speed of a fan motor.
4.) Re-forming power supply capacitors in old equipment by allowing the input voltage to the device to be increased slowly.