I picked up a tote full of goodies at a garage sale a few years back. In it, along with the 3/4 horse, DC motor and controller I had my eye on was a variac. Before that, I'd heard of them, but never got my hands on one.
It looked like something belonging in my toy collection, so it sat on the shelf with other electric/electronic goodies. I was given a few hundred pounds of acrylic and Plexi, so built the contraption in the photo (had to order the meter, but had the other goodies (switch, LED lights, circuit breaker). Now it's a power source for a Plexi - acrylic bender and offers more versatility than the commercial unit I bought.
Unlike my router control, this is a BIG potentiometer (adjustable resister with three wires, vs a rheostat, with two). Mine is only good for five amps ("about 550 watts). As such, it's not the best way to control speeds on routers and other motors.
I forget where I picked up the controller, but remember it was about $20.00. It's important to note it is an electronic control and, instead of reducing the voltage, it keeps the voltage where the motor likes to be and adjusts the amperage.
Hi I'm new to the Forum this is my first post but, have you seen the variac variable AC Transformer on amazon.com? I like $45 brand new you can use it to control pretty much anything. I use mine for Motors to make tools and also to control the voltage to Lichtenberg besides fractal wood burning device to control the output. I think it's the best thing I've ever bought.