but they're actually PWM controllers.
Pulse Width Modulation.
They will control via 'duty cycle' and you can actually measure it with a kill-a-watt meter using the 'power factor' setting.
A lot of people imagine that they're set up like a volume control on an old style radio, or a potentiometer from radio shack, but that's simply not the case.
http://home.howstuffworks.com/dimmer-switch.htm
That's also very similar to how proportional thermostats work, I'd imagine. Yes, their output is rough on fans and light bulbs. Any motor plugged into a pwm will emit the distinctive 'buzz'. I would imagine that heat tape wouldn't know the difference, though. I'm not sure........

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