Yeah, that's the dimmer. With the Accurite thermometer probe attached to the tank, I just start the rheostat at the bottom and gradually increase until I get the desired temp. I keep my reptile room at 80-82 F, so it's a little easier for me as opposed to someone who has warmer daytime and cooler nighttime.

If you're having trouble finding a good median, I'd suggest getting a thermostat to control the temp. You can get a cheap HydroFarm thermostat on Amazon.com for around 28$ shipped. There are many other thermostats to choose from if you have a larger budget.