It controls a UTH or basking lamp by controlling the wattage the item receives. It works just like a lamp dimmer...the lower you have the nob the lower the wattage the UTH or lamp receives. The higher you turn the nob the more power it gets..The only downside to a Rheostat is that you have to constantly adjust it when your house temps change unlike that of a thermostat which like the rheostat controls the amount of power a UTH gets but unlike the rheostat it doesn't go through temp changes based on your house temp. It is set to a certain degrees and keeps the temperature that degree by cutting off the power and turning it back on when necessary..