There are three basic ways to control heat.
On/Off Thermostat - inexpensive. It turns the heating device off when it reaches target temp, turns it back on when it drops below. You need to run a thermostat sensor into the tank, warm side.
Proportional Thermostat - expensive (~ $130 or so). It applies less wattage the closer you get to target temp. Generally far more accurate, and extends the life of heating element (because it never gets full current). Also has a thermostat sensor that you need to run into the tank.
Incadescent Rheostat - get one rated for at least 500W even though you probably don't use that much power. Make sure it is for incadescent. This is a dimmer switch you can use to manually restrict the current to the heating device. Cheap. Extends life of heating element as it doesn't get full current. Only recommended if ambient room temperature is somewhat constant (night drop OK)
You can buy rheostats for reptiles, or you can make one yourself using a dipole incadescent dimmer switch from home depot (use an electrical box). I'll post a pic of mine later if you want.
Be sure to check temp fairly often if you use a rheostat.
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3.6 L. getula californiae - 16 eggs (Cal. King)
1.1 L. getula nigrita (MBK)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata - 14 eggs (Cal. Alligator Lizard)