rheostats and thermostats
I think you are doing the right thing by wanting a UTH even though you may have figured out your problem for now. There are several reasons...
As someone said earlier, having the heat source below the snake (belly heat) help the snake in digestion, which if you've ever had one regurge you will never want to happen again.
Also, they say humidity is not a major concern in cornsnakes, unless they have shedding problems. Now I dont know what the humidity is normally like where you are at, but heat lamps will suck all the moisture out of your viv.
The difference with rheostats and thermostats:
A rheostat is pretty much a dimmer switch that you can plug a UTH (or whatever else) into. Like a dimmer for your lights, you turn the knob and it varies the amount of power supplied to whatever is plugged into it. The bad thing about that is that to keep the same temperature you will have to turn it up some when it is cold and turn it down some when it is hot. A rheostat is a simple device; there are no sensors or anything to tell it how hot your viv is.
Thermostats are pretty efficient working devices that you set at the temperature you want and it does what it has to do to maintain that temperature. The type I have is digital, just like an air condition thermostat. It has a temperature probe I secure on the floor of the viv above the heat source and then set the thermostat at whatever temperature I want it to be. The one I have will vary the amount of power it puts out to make sure it stays where it should. Normally it runs at about 10% power. Some thermostats regulate the temperature by turning on and off. Say you set the temp at 85. It will turn on the power until the temp is around 85, maybe a degree or 2 higher, then it will turn off until then temperature drops to about 83 (just a guess). It turns on and off keeping the temperature "about" where you set it.
As for the UTH, I think I heard of one that comes with some sort of rheostat connected to it, but most of them come seperate. I prefer that because I'd rather have a thermostat over a rheostat. I personally believe a decent thermostat is well work the money. Yes, some them are kinda pricey. I know (have read) numerous others on here agree that a good thermostat is one of the most important pieces you can invest in.
Sorry I got very long winded, but did ask how it works...
Hope this helps and feel free to ask any other questions.
Jimmy