so its it just a thermostat? Or is it connected to something else? Does it turn on the light when the temperature gets to low? Or is it a heater itself?
There is some kind of physics going on. The device has two screws. You cut the negative line of the light electric chord and place one end around one screw, the other end on the other screw, so it's like you interrupt the negative line (usually the ribbed or lined half of an electrical chord) with this device. When there is enough heat, it will disconnect the negative line, not allowing power to go through. To make an electrical device with electricity, there has to be a complete circuit. I'm bad at explaining electricity. But think of it as a loop, Electricity has to go up the positive side of the line and then back down the negative side of the line or nothing will happen (though the positive side can go through you and ground itself to the floor through your feet, if it's powerful enough and if you are wearing something that will ground you, like bare feet in a puddle of water,etc... you can kill yourself, right? So anyway...
So what the device does is breaks the negative line, which cuts the power to the light when it's hot enough. It doesn't require electricity to function, it is physical, some kind of material in there that reacts to small changes in heat.
The device is not the most accurate thing on earth, probably functions within 5 degrees on a water heater, however, with close placement to the light bulb (so it heats up and cools off drastically) I have found you can regulate the temperature very well. After putting it near the light bulb, and turning the system on, you can adjust the dial on the device until it reaches the temperature you want, then watch and see how well it keeps that temperature. If it gets too hot, you can put the device closer to the light bulb or reduce the dial. I usually have it to where the hole in the silver side of the device is about1/4 to 1/2" from the light bulb.
In my small cooler, I used a 40 watt bulb and a computer fan with a 5 volt cell phone charging plug. If you look at the wikipedia article on
molex connectors you will see you need to hook the red line to the power line (usually colored) and black line to the usually black line of the transformer (for a cell phone) If you look at your old unwanted cell phone plug, you can read how many volts it gives, usually 5, sometimes 7. A different kind of device might use a 12 v transformer (battery charger, etc...) you might have one of those in your house as well, and can hook it up with the yellow line. The unused line you just keep out of the connections. It won't be hot when running.
You know, making these cooler-bators isn't that hard, and there are tons of directions in the coop section, under incubators that you can review. But I was confused on how the thermostat device worked myself, then realized it was super simple.