I have one of these. Its very basic, but very reliable too. There is a thermostat in it, when cold, the bulb comes on, and stays on until it reaches the set temperature, and then it will turn off. It is the heat from the lightbulb that warms it, so when the temp falls again, the bulb will come back on until it reaches temp again. You need to get a reliable thermometer and read the temperature at the point the bulb switches off. It should go off at 37.5, or 101F. Good luck with it. What are you hatching first?