There is a temperature sensor (sorry but you'll have to bear with my terminology)that looks like a three way electrical adaptor that turns on at 35 deg and stays on until it senses a rise in temperature to 45 deg.
We would plug our electric heat tape into it and it turns on power to the heat tape when the air temperature drops to 35 deg. We wanted to know when that heat tape was "on" and when it was "off" so we plugged a cord into the other side of this gadget that had a blue light bulb and put the light bulb where we could see it from the front door.
Seems to us it had the word "cube" in the name of it. Weather cube? It was very handy whatever it was called. That and indoor/outdoor thermometers which can tell you the temperature in various storage compartments without us going outside to read them.