Educational thread hijack - thermocouples generate a small voltage by the application of heat. They are used for such things as temperature detection, or as just mentioned above. They work by having 2 dissimilar metals (copper/alumel, chromel/alumel, etc) welded at the couple (the probe part), then putting that part into a heat source and measuring the resulting voltage. You can put several of them in series right next to each other (called a thermopile), so that the small voltages add up. If you then stick the thermopile into a pilot light flame, the output can be used to open an electrical valve that allows gas to flow only when the pilot light is actually lit. If the pilot light goes out, only the very tiny amount of gas that normally supplies the pilot light can build up, and unless things are unusual that small amount won't build up into an explosive mix. The thermopile cools off and the main gas valve closes.
This has nothing to do with the OP, but that's the nature of educational thread hijacks.