Gas heaters with pilot flames are one of the most famous examples for the use of thermocouples. Thermocouples are stacked to make a thermopile, which can generate enough voltage from the heat of the pilot flame to keep the gas valve open which in turn supplies the gas for the pilot flame. If the gas runs out, the flame extinguishes and the voltage across the thermopile decreases, which causes the electric gas valve to close. Many space probes get their electric power from a thermocouple in a radioisotope thermoelectric generator.