How does a thermocouple function in gas appliances?

A thermocouple functions in gas appliances primarily by detecting the presence of a flame. When a flame is present, the thermocouple generates a small voltage due to the difference in temperature between the hot junction (where it is exposed to the flame) and the cold junction (which is at a lower temperature). This voltage is used to signal the gas control valve to remain open, thus allowing gas to flow and maintain combustion.

If the flame goes out, the thermocouple cools, and the voltage drops. This change in voltage triggers the gas valve to close, stopping the flow of gas and thereby preventing any potential gas leaks. This safety mechanism is crucial in gas appliances, ensuring that gas is only supplied when there is a flame present, thus enhancing safety during operation.

Other options do not accurately represent the function of a thermocouple; regulating gas pressure involves components like regulators, cooling the appliance is usually managed by fans or heat exchangers, and while a thermocouple does produce a small voltage, its primary role in this context is not about generating electrical energy but about flame detection and safety.