The thermocouple works by converting the heat generated from the pilot light on a furnace into an electrical signal that tells the gas valve whether to open or close. The thermocouple is technically known as a thermocouple junction. It is an electronic device.
The thermocouple is made up of two metal wires that are welded at the ends and protected by a metal casing. The thermocouple sensor is held in the hottest part of the pilot light flame. When this sensor heats up to a point where the flame is hot enough to ignite the gas for the furnace burners, it sends the signal to open the gas valve. The other end of the thermocouple is attached to the gas valve body. When the electrical signal is sent from one end of the thermocouple to the other, it activates a solenoid at the base of the gas valve that functions using a 24-volt transformer. When the sensor cools off, it closes the valve to keep the gas away from the burners. The thermocouple works as a safety device to assist the thermostat in regulating the amount of gas sent to the burners and to keep the gas from continuously flowing, which can be harmful.