A device that senses heat. It's used in gas furnaces having a standing-pilot light. It determines whether the pilot flame is lit before the main gas valve is opened to supply gas to the burners. The flame must be lit before the valve is opened. The heat of the pilot flame is converted to electricity by the thermocouple. It turns heat into an electrical current. The current is strong enough to open the main gas valve. After being opened, the gas flows to the pilot light. If the thermocouple does not detect a pilot flame, it will turn off the gas supply to the pilot. The electrical current from a 24-volt AC transformer operates the main gas valve.