If two different metals are joined, and heated, a small electric current can be generated. For a temperature rise of around 200° Celsius or 392° fahrenheit, the potential difference created is about nine millivolts. The point that is heated is called a hot junction, and the whole system is called a thermocouple.
In developing engine designs, manufacturers use thermocouples as high-temperature indicators, to determine temperatures of components such as spark plugs and exhaust systems.
