A J thermocouple is a type of thermocouple that is commonly used to measure temperature in a wide range of applications. A J thermocouple consists of two different types of metal wires, usually made of iron (Fe) and constantan, that are welded together at one end to form a sensing junction. When the sensing junction is exposed to heat, a voltage is generated between the two wires, which can be measured and used to determine the temperature.