J-type thermocouple, also known as iron-constantan thermocouple, is also a low-cost metal thermocouple. Its positive electrode (JP) has a nominal chemical composition of pure iron, and its negative electrode (JN) is a copper-nickel alloy, which is often referred to as constantan. Its nominal chemical composition is: 55% copper and 45% nickel, as well as a small amount of very important elements such as manganese, cobalt, and iron. Although it is called constantan, it is different from constantan of nickel-chromium-constantan and copper-constantan, so it cannot be replaced by EN and TN. The measurement temperature range of iron-constantan thermocouple is -200~1200℃, but the commonly used temperature range is 0~750℃. J-type thermocouple has the advantages of good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity, and low price, and is widely used by users. J-type thermocouples can be used in vacuum, oxidation, reduction and inert atmospheres, but the positive iron oxidizes faster at high temperatures, so the use temperature is limited, and it cannot be used directly in a sulfiding atmosphere at high temperatures without protection.

When the two ends of the thermocouple are connected into a loop, when the temperatures of the two junctions are different, a thermal current will be generated in the loop. If there is a temperature difference between the working end and the reference end of the thermocouple, the display instrument will indicate the temperature value corresponding to the thermoelectric potential generated by the thermocouple.

The thermoelectric heat of the thermocouple will increase as the temperature of the measuring end increases. Its size is only related to the thermocouple material and the temperature of the two ends, and has nothing to do with the length and diameter of the thermoelectrode.

The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same, usually consisting of thermoelectrodes, insulating sleeve protection tubes and junction boxes.