Type J thermocouple is a very common and general purpose thermocouple. It has smaller temperature range and a shorter lifespan at higher temperatures. It consist of positive leg made of an Iron wire and negative leg made of an Constantan ( Copper-Nickel ) alloy wire.
Mineral Insulated Thermocouples, commonly referred to as MgO (Magnesium Oxide) thermocouples, are used in many processes and laboratory applications. They are available in all thermocouple element types and a wide variety of sheath diameters and materials.