A Type-S thermocouple inside it's Mullite tube along with protective packing inside.
Type-S is platinium which is thinner, more, accurate, lasts longer, and retains its accuracy far better than Type-K nickel chrome thermocouples do. They also cost $200 rather than $60.
Type-S Thermocouples generate 2/3 less voltage. Because of this:
Type-S thermocouple also require a $15 or less length of shielded wire - two insulated wires surrounded by a metal ribbon jacket to resist electrical interference;
Type-S thermocouples require a compatible kiln controller. Variations of Bartlett V6-CF controllers need to have the one jumper on their circuit board connected to both leads, as opposed to just one with Type-K, and the Thermocouple Type has to be changed from TYPE-K to TYPE-S in the "Hidden Menu".
Hidden Menu is reached by selecting RESET, anf typing 443 enter.