The red thermocouple wire is connected to the negative. Total length is 5 inches. Thermocouples act as temperature sensors during burning. The thermocouples are located inside the furnace and extend from the walls of the furnace to the interior space of the furnace, where they detect temperature changes. There are two general types of thermocouples used in ceramics: K-type thermocouples and S-type thermocouples. K-type thermocouples are the most commonly used thermocouples. They are relatively inexpensive, strong and durable. Type K thermocouples operate in a temperature range of -300°F to 2450°F, so they fit the needs of the majority of ceramic connoisseurs. A type K thermocouple consists of two dissimilar metals: alumel and chromyl. When the two metals meet, they are welded together to form the tip of the thermocouple, where the heat is sensed in your oven. When the alloys are heated, a voltage is produced that is proportional to the temperature difference between the ends of the metallic conductors. In short, these two metals are heated at different rates, and by comparing this difference, a true temperature is determined. The metal chromium is indicated as positive, and is indicated by the yellow symbol +; Alumel is negative and marked with a red symbol. Maintaining this polarity properly connected through the thermocouple wire and oven controller is critical. It is important to know when the thermocouple needs to be replaced to avoid inaccuracies in the temperature readings. As thermocouples age, their accuracy decreases and temperature readings stop. If this is the case, it is time to change the thermocouples. You can accurately test the temperature of your furnace while burning with self-supporting or "witness" funnels. Buying thermocouples is not as difficult as it may seem. Several thermocouples are interchangeable between furnaces. However, it is important to know the length and diameter of the thermocouple you need. The length is determined by the thickness of the furnace wall, and the diameter is the thermocouple's insertion point into your furnace. You'll often find thermocouples shielded in a ceramic casing (such as Scutt furnaces) or in a tube, as in L&L furnaces. The thermocouples can come in braided, as with the Amaco front loader thermocouples, and sometimes Olympic and Paragon furnaces use a metal sheath over the thermocouple.