How does the muffle furnace use a temperature measuring ring to measure temperature?

The furnace temperature of the muffle furnace is generally measured by a thermocouple and displayed on the temperature control meter.

The temperature measurement ring can also be used to measure the temperature of the muffle furnace. During the measurement, put the temperature measuring ring in the corundum sagger and put the lid into the furnace, and then start to raise the temperature. After reaching the set value, keep it warm for 1 hour and then cool the electric furnace. After the furnace cools down, open the lid of the sagger and take out the temperature measuring ring.