In furnace operation, it is necessary to make sure that the fuel or gas is ignited properly by detecting the flame. If there’s no flame detected, the situation can be dangerous if the furnace is flooded with explosive gas, so the fuel/gas supply valve should be closed automatically to avoid catastrophic disaster.  A thermocouple can be use to detect flame, here is the schematic diagram of the circuit:

Flame Detector circuit schematic

A platinum-rhodium thermocouple produce 8mV output at 800 °C. You can see the thermocouple is connected directly to the positive and negative inputs, look like there’s no threshold, but wait, the balance input (pin 5) is connected to the reference output and this gives the threshold. [Circuit’s schematic diagram source: National Semiconductor Application Note]