What is function of a cad cell on an oil furnace?

A cad cell, or cadmium sulfide cell, is a vital component of an oil furnace that serves as a flame sensor. It detects the presence of a flame during the combustion process; if the flame goes out or is not detected, the cad cell sends a signal to the burner control to shut off the fuel supply, preventing unsafe conditions. This ensures safe operation and helps maintain the efficiency of the furnace.