What happens to the resistance when a cad cell detects a flame?

When a cad cell detects a flame, the resistance decreases. This occurs because the cadmium sulfide used in the cad cell is a photoconductive material that changes its electrical resistance in response to light. In the presence of a flame, which emits light, the cad cell's resistance drops significantly, allowing current to flow through the circuit.

This principle is utilized in flame detection systems for oil burners, where the presence of a flame must be confirmed to ensure safe operation. A lower resistance indicates that the flame is present, which in turn triggers the appropriate responses in the burner control system, including maintaining the fuel supply.