The cadmium sulfide cell, or cad cell, is part of the primary control system and is used to sense the presence of the oil burner flame. The cad cell assembly consists of the actual cell, a holder, and cord to connect the cell to the primary control. This detector is mounted in the air tube of the burner in such a way as to sight the flame.

The cad cell is made from a cadmium sulfide coated ceramic disk with a conductive grid over its surface. Electrodes are attached to this surface, and the cell is sealed in glass to protect it. The cad cell in darkness has a very high resistance to the flow of electrical current. Yet, in the presence of visible light, it has a very low resistance. For a properly adjusted burner, during operation, the cad cell resistance should be approximately 300-1000 ohms, but not more than 1600 ohms. A resistance above 1600 ohms signals a problem that should be corrected.