When the burner is off, the cad cell looks into the dark combustion chamber. The bright light of the oil burner flame reduces the resistance in the cad cell, allowing ______ to flow through it.

• A. Electricity
• B. Current
• C. Gas
• D. Heat

Answer: (A)

A cad cell is a light‑sensitive resistor. In the dark, its resistance is high, so little current can flow. When the flame’s bright light shines into it, the resistance drops, allowing electricity to flow through the cad cell. That flow of electrical energy is what the burner control uses to detect that a flame is present. Gas or heat aren’t what pass through the sensor, and the device isn’t generating heat itself—it’s changing its conductance in response to light, which lets electricity flow more readily.