The resistance of a Cadmium-Sulfide cell "photo cell" when dark is __________.

The resistance of a cadmium-sulfide cell, commonly known as a photoresistor or light-dependent resistor (LDR), is significantly high in the dark. This is a key property that enables these devices to function effectively in various applications such as light sensing and automatic lighting systems. When there is no light, the resistance of a cadmium-sulfide cell can reach up to millions of ohms.

The correct answer, which indicates a resistance of 15,000,000 ohms, aligns with typical values found in cadmium-sulfide cells when they are not exposed to light. In dark conditions, the absence of photons leads to a lack of energy for excitation of electrons, maintaining a high resistance state.

This extreme resistance decreases dramatically in bright light, allowing for the detection of light levels. The precise value can vary based on specific cell design, but 15 million ohms is a representative figure for a cadmium-sulfide cell in darkness. Other choices listed are lower values that do not accurately reflect the resistance of cadmium-sulfide cells when they are in dark conditions.