What is the resistance of cadmium-sulfide photo cells when dark?

Cadmium-sulfide photo cells, commonly known as CdS cells, exhibit high resistance when in the dark due to their nature as photoconductive materials. In low-light conditions or complete darkness, these cells can reach resistances in the range of several million ohms.

The correct value is indicative of the fact that the resistance of a CdS cell can be significantly high when there is no incident light, often in the range of tens of millions of ohms. This characteristic allows the photo cell to be effectively used in light-sensing applications, where it may operate as an on/off switch based on the presence or absence of light.

Understanding the typical behavior of CdS cells in darkness is important, especially in applications requiring sensitivity to light levels, such as in automatic lighting systems or devices that require thresholds based on light conditions.