Photocell

A multimeter is set to measure electrical resistance in ohms. The resistance of a CdS photocell shielded from light is too large to be on scale. Illuminating the photocell decreases the resistance to measurable values. When the multimeter has an audible conductivity setting (low resistance causes the meter to produce an audible sound), sound can be heard when the photocell is illuminated. (Companion Demonstration 7.9)