Photo-electric cells may be distingusished as belonging to one of three different types, these are: -
The barrier layer photo-electric cell; in which a voltage is produced when light falls upon the junction of two different materials. - a good example of this type is the Selenium cell fitted to the famed Weston Master (Megatron) light meters.
The photo-conductive cell; in which the resistance of the cell varies in proportion to the amount of light falling on them - a good example of this type is the Cadmium Sulphide (CdS) cell which was the mainstay of most SLR cameras having through the lens (TTL) light metering from the early 1960s to the mid 1980s.
The photo-emissive cell; in which electrons (and hence a current) are liberated from the cathode when irradiated by light.
The first two types of photo-electric cell are mentioned for completeness and interest only and all further discussion will centre around the latter type - the photo-emissive cell which just like a typical radio valve, has an anode and a cathode. The cathode of a photocell consists of a flat metal plate coated with a photo-emissive material. The anode typically takes the form of a metal rod or loop situated in front of the emissive cathode.