Photo electric cells and their applications

A photo cell (photoelectric cell) is a device that converts light energy directly into electrical energy, working on the principle of the photoelectric effect: when light falls on a photosensitive material, the material's electrical properties change in a way that can be measured or exploited. Based on exactly which electrical property changes, photo cells are grouped into three types. A photo emissive cell works by the direct emission of electrons from a metal cathode when it is irradiated by light or other radiation. A photo voltaic cell uses a light-sensitive semiconductor element that itself generates a voltage proportional to the intensity of the incident light. A photo conductive cell uses a semiconductor whose electrical resistance changes according to the amount of radiant energy falling on it.

The photo emissive cell is developed in detail here. It consists of an evacuated glass or quartz bulb containing two electrodes: a semi-cylindrical cathode coated with photosensitive material, and a thin rod-shaped anode held along the axis at the centre of that semi-cylinder. A potential difference is maintained between anode and cathode through a galvanometer. When the curved cathode is illuminated, it emits electrons which are attracted across to the thin central anode, producing a current that the galvanometer registers; for a given cathode material, the size of this current depends on (i) the intensity of the incident radiation and (ii) the potential difference maintained between anode and cathode. …