PHOTO-ELECTRIC CELLS

The photo-electric cell has become one of the most impor­tant servants of industry.

Its characteristic property is sensitivity to light. The light causes changes in electrical pressure.

There are some types of photo-electric cells (photo tubes). The commonest type looks rather like a valve. Within the clear glass bulb there are 'two electrodes: a curved metal sheet (cathode) and a wire in front of it (anode). Light falling on the photosensitive sheet releases electrons.

Photo-electric cells can be used in exactly the same way as an ordinary switch for turning an electric current on or off. Doors can be opened automatically. For instance, if somebody approaches, the interruption of a beam of light focused on a cell brings into action the opening device. The door opens and remains, by the action of a relay, for a given time, after which it shuts again. Sometimes infra-red or ultra-violet light is used; in this case, a cell is being chosen which is sensitive to this "invisible ray".

This "off" and "on" is only one of the applications of the photo-electric cell. If, every time the light-beam is interrupted, the working circuit is made to turn a counter, we get an auto­matic device for registering objects passing at a given spot. This is used in many industries.

In much the same way the cell can be used for automati­cally measuring size.

The photo-electric cell is sensitive not only to the mere switching on and off of light, but also to the degree of light.

In the metal industries, the photo-electric cell can be em­ployed to measure temperature with great accuracy. Heat cont­rol -by photo-electric cells has special applications in welding. The applications of the photo-electric cell are almost limitless.