The cells are mounted in a tiny box and moved over a brilliantly illuminated printed page. The cells will be illuminated and darkened by reflection from the printed letter and in a certain order for each individual letter. Since each cell gives a different sound in the telephone receiver each letter will have a different series of sounds. A sound alphabet is thus made that after a little practice will enable a blind person to read printed text.
Hammond of Radio control fame has employed selenium cells for the purpose of controlling boats at a distance. His method consists in having a number of selenium cells, each being sensitive to a certain colored beam of light and responding to that color only. These cells are connected to separate relays that control the various functions required of the boat or torpedo. Thus by throwing a beam of light of a certain color on the cells any desired relay can be closed and the mechanism controlled at a distance.
The so called Electric Dog constructed by B. F. Meissner, that follows a light carried by a person employs two selenium cells located behind condensing lenses with an opaque plate between them. The cells are mounted in a box fitted with wheels, driven by a motor and steered by an electromagnetic arrangement. The cells are connected to relays so that when light falls on either cell the motor will start and the device will turn towards the lighted side. This results in the box turning till the other cell receives an equal amount of light which closes the other relay and straightens out the steering mechanism to head the machine straight for the light. In this manner the “Dog” will follow a flashlight with unerring precision, keeping a straight course when both lenses are equally illuminated but when one receives more light than the other it will turn towards the lighted side. This illustrates one of the amusing applications to which selenium cells may be adopted.
In cable telegraphy use is made of a siphon recorder or reflecting galvonometer. The latter causes a beam of light to swing to one side or the other according to whether a dot or dash is indicated. By mounting a selenium cell either side of the center of the swing and providing stops it has been possible to actuate relays and print the dots and dashes.
The selenium cell is particularly adapted to burglar alarm work. By arranging a cell in places to be protected and connecting it to a battery and relay an alarm can be turned in should an intruder flash a light on the cell. This method has the advantage of not warning the intruder that an alarm has been given which is an assurance of his capture.
The above covers in a brief form the more important applications of selenium cells in general, being possible with most types of cells that are sufficiently sensitive. However the Fritts cell possesses several interesting features aside from its resistance being altered by light.
As previously mentioned some cells are capable of generating a current. To what extent this property can be developed is a matter of conjecture only. It cannot be denied that they would form an ideal source of current of small values. Compact, sealed, practically unbreakable and perfectly portable are but a few good features of such a battery. The cells are comparatively cheap to construct and once made require no further attention beyond reasonable care. If a number of cells were well developed they might prove valuable in utilizing Solar energy. For this purpose an arrangement would be necessary to protect them from heat, for instance an alum cell. It should be borne in mind that this is a direct transformation of light energy into electrical energy without chemical action of any kind, this is the forerunner of the inevitable Solar Generator.
Another strange property of the cells is their ability to produce sounds when a pulsating current is passed thru them. Under certain conditions a telephonic current may cause them to reproduce speech when connected to a microphone and battery. Weak sounds are also produced when an interrupted beam of light is allowed to fall on a short circuited cell.
It was also mentioned by Fritts that his cells change in sensitiveness with the kind of battery employed with them. They were found rather insensitive with bichromate of potash cells, sensitive with Leclanche cells and extremely sensitive when another current generating cell was employed as a source of current. This fact opens a wide field for experiment. Perhaps a certain type of battery will result in making the selenium cells extremely sensitive and staple. Conjecture on the subject is without bounds. Just what causes a cell to change in sensitiveness with different current sources? Is there some unrecognized force in one current that is not found in the other? The solving of these problems may not be work of the easiest or the soonest done but the accomplishment is worth the labor.