To go into a detailed description of the many applications of selenium cells is not possible in a work of the size of this. However a discussion of the manifold applications will point out in a marked manner the wonderful possibilities of a perfected selenium cell.

The use of selenium cells for photometric purposes was suggested by Clark on the occasion of the first announcement of the light sensitive properties of selenium. It was hoped that the selenium cell would remove the color stumbling block in measuring the brilliancy of light but to date a successful photometer using selenium has not been produced. The proposed method was to connect a selenium cell in series with a galvonometer properly calibrated and to allow the light to be measured to fall on the selenium cell, its brilliancy in candle-power or foot candles to be read direct from the scale on the galvonometer.

A cell of the Fritts type with a gold electrode would not be suitable for the purpose on account of it only passing the green rays but Fritts suggested that a number of cells be used, one with a silver foil electrode to pass blue rays and so on to include all the colors of the spectrum. Naturally the use of a perfectly transparent conductor on the surface of the cell would make it possible to use one cell for the entire range of color and with proper precautions a conducting liquid might solve the problem. This is worthy of attention and experiment.

The application of selenium cells to the transmission of speech over a beam of light has received mention from time to time in the technical press principally in connection with the experiments of Bell and Taintor or Ruhmer. Ruhmer has succeeded in talking a distance of 4½ miles using a speaking arc at the transmitting station. The arc was mounted in the focus of a parabolic mirror, the microphone being connected inductively to the arc circuit by means of an induction coil. The beam of light was picked up by a second parabolic mirror and focused on a selenium cell connected to a battery and telephone receiver. The voice waves impinging on the microphone will act to vary the intensity of the light beam sent out by the arc and in this manner affect the selenium cell at the receiving station which alters the strength of the current thru the receivers and thus reproduces the words.

For experimental purposes a small incandescent lamp may be used at the transmitting station connected in series with a telephone transmitter and battery. The receiving end comprises a selenium cell with proper battery and sensitive telephone receiver. With a little care in adjustment the simple arrangement will transmit the voice across a darkened room without difficulty.

Another application of selenium made by Ruhmer is the automatic control of light buoys. When such buoys are placed in out of the way places it has been necessary to leave the light burning day and night. To prevent the frequent recharging of the gas reservoir this method required, a selenium cell was arranged to turn the gas on and ignite it at nightfall and to extinguish it again in the morning. The cell is connected to a voltmeter the needle of which moves between two contacts. During the day the needle rests against one contact and the gas is turned off by an electromagnet. At nightfall as the resistance of the cell increases the needle falls back till it touches the other contact and operates an electrical mechanism for turning on the gas and igniting it.

Similar applications only await the perfection of a reliable selenium cell in quantities. They could be used to automatically control street lights and lights on advertising signs located along railroads and similar routes of travel.

On the other hand they could be employed in connection with a suitable recording instrument to register the intensity of the sunlight and thus serve as a valuable adjunct to the weather bureau.

They have been used to a limited extent by Prof. Barnard of Lick Observatory in connection with astronomical work and by Minchin in an automatic detector of comets.

The oft heralded successful transmission of photographs over a wire has yet to become an accomplished fact. Photos have been transmitted by Korn with a fair degree of success using selenium cells but such work awaits the perfect cell. Many other investigators have attempted to solve the problem and have proposed various methods, employing images thrown on ground glass screen or thru a photographic negative, a selenium cell or a bank of cells to be affected by the high and low lights of the picture. At the receiving end the current is used to control a light source by one of the many possible methods to reproduce the picture on sensitized paper. These inventions have been variously termed the Telescope, Telectroscope, Telephot, Telephotograph, etc. However with the exception of a very few they have not progressed beyond descriptive matter, drawings and the most preliminary experiments. One interested in the subject would do well to obtain copies of the various patents issued on the art thru a patent lawyer who will do this for a nominal sum.

A scheme has been proposed to utilize the cells in connection with motion pictures to produce the so called talking pictures. This seems entirely feasible provided some form of telephonic relay is used to amplify the reproduced voice currents. The method suggested is to print the spoken words on the motion picture film alongside the pictures in parallel lines of black and white. To do this a beam of light is focused on the film while the picture is being taken, the light being controlled electrically by microphones concealed near the actors. The words spoken acting to vary the intensity of the light, which variations will be printed on the film.

In reproducing the picture a light is arranged to pass thru the voice strip and fall on a selenium cell. In this manner the various shades of black and white imprinted on the film will cause variations in the resistance of the cell and loud talking telephones connected to the cell thru a telephonic relay will reproduce the words. This method has the decided advantage of absolute synchronism lacking in all mechanical devices ever developed for the purpose.

In an attempt to enable the blind to read printed matter by sound Prof. F. C. Brown has devised what is termed a Phonoptican. In his device three or four tiny selenium cells are mounted in a row. The length of the row is equal to the height of the printed letters. Each cell is connected to a telephone receiver thru a separate interrupter. For convenience the telephone receiver used contains as many separate coils and diaphragms as there are cells used. It will be clear that when light falls on one of the cells a certain note will be heard in the receiver and after a little practice the cell affected can be instantly determined.

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.