APPLICATIONS OF SELENIUM CELLS
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.