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Title: Selenium Cells - The Construction, Care and Use of Selenium Cells with - Special Reference to the Fritts Cell
Author: Benson, Thomas W.
Language: English
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SELENIUM CELLS
THE CONSTRUCTION, CARE AND USE
OF SELENIUM CELLS WITH SPECIAL
REFERENCE TO THE FRITTS CELL
BY
THOS. W. BENSON
NEW YORK
SPON & CHAMBERLAIN, 120 LIBERTY STREET
LONDON
E. & F. N. SPON, LIMITED, 57 HAYMARKET, S. W.
1919
Copyright, 1919
BY THOMAS W. BENSON
VAIL-BALLOU COMPANY
BINGHAMTON AND NEW YORK
APPRECIATIVELY
DEDICATED
TO
J. A. STELTZER
FOREWORD
The lack of definite information relative to the construction of
selenium cells has led the writer to put in this form the results of
some of his experiments. The method described was originated by Mr. C.
E. Fritts but the apparatus used was developed by the writer.
Despite the fact that sensitive cells are very difficult to construct
by the methods in vogue the use of apparatus described practically
eliminates failures, the cells, almost without exception, being useful
for one purpose or another.
This is by no means the last word on the subject. Many improvements
are possible and have been pointed out in the text. Should these
instructions but serve to interest others in the fascinating study they
will have served their purpose.
THOS. W. BENSON.
Philadelphia, Pa.
CONTENTS
PAGE
CHAPTER I
SELENIUM, THE ELEMENT 1
Discovery, Naming and Classification. Where Found
and Method of Extraction. Three Forms, Amorphous,
Vitreous and Metallic.
CHAPTER II
CONSIDERATION OF CELL TYPES AND THEIR CHARACTERISTICS 5
Factors in Design. Bildwell Cell. Application of
Selenium and Annealing. Ruhmer Cell. Bell and
Taintor Cell. Mercadier Cell. Gripenberg Cell.
Theory of Operation.
CHAPTER III
THE CONSTRUCTION OF FRITTS SELENIUM CELL 20
Description of Hot Press. Accessories for Press.
Other Apparatus Required and Supplies. Preparing
Copper Plate. Applying Selenium. Treating in Hot
Press. Application of Gold Foil. Assembling Cell.
CHAPTER IV
TESTING AND MATURING SELENIUM CELLS 32
Two States. Testing and Maturing Set. Accessories
for Set. Construction of Rheostat Arm for Wheatstone
Bridge. Measurement of Cells by Bridge
Method. Tests for Light Sensitiveness. Proper
Voltage. Classification of Cells. Testing for
Polarization. Treating with A. C. Raising Resistance
of Cells. Testing for Current Generation. Reconstruction
of Useless Cells. Measurement of Cells by
Substitution Method. Sealing Cells.
CHAPTER V
APPLICATIONS OF SELENIUM CELLS 51
Photometric Applications. Transmission of Speech
Over Beams of Light. Automatic Control of Light
Buoys and Isolated Lights. As Recorder of Sunlight.
Astronomical Applications. Talking Pictures.
The Phonoptican, How the Blind May Read by
Sound. Controlling Mechanisms at a Distance.
Electric Dog. Use in Cable Telegraphy. Burglar
Alarm. Selenium Batteries, Current Generators.
Telephonic Properties. Effect of Different Current
Sources on Sensitiveness of Cells.
CHAPTER VI
THE CARE OF SELENIUM CELLS 62
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Bildwell Cell 7
2. Ruhmer Cell 10
3. Bell and Taintor Cell 12
4. Mercadier Cell 14
5. Gripenberg Cell 15
6. Fritts Cell 17
7. Hot Press with Cell in Place 21
8. Details of Hot Press (elevation) 22
8. Details of Hot Press (plan) 22
9. Platen for Cell; and Template for Applying Selenium 24
10. Fiber Pieces for Enclosing Cell 26
11. Mode of Assembling Cell 30
12. Testing and Maturing Set 33
13. Layout and Wiring Diagram of Testing Maturing Set 35
14. Connections for Buzzer and Induction Coil to Produce
Alternating Current 36
15. Connections for Interior of Rheostat Box 38
16. Circuit for Wheatstone Bridge Measurement 40
17. Circuit for substitution Method of Measurement 47
18. Showing Cell ready for Assembly with Four Complete
Cells in background 49
CHAPTER I
SELENIUM, THE ELEMENT
Over a century ago, 1817 to be exact, the Swedish scientist Berzelius
discovered a new element in the lead chambers used for the manufacture
of sulphuric acid by roasting iron pyrites. Noting its resemblance
to Tellurium, the name for which having been derived from the Greek
for Earth, Tellus, he named the new element Selenium derived from
the Greek for Moon, Selene. The ending ’um being used to indicate a
metal according to the practice of naming newly discovered elements.
Although believed to be a metal for many years, the chemical reaction
of Selenium resembles that of sulphur to such a degree that it is now
accepted to be a non-metal in its amorphous and vitreous forms. In its
third or crystalline state it has many metallic characteristics and in
this form termed metallic selenium. In the Periodic System it occupies
the place between Tellurium and Sulphur.
Designated by the symbol Se, selenium has been found in all parts of
the globe in small quantities, chiefly in combination with copper, lead
and silver forming selenides, in certain pyrites and occasionally in
its pure state. It was found in meteoric iron by Warren in 1909.
An idea of its wide distribution may be gained from the following table:
Mineral Composition Location
Sulphur Selenide in natural sulphur Lispau Islands
Eucarite Selenide of silver and copper Chili
Crooksite Selenide of silver, copper and thallium Norway and
Sweden
Clauthalite Selenide of lead Germany
Lehrbachite Selenide of lead, copper and mercury Germany
Zorgite Selenide of lead and copper Germany
The element is obtained commercially as a by-product from the
manufacture of sulphuric acid, various methods of extracting it from
the chamber mud being employed. The usual process is to heat the well
washed chamber mud with potassium cyanide and nitrate to obtain an
alkaline selenate. The element is then precipitated with hydrochloric
acid or sulphur dioxide.
=Selenium= exists in three well defined forms, Amorphous, Vitreous and
Metallic.
=Amorphous Selenium.= This form is obtained as a finely divided brick
red precipitate when sulphur dioxide is passed thru selenic acid. It is
soluble in sulphuric acid and slightly so in carbon disulphide. It has
a Sp. Gr. of 4.26, with no definite melting point, softening gradually
and running together between 80° and 100° C. In this state it is an
insulator.
=Vitreous Selenium.= When the amorphous selenium is heated to 217° C
and rapidly cooled the vitreous form results. It is now a red vitreous
mass, slightly less soluble in carbon bisulphide. When a thin film is
held up to the light it shows blood red in color. Sp. Gr. 4.28, Atomic
Weight 79.5. This form is practically an insulator having a resistance
of 6 × 10⁹ ohms per Cu. Cent. at 75° C or about 3.8 × 10¹⁰ as great as
that of copper. It can be electrified by friction. Vitreous selenium
has no definite melting point being hard and brittle at 40° C and
softening gradually as the temperature rises, becoming fluid at 210° C.
=Metallic Selenium.= By cooling melted vitreous selenium to 210° C
and holding it at that temperature for a short time the metallic
form results. The element is now a black glossy opaque mass, a fair
conductor of electricity but improving greatly under the influence of
light. It melts between 217° and 220° C, Sp. Gr. 4.788, insoluble in
carbon disulphide but will dissolve in sulphuric acid to form a green
solution. It is the latter form that is used in the construction of
selenium cells.
When selenium is vaporized by heat it gives off dark brown fumes having
an odor similar to rotting cabbage. These fumes are poisonous and care
should be taken that they are not breathed to excess.
CHAPTER II
CONSIDERATION OF CELL TYPES AND THEIR CHARACTERISTICS
=A selenium cell= consists essentially of two electrodes of brass or
copper bridged by a thin layer of metallic selenium. When connected
into a circuit with batteries and other apparatus the current flows
from one electrode to the other thru this selenium bridge. Since the
resistance of the selenium to an electric current depends upon the
amount of light falling upon it the flow of current thru the cell will
be controlled by the brilliancy of the illumination.
=Metallic selenium= being opaque, the light penetrating but ¹/₅₀,₀₀₀th
of an inch as calculated by Marx, it is necessary that the selenium
layer be extremely thin in order that the light may affect an
appreciable proportion of the total conducting area. This condition is
never reached when the electrodes lie parallel to each other with the
selenium between them. However by arranging the electrodes so that the
current flows at right angles to the plane of the selenium surface we
can cause all the current to flow thru the light affected area. This
can only be accomplished by making use of a transparent conductor for
one electrode.
To realize the importance of the above factors a description of the
various types of cells developed by the many investigators in this
field will be of great assistance. The different workers made use of
various arrangements of the electrodes but the cells fall into certain
classes. These types have been named after the inventor or the one most
prominent in the work on them.
=The Bildwell cell= is possibly the best known type. It is made by
winding two bare wires of copper, brass, german silver or platinum on
a sheet of mica or slate. The wires are spaced about ¹/₃₂nd of an inch
apart. The size of the wire is of little importance, the usual practice
being to use #28 wire on a form measuring two by one inches. In Fig. 1
is shown this type of construction using a mica form, the wires being
fastened by passing them thru holes at the ends of the sheet.
[Illustration: FIG. 1. BILDWELL CELL]
The selenium is applied to the cell by melting it over the wires.
The cell is laid on a mica covered copper plate supported over a
bunsen burner. The temperature of the cell is raised to the point
where a stick of selenium when touched to the cell melts. The entire
surface of the cell is coated with the selenium in a very thin layer,
smoothing out the lumps with a sheet of mica or a steel knife. To get
a satisfactory coating the temperature must be regulated closely, if
too low the selenium turns grey and the temperature must be increased
to melt it, if too high the selenium collects in drops due to surface
tension and is as difficult to spread as mercury. The proper state is a
semi-fluid condition which it attains at 220° C when it can be easily
manipulated.
When a satisfactory surface has been obtained the cell is transferred
to a copper plate to cool while the bunsen burner is turned down to
give a temperature of 120° C. When cool the cell is replaced on the
hot copper plate and allowed to heat up again. Shortly the whole
surface will turn grey in color due to the selenium crystallizing. The
temperature is now slowly increased till the selenium shows signs of
melting, this will be indicated by the edges turning black. The bunsen
burner is immediately withdrawn and the edges allowed to recrystallize.
The burner is turned down a trifle and replaced under the hot plate.
The cell is watched carefully for signs of melting and if none appear
it is left so for three or four hours. If it melts again the burner
should be further lowered, just sufficient to keep the cell a trifle
below the melting point of the selenium. The cell is then allowed to
cool by lowering the burner by small amounts extending over a period of
an hour. This prolonged heating and slow cooling is known as annealing.
After the above treatment the cell is complete save for mounting.
The usual method is to mount the cell in a small wooden box fitted
with a glass window to admit the light, leads being brought from the
electrodes to two binding posts mounted on the box. This protects the
cell from moisture and dust.
It will be apparent that with the above method of construction it is
impractical to get the extremely thin layer of selenium necessary if
the light is to affect a relatively large proportion of the total
area. This will be even more clear from an examination of the cross
sectional view of this type of cell as shown in Fig. 1. Here we have a
comparatively thick layer of selenium bridging the space between the
wires. Of this layer only the thin surface film facing the light drops
in resistance while the interior part is unaffected. This means that
should the surface layer drop to even ¹/₅₀₀th of its dark resistance
the total drop of the cell would be much less.
=The Ruhmer cell= is similar in construction to the Bildwell, differing
only in the form of support. A porcelain or glass tube is used to
support the parallel wires as shown in Fig. 2. When porcelain is used
the constructor can fasten the wires at the ends by slipping them into
slots cut with a hack saw. With glass some other means are necessary to
hold the wire while winding.
[Illustration: FIG. 2. RUHMER CELL]
The selenium is applied to the cell in the same manner as the Bildwell
and then annealed. The method of mounting the cells as devised by
Ruhmer is worthy of mention. The cell unit is enclosed in a glass tube
and then the air exhausted. By attaching the leads to an incandescent
lamp base a very convenient arrangement results. The cell is well
protected from all external influences and is therefore more stable and
reliable. This is perhaps the most important improvement in this type
of cell.
We have in the Ruhmer cell conditions almost identical to that in the
Bildwell, namely a large area of conducting selenium that is beyond
the range of the light and hence not affected thereby. This is offset
to a certain extent by the large area exposed to the light as these
cells can be employed with a parabolic reflector to cause the light
to fall on all sides. This type of cell was employed by Ruhmer in his
experiments with the Photophone. He succeeded in transmitting speech
for a distance of four miles using a speaking arc at the transmitting
station.
The cell developed by Bell and Taintor in their experiments is rather
novel in the arrangement of the electrodes. As shown in Fig. 3 the
electrodes take the form of brass disks separated by thin mica disks
supported by two brass rods, the whole being clamped together by nuts
on the ends of the rods. The disks are one inch in diameter, eighteen
or twenty being sufficient for a small cell. By drilling the holes
in the disks of different sizes and assembling them as shown it is
possible to have alternate disks connected to the same rod. After
assembling and clamping the cell skeleton is chucked in a lathe and the
surface turned smooth and polished.
[Illustration: FIG. 3. BELL AND TAINTOR CELL]
The selenium is applied by heating the form and melting it on, by
rolling the cell back and forth over the hot plate it is possible to
get an extremely thin film of selenium on the smooth surface offered by
the cell. The coating is then annealed in the regular manner. This cell
can well be mounted in a glass tube and the air exhausted.
The advantage gained by this form of construction is the thin film of
selenium obtainable due to there being no spaces between the electrodes
into which the selenium can flow. A cross section of this type is given
in the illustration. It will be seen that although the main defect of
the cells mentioned previously has been reduced to some extent still it
has not been removed entirely.
=The Mercadier cell= is similar in many respects to the Bell but is
easier of construction. Two strips of thin copper one half inch wide
are wound into a spiral being separated from each other by strips of
mica. One face of the flat spiral is filed flat and then polished. The
selenium is melted onto the cell and then smoothed off with a strip of
mica. The cell then being annealed as described previously.
We have here a condition analogous to that in the Bell cell, the
only advantage being ruggedness and simplicity against a loss in
active surface area. For experimental purposes this cell is entirely
satisfactory for if it does not prove sensitive the selenium coating
can be filed off and another applied.
[Illustration: FIG. 4. MERCADIER CELL]
We come now to the consideration of cells wherein the current flows at
right angles to the surface of the selenium. This implies the use of at
least one electrode on the surface of the selenium. The Gripenberg cell
has both electrodes on the surface. As shown in Fig. 5 the electrodes
are made by depositing a thin film of gold on a glass plate and with a
sharp tool removing narrow strips of it to form a grid, alternate bars
of which are connected to the same terminal. The grid arrangement is
shown in the detail illustration.
[Illustration: CROSS SECTION OF ASSEMBLED CELL]
[Illustration: FIG. 5. GRIPENBERG CELL]
The selenium is not applied to the grid in the molten state. A thin
plate of metallic selenium is obtained by melting the metal on a
glass plate and then applying pressure with a cold glass plate. After
annealing the selenium will be found to adhere closely to the hot plate
in a thin film. The cell is then assembled by placing the plate with
the gold grid in a frame having a portion of one side cut out to form
a window. The plate with the selenium adhering is placed over the grid
and forced into contact with same by means of a small screw.
A study of the cross section of this cell shows that the current
inflowing from one electrode to the other must pass the surface of the
selenium. Since the gold film is semi-transparent the light passes
thru it to affect the selenium and will have a maximum effect upon the
resistance of the cell. This object is attained at the expense of loss
of illumination since the gold film cuts off all but the green rays
of light. The advantage just mentioned more than outweighs this loss.
Could a transparent conductor be found this defect would be removed
entirely.
=The Fritts Cell= is little known except by name but is superior to the
others both in simplicity of construction and correctness of design. In
this case the selenium is melted directly on a copper plate that serves
as one electrode. While soft, pressure is applied by a non-adherent
plate to obtain the thin film necessary. The selenium enters into
chemical combination with the copper and adheres firmly to it. After
cooling the selenium film is covered with gold leaf to form the other
electrode. The cell unit can be mounted between two strips of fibre as
shown in Fig. 6, a sheet of very thin mica serving as a protection to
the gold foil surface.
[Illustration]
[Illustration: CROSS SECTION OF ASSEMBLED CELL]
[Illustration: FIG. 6. FRITTS CELL]
We have here the ideal condition. All the current must flow thru the
light affected area in passing from one electrode to the other. The
light passes thru the semi-transparent gold foil to effect the change
in resistance of the selenium film. The only disadvantage lies in
the diminishing of the light strength by the gold film. Despite this
however the Fritts cell has proved the most sensitive cell made having
in one case a ratio of 337 to 1, that is, the resistance in the light
is but ³/₁₀ths of one per cent of that in the dark.
The six types of cells described cover all the types worthy of special
mention and will enable one to select a cell for experimental purposes.
In view of the fact that the Gripenberg and Fritts cells are superior
in point of design the selection of the cell becomes a question of
mechanical difficulties to be overcome. The construction of the grid in
the Gripenberg type is rather difficult unless an engraving machine is
obtainable whereas the Fritts cell requires very simple apparatus for
its construction.
A factor often overlooked in considering the design of selenium cells
is the relation that light and electricity bear to each other. These
are manifestations of the same force and their interaction can be taken
advantage of in the Fritts cell due to it being possible to cause the
current to flow in the same or opposite direction to that of the light
vibrations. The importance of this will be covered in detail later.
No mention has been made of just how the light affects the selenium to
reduce its resistance. This is still a moot point, one theory being
that the light being electromagnetic in nature causes the molecules of
the selenium to cohere in a manner similar to that of the radio coherer
used in the early days of radio telegraphy. However the conduction thru
a selenium cell is similar to that in an electrolyte and differs from
metallic conduction. Considering this the light may act to ionize the
selenium in some manner and make possible the more rapid interchange of
the ions from the opposite terminals. Although a thorough understanding
of the action taking place may lead to the improvement of selenium
cells it is not within the scope of the present work to consider the
various phases of this part of the problem, confining itself as it does
more to the practical production of the cells.
CHAPTER III
THE CONSTRUCTION OF FRITTS SELENIUM CELL
The secret of the successful construction of the Fritts selenium cell
lies entirely in the method of applying the selenium to the copper
plate. The selenium is melted on the plate and pressure applied at the
same time, due to the simultaneous action of heat, chemical affinity
and pressure, crystallization takes place and makes prolonged annealing
unnecessary. One side of the selenium layer enters into chemical
combination with the copper plate forming a selenide while the other
is uncombined resulting in a film that is polarized or has different
electrical and physical conditions at front and back.
To construct the cells use is made of what may be termed a hot press.
This is simply a device for applying pressure to the selenium film
while in a molten state. At Fig. 7 is given a photograph of the
apparatus while Fig. 8 shows the various details of the press. The
dimensions can be changed if desired but the press should be capable of
exerting a pressure of 50 pounds on the bed.
[Illustration: FIG. 7. HOT PRESS WITH CELL IN PLACE]
[Illustration: FIG. 8. DETAILS OF HOT PLATE PRESS (ELEVATION)]
[Illustration: FIG. 8. DETAILS OF HOT PLATE PRESS (PLAN)]
A base of any convenient size, say, 16 x 8 inches, has fastened to one
end a tripod or similar elevated support. A slab of slate 4 x 8 inches
and ⅞th of an inch thick is drilled at the ends to take the bolts
holding the pivot and guide posts shown.
The posts are made from ³/₁₆th inch strap iron ¾ inch wide. The pivot
post being made by giving the metal strip a quarter turn with a large
wrench while holding one end of the strip in a vise. The guide post is
formed by slotting the end of the bent strip to pass the lever.
The lever is made from ¼ inch iron, ¾ inch wide and 20 inches long.
One end is drilled to pass the bolt pivoting it to the pivot post. A
saddle bent from ⅛ inch sheet iron is pivoted to the lever as shown so
as to be directly over the center of the slate slab. The saddle serves
to equalize the pressure at point of contact with the cell undergoing
treatment and gives a direct vertical thrust.
The weight used with the press is made by pouring lead into a round
form 2¼ inches in diameter and 5 inches high. Any other weight of
eight pounds can be used. A hook bent from a strip of brass or iron is
inserted in the lead while in a molten state and serves to hang the
weight on the lever.
[Illustration: FIG. 9. PLATEN FOR CELL]
[Illustration: FIG. 9. TEMPLATE FOR APPLYING SELENIUM]
A bunsen burner is utilized to heat the slate slab. Some convenient
means should be employed to control the height of the flame, either a
valve mounted on the base or a screw clamp on the rubber hose supplying
the gas.
The cell while undergoing treatment rests on a small iron block, the
details of which are given in Fig. 9. The block is 1½ inches square
and ½ inch thick. A hole is drilled at one side to take the bulb of
a chemical thermometer. This block serves two purposes, it allows of
accurate determination of the cell temperature and simplifies the
assembly of the cell for treatment. The thermometer should have a range
up to 220° C.
This completes the hot press proper. In addition will be required a 2
or 4 oz. mortar and pestle to powder the selenium, a pair of tweezers,
a small palette knife and a template for applying the selenium to the
copper plate. The latter is made from a sheet of mica or thin cardboard
with rectangular hole cut in the center as shown in Fig. 9. This sheet
is glued to two small strips of wood or fibre. The template just fits
over the copper plates and confines the selenium powder to the center
of the plate.
The supplies required are chemically pure selenium, specify
electrolytic selenium for electrical purposes when ordering and the
proper material will be supplied. It comes in the form of small
black sticks and looks like sealing wax. A book of gold leaf as used
by decorators is required for the front electrode of the cell. The
patented form in which the foil is attached to a sheet of thin paper
is the easiest to handle. The copper plates for the cell are cut from
⅛ inch copper or brass 1½ inches long and 1 inch wide. These plates
should be perfectly flat and the edges free from burrs. Some clear mica
will be required for the front of the cell to protect it from dust and
moisture.
[Illustration: FIG. 10. FIBRE PIECES FOR ENCLOSING CELL]
The fibre sheets to enclose the finished cell are shown in Fig. 10.
These are cut to the dimensions shown from ⅛ inch fibre.
The construction of a cell is accomplished in the following manner.
The copper plate is first thoroughly cleaned with fine sandpaper and
polished. Coat the plate with a suitable flux and heat in the bunsen
flame, flowing solder over the plate to tin it. While the solder is
still molten throw off as much of the solder as possible and quickly
wipe the cell with a dry rag. This should result in a thin even film of
solder.
Grind the selenium up into a fine powder in the mortar and place in a
well stoppered bottle to protect from dust and moisture.
Lay the tinned copper plate on the table, tinned side up and place the
template over it. With the palette knife place a small quantity of
the selenium on the plate and smooth it out into an even layer ¹/₃₂
of an inch thick. Remove the template carefully so as not to disturb
the selenium and lay the plate on one end of the iron block having the
thermometer well in it. Cover the selenium with a sheet of mica and lay
another smooth block ¼ inch thick on top of the mica sheet. The whole
is conveyed to the hot press and placed in the center of the slate
slab, the lever being let down into place with the saddle centering
over the cell.
Place the thermometer in the well, hang the weight on the lever and
light the bunsen burner, adjusting it so an even blue flame results
without roaring.
The temperature as indicated by the thermometer will rise slowly due
to the bulk of the slate to be heated. At 150° C the selenium will
soften and the lever settle slightly. Continue the heating till the
temperature reaches 220° C and then remove the bunsen burner.
The cell is allowed to remain in the press until the temperature has
dropped to 60° C when the lever can be lifted and the cell taken out.
The mica will come off without trouble leaving a thin even film of
metallic selenium with a shiny grey surface adhering to the plate.
Now go around the edges of the cell with a knife or small file and
remove all selenium that may have flowed over the edges of the plate
and brush the cell to remove all traces of dust and particles of
selenium and copper. Lay the cell face up on a sheet of glass and flood
the surface with alcohol with an eye dropper or pippette. Having cut
a strip of gold foil 1 inch wide lay it on the cell and smooth out
any creases with the fingers. Now go over the entire surface of the
cell with the finger tips pressing rather hard on the paper backing
on the foil. Continue this till the alcohol that seeped through the
paper has evaporated. Then flood the back of the paper and repeat the
pressing. When the second application of alcohol has dried the paper
can be lifted from the cell without difficulty leaving the foil on the
selenium. Should any of the foil adhere to the paper replace it and
treat again with alcohol and pressure.
The above applies to the patent foil. If the foil is loose it can be
cut by placing a sheet between two sheets of paper. A camel hair brush
is rubbed through the hair to electrify it and the foil picked up by
touching the brush to it. The foil is laid on the cell wet with alcohol
and smoothed out with the brush. On drying the foil will adhere to the
selenium.
When a good foil surface is obtained go carefully around the edge with
a small knife and remove any foil that extends over the edge of the
selenium film, otherwise direct contact might be made between the foil
and copper plate, thus short circuiting the cell.
Now take a strip of paper ⅛ inch wide and coat one side lightly with
thick shellac. Wrap the paper around one end of the cell as shown in
Fig. 11. A second sheet of paper is placed over the end of the cell
and attached in the same manner. The paper prevents the copper plate
coming in contact with the terminal leading to the gold foil as will be
evident when the cell is to be assembled in the fibre strips.
[Illustration: FIG. 11. MODE OF ASSEMBLING CELL]
The method of completing the cell assembly is shown in Fig. 11. Take
the sheet of fibre with the hole cut in it and place a ⁸/₃₂ bolt 1 inch
long in one of the holes. A sheet of thin clear mica is laid on the
fibre and short strip of tinfoil slipped over the bolt. A nut is then
run on to clamp the tinfoil and make good contact. The tinfoil should
be cut off even with the edge of the opening in the fibre. Another bolt
is put into the other hole and a longer strip of tinfoil clamped to it
also by means of a nut.
Turn back the long strip of foil and lay the cell between the bolts
with the paper wrapped end nearest the post with the short tinfoil
strip. The paper will prevent contact between the plate and this post
but make sure that the tinfoil strip makes contact with the gold foil.
The long strip of foil is laid over the back of the plate and the other
fibre strip dropped over the bolts. Nuts are then run on to clamp the
whole together.
The cell is now complete and should be tested as described in the
following chapter before sealing the edges with sealing wax.
CHAPTER IV
TESTING AND MATURING SELENIUM CELLS
=The Fritts Cell=, as mentioned by the inventor, when first made
has one of two states or conditions. In one the resistance is very
high, in the other very low, being but a few ohms or a fraction of an
ohm. In the latter state it is insensitive and possesses no definite
characteristic until matured. This maturing will come about gradually
if the cell is used in experimenting but can be greatly hastened by the
proper treatment. By subjecting them to an alternating or pulsating
current the resistance can be increased rapidly.
To facilitate the maturing and enable the resistance of the cell to be
determined at various stages of the treatment the maturing and testing
set shown in Fig. 12 can be made use of. The apparatus allows of the
measurement of the cell by the Wheatstone bridge and substitution
methods both with the cell lighted and dark and permits the application
of alternating current as desired. It consists of a base on which
is conveniently mounted a closed lamp box to light the cell when
necessary and the various switches and binding posts arranged for quick
manipulation of the circuit.
[Illustration: FIG. 12. TESTING AND MATURING SET]
The details of the device are given in Fig. 13. The base measuring 20
by 12 inches has mounted at the rear center a wooden box containing a
75 Watt, type C, Mazda lamp. Where lighting current cannot be obtained
a 12 volt automobile headlight bulb may be used to illuminate the cell.
The lamp in either case is connected to the switch mounted just to the
left of the box.
The front of the lamp housing has a hole measuring 1 by ¾ inches cut in
it on a level with the lamp filament. Two clips cut from spring brass
of the shape shown in the detail drawing are mounted on the front of
the box in such a position that when the posts on the back of the cell
are slipped into the holes in the ends of the strips the window of the
cell will be opposite the opening in the lamp housing. Connection is
made to the cell by means of these clips.
[Illustration: FIG. 13. LAYOUT AND WIRING DIAGRAM OF TESTING AND
MATURING SET]
At the extreme left of the base a double pole fuse switch is mounted
to control the alternating current to treat the cell. Instead of fuses
two tubular incandescent lamps are screwed into the receptacles on the
switch to limit the A. C. to a value that will not endanger the cell
by overheating it. Should lighting current not be available use can be
made of a buzzer and telephone induction coil connected to a dry cell
as shown in Fig. 14 to furnish current for treating the cell.
[Illustration: FIG. 14. CONNECTIONS FOR BUZZER AND INDUCTION COIL TO
PRODUCE ALTERNATING CURRENT]
A small two point battery switch and four double spring binding posts
are also mounted on the base and wired as shown by the dotted lines.
A fairly sensitive galvonometer is employed to indicate when the
bridge is balance. A telephone receiver might be used for the purpose,
opening and closing the circuit to cause clicking in the receiver,
the bridge being balance when the noise is reduced to a minimum. The
small center zero ammeters with the shunt removed make excellent
galvonometers.
Ratio arms and a rheostat box having a maximum resistance of 100,000
ohms complete the apparatus necessary to make the tests on the cells.
A laboratory set with this range is rather expensive and since our
measurements need not be extremely accurate a good resistance box may
be made from resistance units as used for motor starting and signal
work. These units consist of an iron tube covered with asbestos on
which is wrapped the resistance wire, the wire in turn being covered
with a vitreous insulating material baked in place. These units can
be purchased quite reasonable from any large electrical supply house
in any resistance up to 150,000 ohms. For our purpose 15 units will
be required, five, 200 ohm; five, 2000 ohm and five, 20,000 ohm; all
tapped at the center, for the rheostat arm. For the ratio arms a
single 1000 ohm unit tapped at the center is used. The rheostat arm
units should be mounted in a box and heavy leads run to 12 single
pole switches mounted on the top of the box. The method of wiring
the resistances is shown in Fig. 15, the switches being marked so
the resistance in the circuit can be quickly determined. With this
arrangement any resistance from 100 to 111,000 ohms can be obtained in
steps of 100 ohms by opening the proper switches.
[Illustration: FIG. 15. CONNECTIONS FOR INTERIOR OF RHEOSTAT BOX]
The ratio arms are made by bringing leads from the ends and center
of the 1000 ohm unit. Ratios other than one to one are not advised,
for the cells are so sensitive to external influences that one system
of measurement must be adopted and adhered to if the cells are to be
compared. With the one to one ratio half the current flows thru the
cells when the bridge is balanced.
The current for testing the cells can be obtained from a dozen three
cell flashlight batteries. The battery should be connected to a
multipoint switch so that any number of cells can be switched into the
current as desired.
To prepare the various instruments for testing the cells with the
bridge circuit connect as shown in Fig. 16. The diagrammatic wiring
is shown in the insert in the illustration. The positive or carbon of
the battery is connected to post A. The source of alternating current
whether from the lightning mains or an induction coil is connected to
the double pole switch, current for the lamp in the enclosed box being
supplied to it thru the single pole switch.
[Illustration: FIG. 16. CIRCUIT FOR WHEATSTONE BRIDGE MEASUREMENT]
To test the cell, open both knife switches and place switch S on
the left hand point. Place the cell in the clips with the terminal
connected to the gold leaf in the clip connected to binding post
A. This makes the gold leaf the anode of the cell. Now close the
battery switch applying about 6 volts to the bridge. Balance the
bridge by adding or removing resistance in the rheostat arm until the
galvonometer gives no deflection. The resistance of the cell in the
dark is read direct from the markings on the open switches on the
rheostat box.
Now close the single pole knife switch lighting the cell and again
balance the bridge. The latter reading will be the light resistance of
the cell, gold anode.
For convenience in recording the data a sheet of paper should be ruled
into columns headed, date, voltage applied, resistance dark, resistance
lighted (gold anode), resistance dark, resistance lighted (copper
anode) and remarks. The cells should be numbered and the corresponding
data sheet headed with the same number so that changes in the cell can
readily be detected and improvements noted. A separate sheet should be
provided for each cell.
Now increase the voltage to 9 volts and take another set of readings
with the cell both dark and lighted. The values obtained are recorded
and the voltage increased further. The cell is tested with gradually
increasing voltages till the maximum voltage possible without heating
the cell has been applied. Heating of the cell can usually be noted by
a faint crackling noise being given but it is advisable to feel the
surface of the mica occasionally to detect any heating.
It may be found on examining the data obtained that the resistance of
the cell has varied with different voltages. We can classify the cells
by calling a cell in which the resistance increases with an increasing
voltage the A type, those in which the resistance falls off with an
increase of voltage the B type and those in which changes of voltage
cause little or no change of resistance the C type.
Now reverse the cell in the clips making the copper plate the anode and
repeat the above series of tests and record the values in the proper
columns on the ruled sheet. It will be found in many cases that the
reversal of the current thru the cell has increased the resistance. A
cell showing this characteristic is polarized while one in which the
reversal of the current does not alter the resistance to any great
extent is non-polarized.
The cell is now to be treated with alternating current for a period of
five minutes by opening the battery switch, cutting the galvanometer
out of the circuit and closing the double pole switch. Current will
flow thru the ratio arms and the cell. Care should be taken that the
current is not heavy enough to heat the cell excessively. If this
occurs reduce the current by adding resistance till heating is not
detected.
The cell is again tested according to the directions already given
both for its light and dark resistances with the current flowing in
both directions thru the cell. If the difference between the dark and
light resistance has increased it is safe to say that the cell will be
sensitive to light and the treatment should be continued to develop
this property. The quickest method of doing this is to select from the
various readings the voltage that gives the greatest difference in the
dark and light resistance. Use this voltage to make one test after each
treatment with A. C. till the sensitiveness of the cell to light has
reached a fair value. When the resistance in the dark is ten times as
great as in the light the cell is suitable for experimental working and
treatment can be discontinued.
Should the cell have practically no resistance when first put in the
testing set it is useless to go thru the entire series of tests till
the resistance has been raised. Treat the cell with A. C. repeatedly
till the resistance is brought up to a fair amount, at least 500 ohms,
then make the tests outlined previously.
Sometimes even prolonged treatment will fail to raise the resistance,
in this case test the cell for polarization by reversing in the clips.
If strongly polarized the cell may be a good generator, i.e., will
give a current under the influence of light. To test this, connect the
galvonometer to posts A and D of the set, put battery switch on left
hand point and light the cell by closing the single pole knife switch.
If the galvonometer gives a deflection it indicates that current is
being generated in the cell. This cell should be reserved for use as
a generator, this property being increased by short circuiting the
terminals and exposing the cell to light periodically until it will
generate a fair amount of current. The current generated by these cells
is a true photo-electric current, no chemical action taking place, the
light rays being converted directly into electricity by some unknown
action of the cell. The current flows from the copper plate thru the
external circuit back to the gold foil terminal.
Should the cell after prolonged treatment with A.C., fail to increase
in resistance or having increased in resistance and remain insensitive
to light and not prove to be a current generator the cell must be
classed as useless. It is seldom indeed that this occurs for a cell
that shows even a slight change in resistance when lighted should be
treated with A.C. from time to time and used in experiments and will
eventually increase in sensitiveness sufficiently to be of value. When
a cell proves intractable, the gold foil may be carefully removed by
means of a stiff brush and the cell put back in the hot press to be
reconstructed. Retreating with heat and pressure will often give a very
sensitive cell.
Even when the cells are being used for experimental work the
maturing process will be going on. The point of maturity and maximum
sensitiveness of any cell has been reached when the dark resistance of
the cell remains constant over a period of time. The maturing seems to
affect the cell by increasing the dark resistance, the light resistance
remaining practically the same thruout. From this it will be seen that
a cell with a very low resistance when first made may under proper
treatment become extremely sensitive to light.
As previously mentioned the testing set may be used for measuring the
resistance of the cells by the substitution method. The apparatus is
connected as shown in Fig. 17 when so used, a diagrammatic circuit
being given in the insert. When the battery switch is on the left hand
point the cell is in series with the battery and galvonometer. When
the switch is moved to the right hand point the rheostat is in series
with the battery and galvonometer. The method of measuring is to first
determine the galvonometer deflection with the cell in the circuit and
then by switching the rheostat into the circuit and adjusting it so
the galvonometer gives the same deflection enabling the resistance of
the cell to be read directly. The same series of tests should be gone
through with this method as with the bridge circuit.
[Illustration: FIG. 17. CIRCUIT FOR SUBSTITUTION METHOD OF MEASUREMENT]
The substitution method has the advantage of indicating roughly
the sensitiveness of the cell to light. With the cell darkened and
the current flowing thru it the galvonometer will give a certain
deflection, on lighting the cell the deflection will increase
indicating a lowering of the resistance. The determination of cell
sensitiveness is more rapid and this circuit can be employed when the
cell has a low resistance at first and it is necessary to increase it
before tests can be made. With the substitution method of measurement
it is advisable to use a rather insensitive galvonometer or meter since
the current at times may reach a value that would damage a delicate
instrument. A tangent galvonometer with heavy windings will be found
most suitable.
To determine if a cell is sensitive enough to close a relay the
substitution circuit may be used with a relay connected in place of the
galvonometer. If the cell is suitable for the purpose the relay will
close when the cell is lighted.
[Illustration: FIG. 18. SHOWING CELL READY FOR ASSEMBLY WITH FOUR
COMPLETED CELLS IN BACKGROUND]
For practical purposes the sensitiveness of a cell to light and its
current generating properties are the only ones of value but other
properties of these cells will be mentioned later. These may be
developed by proper treatment and the results of experiments along
these lines can hardly be foreseen with any degree of accuracy at the
present stage of the development of the cells.
When satisfied that a cell is properly made and fairly sensitive it can
be permanently sealed by pouring melted sealing wax around the edges
and smoothing with a hot knife. A small sheet of paper should be pasted
on the back of the cell with the more important characteristics marked
thereon as well as the most suitable voltage. The terminal connected to
the gold foil can be indicated by a positive (+) sign for convenience
in properly connecting the cell into the circuit.
CHAPTER V
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.
CHAPTER VI
THE CARE OF SELENIUM CELLS
A selenium cell will give much better service and have a longer life
if a little care is taken with it. These suggestions if followed will
prevent to a great extent irreparable damage to the cells.
Keep them cool. Do not allow them to become heated to any extent or the
gold foil will combine with the selenium to form a gold selenide and
destroy the sensitiveness of the cell. This condition will be indicated
by dark brown splotches on the face of the cell.
Use as small current values as possible to accomplish a desired result.
This demands the use of high resistance relays. Resistances may be
employed in the circuit to limit the current but a high resistant relay
is the proper solution.
Use the proper voltage as determined by the testing set.
Do not expose light sensitive cells to bright light for excessively
long periods of time. It fatigues the cell and causes it to become
insensitive temporarily.
When using generator cells do not pass a current from an outside source
thru them while lighted. This destroys temporarily the generating
powers of the cell.
Keep the cells dry. If not of the sealed type keep them in a box having
a few lumps of calcium chloride in the bottom to absorb moisture.
When not in use they should be kept in a light tight box but should be
exposed to light each day or so regardless of whether they are used or
not. This aids maturing and retains the sensitiveness of the cells.
Should their resistance drop to any great extent after continued use
it can again be raised by treating with an alternating or pulsating
current as described under treating and testing cells.
THE END
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