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.