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.

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 ?th 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 ? 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.

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 ? 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.

The fibre sheets to enclose the finished cell are shown in Fig. 10. These are cut to the dimensions shown from ? 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 ? 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.

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.