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.
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.