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.
Fig. 11. Mode of Assembling Cell