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.
CROSS SECTION OF ASSEMBLED CELL
