Fig. 6. Fritts Cell
We have here the ideal condition. All the current must flow thru the light affected area in passing from one electrode to the other. The light passes thru the semi-transparent gold foil to effect the change in resistance of the selenium film. The only disadvantage lies in the diminishing of the light strength by the gold film. Despite this however the Fritts cell has proved the most sensitive cell made having in one case a ratio of 337 to 1, that is, the resistance in the light is but ³/₁₀ths of one per cent of that in the dark.
The six types of cells described cover all the types worthy of special mention and will enable one to select a cell for experimental purposes. In view of the fact that the Gripenberg and Fritts cells are superior in point of design the selection of the cell becomes a question of mechanical difficulties to be overcome. The construction of the grid in the Gripenberg type is rather difficult unless an engraving machine is obtainable whereas the Fritts cell requires very simple apparatus for its construction.
A factor often overlooked in considering the design of selenium cells is the relation that light and electricity bear to each other. These are manifestations of the same force and their interaction can be taken advantage of in the Fritts cell due to it being possible to cause the current to flow in the same or opposite direction to that of the light vibrations. The importance of this will be covered in detail later.
No mention has been made of just how the light affects the selenium to reduce its resistance. This is still a moot point, one theory being that the light being electromagnetic in nature causes the molecules of the selenium to cohere in a manner similar to that of the radio coherer used in the early days of radio telegraphy. However the conduction thru a selenium cell is similar to that in an electrolyte and differs from metallic conduction. Considering this the light may act to ionize the selenium in some manner and make possible the more rapid interchange of the ions from the opposite terminals. Although a thorough understanding of the action taking place may lead to the improvement of selenium cells it is not within the scope of the present work to consider the various phases of this part of the problem, confining itself as it does more to the practical production of the cells.
CHAPTER III
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.