The advantage gained by this form of construction is the thin film of selenium obtainable due to there being no spaces between the electrodes into which the selenium can flow. A cross section of this type is given in the illustration. It will be seen that although the main defect of the cells mentioned previously has been reduced to some extent still it has not been removed entirely.
[The Mercadier cell] is similar in many respects to the Bell but is easier of construction. Two strips of thin copper one half inch wide are wound into a spiral being separated from each other by strips of mica. One face of the flat spiral is filed flat and then polished. The selenium is melted onto the cell and then smoothed off with a strip of mica. The cell then being annealed as described previously.
We have here a condition analogous to that in the Bell cell, the only advantage being ruggedness and simplicity against a loss in active surface area. For experimental purposes this cell is entirely satisfactory for if it does not prove sensitive the selenium coating can be filed off and another applied.
Fig. 4. Mercadier Cell
We come now to the consideration of cells wherein the current flows at right angles to the surface of the selenium. This implies the use of at least one electrode on the surface of the selenium. The Gripenberg cell has both electrodes on the surface. As shown in [Fig. 5] the electrodes are made by depositing a thin film of gold on a glass plate and with a sharp tool removing narrow strips of it to form a grid, alternate bars of which are connected to the same terminal. The grid arrangement is shown in the detail illustration.
CROSS SECTION OF ASSEMBLED CELL