Fig. 2. Ruhmer Cell
The selenium is applied to the cell in the same manner as the Bildwell and then annealed. The method of mounting the cells as devised by Ruhmer is worthy of mention. The cell unit is enclosed in a glass tube and then the air exhausted. By attaching the leads to an incandescent lamp base a very convenient arrangement results. The cell is well protected from all external influences and is therefore more stable and reliable. This is perhaps the most important improvement in this type of cell.
We have in the Ruhmer cell conditions almost identical to that in the Bildwell, namely a large area of conducting selenium that is beyond the range of the light and hence not affected thereby. This is offset to a certain extent by the large area exposed to the light as these cells can be employed with a parabolic reflector to cause the light to fall on all sides. This type of cell was employed by Ruhmer in his experiments with the Photophone. He succeeded in transmitting speech for a distance of four miles using a speaking arc at the transmitting station.
The cell developed by Bell and Taintor in their experiments is rather novel in the arrangement of the electrodes. As shown in [Fig. 3] the electrodes take the form of brass disks separated by thin mica disks supported by two brass rods, the whole being clamped together by nuts on the ends of the rods. The disks are one inch in diameter, eighteen or twenty being sufficient for a small cell. By drilling the holes in the disks of different sizes and assembling them as shown it is possible to have alternate disks connected to the same rod. After assembling and clamping the cell skeleton is chucked in a lathe and the surface turned smooth and polished.
Fig. 3. Bell and Taintor Cell
The selenium is applied by heating the form and melting it on, by rolling the cell back and forth over the hot plate it is possible to get an extremely thin film of selenium on the smooth surface offered by the cell. The coating is then annealed in the regular manner. This cell can well be mounted in a glass tube and the air exhausted.