To realize the importance of the above factors a description of the various types of cells developed by the many investigators in this field will be of great assistance. The different workers made use of various arrangements of the electrodes but the cells fall into certain classes. These types have been named after the inventor or the one most prominent in the work on them.
The Bildwell cell is possibly the best known type. It is made by winding two bare wires of copper, brass, german silver or platinum on a sheet of mica or slate. The wires are spaced about ¹/₃₂nd of an inch apart. The size of the wire is of little importance, the usual practice being to use #28 wire on a form measuring two by one inches. In [Fig. 1] is shown this type of construction using a mica form, the wires being fastened by passing them thru holes at the ends of the sheet.
Fig. 1. Bildwell Cell
The selenium is applied to the cell by melting it over the wires. The cell is laid on a mica covered copper plate supported over a bunsen burner. The temperature of the cell is raised to the point where a stick of selenium when touched to the cell melts. The entire surface of the cell is coated with the selenium in a very thin layer, smoothing out the lumps with a sheet of mica or a steel knife. To get a satisfactory coating the temperature must be regulated closely, if too low the selenium turns grey and the temperature must be increased to melt it, if too high the selenium collects in drops due to surface tension and is as difficult to spread as mercury. The proper state is a semi-fluid condition which it attains at 220° C when it can be easily manipulated.
When a satisfactory surface has been obtained the cell is transferred to a copper plate to cool while the bunsen burner is turned down to give a temperature of 120° C. When cool the cell is replaced on the hot copper plate and allowed to heat up again. Shortly the whole surface will turn grey in color due to the selenium crystallizing. The temperature is now slowly increased till the selenium shows signs of melting, this will be indicated by the edges turning black. The bunsen burner is immediately withdrawn and the edges allowed to recrystallize. The burner is turned down a trifle and replaced under the hot plate. The cell is watched carefully for signs of melting and if none appear it is left so for three or four hours. If it melts again the burner should be further lowered, just sufficient to keep the cell a trifle below the melting point of the selenium. The cell is then allowed to cool by lowering the burner by small amounts extending over a period of an hour. This prolonged heating and slow cooling is known as annealing.
After the above treatment the cell is complete save for mounting. The usual method is to mount the cell in a small wooden box fitted with a glass window to admit the light, leads being brought from the electrodes to two binding posts mounted on the box. This protects the cell from moisture and dust.
It will be apparent that with the above method of construction it is impractical to get the extremely thin layer of selenium necessary if the light is to affect a relatively large proportion of the total area. This will be even more clear from an examination of the cross sectional view of this type of cell as shown in [Fig. 1]. Here we have a comparatively thick layer of selenium bridging the space between the wires. Of this layer only the thin surface film facing the light drops in resistance while the interior part is unaffected. This means that should the surface layer drop to even ¹/₅₀₀th of its dark resistance the total drop of the cell would be much less.
The Ruhmer cell is similar in construction to the Bildwell, differing only in the form of support. A porcelain or glass tube is used to support the parallel wires as shown in [Fig. 2]. When porcelain is used the constructor can fasten the wires at the ends by slipping them into slots cut with a hack saw. With glass some other means are necessary to hold the wire while winding.