state selenium is practically a non-conductor of
electricity, its resistance being forty thousand million times greater than
copper. Its practical value lies in the property which it possesses, that
when in a prepared condition it is capable of varying its electrical
resistance according to the amount of light to which it is exposed, the
resistance decreasing as the light increases.

Selenium is prepared by heating it to a temperature of 120deg C., keeping
it there for some hours, and allowing it to cool slowly, when it assumes a
crystalline form and changes from a bluish grey to a dull slate colour. A
selenium cell in its simplest form consists merely of some prepared
selenium placed between two or more metal electrodes, the selenium acting
as a high resistance conductor between them. The form given by Bell and
Tainter to the cells used in their experiments is given in Figs. 53 and
53a. It consists of a number of rectangular brass plates P, P', separated
by very thin sheets of mica M, the mica sheets being slightly narrower than
the brass plates, the whole being clamped together in the frame F by the
two bolts B. {110} By means of a sand-bath the cell is raised to the
desired temperature, and selenium is rubbed over the surface, which melts
and fills the small spaces between the brass plates. All the plates P are
connected together to form one terminal, and the plates P' to form the
other. By using very thin mica sheets, and a large number of elements, a
very narrow transverse section of selenium, together with a large active
surface, can be obtained.

The cell used for commercial purposes is usually constructed as follows. A
small rectangular piece of porcelain, slate, mica, or other insulator, is
wound with many turns of fine platinum wire. The wire is wound double, as
shown in Fig. 54, the spaces between the turns being filled with prepared
selenium. A thin glass cover is sometimes placed over the cell to protect
the surface from injury.

[Illustration: FIG. 53.

P, P', plates; M, mica; S, selenium.]

[Illustration: FIG. 53a.]

A strong light falling upon a cell lowers its resistance, and _vice versa_,
the resistance of a cell being at its highest when unexposed to light; the
light is apparently absorbed and made to do work by varying the electrical
resistance of the selenium. Selenium cells vary very considerably as
regards their quality as well as in their electrical resistance, it being
possible to obtain