A selenium actinometer has been described in the _Comptes Rendus_ in a
communication from M. Morize, of Rio de Janeiro. The instrument is used to
measure the actinic power of sunlight when the sun is at various
altitudes; but the same principle is applicable to other light sources.
The sensitive part of the apparatus consists of a cylinder formed of 38
disks of copper, isolated from each other by as many disks of mica. The
latter being of smaller diameter than the copper disks, the annular spaces
between the two are filled with selenium, by the simple process of rubbing
a stick of this substance over the edges, and afterward gently warming.
The selenium then presents a grayish appearance, and is ready for use.
Connection is made by conductors, on opposite sides, with the odd and even
numbers of the disks, which diminishes the resistance of the selenium. The
cylinder thus formed is insulated by glass supports in the inside of a
vacuum tube, for the purpose of preserving it from the disturbing
influence of dark rays. The whole is placed upon a stand, and shielded
from reflected light, but fully exposed to that which is to be measured
for actinic intensity. If now a constant current of electricity is passed
through the apparatus, as indicated by a galvanometer, the variations of
the latter will show the effect produced upon the selenium. A scale must
be prepared, with the zero point at the greatest possible resistance of
the selenium, which corresponds with absolute darkness. The greatest
effect of the light would be to annul the resistance of the selenium.
Consequently, the cylinder must be withdrawn from the circuit to represent
this effect; and the maximum deviation of the galvanometer is then to be
observed, and marked 100. By dividing the range of the galvanometer thus
obtained into 100 equal parts, the requisite actinometric scale will be
established. In practice, the Clamond battery is used to supply the
constant current required.
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