THE CHEMICAL NEWS,
EDITED BY WILLIAM CROOKES, F.R.S., &e.
No. 1127.—JULY 1, 1881.
SELENIUM AND ITS APPLICATIONS TO THE
PHOTOPHONE AND TELEPHOTOGRAPHY.*
By SHELFORD BIDWELL, M.A, LL.B, M.R.I.
BEFORE entering upon my subje®, I must claim your in.
dulgence upon two grounds. A week ago I had not the
remotest idea that I was to have the honour of addressing
you here this evening; the time which I have had for pre.
paration has, therefore, been exceedingly limited. Inthe
second place, it is my desire (in accordance with the
traditions of this Institution) not merely to give a description
of the experiments in which I have for the last few
months been engaged, but, as far as possible, to reproduce
them before vou. Now,these experiments are mostly of a very
delicate nature. In the quiet of a laboratory—where time
is praically unlimited, and where an operation, if it
should fail at first, may be repeated an indefinite number
of times—success is tolerably certain to be finaily ob.
tained ; but in exhibiting delicate experiments before an
audience, one is working under the most unfavourable
conditions, and, in case of failure in the first instance, the
attempt cannot generally be repeated. Moreover, the
substance with which we are chiefly concerned, selenium,
is apparently extremely capricious in its behaviour, This
appearance is, of course, really due to our present ignorance
of its properties; but the fa& remains that, on ac.
count of the great uncertainty of its action, it is a very
difficult substance to deal with.
Selenium is a rare chemical element which was
discovered in the beginning of the present century. In
many of its properties it closely resembles sulphur, and,
like sulphur and some other substances, it is capable of
existing in more than one form.
The ordinary form is that called vitreous. Selenium in
this condition is as absolutely structureless as glass, and
in appearance resembles nothing so much as bright black
sealing-wax, with, perhaps, somewhat of a metallic lustre;
its real colour, however, when seen in thin films, is ruby
red, Its melting-point is a little higher than 100° C. In
its second modification selenium is crystalline. When in
this form its surface is dull, its fracture is metallic (not
unlike that of cast-iron), its colour is grey or leaden, and
it is quite opaque to light; its melting-point also is considerably
higher, being 217° C.
Vitreous selenium, if melted and kept for a certain
length of time at a temperature between its own fusing.
* A Paper read before the Royal Institution of Great Britain
Friday, March 11, 1881.
| point and that of crystalline selenium, will crystallise;
and I think I am right in saying, from casual observation,
though I have made no experiments to verify the point,
that the length of time necessary for crystallisation depends
upon the degree of temperature, being proportionately
shorter as the temperature approaches 217° C,
Vitreous selenium is an exceedingly bad conducor of
ele@ricity ; it is, indeed, an almost perfe& insulator,
Crystalline selenium is a moderately good condu&or, and
it possesses this very remarkable property, which has been
1tilised in the photophone and other inventions, that it
conduéts better in the light than in the dark, the change
in its resistance to the passage of a current of ele@ricity
through it varying, according to Professor W. G. Adams,
as the square root of the illuminating power.
Let a galvanometer be conneéed to the two poles of a
battery by means of two copper wires. The passage of a
current of electricity will at once be denoted by the deflec.
tion of the magnetic needle ; or, if a little mirror is attached
to the needle, and a beam of light be reflected from
it upon a scale, the movement of the spot of light will
ndicate the movement of the needle. Let, now, one of
the wires be cut, and the two ends be joined together by
a piece of crystalline selenium, The spot of light will
again move, but its deflection will be very much less than
it was before, showing that the resistance of the selenium
is very much greater than that of the wire. Moreover, if
the piece of selenium be alternately exposed to and
screened from a beam of light, the deflection will be greater
when it is in the light than when it is in the dark, showing
a corresponding variation in its resistance. This remarkable
property of selenium was first announced and exhibited
by Mr, Willoughby Smith in 1873. But the effets
produced by the simple arrangement which I have just
described are small, and very delicate instruments are required
for their observation.
Since that date several devices have been proposed for
exaggerating the effe@, but they all depend upon the fa&
that the amount of the variation increases with the extent
of the selenium surface acted upon. It has lately been the
fashion to call these arrangements ‘* cells,” which, in most
cases at all events, seems to be a very inappropriate name.
It has been suggested that they should be termed
i rheostats,” a name which well expresses the purposes
for which they are generally used, and is less likely to lead
to confusion than the other. In deference to custom,
however, I shall call them by the usual name.
The simplest selenium cell which could be devised
would be made by placing two short pieces of copper wire
parlalel to each other, and very near together, and con-
