<h2><SPAN name="CHAPTER_XX" id="CHAPTER_XX"></SPAN>CHAPTER XX.</h2>
<h3>SOME CURIOSITIES.</h3>
<p>Until within recent years it was never supposed
that a sunbeam would ever laugh except
in poetry. But the modern scientist has taken
it out of the realm of poetry and put it into the
prosy play of every-day life. The Radiophone,
invented by A. G. Bell, is an instrument by
which articulate or other sounds are transmitted
through the medium of a ray of light. It
has as yet no practical application and has
never gone beyond the experimental stage, but
as a bit of scientific information it is very interesting.</p>
<p>If we introduce into an electric circuit a
piece of selenium, prepared in a certain way,
its resistance as an electric conductor undergoes
a radical change when a beam of sunlight
is thrown upon it. For instance, a selenium
cell, so called, that in the dark would measure
300 ohms resistance, would have only about
150 ohms when exposed to sunlight. This
amount of variation in a short circuit of low
resistance would produce a considerable change<span class="pagenum"><SPAN name="Page_172" id="Page_172"></SPAN></span>
in the strength of a current passing through
it from a battery of a given voltage.</p>
<p>If now we connect a selenium cell to one
pole of a battery, and thence through a telephone
and back to the other pole, we have completed
an electric circuit, of which the selenium
cell is a part, and any variation of resistance
in this cell, if made suddenly, will
be heard in the telephone. Let the diaphragm
of a telephone transmitter have a very light,
thin mirror on one side of it, and a beam of
sunlight be thrown upon it and reflected from
that on to the selenium cell, which may be
some distance away. Then, if the diaphragm
is thrown into vibration by an articulate word
or other sound, the light-ray is also thrown
into vibration, which causes a vibratory change
of resistance in the selenium cell in sympathy
with the light-vibrations; and this in turn
throws the electric current into a sympathetic
vibratory state which is heard in the telephone.
So that if a person laughs or talks or
sings to the diaphragm, the sunbeam laughs,
talks and sings and tells its story to the electric
current, which impresses itself upon the
telephone as audible sounds—articulate or
otherwise. Instead of the telephone, battery
and selenium cell, a block of vulcanite or certain
other substances may be used as a receiver;
as a light-ray thrown into vibration<span class="pagenum"><SPAN name="Page_173" id="Page_173"></SPAN></span>
has the power to produce sound or sympathetic
vibration in certain substances.</p>
<p>Another curious application of the selenium
cell has been attempted, but has scarcely
gone beyond the domain of theory. This
apparatus, if perfected, might be called a
Telephote. It is an apparatus by which an
illuminated picture at one end of a line of
many wires is reproduced upon a screen at
the other end. The light is not actually transmitted,
but only its effects. Suppose a picture
is laid off into small squares and there is a
selenium cell corresponding to each square
and for each selenium cell there is a wire that
runs to a distant station in which circuit there
is a battery. At the distant station there are
little shutters, one for each wire, that are controlled
by the electric current and so adjusted
that when the cell at the transmitting-end is
in the dark the shutter will be closed. Now if
a strong light be thrown upon the picture at
the transmitting-end, and each square of the
picture reflects the light upon its corresponding
selenium cell, the high lights of the
picture will reflect stronger light than the
shadows, and therefore the wires corresponding
to the high-light squares will have a
stronger current of electricity flowing through
them, because the resistance of the circuit is
less than the ones connected with the darker
shadows. So that the degree of current-<span class="pagenum"><SPAN name="Page_174" id="Page_174"></SPAN></span>strength
in the various wires will correspond
to the intensity of light reflected by the different
sections of the picture. The shutters
are so adjusted that the amount of opening
depends upon the strength of current. The
shutters corresponding to the high lights of
the picture will open the widest and throw the
strongest light upon the screen, from a source
of light that is placed behind the shutters.
The shutters that open the least will be those
that are operated upon by the shadows of the
picture. Inasmuch as a picture thrown on a
screen from a source of light is wholly made
up of lights and shadows, the theory is that
this apparatus perfectly constructed would
transmit any picture to a distance, through
telegraph-wires. It must not be understood
that the rays of light are transmitted through
the wires as sound-vibrations are. Light, per
se, can be transmitted only through the luminiferous
ether, as we have seen in the chapter
on light in Volume II.</p>
<p>While we are talking about these curious
methods of telegraphic transmission, I wish to
refer to an apparatus constructed by the writer
in 1874-5, for the purpose of receiving musical
tones or compositions transmitted from a distance
through a wire by electricity. (A cut
of this apparatus is shown on page 875 of
"Electricity and Electric Telegraph," by
Prescott, issued in 1877.) It consists of a disk<span class="pagenum"><SPAN name="Page_175" id="Page_175"></SPAN></span>
of metal rotated by a crank mounted on a
suitable stand. The electric circuit passes
through the disk to the hand of the operator
in contact with it, thence running through the
line-wire to the distant station. Now, if a
tune is played at that station, upon an electrical
key-board, as described in a previous
chapter, and the disk rotated with the fingers
in contact with it, the tune or other sounds will
be reproduced at the ends of the fingers. After
the telephone was invented and put into use I
used this revolving disk as a receiver for
speech as well as music, and by this means
persons may carry on an oral conversation
through the ends of their fingers. This apparatus
has been confounded in the minds of
some people with Edison's electromotograph.
The phenomena of the electromotograph were
produced by chemical effects, while that of the
apparatus just described is electrostatic in its
action. The electrostatic disk was made in
the winter of 1873-4, while Edison's electrochemical
discovery was made some time later.</p>
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