SOME CURIOSITIES.
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.
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 in the strength of a current passing
through it from a battery of a given voltage.
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 has the power to produce sound or sympathetic vibration in
certain substances.
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-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.
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 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.