THE THERMOPHONE. THE SELENIUM CELL. THE SPEAKING ARC.
The telephone is an instrument for the transmission of sounds to a
distance by the agency of electricity, wherein the speaker talks to an
elastic plate of thin sheet-iron, which vibrates and transmits its every
movement, electrically, causing it to vibrate in an identical manner and
emit the same sounds.
The transmission of the vibrations depends upon well known principles of
electricity, and consists, not of an actual transmission of the sounds,
but the passage of electric waves, or impulses, which keep perfect
accord and agree in phase and period with the atmospheric waves produced
by the voice. These in turn, through the medium of an electromagnet,
cause vibrations of a plate or membrane, which agitates the air in a
manner similar to the original disturbance, and thus emits sounds.
The parts of the apparatus which take up the sound waves and change them
into electric currents compose the transmitter. In the form of
transmitter most commonly used, the motions of the diaphragm cause
variations in the strength of a current flowing from a battery by
varying the resistance in the path of the electric current.
The sounds are directed to the mouthpiece, which causes the vibrations
of the air to strike the diaphragm, on the back and center of which is
fastened a small cup shaped piece of carbon. A second cup is mounted in
a rigid position directly in back of the first. The space between is
filled with small polished granules of carbon.
[Illustration: FIG. 131.—Diagram of a telephone transmitter.]
When these are in a perfectly free and loose state their resistance to
an electric current is very great, and they allow almost none to flow.
When slightly compressed their resistance is greatly lowered, and they
permit the current to pass. The vibrations of the diaphragm exert a
varying pressure upon the granules, with a corresponding variation in
their resistance and the amount of current flowing.
[Illustration: FIG. 132.—Diagram showing the principle and
construction of the telephone receiver.]
The receiver, as has already been explained, consists of a thin iron
disk, placed very near but not quite touching the end of a small bar of
steel, permanently magnetized, about which is wound a coil of fine
insulated wire. The ends of this coil are connected to the wires leading
from the transmitter and battery. The varying currents of electricity,
produced by the transmitter, generate corresponding changes in the
magnetism of the receiving instrument, and thus, by alternately
attracting and repelling the diaphragm, cause it to vibrate and emit
sounds.
[Illustration: FIG. 133.—The photophone.]
Alexander Graham Bell, the ingenious inventor of the telephone, with the
aid of Sumner Tainter was the first who achieved success in the attempts
to transmit speech without the aid of connecting wires between the
source of intelligence and the receptor.
In 1873 Willoughby Smith announced that the element selenium possesses
the abnormal property of changing its electrical resistance under the
influence of light. Bell and Tainter took advantage of this discovery,
and devised selenium cells, in which selenium is formed into narrow
strips between the edges of broad conducting plates of brass. The
resistance of the cell in the darkness is approximately twice the
resistance when illuminated.
[Illustration: FIG. 134.—Photophone receiving apparatus.]
[Illustration: FIG. 135.—Photophone transmitting apparatus, using
acetylene flame to furnish light.]
This property of the cell was immediately applied to the construction of
the photophone, an instrument which transmits sounds to a distance by
means of a beam of light reflected to a distant spot from a thin mirror
thrown into vibration by the voice. Over fifty different forms were
devised but the most successful consisted of a transmitter composed of a
glass disk, silvered to reflect a pencil of light focused from the sun,
or an arc lamp. This glass disk was used as a diaphragm similar to that
of an ordinary telephone transmitter, except that the rear side of it
was made free to reflect the beam of light. Bell used for this purpose
disks about two inches in diameter and the thickness of ordinary paper.
The receiver consisted of a parabolic reflector, with a selenium cell
placed at its focus. In series with the cell was placed a battery and
telephone receiver.
When the membrane was set into vibration by the sound waves, it became
alternately concave and convex, the normally parallel rays of light
correspondingly converging and diverging. The receiving station was thus
under the influence of light rays of rapidly varying intensity in
perfect phase with the vibrations of the voice. The reflector
concentrated the rays on the selenium cell, and their varying strength
changed its resistance and caused a pulsating current to flow through
the receiver and reproduce the speech produced at the transmitter.
In another arrangement employed by Bell and Tainter, they used the rays
of a powerful electric arc lamp, and by varying the electric current
supplying the arc caused the light to fluctuate and produce the same
results at the receiver.
These ingenious inventors also devised a method of transmitting speech
called the thermophone. The transmitter remained the same as in the
photophone–a thin silvered membrane, or glass diaphragm, stretched
across the back of a mouthpiece, and arranged to reflect the rays of the
sun, or the light of an arc lamp.
The receiver was a small glass bulb containing a plate of mica covered
with lampblack, or little charred pieces of cork. The glass bulb was
placed in the focus of a reflector, which collected the rays and
concentrated them. The variations in the intensity of the heat
radiations caused the air in the bulb to expand or contract with each
vibration. Rubber tubes extended from the bulb to the ears of the
observer, and the pulsations of air, traveling through the tube as sound
waves, would strike the ear-drum and reproduce the speech.
[Illustration: FIG. 136.—Powerful searchlight arranged to transmit
speech over a beam of light.]
Both of these methods were later very much improved by the employment of
Koenig's manometric flame in place of the silvered mirror as a
transmitter. As explained in the last chapter, speech delivered into the
mouthpiece causes the gas to become compressed or rarefied in direct
accordance with the sound waves, and the flame rises and falls with a
rapidity too great to be detected by the naked eye. These rapid
alterations in the intensity of the illumination of the flame act on the
selenium cell, and reproduce the original voice perfectly in the
telephone receiver.
[Illustration: FIG. 137.—The Electric arc.]
It is obvious, however, that such methods as these are only capable of
transmitting speech over very limited distances, and if greater ranges
are to be traversed much more powerful transmitters must be employed for
the purpose.
Ernest Ruhmer, after long and laborious researches, finally succeeded in
transmitting speech many miles by taking advantage of the speaking arc,
discovered by Simon, who observed that an arc lamp gave out a loud
rattling noise if its current supply was interfered with. An electric
arc consists of two carbon rods, connected to a generator. When the
carbons are brought into contact for a moment and then drawn apart to a
short distance, a kind of electric flame or arc is produced between the
points of carbon, and a brilliant white light is emitted by the white
hot points of the carbon electrodes.
Ruhmer immediately made the arc serve as a telephone receiver and speak
by utilizing the pulsating current of a telephone transmitter to vary
the current supplying the arc.
[Illustration: FIG. 138.—Circuit showing how a singing arc is arranged.]
The arc could thus be made to sing, whistle or reproduce music and the
human voice perfectly, if the sounds were clearly conveyed into the
transmitter. Further investigations showed that every alteration of
current caused by the action of the transmitter also caused an
alteration in the intensity of the light and radiation of the lamp. The
speaking arc could therefore be used as a perfect photophone transmitter
by directing the rays toward the receiving station with the aid of a
parabolic reflector.
In 1902 Ruhmer performed a series of experiments in Germany on the
Wannsee, near Berlin. A large motorboat, the Germania, was fitted with
an electric searchlight connected with a microphone, so as to form a
speaking arc. The receiving station was located on the shore, so that
the distance between the stations could be easily enlarged by moving the
boat.
The receptor was an ordinary selenium cell, placed at the focus of a
large reflector and connected with a telephone receiver and battery. No
difficulty was encountered in reproducing the speech over varying
distances up to about three miles.