The applications of any of the wireless telephone systems thus far
described are very limited, for at the best they only operate under the
most favorable conditions, and then over rather limited distances. In
the case of any system whereby the speech must be transmitted over a
beam of light, the great resulting limitations are that the transmission
can only take place in a straight line over water or clear country, and
that stormy weather or a fog will interrupt communication.
None of these objections are present, however, when recourse is had to
Hertzian, or electromagnetic waves. Wireless transmission of speech has
therefore followed in the wake of wireless telegraphy, and the methods
and apparatus employed are very similar.
Some who have followed the text closely might reasonably ask why it
would not be possible to establish wireless telephony by simply
connecting a telephone transmitter in some manner to an ordinary
wireless telegraph, and by directing speech into the latter, vary the
strength of the oscillations emitted.
Such a system, at first thought, seems very plausible, and many
experimenters have devised countless methods trying to attain this
result, only to meet with ultimate failure. The reason is very simple.
Suppose that an induction coil, having a high-speed interruptor, and
therefore able to produce a very rapid stream of sparks at the gap, is
connected to the aerial and ground in the usual manner and a telephone
transmitter placed in series with the ground wire. When the coil is set
in operation the sparks jump across the gap, each spark setting up a
train of oscillations. If speech is conveyed into the transmitter, the
resistance in the path of the oscillations will be varied and
correspondingly also the strength of the waves emitted. The sounds will
be reproduced to a certain extent by the receptor. Whistling, certain
musical tones, and words containing many vowels are sometimes heard in
the receptor, with sufficient distinctness to be recognizable. The voice
cannot, however, be heard at all times, and the system is of no real
value other than an interesting experiment.
[Illustration: FIG. 139.—A "logical" form of wireless telephone which
is impracticable.]
[Illustration: FIG. 140.—De Forest wireless telephone equipment.]
The reason is very simple and readily explained. For the sake of
clearness we will suppose that the speed of the interruptor attached to
the coil is 100 per second. It will therefore produce 100 sparks per
second at the spark gap if the electrodes are close together. The
passage of the sparks is not continuous, each one only occupying a very
small space of time. The pause between each is very distinct, although
it could not be detected with the naked eye. The ten straight lines in
Fig. 141 represent ten sparks which cover a period of one-tenth of a
second, since they pass at the rate of 100 per second. Each spark
produces a train of oscillations, which surge back and forth in the
aerial, rapidly dying out, however, or becoming damped in the manner
already explained.
[Illustration: FIG. 141.–Wireless telephone receiving apparatus
(induction method).]
[Illustration: FIG. 142.—Fessenden wireless telephone transmitting
phonograph music.]
It may now readily be realized that there are long pauses between the
sparks when there are no oscillations in the aerial, and, consequently,
no electromagnetic waves passing between the transmitter and receptor
during those periods.
The wavy line, C, represents the vibrations of the human voice when
producing speech. Part of it has been represented by a continuous line,
and part by a dotted line. The portions represented by the dotted line
occur when there are no oscillations in the aerial, and consequently
these portions are not transmitted. The continuous portions are the only
ones reaching the receptor. Literally, there are "holes in the voice,"
and the result is a jumble of sounds, sometimes bearing a resemblance to
speech, but usually untranslatable.
[Illustration: FIG. 143.—Diagram illustrating the reason why damped
oscillations will not carry the voice.]
The fault lies in the method of producing the oscillations which are
damped and therefore do not exist continuously. If they could be made to
keep on swinging and at a sufficiently high speed so that their tone
would be inaudible and not confuse the speech, the problem would be
solved. In other words, three things are necessary for the successful
operation of a wireless telephone.
1. A means of producing and radiating a stream of undampened electrical
waves sufficiently continuous to transmit the upper harmonics of the
voice, on which the quality and recognition of the speech depends.
2. Means for varying or modulating the stream of electrical waves in
accordance with the sound waves.
3. A receiver, continuously responsive and capable of corresponding
with sufficient rapidity to the speech harmonics.
In order to obtain the desired result, recourse is had to an arc lamp as
a generator of undamped high frequency oscillations.
When an arc is properly connected with a condenser and an impedance coil
it will emit a musical note. The note is due to rapid changes in the
arc, a very important factor which led to its recognition as a a value
in wireless telephony.
[Illustration: FIG. 144.—How the sound waves of the voice are impressed
upon undamped oscillations.]
When the condenser and inductance are shunted across an arc supplied
with direct current, the condenser immediately becomes charged, and the
current through the arc is diminished. The potential difference across
the latter is therefore instantly increased, tending to further charge
the condenser. This increase of charge reacts on the arc, increasing its
current. The condenser discharges, through the inductance coil, and
becomes charged in the opposite direction, just like a spring, which
released, goes beyond its normal position and then returns.
The operation is repeated many times per second (usually over
1,000,000), setting up persistent undamped oscillations.
[Illustration: FIG. 145.—Arrangement of the speaking arc.]
Perhaps a better conception of how it is possible for a continuous
current, such as that which supplies the arc, to change into alternating
current, vibrating backward and forward, may be gained by comparison
with the steady forward motion of a violin bow, which produces a to and
fro motion of the strings.
[Illustration: FIG. 146.—Diagram showing how a wireless telephone
transmitting system is arranged.]
It was later discovered by Poulsen that if one of the arc electrodes was
kept cool by making it of copper and passing water through it that the
efficiency was greatly increased. A further improvement was obtained by
burning the arc in an atmosphere of coal gas or hydrogen. By surrounding
the arc with a powerful magnetic field, its resistance is greatly
increased and the voltage raised.
[Illustration: FIG. 147.—Poulsen wireless telephone equipment. The
condenser shunted around the are usually consists of a number of metal
plates, placed above one another in a tank of insulating oil. The
inductance is simply a single helix or bare wire.]
By connecting a telephone transmitter to the arc in the same manner that
it is connected to the speaking arc, the oscillations can be varied in
accordance with the vibrations of the voice. The apparatus is connected
to the aerial and the earth through the medium of a loose-coupled helix,
formed by providing the helix in series with the arc and condenser, with
a secondary winding.
[Illustration: FIG. 148.—The Majorana wireless telephone transmitter.]
The ordinary carbon transmitter, in its common form, is unsuited for
wireless telephonic work, on account of its inability to handle large
amounts of power. Many modifications have been designed, the usual
procedure being to make it on a larger scale.
One of the most interesting forms, and also probably the best, is that
devised by an Italian inventor, Majorana.
Its action will be clear from the illustration. T is a tube in which
water or some other liquid is allowed to flow in the direction of the
arrow. The bottom of the tube is contracted so that the stream will
issue in a fine jet. The tube is made of strong, rigid material, except
at one point, D, where an opening is covered with a thin elastic
diaphragm. This diaphragm is connected by means of a short rod to a
second diaphragm, which is provided with a mouthpiece. The water
normally flows out of the jet in a smooth, unbroken column, breaking
into drops at about the point A. As soon as it is disturbed in any way,
however, the distance from the outlet of the tube and the point where
the drops commence becomes shortened. The vibrations of the voice,
thrown into the mouthpiece and striking the diaphragm, are transmitted
to the membrane through the medium of the little rod, and so cause
corresponding changes in the pressure of the fluid in the tube. Each
variation or disturbance in the pressure increases or decreases the
length of the stream before it breaks into drops.
A pair of fine wires are inserted in the stream where the contractions
are the strongest.
Connection is established between the wires by the liquid. If the stream
is narrow its resistance will be greater than if it were expanded at
that point. The contracted portion of the liquid will jump up and down
with the vibrations of the voice, and thus alter the amount of current
flowing.
The receiving apparatus consists of some form of detector and a
telephone receiver and battery. The usual form of detector employed is
the electrolytic. The currents generated in the receiving aerial by the
incoming waves vary in amplitude with those of the transmitting aerial,
and, being in perfect accordance with the vibrations conveyed into the
transmitter, cause the detector and telephone transmitter to reproduce
the speech perfectly.
Experiments in wireless telephony have developed an interesting type of
detector, known as the "Audion." This consists of a six-volt,
low-candlepower, incandescent lamp, having a small, nickel plate
fastened a short distance from the filament, and a "grid" bent from wire
placed midway between the two. When the filament is lighted from a
battery, it throws off a stream of extremely small particles charged
with electricity and called "ions."
These ions pass through the grid and discharge against the plate. When
the aerial is connected to the "grid," and the plate to the ground, the
stream of ions carries that part of the alternating current in the
aerial which flows in the same direction, across, but does not allow the
current tending to pass in the opposite direction. In reality it is a
valve, or "rectifier," opening one way and closing the other; thus
changing the current into an intermittent, direct current, capable of
manifesting itself in a telephone receiver.
[Illustration: FIG. 149.—Showing the brush discharge from a Marconi
transatlantic aerial at night.]
The Audion is a very sensitive device, and is much employed for wireless
telephone purposes.
With such a system it has been found possible to transmit speech and
music to a distance of two hundred miles. In fact, even greater
distances have been covered, and there does not seem to be any good
reason why it is limited to any range.
Transmission by wireless telephone is considerably more distinct than by
wire line, and the fine inflections of the voice are brought out much
better.
Unlike the ordinary line telephone, no rumbling or roaring noises are
heard which confuse the speech, and there is absolute silence in the
wireless telephone receiver, except when talking is going on. Any noises
or sounds produced in the transmitting station, such as walking about
the room, or the breathing of the person speaking into the transmitter,
are reproduced faithfully at the receiving station many miles away.