[Illustration: =MODERN TELEGRAPHIC TYPEWRITING ATTACHMENT=]
[Illustration: =A GOOD TYPE OF DYNAMO=]
_What are some of the chief modern applications of electricity?_
The field of applied electricity is one of the most extensive in modern
science, invention and industry. Electricity in some form is now
utilized in connection with lighting, telegraphy, the telephone,
heating, motor boats, railways, aëroplanes, in metallurgy and the arts,
clocks, bells and alarms, wireless telegraphy and telephony, submarine
telegraphy, automobiles, cooking and domestic science, in medicine and
in military science.
_Give a brief account of wireless telegraphy._
In the case of ordinary telegraphy we always make use of extended
metallic wires or conductors from the place from which the message is
sent to the neighborhood to which it is desired to send it. In the case
of _wireless_ telegraphy no such conductors exist.
Among the most interesting of the many systems of wireless telegraphy
now in vogue the modern Marconi, the De Forrest, the Fessenden, and the
Poulsen are noteworthy. It is, however, with the name of Marconi that
the introduction of wireless telegraphy will always be directly
associated.
In 1888 Hertz had demonstrated in a remarkable series of experiments the
existence of electro-magnetic waves, and had even shown how these might
be produced, detected, and made to exhibit all the chief phenomena of
wave-motion. Marconi’s great achievement lay in so controlling and
regulating the dispatch and receipt of such waves as to make them
record signals on a specially designed apparatus in accordance with the
well-known Morse telegraphic system. His method, as first patented in
1896, was briefly as follows:
THE TRANSMITTER, by which the electromagnetic waves were generated and
sent off into space in all directions, consisted of a battery connected
through a key to the primary of an induction coil whose secondary
terminals were joined to two brass balls between which there was a short
air-gap. From one of these balls a wire was taken to earth, and from the
other an aërial wire was led some distance up in the air. The closing of
the primary circuit led to sparks passing across the air-gap, which
produced electro-magnetic waves in the ether in exactly the same way as
the dropping of a stone into a pool produces a series of concentric
ripples.
THE COHERER.--To receive and interpret these waves Marconi employed a
“coherer” in circuit with a battery and having connection with an aërial
wire on the one side and an earth wire on the other. The coherer
consisted of a small glass tube not more than, say, two inches long by
one-quarter inch in diameter, into the ends of which were fused two
platinum wires leading to small metallic electrodes. These electrodes
were brought quite near each other, and in the narrow gap between them
was placed powdered metallic silver, antimony, etc. The resistance
offered by this powder was so high, on account of small air-gaps between
the particles, that no current could pass through.
Electro-magnetic waves, however, possess the peculiar property of
breaking down the resistance of this powder whenever they impinge upon
it. Hence as soon as a wave reached the coherer, the resistance
practically vanished and a current passed round the circuit. It was a
mere detail to arrange that this current should actuate a relay
connected with a telegraphic instrument which would record the signal,
and that a hammer would at the same time tap the coherer so as to
agitate the powder and “decohere” it, setting up the resistance again
for a fresh signal.
IMPROVEMENTS.--Since this system was devised many most important
improvements have taken place. One of the most noticeable of these was
Sir Oliver Lodge’s invention of tuning and syntonizing apparatus by
which a transmitter and receiver are tuned to the same periodic
oscillation, and thus a number of messages might be operated in the same
field without interference. Lodge accomplished this to some extent by
adding inductance coils and condensers to the circuits. Various other
methods have been adopted to secure syntonization; but the resonance
effects obtained are not great enough to make selective signaling
certain.
THE GENERATOR.--In the modern Marconi system the energy for the
transmitter is obtained from a generator working at one hundred and ten
volts. The current is led through a key and an improved form of
interruptor to the primary of the induction coil, whose secondary
terminals communicate with the spark-gap. The spark-gap is in series
with a condenser and the primary of a high tension transformer, of which
latter one secondary terminal leads to the aërial and the other to the
earth wire.
[Illustration: =THE WIRELESS MESSAGE OVER LAND AND SEA=]
THE DETECTOR.--In the receptor the metallic coherer has been discarded
for a magnetic detector. This instrument consists of a small glass tube
through which travels an endless band of iron wires, moving round two
grooved pulleys. Close to the tube are two permanent magnets, and round
it is wound a primary coil consisting of one layer of wire. One end of
this coil is led straight to earth; the other passes through a condenser
to a tuning inductance coil leading in one direction to earth and in the
other to the aërial. Above the primary coil on the glass tube a
secondary coil is wound and connects with a telephone receiver. The
action is simple. The electro-magnetic waves, reaching the aërial, set
up oscillatory currents in the primary which act upon the magnetic
field. Currents are thus generated in the secondary, which record the
message in the telephone receiver by a series of taps corresponding to
the Morse dashes and dots.
[Illustration: _Courtesy of Marconi Wireless Co._
=TELEPHONING FROM NEW YORK TO SAN FRANCISCO BY WIRELESS=]
The _De Forrest system_ is very largely used in the United States,
Japan, and elsewhere, and in its more recent modifications secures a
high efficiency by means of a number of ingenious improvements.
_Describe the wireless telephone._
As in wireless telegraphy, all modern systems of wireless telephony are
based upon the action of electro-magnetic waves. It is impossible here
to discuss all the various methods that have been devised, but the
leading principles employed may be indicated, with special reference to
some of the best-known systems. They may be classified according to the
methods in which the waves are produced.
SPARK DISCHARGE SYSTEMS.--These rely for the generation of the Hertzian
waves upon a spark discharge across an air-gap. The _De Forrest system_
is perhaps the most popular of this type. In this system the spark
discharge is utilized to produce waves of a frequency not less than one
hundred thousand per second, the resulting sound being inaudible at the
receiving station.
A microphone transmitter is employed with this apparatus. When the
operator speaks into the transmitter, the variations of resistance act
upon the waves in such a way as to produce a new series of waves of such
frequency as to be audible at the receiver.
The receiving apparatus includes the usual antenna, and closed secondary
circuit, comprising an inductance and a variable capacity, across the
terminals of which an Audion delicate detector is introduced. This
instrument depends upon the motions of the ions in a rarefied gas. It is
one of the most sensitive detectors yet invented, and offers the great
advantage of a practically total absence of time lag in recovery.
SINGING-ARC SYSTEMS.--Duddell’s discovery of the singing arc in 1909 has
been quickly followed by its application to radio-telephony and
radio-telegraphy, first by Poulsen and subsequently by Fessenden, Stone,
De Forrest, and others. Under certain conditions the electric arc can be
made to emit a musical note, while at the same time it transforms a
portion of the energy of its own direct current into oscillations. These
are led into an oscillation circuit containing a condenser and
inductance, and associated with an antenna and earth line. The
microphone transmitter may be included in a circuit associated with the
inductance, in which case the voice acting on the resistance of the
transmitter causes variations in the oscillating currents; or it may be
associated with some part of the direct-current circuit, in which case
it acts by affecting the current passing across the arc.
Any form of receiver may be used with this arc apparatus. The great
advantage of this method is that the arc produces continuous
oscillations of constant amplitude, and that the wave-length and
frequency of the oscillations are subject to better regulation and
control.
ADVANTAGES.--The advantages of wireless telephony over wireless
telegraphy are many. One is that no skilled operator is required to
translate the dot-and-dash signals; for in the latter one hears only
long and short buzzes, whereas in the former one hears the actual spoken
words. By means of wireless telephony the transmission of intelligence
is far more direct and expeditious, and in times of emergency this not
unfrequently becomes a very vital question indeed. An important
characteristic of wireless telephonic communication is the exceptional
clearness of the articulation, owing to the absence of the electrostatic
capacity of metallic lines and cables which is always present in wire
telephony.
Stronger currents, improved sending and receiving apparatus, and the
application of new principles have now greatly extended the speaking
range; and only recently distinct communication has been established by
wireless telephony between New York and San Francisco. The use of the
wireless telephone will be greatly extended, especially in naval,
military, and shipping communication.