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The Story Of Electricity

J >> John Munro >> The Story Of Electricity




Other fishes--the silurus, malapterurus, and so on--are likewise
endowed with electric batteries for stunning and capturing their
prey. The action of the organs is still a mystery, as, indeed, is
the whole subject of animal electricity. Nobili and Matteucci
discovered that feeble currents are generated by the excitation of
the nerves and the contraction of the muscles in the human
subject.

Electricity promises to become a valuable remedy, and currents--
continuous, intermittent, or alternating--are applied to the body
in nervous and muscular affections with good effect; but this
should only be done under medical advice, and with proper
apparatus.

In many cases of severe electric shock or lightning stroke, death
is merely apparent, and the person may be brought back to life by
the method of artificial respiration and rhythmic traction of the
tongue, as applied to the victims of drowning or dead faint.

A good lightning conductor should not have a higher electrical
resistance than 10 ohms from the point to the ground, including
the "earth" contact. Exceptionally good conductors have only about
5 ohms. A high resistance in the rod is due either to a flaw in
the conductor or a bad earth connection, and in such a case the
rod may be a source of danger instead of security, since the
discharge is apt to find its way through some part of the building
to the ground, rather than entirely by the rod. It is, therefore,
important to test lightning conductors from time to time, and the
magneto-electric tester of Siemens, which we illustrate in figures
98 and 99, is very serviceable for the purpose, and requires no
battery. The apparatus consists of a magneto-electric machine AT,
which generates the testing current by turning a handle, and a
Wheatstone bridge. The latter comprises a ring of German silver
wire, forming two branches. A contact lever P moves over the ring,
and is used as a battery key. A small galvanometer G shows the
indications of the testing current. A brass sliding piece S puts
the galvanometer needle in and out of action. There are also
several connecting terminals, b b', l, &c., and a comparison
resistance R (figure 98). A small key K is fixed to the terminal l
(figure 99), and used to put the current on the lightning-rod, or
take it off at will. A leather bag A at one side of the wooden
case (figure 99) holds a double conductor leading wire, which is
used for connecting the magneto-electric machine to the bridge. On
turning the handle of M the current is generated, and on closing
the key K it circulates from the terminals of the machine through
the bridge and the lightning-rod joined with the latter. The
needle of the galvanometer is deflected by it, until the
resistance in the box R is adjusted to balance that in the rod.
When this is so, the galvanometer needle remains at rest. In this
way the resistance of the rod is told, and any change in it noted.
In order to effect the test, it is necessary to have two earth
plates, E1 and E2, one (El) that of the rod, and the other (E2)
that for connecting to the testing apparatus by the terminal b1
(figure 99). The whole instrument only weighs about 9 lbs. In
order to test the "earth" alone, a copper wire should be soldered
to the rod at a convenient height above the ground, and terminal
screws fitted to it, as shown at T (figure 99), so that instead of
joining the whole rod in circuit with the apparatus, only that
part from T downwards is connected. The Hon. R. Abercrombie has
recently drawn attention to the fact that there are three types of
thunderstorm in Great Britain. The first, or squall thunderstorms,
are squalls associated with thunder and lightning. They form on
the sides of primary cyclones. The second, or commonest
thunderstorms, are associated with secondary cyclones, and are
rarely accompanied by squalls The third, or line thunderstorms,
take the form of narrow bands of rain and thunder--for example,
100 miles long by 5 to 10 miles broad. They cross the country
rapidly, and nearly broadside on. These are usually preceded by a
violent squall, like that which capsized the Eurydice.

The gloom of January, 1896, with its war and rumours of war, was,
at all events, relieved by a single bright spot. Electricity has
surprised the world with a new marvel, which confirms her title to
be regarded as the most miraculous of all the sciences. Within the
past twenty years she has given us the telephone of Bell, enabling
London to speak with Paris, and Chicago with New York; the
microphone of Hughes, which makes the tread of a fly sound like
the "tramp of an elephant," as Lord Kelvin has said; the
phonograph of Edison, in which we can hear again the voices of the
dead; the electric light which glows without air and underwater,
electric heat without fire, electric power without fuel, and a
great deal more beside. To these triumphs we must now add a means
of photographing unseen objects, such as the bony skeletons in the
living body, and so revealing the invisible.

Whether it be that the press and general public are growing more
enlightened in matters of science, or that Professor Rontgen's
discovery appeals in a peculiar way to the popular imagination, it
has certainly evoked a livelier and more sudden interest than
either the telephone, microphone, or phonograph. I was present
when Lord Kelvin first announced the invention of the telephone to
a British audience, and showed the instrument itself, but the
intelligence was received so apathetically that I suspect its
importance was hardly realised. It fell to my own lot, a few years
afterwards, to publish the first account of the phonograph in this
country, and I remember that, between incredulity on the one hand,
and perhaps lack of scientific interest on the other, a
considerable time elapsed before the public at large were really
impressed by the invention. Perhaps the uncanny and mysterious
results of Rontgen's discovery, which seem to link it with the
"black arts," have something to do with the quickness of its
reception by all manner of people.

Like most, if not all, discoveries and inventions, it is the
outcome of work already done by other men. In the early days of
electricity it was found that when an electric spark from a
frictional machine was sent through a glass bulb from which the
air had been sucked by an air pump, a cloudy light filled the
bulb, which was therefore called an "electric egg". Hittorf and
others improved on this effect by employing the spark from an
induction coil and large tubes, highly exhausted of air, or
containing a rare infusion of other gases, such as hydrogen. By
this means beautiful glows of various colours, resembling the
tender hues of the tropical sky, or the fleeting tints of the
aurora borealis, were produced, and have become familiar to us in
the well-known Geissler tubes.

Crookes, the celebrated English chemist, went still further, and
by exhausting the bulbs with an improved Sprengel air-pump,
obtained an extremely high vacuum, which gave remarkable effects
(page 120). The diffused glow or cloudy light of the tube now
shrank into a single stream, which joined the sparking points
inserted through the ends of the tube as with a luminous thread A
magnet held near the tube bent the streamer from its course; and
there was a dark space or gap in it near the negative point or
cathode, from which proceeded invisible rays, having the property
of impressing a photographic plate, and of rendering matter in
general on which they impinged phosphorescent, and, in course of
time, red-hot. Where they strike on the glass of the tube it is
seen to glow with a green or bluish phosphorescence, and it will
ultimately soften with heat.

These are the famous "cathode rays" of which we have recently
heard so much. Apparently they cannot be produced except in a very
high vacuum, where the pressure of the air is about 1-100th
millionth of an atmosphere, or that which it is some 90 or 100
miles above the earth. Mr Crookes regards them as a stream of airy
particles electrified by contact with the cathode or negative
discharging point, and repelled from it in straight lines. The
rarity of the air in the tube enables these particles to keep
their line without being jostled by the other particles of air in
the tube. A molecular bombardment from the cathode is, in his
opinion, going on, and when the shots, that is to say, the
molecules of air, strike the wall of the tube, or any other body
within the tube, the shock gives rise to phosphorescence or
fluorescence and to heat. This, in brief, is the celebrated
hypothesis of "radiant matter," which has been supported in the
United Kingdom by champions such as Lord Kelvin, Sir Gabriel
Stokes, and Professor Fitzgerald, but questioned abroad by
Goldstem, Jaumann, Wiedemann, Ebert, and others.

Lenard, a young Hungarian, pupil of the illustrious Heinrich
Hertz, was the first to inflict a serious blow on the hypothesis,
by showing that the cathode rays could exist outside the tube in
air at ordinary pressure. Hertz had found that a thin foil of
aluminium was penetrated by the rays, and Lenard made a tube
having a "window" of aluminium, through which the rays darted into
the open air. Their path could be traced by the bluish
phosphorescence which they excited in the air, and he succeeded in
getting them to penetrate a thin metal box and take a photograph
inside it. But if the rays are a stream of radiant matter which
can only exist in a high vacuum, how can they survive in air at
ordinary pressure? Lenard's experiments certainly favour the
hypothesis of their being waves in the luminiferous ether.

Professor Rontgen, of Wirzburg, profiting by Lenard's results,
accidentally discovered that the rays coming from a Crookes tube,
through the glass itself, could photograph the bones in the living
hand, coins inside a purse, and other objects covered up or hid in
the dark. Some bodies, such as flesh, paper, wood, ebonite, or
vulcanised fibre, thin sheets of metal, and so on, are more or
less transparent, and others, such as bones, carbon, quartz, thick
plates of metal, are more or less opaque to the rays. The human
hand, for example, consisting of flesh and bones, allows the rays
to pass easily through the flesh, but not through the bones.
Consequently, when it is interposed between the rays and a
photographic plate, the skeleton inside is photographed on the
plate. A lead pencil photographed in this way shows only the black
lead, and a razor with a horn handle only the blade.

Thanks to the courtesy of Mr. A. A. Campbell Swinton, of the firm
of Swinton & Stanton, the well-known electrical engineers, of
Victoria Street, Westminster, a skilful experimentalist, who was
the first to turn to the subject in England, I have witnessed the
taking of these "shadow photographs," as they are called, somewhat
erroneously, for "radiographs" or "cryptographs" would be a better
word, and shall briefly describe his method. Rontgen employs an
induction coil insulated in oil to excite the Crookes tube and
yield the rays, but Mr. Swinton uses a "high frequency current,"
obtained from apparatus similar to that of Tesla, and shown in
figure 100, namely, a high frequency induction coil insulated by
means of oil and excited by the continuous discharge of twelve
half-gallon Leyden jars charged by an alternating current at a
pressure of 20,000 volts produced by an ordinary large induction
coil sparking across its high pressure terminals.

A vacuum bulb connected between the discharge terminals of the
high frequency coil, as shown in figure 101, was illuminated with
a pink glow, which streamed from the negative to the positive
pole--that is to say, the cathode to the anode, and the glass
became luminous with bluish phosphorescence and greenish
fluorescence. Immediately under the bulb was placed my naked hand
resting on a photographic slide containing a sensitive bromide
plate covered with a plate of vulcanised fibre. An exposure of
five or ten minutes is sufficient to give a good picture of the
bones, as will be seen from the frontispiece.

The term "shadow" photograph requires a word of explanation. The
bones do not appear as flat shadows, but rounded like solid
bodies, as though the active rays passed through their substance.
According to Rontgen, these "x" rays, as he calls them, are not
true cathode rays, partly because they are not deflected by a
magnet, but cathode rays transformed by the glass of the tube; and
they are probably not ultra-violet rays, because they are not
refracted by water or reflected from surfaces. He thinks they are
the missing "longitudinal" rays of light whose existence has been
conjectured by Lord Kelvin and others--that is to say, waves in
which the ether sways to and fro along the direction of the ray,
as in the case of sound vibrations, and not from side to side
across it as in ordinary light.

Be this as it may, his discovery has opened up a new field of
research and invention. It has been found that the immediate
source of the rays is the fluorescence and phosphorescence of the
glass, and they are more effective when the fluorescence is
greenish-yellow or canary colour. Certain salts--for example, the
sulphates of zinc and of calcium, barium platino-cyanide,
tungstate of calcium, and the double sulphate of uranyle and
potassium--are more active than glass, and even emit the rays
after exposure to ordinary light, if not also in the dark.
Salvioni of Perugia has invented a "cryptoscope," which enables us
to see the hidden object without the aid of photography by
allowing the rays to fall on a plate coated with one of these
phosphorescent substances. Already the new method has been applied
by doctors in examining malformations and diseases of the bones or
internal organs, and in localising and extracting bullets,
needles, or other foreign matters in the body. There is little
doubt that it will be very useful as an adjunct to hospitals,
especially in warfare, and, if the apparatus can be reduced in
size, it will be employed by ordinary practitioners. It has also
been used to photograph the skeleton of a mummy, and to detect
true from artificial gems. However, one cannot now easily predict
its future value, and applications will be found out one after
another as time goes on.





CHAPTER X.

THE WIRELESS TELEGRAPH.


Magnetic waves generated in the ether (see pp. 53-95) by an
electric current flowing in a conductor are not the only waves
which can be set up in it by aid of electricity. A merely
stationary or "static" charge of electricity on a body, say a
brass ball, can also disturb the ether; and if the strength of the
charge is varied, ether oscillations or waves are excited. A
simple way of producing these "electric waves" in the ether is to
vary the strength of charge by drawing sparks from the charged
body. Of course this can be done according to the Morse code; and
as the waves after travelling through the ether with the speed of
light are capable of influencing conductors at a distance, it is
easy to see that signals can be sent in this way. The first to do
so in a practical manner was Signer Marconi, a young Italian
hitherto unknown to fame. In carrying out his invention, Marconi
made use of facts well known to theoretical electricians, one of
whom, Dr, Oliver J. Lodge, had even sent signals with them in
1894; but it often happens in science as in literature that the
recognised professors, the men who seem to have everything in
their favour--knowledge, even talent--the men whom most people
would expect to give us an original discovery or invention, are
beaten by an outsider whom nobody heard of, who had neither
learning, leisure, nor apparatus, but what he could pick up for
himself.

Marconi produces his waves in the ether by electric sparks passing
between four brass balls, a device of Professor Righi, following
the classical experiments of Heinrich Hertz. The balls are
electrified by connecting them to the well-known instrument called
an induction coil, sometimes used by physicians to administer
gentle shocks to invalids; and as the working of the coil is
started and stopped by an ordinary telegraph key for interrupting
the electric current, the sparking can be controlled according to
the Morse code. In our diagram, which explains the apparatus, the
four balls are seen at D, the inner and larger pair being partly
immersed in vaseline oil, the outer and smaller pair being
connected to the secondary or induced circuit of the induction
coil C, which is represented by a wavy line. The primary or
inducing circuit of the coil is connected to a battery B through a
telegraph signalling key K, so that when this key is opened and
closed by the telegraphist according to the Morse code, the
induction coil is excited for a longer or shorter time by the
current from the battery, in agreement with the longer and shorter
signals of the message. At the same time longer or shorter series
of sparks corresponding to these signals pass across the gaps
between the four balls, and give rise to longer or shorter series
of etheric waves represented by the dotted line. So much for the
"Transmitter." But how does Marconi transform these invisible
waves into visible or audible signals at the distant place? He
does this by virtue of a property discovered by Mr. S. A. Varley
as far back as 1866, and investigated by Mr. E. Branly in 1889.
They found that powder of metals, carbon, and other conductors,
while offering a great resistance to the passage of an electric
current when in a loose state, coheres together when electric
waves act upon it, and opposes much less resistance to the
electric current. It follows that if a Morse telegraph instrument
at the distant place be connected in circuit with a battery and
some loose metal dust, it can be adjusted to work when the etheric
waves pass through the dust, and only then. In the diagram R is
this Morse "Receiver" joined in circuit with a battery B1; and a
thin layer of nickel and silver dust, mixed with a trace of
mercury, is placed between two cylindrical knobs or "electrodes"
of silver fused into the glass tube d, which is exhausted of air
like an electric glow lamp. Now, when the etheric waves proceeding
from the transmitting station traverse the glass of the tube and
act upon the metal dust, the current of the battery B1 works the
Morse receiver, and marks the signals in ink on a strip of
travelling paper. Inasmuch as the dust tends to stick together
after a wave passes through it, however, it requires to be shaken
loose after each signal, and this is done by a small round hammer
head seen on the right, which gives a slight tap to the tube. The
hammer is worked by a small electromagnet E, connected to the
Morse instrument, and another battery b in what is called a
"relay" circuit; so that after the Morse instrument marks a
signal, the hammer makes a tap on the tube. As this tap has a
bell-like sound, the telegraphist can also read the signals of the
message by his ear.

Two "self-induction bobbins," L Ll, a well-known device of
electricians for opposing resistance to electric waves, are
included in the circuit of the Morse instrument the better to
confine the action of the waves to the powder in the tube.
Further, the tube d is connected to two metal conductors V Vl,
which may be compared to resonators in music. They can be adjusted
or attuned to the electric waves as a string or pipe is to
sonorous waves. In this way the receiver can be made to work only
when electric waves of a certain rate are passing through the
tube, just as a tuning-fork resounds to a certain note; it being
understood that the length of the waves can be regulated by
adjusting the balls of the transmitter. As the etheric waves
produced by the sparks, like ripples of water caused by dropping a
stone into a pool, travel in all directions from the balls, a
single transmitter can work a number of receivers at different
stations, provided these are "tuned" by adjusting the conductors V
Vl to the length of the waves.

This indeed was the condition of affairs at the time when the
young Italian transmitted messages from France to England in
March, 1899, and it is a method that since has been found useful
over limited distances. But to the inventor there seemed no reason
why wireless telegraphy should be limited by any such distances.
Accordingly he immediately developed his method and his apparatus,
having in mind the transmission of signals over considerable
intervals. The first question that arose was the effect of the
curvature of the Earth and whether the waves follow the surface of
the Earth or were propagated in straight lines, which would
require the erection of aerial towers and wires of considerable
height. Then there was the question of the amount of power
involved and whether generators or other devices could be used to
furnish waves of sufficient intensity to traverse considerable
distances.

Little by little progress was made and in January, 1901, wireless
communication was established between the Isle of Wight and Lizard
in Cornwall, a distance of 186 miles with towers less than 300
feet in height, so that it was demonstrated that the curvature of
the Earth did not seriously affect the transmission of the waves,
as towers at least a mile high would have been required in case
the waves were so cut off. This was a source of considerable
encouragement to Marconi, and his apparatus was further improved
so that the resonance of the circuit and the variation of the
capacity of the primary circuit of the oscillation transformer
made for increased efficiency. The coherer was still retained and
by the end of 1900 enough had been accomplished to warrant Marconi
in arranging for trans-Atlantic experiments between Poldhu,
Cornwall and the United States, stations being located on Cape Cod
and in Newfoundland. The trans-Atlantic transmission of signals
was quite a different matter from working over 100 miles or so in
Great Britain. The single aerial wire was supplanted by a set of
fifty almost vertical wires, supported at the top by a horizontal
wire stretched between two masts 157 1/2 feet high and 52 1/2 feet
apart, converging together at the lower end in the shape of a
large fan. The capacity of the condenser was increased and instead
of the battery a small generator was employed so that a spark 1
1/2 inches in length would be discharged between spheres 3 inches
in diameter. At the end of the year 1901 temporary stations at
Newfoundland were established and experiments were carried on with
aerial wires raised in the air by means of kites. It was here
realized that various refinements in the receiving apparatus were
necessary, and instead of the coherer a telephone was inserted in
the secondary circuit of the oscillation transformer, and with
this device on February 12th the first signals to be transmitted
across the Atlantic were heard. These early experiments were
seriously affected by the fact that the antennae or aerial wires
were constantly varying in height with the movement of the kites,
and it was found that a permanent arrangement of receiving wires,
independent of kites or balloons, was essential. Yet it was
demonstrated at this time that the transmission of electric waves
and their detection over distances of 2000 miles was distinctly
possible.

A more systematic and thorough test occurred in February, 1902,
when a receiving station was installed on the steamship
Philadelphia, proceeding from Southampton to New York. The
receiving aerial was rigged to the mainmast, the top of which was
197 feet above the level of the sea, and a syntonic receiver was
employed, enabling the signals to be recorded on the tape of an
ordinary Morse recorder. On this voyage readable messages were
received from Poldhu up to a distance of 1551 miles, and test
letters were received as far as 2099 miles. It was on this voyage
that Marconi made the interesting discovery of the effect of
sunlight on the propagation of electric waves over great
distances. He found that the waves were absorbed during the
daytime much more than at night and he eventually reached the
conclusion that the ultraviolet light from the sun ionized the
gaseous molecules of the air, and ionized air absorbs the energy
of the electric waves, so that the fact was established that clear
sunlight and blue skies, though transparent to light, serve as a
fog to the powerful Hertzian waves of wireless telegraphy. For
that reason the transmission of messages is carried on with
greater facility on the shores of England and Newfoundland across
the North Atlantic than in the clearer atmosphere of lower
latitudes. But atmospheric conditions do not affect all forms of
waves the same, and long waves with small amplitudes are far less
subject to the effect of daylight than those of large amplitude
and short wave length, and generators and circuits were arranged
to produce the former. But the difficulty did not prove
insuperable, as Marconi found that increasing the energy of the
transmitting station during the daytime would more than make up
for the loss of range.

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