of electrical oscillations with those by which sound is produced. It is
well known that a vibrating tuning-fork will set another fork of the
same pitch to vibrate also by mere approximation. A and B (Fig. 280_k_)
are two exactly similar Leyden jars, the inner and outer coatings of
each being connected by a wire enclosing a considerable area in its
circuit, which in the case of A contains an air gap across which sparks
pass when the coatings are connected with the poles of an electrical
machine. The circuit of B is provided with an adjustable sliding piece
C, and the coatings are almost connected with each other by a strip of
tinfoil hanging over the rim but not quite reaching to the outer
coating. When the jars are placed so that their wire circuits are
parallel, and sparks are passing across the air interval of A’s circuit,
a position of the slider on the other can be found when sparks also pass
between the tin-foil and the outer coating. But if the slider be moved
from this position, the two circuits will no longer be in unison, and
the sparks in B will cease. This response of the oscillations in one jar
to those set up in another of the same vibratory period is called
_electrical resonance_.
Dr. Hertz, a professor in the University of Bonn, has opened out new
paths to investigators by a brilliant series of researches which have
shown that in the dielectric surrounding an electrical system executing
very rapid oscillations there are waves of electro-motive and magnetic
force. These researches are not capable of any condensed description
here, and the reasoning is of a kind that appears mainly to the expert
physicist. One of his modes of investigation required oscillations of
extreme rapidity, and he obtained them by attaching to each pole of an
induction coil a metal plate, and between these plates, which were in
the same vertical plane, passed a stout wire interrupted by an air gap
in its centre provided with small brass balls. The rate of oscillation
of this arrangement was calculated as the hundred-millionth part of 1·4
second. In conjunction with this system Hertz made use of a very simple
apparatus he called a resonator, which consisted merely of a piece of
copper wire bent into a circle of about 28 inches diameter. The ends of
the wire did not, however, meet, but were fitted with two balls, or with
a ball and a point, and an arrangement by which the air gap between them
could be very finely adjusted and measured. This resonator was, of
course, prepared as to be in electrical tune with the original vibrator,
and with it Hertz was able to examine the condition of the surrounding
space. When held in the hand near the vibrator he found that sparks
crossed the air space in the resonator, and that the length of the air
space across which the sparks would pass varied with the position of the
resonator. When the plane of the resonator was parallel with the metal
planes of the vibrator and its axis in the horizontal line drawn
perpendicularly through the vibrator’s air space, the sparks passed
readily when the air space of the resonator was at the same time
vertically above or below its centre, but they ceased entirely when it
was level with the centre. He obtained these sparks when the resonator
was held—in free space, be it understood—in the above-mentioned position
even at a distance from the vibrator of 13 yds., the length of the
apartment. By examining the results with other positions of his
resonator and by other and varied experiments, Hertz was able to prove
the existence of definite waves of electro-magnetic and electro-motive
forces, to measure their lengths, and to show that they are capable of
reflection, refraction, and even polarization by the same laws that hold
with the extremely short but enormously rapid vibrations constituting
light. It may here be mentioned that the existence of currents in the
resonator can be shown by a Geissler tube being made to take the place
of the air space, which tube is thus lighted up without any metallic or
visible connection with any electrical apparatus whatever, the only
requisite conditions being that its circuit be tuned to the vibrator,
and in a certain position in relation to the axis of the spark space of
the latter. Hertz has also shown that electro-magnetic disturbances
(transversal waves) are propagated in space with a determinate velocity
akin to that of light, and in short the outcome of his investigations,
as well as of those undertaken by others, has been a vindication of
Clerk Maxwell’s splendid theory by which light is regarded as an
electro-magnetic action. Professor Righi of Bologna, having succeeded in
obtaining shorter electrical waves than anyone before—namely, 4/10ths of
an inch instead of about 20 inches—was able with them to repeat all the
phenomena of optics such as reflection, refraction, circular
polarization, interference, &c. It appears then almost certain that
light and electro-magnetic waves or radiations are but one and the same
affection of a pervading medium we call the ether.
By following up in certain directions lines of research suggested by the
investigations of Maxwell, Lodge, Hertz and others, and by an unreserved
acceptance of the ether theory of light, electricity and magnetism, some
wonderful practical results have recently been obtained by M. Nikola
Tesla, an electrical engineer now resident in New York. The experiments
shown by Tesla in his public lectures have excited great interest in
scientific circles, and have by many persons been witnessed with
something like astonishment.
[Illustration:
FIG. 280_l_.—_The Tesla Oscillator._
]
[Illustration:
FIG. 280_m_.—_M. Nikola Tesla._
]
One of the first objects of M. Tesla was to obtain alternating currents
of high tension and great frequency. It may be seen from Fig. 272 that
the movement of coils of wire in a magnetic field generates currents,
and it has been stated that these currents are in alternately opposite
directions as the coils approach or recede from the magnetic poles. In
the machine represented in Fig. 280_a_, each revolution would produce 16
reversals of current. Tesla constructed a rotatory machine which gave
20,000 alternations of current in one second, because it had 400 poles
and could be rotated at a very high speed. But of course the number of
poles and the speed of the machine could not be increased beyond certain
practical limits. By a happy application of the known principle of
harmonic oscillations, in which all the rotatory movements of
fly-wheels, coils and poles could be dispensed with, Tesla simplified
the alternate current generator, reducing the moving parts to the
minimum at the same time that he obtained a greater number of
alternations and almost perfect regularity in their periodicity. The way
in which this has been accomplished may be gathered from a careful
inspection of Fig. 280_l_ compared with the following explanation. This
illustration, it should be understood, is merely a diagram in which
details of mechanism are altogether omitted, and only so much shown as
will serve to explain the principle. We shall take the mechanical part
first, and direct the reader’s attention to the means by which an iron
rod is made to perform very rapid to-and-fro movements in the direction
of its length, and to do that with perfectly isochronous periods, which
may be made longer or shorter at will, and which are quite independent
of very considerable variations in the motive power. The diagram
represents the apparatus in section, and the central part of it marked
by letters P and P´ is a piston through which passes what may be called
a piston-rod A, which projects some distance out of the cylinder at both
ends. The piston is shown in the diagram in its central position, where
the impelling power has no action to move it as will presently be seen.
This moving power we may assume to be the compressed air applied through
the ports I I´. Just to the right of the upper one of these on the
diagram will be observed in the piston a slot S opening into a hollow T,
which communicates directly with the space on the left of the piston.
The same arrangement, with directions reversed, is seen on the other
side of the piston. If now the piston were pushed a little to the left
of the position shown in the diagram, the compressed air rushing from I
through the slot into the opening S T would impel the piston towards the
right, and it would be carried onward by its inertia beyond the position
shown in the figure towards the right, but in doing this the access of
the compressed air on the left would be cut off, and the slot
communicating with the space on the right hand would allow the
compressed air to act in the space P, checking the further advance of
the piston to the right, acting like a spring or elastic cushion, and
again driving the piston to the left, during which movement the air that
has done its work is allowed to escape at the outlet O O. The same cycle
of operations will be rapidly repeated, but the rate of oscillation
admits of control, for the larger the air chamber in which the air is
compressed by the momentum of the piston and rod, the less will it be
compressed and the less powerfully it will resist, while with a smaller
capacity of air-chamber the more powerful will be the back spring of the
imprisoned air. On the other hand, the mass that is moved may be
increased; that is the weight of the rod, &c., may be increased. In any
case the oscillations will be perfectly regular, because the force which
tends to bring the piston to its position of equilibrium will be always
proportionate to its distance from that point. So that we have here a
rod shooting in and out shuttle-wise with the utmost regularity and with
almost any desired rapidity, controllable under precisely the like
conditions as the stretched string already mentioned, for as the tension
of the string is the measure of the force with which it strives to
regain its position of equilibrium, so the compression of air in the
chamber behind the piston; and as the loaded string vibrates slower, so
will the loaded piston. So much for the mechanical part of this machine,
for we may omit all details of valves, &c. The electrical arrangement is
very simple and of the greatest efficiency. On each projecting end of
the piston are wound coils of insulated copper wire, which being shot in
and out across a powerful magnetic field between the jaws of very large
electro-magnets M M´ cut the “lines of force” to the best advantage, and
from these coils alternating currents of high tension and frequency are
gathered up. The vibrating rod is steadied by working in bearings (not
shown). The electro-magnets are actuated as usual by coils of insulated
wire surrounding their iron cores. In the motion of the moving coils
there are electrical forces called into play which in mechanical effect
control the movement in the same way as the air-springs, and as these
electrical forces admit of certain adjustments and have calculable
effects, the _mechanical period_ of the machine and the _electrical_ one
can be made to accord, and thus to, as it were, sustain each other, and
assure a perfectly isochronous periodicity, even with considerable
variations of the impelling force. Though we have supposed compressed
air as the actuating agent, steam has been applied in some slightly
modified forms of the machine, and sometimes at the high pressure of 350
lbs. per square inch. Such is Tesla’s alternating current producer, or
the _Tesla Oscillator_, as it has been called. This, of course, is a
very different thing from the vibrator of disruptive discharge already
mentioned in connection with the experiments of Professor Hertz. Tesla
also uses the disruptive discharge, and what with the high frequency and
the great tension of his currents, he obtains electric oscillations of
hitherto unequalled rapidity, calculable at thousands of millions per
second. He claims, indeed, to be able to agitate the ether at rates of
undulation comparable with those of light itself (500 billions per
second). Some of the experiments he has shown certainly lend support to
such an explanation. The lighting of electric lamps with but one
metallic connection, and that held in a person’s hand, and causing
Geissler tubes to light up without any metallic connections whatever,
and making gas at ordinary pressures luminous, a lump of charcoal
contained in a closed glass vessel to become red-hot while the vessel is
merely held in the hand, are certainly phenomena that cannot be
explained on the old lines. The space between two large surfaces of
metal 15 feet apart, and forming the poles of an oscillatory system, is
shown to be full of light-forming influences, as when phosphorescent
substances contained in closed glass vessels glow intensely, the glass
being apparently no obstacle. According to Tesla, you make space and
matter equally permeable to ethereal undulations when these are tuned,
so to speak, to the proper frequency.
Many of the strange effects Tesla has shown are referable to the
principle of electric resonance; such are the powers of a coil with no
metallic connections with any other apparatus and removed, by a distance
of many feet, from any current-conveying wires. Tesla’s workshop was an
apartment 40 feet long and 20 wide, and the wires connecting the poles
of his oscillator were carried round the walls, while in the centre of
the workshop stood a very large but entirely insulated coil, between the
terminals of which an ordinary incandescent lamp was placed. This lamp
was brilliantly illuminated when the oscillator was in action. The
electric qualities of this coil were so adjusted that its currents came
into tune with the ethereal vibrations propagated from the conductor
round the room. But further, a single hoop of copper wire of the proper
diameter and thickness could be brought into unison with the coil, and
when held in the hand over the latter, even at a considerable distance,
incandescent lamps attached to it were lighted up by the induced
currents. Many other novel experiments have been shown by M. Tesla, but
they need not here be described, as they have yet to be connected with
the logical study of the entire class of phenomena. M. Tesla speaks
somewhat sanguinely of being ultimately able to convey signals, and even
power, to a distance, not merely with one wire but with no wires at all!
Another thing he looks forward to is to set the electricity, or rather
the ether that interpenetrates the matter of the whole earth, into a
state of agitation. This seems what is commercially termed “a large
order;” but we have seen that every Leyden jar, every coil, and in fact
every electrical system, has its own period, and if by any possibility
we could discover, or by chance hit upon the earth’s electric vibration
period, it is not antecedently impossible that even the comparatively
small efforts of such oscillatory vibrations as we could produce, would
by their accumulation agitate the earth’s ether. It is well known that
very small impulses, so tuned as to correspond with the natural period
of a considerable mass, will produce striking mechanical effects. Thus,
a troop of soldiers passing over a bridge have often been known to break
down a structure that would have supported their mere weight many times
over, because they were all marching together and with a step
corresponding in time with the oscillatory period of the bridge. It is
now always enjoined in the military orders that troops in crossing a
bridge must “break step.” Another familiar illustration of the
accumulation of small synchronous impulses is the experiment of singing
into a glass goblet the note corresponding with its vibration period.
The singer merely by sustaining this note for a short time often
succeeds in shivering the glass into fragments. M. Tesla believes that
he has already succeeded in agitating the earth’s ether to some extent;
he does at least obtain flaming purple streamers passing into the air
from one end of a coil, while the other is connected with the earth.
These discoveries and theories appear likely to lead to many unforeseen
results, valuable for both science and its applications, and such as may
far surpass the expectations of those who take less enthusiastic views
of the matter than M. Tesla and his friends do. The theoretical
properties of the ether and the conditions of it, which are held capable
of making it the scene and the medium of all the hitherto so-called
ponderable and imponderable forces, have not been completely worked out.
The experiments that have been already made show that disturbances of
very different kinds may be propagated in the ether by undulations of
any length from less than 1/60000th part of an inch, as in the case of
violet light already spoken of, to the 1,200 miles attributed to certain
electrical conditions.
The foregoing sentences, describing the discoveries of Hertz and others,
had not long been penned before it had become possible to announce that
they had borne fruit in as extraordinary an invention as could have
distinguished the close of an extraordinary century. It is the
realization of what the most accomplished electrician would not long
before have pronounced a dream—namely, _wireless telegraphy_. The
general principle of it should not be obscure after the account of the
“Hertzian waves”; but our space does not permit a description of details
of its working out in a practical form by a young Italian electrician,
Signor Marconi. We have already seen that a Geissler tube, when its
circuit is properly attuned, can be lighted up by the magneto-electric
disturbance propagated without material contacts, and this itself would
constitute a method of signalling to a distance. On the same principle,
a discharge may be determined by the “wave” between conductors in
certain adjustable conditions of electric tension, and in this way local
circuits may be brought into play, and ordinary telegraphic effects
produced, as described in the following article. The actual apparatus to
receive the ethereal impulses is extremely simple—merely a little fine
metallic dust (nickel and silver) in a glass tube included in the
resonator circuit by a wire at each end, touching the dust. This gathers
together, or coheres (hence the apparatus is called the _coherer_),
under the magneto-electric influence, a local battery discharge then
passes, completing a circuit, and the dust has to be shaken loose again
by a mechanical agitation. Marconi has been able to signal over a
distance of forty-three miles.
[Illustration:
FIG. 281.—_Portrait of Professor Morse._
]
THE ELECTRIC TELEGRAPH.
More than two centuries ago a learned Italian Jesuit, named Strada, gave
a fanciful account of a method by which he supposed two persons might
communicate with each other, however far they might be separated. He
conceived two needles magnetized by a loadstone of such virtue, that the
needles balanced on separate pivots ever afterwards pointed in parallel
directions; and if one were turned to any point, the other also
sympathetically moved in complete accordance with it. The happy
possessors of these sympathetic needles, each having his needle mounted
on a dial marked with the same letters and words similarly inscribed,
would be able to communicate their thoughts to each other at
preconcerted hours, by movements and pauses of the wonderful needles.
The poet Akenside, when describing, in his “Pleasures of the
Imagination,” the effect of association in bringing ideas before our
minds, illustrates his point by a happy allusion to Strada’s conceit.
Here is the passage:
“For when the different images of things,
By chance combined, have struck the attentive soul
With deeper impulse, or, connected long,
Have drawn her frequent eye; howe’er distinct
The external scenes, yet oft the ideas gain
From that conjunction an eternal tie
And sympathy unbroken. Let the mind
Recall one partner of the various league—
Immediate, lo! the firm confederates rise.
‘Twas thus, if ancient fame the truth unfold,
Two faithful needles, from the informing touch
Of the same parent stone, together drew
Its mystic virtue, and at first conspired
With fatal impulse quivering to the pole.
Then—though disjoined by kingdoms, though the main
Rolled its broad surge betwixt, and different stars
Beheld their wakeful motions—yet preserved
The former friendship, and remembered still
The alliance of their birth. Whate’er the line
Which one possessed, nor pause nor quiet knew
The sure associate, ere, with trembling speed,
He found its path, and fixed unerring there.”
In our own day this fancy of Strada’s has been literally and completely
realized in all save the convenient portability of the sympathetic
dials; but this and the other forms of apparatus which are now so
familiar in electric telegraphy were produced by no sudden inspiration
occurring to a single individual. Great inventions are ever the outcome
not of the labours of one but of a hundred minds, and the progress of
the electric telegraph might be traced, step by step, from the first
suggestions, made more than a century ago, of employing, for the
communication of intelligence at a distance, the imperfect electric
means then known. The men who then attempted to utilize the mysterious
agency of electricity failed to produce a practical telegraph, because
the conditions of electrical excitation known at that time gave no scope
for the realization of their project. Not the less do they deserve our
grateful remembrance for the faith and energy with which they strove to
overcome the difficulties of their task. Voltaic electricity was first
proposed as the means of conveying signals to a distance in 1808,
immediately after the discovery of the power of the pile to decompose
water; and the method of communicating the signals was based upon this
property. Sömmering proposed to arrange thirty-five pairs of electrodes,
formed by gold pins passed through the bottom of a glass vessel
containing acidulated water. Each pair of pins was marked by a letter of
the alphabet or a numeral, and attached to distinct wires, which could
be put into connection with a pile at the sending station. The signals
were made by the gas evolved from these electrodes indicating the letter
intended. The number of wires required and the slowness of working were
great objections, and this system never came into practical use,
although it was afterwards proposed to diminish the number of the wires
from thirty-five to two—by so varying the amounts of gas given off and
the periods of time as to form an intelligible system of signals. Ten or
twelve years after, Mr. Ronalds, of Hammersmith, invented an ingenious
system by which letters on a dial could be pointed out at a distance by
frictional electricity. Two dials, on which the letters, &c., were
marked, were each placed behind a screen having an aperture, which
permitted only one letter to be seen at once; and the dial was mounted
on the seconds arbor of a clock with a dead-beat escapement. A pair of
pith balls hung in front, insulated and connected by means of an
insulated wire with the similar pair at the other end of the line, where
the other clock and dial were placed. The clocks were regulated to go as
nearly as possible at the same rate, so that at each end of the line the
same letters were simultaneously displayed. It was easy, however, at any
time to start the clocks together at the same letter by a signal
previously agreed upon, and all that was really required was a
synchronous motion of the discs during the time the signals were being
sent. The insulated wire received from a small electrical machine a
charge, which caused the pith balls at both ends to diverge; and the
moment the wire was discharged, the balls collapsed suddenly and
simultaneously, and this discharge was effected by the sender of the
message at the instant that the letter he wished to indicate appeared at
the opening in front of his dial. Since the same letter was at the same
instant visible at the other end also, it was indicated to the receiver
of the message by the collapse of the pith balls. Ronalds worked this
telegraph experimentally with a wire 525 ft. long, but it was never
adopted practically. On communicating to the Admiralty the power of his
invention, he was informed that “_telegraphs of any kind were wholly
unnecessary, and no other than the one in use would be adopted_.”