MINOR USES OF ELECTRICITY.
The electric "trembling bell," now in common use, was first
invented by John Mirand in 1850. Figure 83 shows the scheme of the
circuit, where
B is a small battery, say two or three "dry" or Leclanche cells,
joined by insulated wire to P, a press-button or contact key, and
G an electromagnetic gong or bell. On pressing the button P, a
spring contact is made, and the current flowing through the
circuit strikes the bell. The action of the contact key will be
understood from figure 84, where P is the press-button removed to
show the underlying mechanism, which is merely a metal spring A
over a metal plate B. The spring is connected by wire to a pole of
the battery, and the plate to a terminal or binding screw of the
bell, or vice versa. When the button P is pressed by the finger
the spring is forced against the plate, the circuit is made, and
the bell rings. On releasing the button it springs back, the
circuit is broken, and the bell stops.
Figure 85 shows the inner mechanism of the bell, which consists of
a double-poled electromagnet M, having a soft iron armature A
hinged on a straight spring or tongue S, with one end fixed, and
the other resting against a screw contact T. The hammer H projects
from the armature beside the edge of the gong E.
In passing through the instrument the current proceeds from one
terminal, say that on the right, by the wire W to the screw
contact T, and thence by the spring S through the bobbins of the
electromagnet to the other terminal. The electromagnet attracts
the armature A, and the hammer H strikes the gong; but in the act
the spring S is drawn from the contact T, and the circuit is
broken. Consequently the electromagnet, no longer excited, lets
the armature go, and the spring leaps back against the contact T,
withdrawing the hammer from the gong. But the instrument is now as
it was at first, the current again flows, and the hammer strikes
the gong, only to fly back a second time. In this way, as long as
the button is pressed by the operator, the hammer will continue to
tap the bell and give a ringing sound. Press-buttons are of
various patterns, and either affixed to the wall or inserted in
the handle of an ordinary bell-pull, as shown in figure 86.
The ordinary electric bell actuated by a battery is liable to get
out of order owing to the battery spending its force, or to the
contacts becoming dirty. Magnetoelectric bells have, therefore,
been introduced of late years. With these no battery or
interrupting contacts are required, since the bell-pull or press-
button is made in the form of a small dynamo which generates the
current when it is pulled or pushed. Figure 87 illustrates a form
of this apparatus, where M P is the bell-pull and B the bell,
these being connected by a double wire W, to convey the current.
The bell-pull consists of a horseshoe magnet M, having a bobbin of
insulated wire between its poles, and mounted on a spindle. When
the key P is turned round by the hand, the bobbin moves in the
magnetic field between the poles of the magnet, and the current
thus generated circulates in the wires W, and passing through an
electromagnet under the bell, attracts its armature, and strikes
the hammer on the bell. Of course the bell may be placed at any
distance from the generator. In other types the current is
generated and the bell rung by the act of pulling, as in a common
house-bell.
Electric bells in large houses and hotels are usually fitted up
with indicators, as shown in figure 88, which tell the room from
which the call proceeds. They are serviceable as instantaneous
signals, annunciators, and alarms in many different ways. An
outbreak of fire can be announced by causing the undue rise of
temperature to melt a piece of tallow or fusible metal, and thus
release a weight, which tails on a press-button, and closes the
circuit of an electric bell. Or, the rising temperature may expand
the mercury in a tube like that of a thermometer until it connects
two platinum wires fused through the glass and in circuit with a
bell. Some employ a curving bi-metallic spring to make the
necessary contact. The spring is made by soldering strips of brass
and iron back to back, and as these metals expand unequally when
heated, the spring is deformed, and touches the contact which is
connected in the circuit, thus permitting the current to ring the
bell. A still better device, however, is a small box containing a
thin metallic diaphragm, which expands with the heat, and sagging
in the centre, touches a contact screw, thus completing the
circuit, and allowing the current to pass.
These automatic or self-acting fire-alarms can, of course, be
connected in the circuit of the ordinary street fire-alarms, which
are usually worked by pulling a handle to make the necessary
contact.
From what has been said, it will be easy to understand how the
stealthy entrance of burglars into a house can be announced by an
electric bell or warning lamp. If press-buttons or contact-keys
are placed on the sashes of the windows, the posts of the door, or
the treads of the stair, so that when the window or door is
opened, or the tread bends under the footstep, an electric circuit
is closed, the alarm will be given. Of course, the connections
need only be arranged when the device is wanted. Shops and offices
can be guarded by making the current show a red light from a lamp
hung in front of the premises, so that the night watchman can see
it on his beat. This can readily be done by adjusting an
electromagnet to drop a screen of red glass before the flame of
the lamp. Safes and showcases forcibly opened can be made to
signal the fact, and recently in the United States a thief was
photographed by a flashlight kindled in this way, and afterwards
captured through the likeness.
The level of water in cisterns and reservoirs, can be told in a
similar manner by causing a float to rise with the water and make
the required contact. The degree of frost in a conservatory can
also be announced by means of the mercury "thermostat," already
described, or some equivalent device. There are, indeed, many
actual or possible applications of a similar kind.
The Massey log is an instrument for telling the speed of a ship by
the revolutions of a "fly" as it is towed through the water, and
by making the fly complete a circuit as it revolves the number of
turns a second can be struck by a bell on board. In one form of
the "electric log," the current is generated by the chemical
action of zinc and copper plates attached to the log, and immersed
in the sea water, and in others provided by a battery on the ship.
Captain M'Evoy has invented an alarm for torpedoes and torpedo
boats, which is a veritable watchdog of the sea. It consists of an
iron bell-jar inverted in the water, and moored at a depth below
the agitation of the waves. In the upper part of the jar, where
the pressure of the air keeps back the water, there is a delicate
needle contact in circuit with a battery and an electric bell or
lamp, as the case may be, on the shore. Waves of sound passing
through the water from the screw propeller of the torpedo, or,
indeed, any ship, make and break the sensitive contact, and ring
the bell or light the lamp. The apparatus is intended to alarm a
fleet lying at anchor or a port in time of war.
Electricity has also been employed to register the movements of
weathercocks and anemometers. A few years ago it was applied
successfully to telegraph the course marked by a steering compass
to the navigating officer on the bridge. This was done without
impeding the motion of the compass card by causing an electric
spark to jump from a light pointer on the card to a series of
metal plates round the bowl of the compass, and actuate an
electric alarm.
The "Domestic Telegraph," an American device, is a little dial
apparatus by which a citizen can signal for a policeman, doctor,
messenger, or carriage, as well as a fire engine, by the simple
act of setting a hand on the dial.
Alexander Bain was the first to drive a clock with electricity
instead of weights, by employing a pendulum having an iron bob,
which was attracted to one side and the other by an electromagnet,
but as its rate depends on the constancy of the current, which is
not easy to maintain, the invention has not come into general use.
The "butterfly clock" of Lemoine, which we illustrate in figure
89, is an improved type, in which the bob of soft iron P swings to
and fro over the poles of a double electro magnet M in circuit
with a battery and contact key. When the rate is too slow the key
is closed, and a current passing through the electromagnet pulls
on the pendulum, thus correcting the clock. This is done by the
ingenious device of Hipp, shown in figure 90, where M is the
electromagnet, P the iron bob, from which projects a wire bearing
a light vane B of mica in the shape of a butterfly. As the bob
swings the wire drags over the hump of the metal spring S, and
when the bob is going too slowly the wire thrusts the spring into
contact with another spring T below, thus closing the circuit, and
sending a current through the magnet M, which attracts the bob and
gives a fillip to the pendulum.
Local clocks controlled from a standard clock by electricity have
been more successful in practice, and are employed in several
towns--for example, Glasgow. Behind local dials are electromagnets
which, by means of an armature working a frame and ratchet wheel,
move the hands forward every minute or half-minute as the current
is sent from the standard clock.
The electrical chronograph is an instrument for measuring minute
intervals of time by means of a stylus tracing a line on a band of
travelling paper or a revolving barrel of smoked glass. The
current, by exciting an electromagnet, jerks the stylus, and the
interval between two jerks is found from the length of the trace
between them and the speed of the paper or smoked surface.
Retarded clocks are sometimes employed as electric meters for
registering the consumption of electricity. In these the current
to be measured flows through a coil beneath the bob of the
pendulum, which is a magnet, and thus affects the rate. In other
meters the current passes through a species of galvanometer called
an ampere meter, and controls a clockwork counter. In a third kind
of meter the chemical effect of the current is brought into play--
that of Edison, for example, decomposing sulphate of copper, or
more commonly of zinc.
The electric light is now used for signalling and advertising by
night in a variety of ways. Incandescent lamps inside a
translucent balloon, and their light controlled by a current key,
as in a telegraph circuit, so as to give long and short flashes,
according to the Morse code, are employed in the army. Signals at
sea are also made by a set of red and white glow-lamps, which are
combined according to the code in use. The powerful arc lamp is
extremely useful as a "search light," especially on men of war and
fortifications, and it has also been tried in signalling by
projecting the beam on the clouds by way of a screen, and
eclipsing it according to a given code.
In 1879, Professor Graham Bell, the inventor of the speaking
telephone, and Mr Summer Tamter, brought out an ingenious
apparatus called the photophone, by which music and speech were
sent along a beam of light for several hundred yards. The action
of the photophone is based on the peculiar fact observed in 1873
by Mr J E Mayhew, that the electrical resistance of crystalline
selenium diminishes when a ray of light falls upon it. Figure 91
shows how Bell and Tamter utilised this property in the telephone.
A beam of sun or electric light, concentrated by a lens L, is
reflected by a thin mirror M, and after traversing another lens L,
travels to the parabolic reflector R, in the focus of which there
is a selenium resistance in circuit with a battery S and two
telephones T T'. Now, when a person speaks into the tube at the
back of the mirror M, the light is caused to vibrate with the
sounds, and a wavering beam falls on the selenium, changing its
resistance to the current. The strength of the current is thus
varied with the sonorous waves, and the words spoken by the
transmitter are heard in the telephones by the receiver. The
photophone is, however, more of a scientific toy than a practical
instrument.
Becquerel, the French chemist, found that two plates of silver
freshly coated with silver from a solution of chloride of silver
and plunged into water, form a voltaic cell which is sensitive to
light. This can be seen by connecting the plates through a
galvanometer, and allowing a ray of light to fall upon them. Other
combinations of the kind have been discovered, and Professor
Minchin, the Irish physicist, has used one of these cells to
measure the intensity of starlight.
The "induction balance" of Professor Hughes is founded on the
well-known fact that a current passing in one wire can induce a
sympathetic current in a neighbouring wire. The arrangement will
be understood from figure 92, where P and P1 are two similar coils
or bobbins of thick wire in circuit with a battery B and a
microphone M, while S and S1 are two similar coils or bobbins of
fine wire in circuit with a telephone T. It need hardly be said
that when the microphone M is disturbed by a sound, the current in
the primary coils P P1 will induce a corresponding current in the
secondary coils S S1; but the coils S S1 are so wound that the
induction of P on S neutralises the induction of P1 on S1; and no
current passes in the secondary circuit, hence no sound is heard
in the telephone. When, however, this balance of induction is
upset by bringing a piece of metal--say, a coin--near one or other
of the coils S S1, a sound will be heard in the telephone.
The induction balance has been used as a "Sonometer" for measuring
the sense of hearing, and also for telling base coins. The writer
devised a form of it for "divining" the presence of gold and
metallic ores which has been applied by Captain M'Evoy in his
"submarine detector" for exploring the sea bottom for lost anchors
and sunken treasure. When President Garfield was shot, the
position of the bullet was ascertained by a similar arrangement.
The microphone as a means of magnifying feeble sounds has been
employed for localising the leaks in water pipes and in medical
examinations. Some years ago it saved a Russian lady from
premature burial by rendering the faint beating of her heart
audible.
Edison's electric pen is useful in copying letters. It works by
puncturing a row of minute holes along the lines of the writing,
and thus producing a stencil plate, which, when placed over a
clean sheet of paper and brushed with ink, gives a duplicate of
the writing by the ink penetrating the holes to the paper below.
It is illustrated in figure 93, where P is the pen, consisting of
a hollow stem in which a fine needle actuated by the armature of a
small electromagnet plies rapidly up and down and pierces the
paper. The current is derived from a small battery B, and an
inking roller like that used in printing serves to apply the ink.
In 1878 Mr. Edison announced his invention of a machine for the
storage and reproduction of speech, and the announcement was
received with a good deal of incredulity, notwithstanding the
partial success of Faber and others in devising mechanical
articulators. The simplicity of Edison's invention when it was
seen and heard elicited much admiration, and although his first
instrument was obviously imperfect, it was nevertheless regarded
as the germ of something better. If the words spoken into the
instrument were heard in the first place, the likeness of the
reproduction was found to be unmistakable. Indeed, so faithful was
the replica, that a member of the Academy of Sciences, Paris,
stoutly maintained that it was due to ventriloquism or some other
trickery. It was evident, however, that before the phonograph
could become a practical instrument, further improvements in the
nicety of its articulation were required. The introduction of the
electric light diverted Mr. Edison from the task of improving it,
although he does not seem to have lost faith in his pet invention.
During the next ten years he accumulated a large fortune, and was
the principal means of introducing both electric light and power
to the world at large. This done, however, he returned to his
earlier love, and has at length succeeded in perfecting it so as
to redeem his past promises and fulfill his hopes regarding it.
The old instrument consisted, as is well known, of a vibrating
tympan or drum, from the centre of which projected a steel point
or stylus, in such a manner that on speaking to the tympan its
vibrations would urge the stylus to dig into a sheet of tinfoil
moving past its point. The foil was supported on a grooved barrel,
so that the hollow of the groove behind it permitted the foil to
give under the point of the stylus, and take a corrugated or wavy
surface corresponding to the vibrations of the speech. Thus
recorded on a yielding but somewhat stiff material, these
undulations could be preserved, and at a future time made to
deflect the point of a similar stylus, and set a corresponding
diaphragm or tympan into vibration, so as to give out the original
sounds, or an imitation of them.
Tinfoil, however, is not a very satisfactory material on which to
receive the vibrations in the first place. It does not precisely
respond to the movements of the marking stylus in taking the
impression, and does not guide the receiving stylus sufficiently
well in reproducing sounds. Mr. Edison has therefore adopted wax
in preference to it; and instead of tinfoil spread on a grooved
support, he now employs a cylinder of wax to take the print of the
vibrations. Moreover, he no longer uses the same kind of diaphragm
to print and receive the sounds, but employs a more delicate one
for receiving them. The marking cylinder is now kept in motion by
an electric motor, instead of by hand-turning, as in the earlier
instrument.
The new phonograph, which we illustrate in figure 94, is about the
size of an ordinary sewing machine, and is of exquisite
workmanship, the performance depending to a great extent on the
perfection and fitness of the mechanism. It consists of a
horizontal spindle S, carrying at one end the wax cylinder C, on
which the sonorous vibrations are to be imprinted. Over the
cylinder is supported a diaphragm or tympan T, provided with a
conical mouthpiece M for speaking into. Under the tympan there is
a delicate needle or stylus, with its point projecting from the
centre of the tympan downwards to the surface of the wax cylinder,
so that when a person speaks into the mouthpiece, the voice
vibrates the tympan and drives the point of the stylus down into
the wax, making an imprint more or less deep in accordance with
the vibrations of the voice. The cylinder is kept revolving in a
spiral path, at a uniform speed, by means of an electric motor E,
fitted with a sensitive regulator and situated at the base of the
machine. The result is that a delicate and ridgy trace is cut in
the surface of wax along a spiral line. This is the sound record,
and by substituting a finer tympan for the one used in producing
it, the ridges and inequalities of the trace can be made to
agitate a light stylus resting on them, and cause it to set the
delicate tympan into vibrations corresponding very accurately to
those of the original sounds. The tympan employed for receiving is
made of gold-beater's skin, having a stud at its centre and a
springy stylus of steel wire. The sounds emitted by this device
are almost a whisper as compared to the original ones, but they
are faithful in articulation, which is the main object, and they
are conveyed to the ear by means of flexible hearing-tubes.
These tympans are interchangeable at will, and the arm which
carries them is also provided with a turning tool for smoothing
the wax cylinder prior to its receiving the print. The cylinders
are made of different sizes, from 1 to 8 inches long and 4 inches
in diameter. The former has a storage capacity of 200 words. The
next in size has twice that, or 400 words, and so on. Mr. Edison
states that four of the large 8-inch cylinders can record all
"Nicholas Nickleby," which could therefore be automatically read
to a private invalid or to a number of patients in a hospital
simultaneously, by means of a bunch of hearing-tubes. The
cylinders can be readily posted like letters, and made to deliver
their contents viva voce in a duplicate phonograph, every tone and
expression of the writer being rendered with more or less
fidelity. The phonograph has proved serviceable in recording the
languages and dialects of vanishing races, as well as in teaching
pronunciation.
The dimensions, form, and consequent appearance of the present
commercial American phonograph are quite different from that above
described, but the underlying principles and operations are
identical.
A device for lighting gas by the electric spark is shown in figure
95, where A is a flat vulcanite box, containing the apparatus
which generates the electricity, and a stem or pointer L, which
applies the spark to the gas jet. The generator consists of a
small "influence" machine, which is started by pressing the thumb-
key C on the side of the box. The rotation of a disc inside the
box produces a supply of static electricity, which passes in a
stream of sparks between two contact-points in the open end of the
stem D. The latter is tubular, and contains a wire insulated from
the metal of the tube, and forming with the tube the circuit for
the electric discharge. The handle enables the contrivance to be
readily applied. The apparatus is one of the few successful
practical applications of static electricity.
Other electric gas-lighters consist of metal points placed on the
burner, so that the electric spark from a small induction coil or
dynamo kindles the jet.
A platinum wire made white-hot by the passage of a current is
sometimes used to light lamps, as shown in figure 96, where W is a
small spiral of platinum connected in circuit with a generator by
the terminals T T. When the lamp L is pressed against the button B
the wire glows and lights it.
Explosives, such as gunpowder and guncotton, are also ignited by
the electric spark from an induction coil or the incandescence of
a wire. Figure 97 shows the interior of an ordinary electric fuse
for blasting or exploding underground mines. It consists of a box
of wood or metal primed with gunpowder or other explosive, and a
platinum wire P soldered to a pair of stout copper wires W,
insulated with gutta-percha. When the current is sent along these
wires, the platinum glows and ignites the explosive. Detonating
fuses are primed with fulminate of mercury.
Springs for watches and other purposes are tempered by heating
them with the current and quenching them in a bath of oil.
Electrical cautery is performed with an incandescent platinum wire
in lieu of the knife, especially for such operations as the
removal of the tongue or a tumour.
It was known to the ancients that a fish called a torpedo existed
in the Mediterranean which was capable of administering a shock to
persons and benumbing them. The torpedo, or "electric ray," is
found in the Atlantic as well as the Mediterranean, and is allied
to the skate. It has an electric organ composed of 800 or 1000
polygonal cells in its head, and the discharge, which appears to
be a vibratory current, passes from the back or positive pole to
the belly or negative pole through the water. The gymotus, or
Surinam eel, which attains a length of five or six feet, has an
electric organ from head to tail, and can give a shock sufficient
to kill a man. Humboldt has left a vivid picture of the frantic
struggles of wild horses driven by the Indians of Venezuela into
the ponds of the savannahs infested by these eels, in order to
make them discharge their thunderbolts and be readily caught.
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.