SUNDRY ELECTRICAL APPLIANCES—MR. EDISON’S INVENTIONS—THE ELECTRIC
LIGHT—THE GYROSCOPE—A NEW ELECTROPHORUS—ELECTRIC TOYS.
THE ELECTRO-MOTOGRAPH—although perhaps even yet scarcely
developed—has already proved a very useful invention. The idea of it
first occurred to Mr. Edison in 1873, when he was prosecuting some
researches in chemical telegraphy. “One day,” says Mr. Fox, in his
account of the invention, “as he sat pondering over his work, he
happened to take in hand the metallic point through which, as it rested
upon the paper, the current was wont to pass. When again he closed the
circuit to let the current through the paper, he held the metallic
point loosely, and unintentionally allowed it to rest upon the paper.
Every time he moved the metallic strip on the paper the latter became
wonderfully smooth. Edison was determined to find the reason of this,
and he decided that the electricity very much lessened the friction
of the metal on the paper. He made many experiments, and brought the
subject before the Royal Society in 1874, but nothing came of the idea
till 1876, when Edison was perfecting his musical telephone.
“The new appliance is, in fact, the same invention revived and now
perfected by the original inventor, and brought to complete practical
success under the title of the ‘electro-motograph.’ The action of the
‘electro-motograph’ depends on the fact, discovered during former
experiments, and employed imperfectly in the musical telephone, that
the friction of moving bodies varies in greater or less degree with
their electrical condition. In the electro-motograph a cylinder made
of prepared chalk, and saturated with a strong solution of caustic
alkali, is set upon supports, so that it can be turned upon its axis.
A strip of metal fastened to the mica diaphragm rests on the cylinder,
and is pressed so firmly by its spring upon the cylinder that when
it is turned by means of the handle the friction of the strip on the
cylinder tends to pull the diaphragm out of shape, causing it to bulge
inward as long as the cylinder is in motion. If now, while this motion
of the cylinder is maintained, an electric current passes through the
strip of metal, and then through the chalk cylinder to earth, the
amount of this friction is varied or it is destroyed altogether, and
the strip slides freely on the cylinder. This was the basis of the
former invention. The release from friction by a change in electric
condition in the first instrument failed simply from ignorance of some
slight matters of detail, that in the electro-motograph are corrected
and made practical. In the musical telephone the releasing of the
frictional resistance by electric action caused the sounding-board
of a guitar to vibrate, and thus set up sonorous vibrations. In the
electro-motograph the mica disc takes the place of the guitar, and,
by the improved construction of the apparatus, intricate and complex
vibrations, such as are produced in speaking, are reproduced in their
original or even in greater volume. When the apparatus is at rest
the diaphragm is motionless, and electric currents shot through the
apparatus produce no effect. In the same manner the mere turning of
the cylinder without electric action produces no effect, except to
pull the diaphragm slightly out of shape. If while the cylinder is
being turned an electric impulse arrives, the pull on the diaphragm,
caused by the friction of the strip on the cylinder, is more or less
released, and the diaphragm is free to vibrate or spring back into its
original condition. If now, the electric impulses follow one another in
regular order in correspondence with the sonorous vibrations imparted
to the transmitting telephone, the alternate slipping and catching of
the metal strip on the cylinder will follow in the same order, and
thus the diaphragm will be made to vibrate in unison with the original
vibrations, and thus reproduce the original words. As the mica disc is
much larger than the disc of the transmitting instrument, the amplitude
of its swing may be much greater, and consequently it will repeat the
words with greater power. The electro-motograph is practically an
apparatus for transforming electric action received from a distance
into mechanical work. The amount of electric action has nothing to do
with the amount of the mechanical work performed, because the movement
of the cylinder is controlled by power independently of the electric
action, the electricity merely releasing this power by destroying the
friction in greater or less degree. The electric action set up by the
sonorous vibrations at the transmitting end of the line may be very
slight, while the mechanical action at the distant end may be powerful,
and in this manner the amplitude of the vibrations may be increased to
an indefinite extent, and a whisper may reappear as a loud shout.
“The electro-motograph is not only a solution of the telephone,
making it capable of sounds of every quality and pitch and in greatly
increased volume, but by this conversion of electrical action into
mechanical work at a distance makes it possible to unite the telephone
and phonograph. Telephonic messages by the electro-motograph may
be impressed upon a self-acting (clock-work) phonograph, the same
current starting and stopping the phonograph after the manner of the
stock-reporting machines, and afterward the phonograph may be made to
repeat the message impressed upon it.”
[The above extract, which explains the principle fully, has been
taken from a long article on the subject which formerly appeared in
_Scribner’s Magazine_.]
The uses to which the electro-motograph may be applied are various.
It can produce mechanical motion even at a distance, and is useful to
lessen friction by machinery; and in this way its service to railways
and other locomotive systems may be estimated. It is a great help to
telegraphy by increasing the speed of transmission, and can ascertain
the beatings of the heart of the apparently dead. It amplifies sound in
a much greater degree than the microphone, by which even a fly can be
heard moving. In fact, the limit of the usefulness of this wonderful
machine has not been reached.
Another very ingenious apparatus has been developed by Professor Bell.
This is for the purpose of ascertaining the position of bullets in the
body. The following is condensed from the _Times_:—
“Two conductors are used, and the ball completes the circuit. Professor
Bell inserts a fine needle in the suspected region. It is connected by
wire with one of the binding screws of a telephone, which the surgeon
holds to his ear; the other binding screw being connected with a
metallic mass applied to the skin. When the needle point touches the
ball, an electric couple is formed, and the current generates the sound
in the telephone. The surgeon may then use his knife with confidence,
guided by the needle. He may make several insertions of the needle if
necessary without danger, and any pain may be obviated by etherization.
This simple method (which should prove useful on the field of battle)
was tried with success with a lead ball introduced into a piece of
beef. Contact of the needle with bone had no effect, but a very
distinct sound was heard each time the ball was reached. A modification
consists in inserting a vibrator in the circuit; this gives a musical
note in the telephone at each contact of ball and needle. Again, if the
circuit include a battery, the telephone sounds may be heard by several
persons at once. A sound is heard, in this case, whenever the needle
enters the skin; but, on reaching the ball, it is much intensified,
owing to lessened resistance. A galvanometer may be used in place of
the telephone.”
Mr. C. Vernon Boys has exhibited and described a very ingenious
new integrating machine of his invention, and its application as a
measurer of the electric energy in the circuit of an electric lamp
or a dynamo-electric motor. Mr. Boys’ mechanical integrator belongs
to the class termed tangent machines, and consists essentially of a
small disc or wheel running along the surface of a drum or cylinder.
When the wheel runs straight along the drum parallel to its axis
there is no rotation of the latter, but when the wheel is inclined
to the axis the drum rotates, and the integral is represented by the
amount of rotation. Continuous action is secured in giving the drum
a reciprocating motion along its axis, so that when the wheel has
travelled to one end of the cylinder it can travel back again. The
new integrator is especially adapted for measuring forces which are
either delicate or variable. It is applied by causing the varying
force to be measured to vary in a corresponding manner the inclination
of the wheel to the axis of the rotating cylinder. In this way it
can be used to find the work done by a fluid pressure reciprocating
engine, or the energy transmitted by a shaft or belt from one part of
a factory to another. By making the wheel very small and light, the
strength of an electric current may be continuously measured, if the
disc is inclined by means of the needle of a galvanometer in circuit.
Mr. Boys has constructed on the same principle an electric energy
meter, which integrates the product of the strength of current and
the difference of potential between two points with respect to time.
In it the current is passed through a pair of concentric solenoids or
coils of wire, and in the annular space between these is hung a third
solenoid, the upper half of which is wound in the opposite direction
to its lower half. By the use of what Mr. Boys calls “induction traps”
of soft iron, the magnetic force is confined to a small portion of
the suspended solenoid, and by this means the attracting force of
the fixed solenoids upon it is independent of position. The middle
solenoid is hung from the end of a balance beam, and its motion is
retarded by a counterweight, which admits of regulating the meter to
give standard measure as a clock gives standard time. The motion of the
beam is caused to incline the integrating wheel, and the rotation of
the cylinder gives the energy expended in foot-pounds by means of an
indicator or diagram, as the case may be. The object in giving an equal
number of turns in opposite directions to the suspended solenoid is to
render the instrument insensible to external magnetic forces.
We have, in a former portion of this work, explained the construction
of the telephone and phonograph with other inventions to make sounds
audible at a distance, so we need not repeat the explanations
here. A brief reference to them will, however, be found in this
chapter, in which the electro-magnet and the methods of lighting by
magneto-electric machines are treated of. We will proceed to give some
particulars concerning the electric light before considering the means
by which it is produced, as such an arrangement is more convenient.
The light is very easily produced by uniting and then separating the
terminals of a strong battery. The passage of the electric current
induces intense heat and a most brilliant light. But if this were
continued the wires would melt, and therefore some non-fusible
substance is placed at the ends of the wires, which will be at once a
conductor and infusible. Now in gas-carbon (the deposited substance
found in gas retorts) we have a substance suited to these conditions.
The carbon is heated to an intense brightness, and particles of it are
passed across the arc of flame almost in a state of fusion. Combustion
does not actually take place, because it has been proved that the wires
will give out light under water, and in the vacuum of an air-pump the
light is even increased, so that had the oxygen of the air any part in
the production of the light it would not remain unaffected under these
conditions. The heat arising from this Voltaic arc is intense, and
even platinum may be fused with the assistance of the gas carbon. The
carbon points are of course liable to be worn away, and one side more
than another. The positive pole is generally more concave than the
other, for it sheds its particles in a greater degree, and is the more
intensely heated. The electric light first appeared in public at the
opera in Paris in 1836, to illustrate a sunrise, but it was not till
1843 that it was experimented upon in the open air. We need not trace
it farther at present, for a full account of its origin, rise, and
progress is published in a small shilling volume by Messrs. Ward, Lock,
& Co. We will proceed to the methods of bringing out the light.
[Illustration: Fig. 267.—The Maxim light.]
[Illustration: Fig. 268.—Mechanism of Maxim’s lamp.]
There are various lamps, many of which required a regulator in
consequence of the wearing away of the carbon points, as already
explained. We append two illustrations of the Maxim lamp, the invention
of Hiram Maxim, of New York. In both cuts the letters refer to the same
portions.
In the first illustration (fig. 267), A and B are the positive and
negative carbon-holders respectively, and the carbon points are
controlled by an armature, which is, in its turn, adjusted by the
screw, D. When it happens that the magnetic force is reduced
the spring acts and permits the points to approach again, and the light
is rekindled; the carbons are then locked till required to move. The
second illustration (fig. 268) shows a section of the lamp with the
wheel arrangements for controlling the advance of the carbon points as
they waste away.
[Illustration: Fig. 269.—Wallace lamp]
[Illustration: Fig. 270.—Houston lamp.]
In the “Brush” light, which is in use in London, and is fitted for
large spaces, the carbon points are held by a regulator side by side,
and they last eight hours without renewal. The power is generated
by an electro-dynamic engine. We give illustrations of the lamps of
Wallace and Houston (figs. 269, 270). The current is conveyed through
_b_ and the magnet, _m_. The armature, _a_, separates the electrodes,
and the weakened current is restored by _b_, and the light continues.
The pillar, _p_, is hollow, with a wire running through it. The
positive electrode is supported by J, the negative by C; V is a button
which comes in contact with the lever, T, when the carbon points are
exhausted, and cuts the lamp out of the circuit by passing it direct
through mercury cups.
The Jablochkoff candle and chandelier are also represented (figs.
271, 272). The candles consist of carbons connected at the top, but
otherwise insulated, and fixed in a socket. They do not last very
long without renewal. The exhibition at the Crystal Palace will be
essentially an Electric Light Exhibition, and all the latest forms can
be studied there. The great attraction will doubtless be, as at Paris,
the varied and numerous inventions of Mr. Edison. The early career of
that American “magician” is now tolerably well known; his tremendous
energy and application are fully appreciated. With only a few months
schooling all his life he has taken a foremost place in the scientific
world. In ten years he has invented the phonograph, the electric pen,
a system of fast telegraphy, the electro-motograph, the telephone,
a tasimeter, and other useful applications of electricity, besides
solving the problem of electric light for domestic purposes.
Mr. Edison’s electric light[18] requires something more than a passing
notice, and we will therefore endeavour to give a sketch of the general
subject. Now that the electric light has been made available for
domestic purposes, and the very simple lamp (consisting of an exhausted
glass globe, two platinum wires, and a piece of charred paper) can be
obtained, people will no doubt soon largely adopt electric lighting
in their houses. The light has found a success at the theatre, in the
streets, and in the train; there is no reason why it should not be
adopted generally, being more economical and more healthy than gas.
[Illustration: Fig. 271. Electric candle.]
[Illustration: Fig. 272.—Chandelier.]
If we sever an electric wire, and bring the ends, tipped with carbon,
into juxtaposition, we obtain a brilliant light. This is the Voltaic
arc we have already mentioned, produced by the incandescence of
finely-divided matter; it was the first method of illuminating by
electricity, and was discovered by Sir Humphrey Davy, who obtained a
very brilliant light, but at great expense—about a guinea a minute! But
the Daniell and Grove batteries and generators, and modern improvements
in 1860, brought the use of the electric light into prominence. Faraday
lighted a lighthouse with its assistance.
But when the GRAMME GENERATOR was invented the needed impetus
was applied. The Jablochkoff candles followed, and now we have the
electric light in full operation. So far we have sketched the history
of illumination by the Voltaic arc, and descriptions of the various
apparatus will be found at the end of this chapter. But the method of
lighting with an incandescent solid was introduced in 1845 by Starr
and Peabody, who took out a patent for the use of platinum. Later on
Drs. Draper and Despretz made experiments with platinum and carbon.
The latter gentleman sealed the carbon in an exhausted globe, and then
introduced nitrogen in place of the air. But the method died out and
was forgotten, and in 1873 a medal was actually given by the Academy of
St. Petersburg for the “discovery” to Messrs. Sawyer and Mann.
In 1878 Paris was lighted with the electric candles of Jablochkoff.
This application of electricity stirred up our transatlantic cousins,
and Mr. Edison was requested—backed up by many influential persons—to
make the investigation whether the light could be produced for domestic
purposes. The celebrated electrician undertook the commission, and
certainly came unprejudiced to the encounter, for he had not at that
time even seen an electric light.
He perceived at once that “permanence in the lamp and the subdivision
of the light” were the two desiderata. He put the Voltaic arc aside
as unsuitable, and addressed himself to the problem of obtaining the
desired results from an incandescent solid. The subdivision of the
light is really an important point, and a comparison between divided
and undivided burners is in favour of the more diffused light in a
number of burners. This subdivision Edison worked hard to secure, and,
as it is said of him, “With a steadfast faith in the fulness of nature,
a profound conviction that if a new substance were demanded for the
carrying out of some beneficial project, that substance need only be
sought for, he set to work.”
Mr. Edison found difficulties in his way. One was the apparent
impossibility of illuminating by means of an incandescent solid, for
even platinum will melt at a heat too low for use. But this apparent
impossibility was overcome by the inventor’s genius. He, after many
trials, found that if he raised the platinum to a white heat _in a
vacuum_ he would practically obtain _a new metal_ which would sustain
the required heat.
[Illustration: Fig. 273.—Edison’s platinum lamp.]
“In making an electric lamp without a regulator,” says Mr. Upton,
“two things are essential,—great resistance in the wire, and a small
radiating surface. Mr. Edison sought to combine these two essential
conditions by using a considerable quantity of insulated platinum
wire wound like thread on a spool.” This platinum, as shown in the
accompanying cut (fig. 273), was suspended in a glass bulb _in vacuo_,
the air contained in it being expelled by electricity, heating it,
and suddenly cooling the platinum, and squeezing out the air by the
process. But, after all, the great difficulty of the inventor was to
insulate his wires so perfectly that they would not meet and become a
conductor. For, to perfect his lamp, this non-conducting principle was
a necessity, otherwise the current would flow across instead of going
all along the wires. He had previously made many uses of carbon, which
we know is infusible. He tried lampblack tar, but it contained air, and
would not do.
Thread answered his purpose, but was too fragile and uneven in texture.
It suddenly occurred to him that paper—_charred paper_—cut into a
thread-like form would satisfy all his conditions.
The problem was solved—the lamp was a fact. But how can paper, so
easily burned, answer? We will endeavour to explain. “A piece of
charred paper, cut into horse-shoe shape, so delicate that it looked
like a fine wire firmly clamped to the two ends of the conducting and
discharging wires, so as to form part of the electric circuit, proved
to be the long-sought combination.”
We will now explain the construction of this little lamp, which is
shown in the illustration (fig. 274) one-half of its actual size. The
illuminating is equal to ten or twelve ordinary gas jets.
[Illustration: Fig. 274.—Edison’s electric lamp.]
The manner in which the paper is prepared is, like many other very
important inventions, extremely simple, and, we may add, almost
costless. Cardboard will furnish us with the loops, and these
“horseshoes” are placed in layers in an iron box with tissue paper
between each. The box is then hermetically sealed, and made red hot.
The carbonized paper remains till all the air has been got rid of,
and although it will burn freely to ashes in atmospheric air, _in
the vacuum prepared for it_ it is never consumed. That is the plain
fact—the secret of the Edison lamp.
A vacuum can now be produced almost perfect. It is of course impossible
to extract every tiny particle of air from the globes, but by the
Sprengel pump, in which mercury is employed, excellent vacuums are
obtained. Several very curious phenomena have been observed in
these vacuums, and the Royal Society has been engaged upon their
consideration. Another advantage of the vacuum, as applied by Edison,
is that little or no heat is generated. The electricity is all, or very
nearly all, converted into light. Thus the glass globes remain almost
unheated, and are unbroken.
The electric current passes along the wire, W, and at a
certain place marked B, the copper is soldered to a platinum
wire, which enters at C, and so by platinum clamps into the
horse-shoe, L. The return wire is similarly arranged; the
carbon is enclosed in a glass bulb, GG, and all the air is
extracted by the pumps; the end is then sealed up by melting it at
F.
The world is now in possession of a lamp for household use, and we are
surprised that it is not more extensively adopted in England. There
are some Swan lamps used in parts of the British Museum, and when we
have explained the application of the light, and the uses to which the
motive power can be applied, we shall, we believe, convince the most
conservative gas bill advocate that Edison’s lamp is cheaper, safer,
and far better in illuminating power than gas, if the success of the
electric lamp can be assured.
We need not dwell upon the construction of the “pumping station,” for
that is virtually what the magneto-electric generator is. Several of
these stations can be established in various parts of the city, and
each station will supply a district with electricity. The wires are
laid in a tight box along the street, beneath the footpath, or other
convenient position, and we are informed that the frost rather improves
their electrical condition. Here is one advantage over gas.
From the main wires smaller ones enter the houses, and are carried
through a “meter” containing a safety valve. There are two wires—a
distributing wire and a waste—coloured, one red and the other green,
which communicate respectively with the main supply and return wires
to the “pumping station” or generator. The electricity is admitted
between carbon points and flows round a magnet, the armature of which
is held above it by a spring. If too much force be put on and any
danger incurred, the magnet will attract the armature, and the current
will cease. A snap connected by a small wire will then be closed by the
electricity, and melting from the heat will cut off all the current.
In ordinary circumstances the electricity passes through regulators
(wire wound on spools) and on to a copper plate, “through a solution
of copper salt.” Thus for every unit of current a certain quantity of
copper is deposited. A certain standard amount represents five cubic
feet, and the bills, based on the accumulation of copper, are made out
like gas bills.
When the lamp is required a small handle is turned, and is instantly
lighted; the reverse motion cuts off the current. “By touching a knob
in the bedroom the whole house can be simultaneously lighted up” if
desirable. No matches are necessary, as the lamps light themselves.
By adding a small electro-motor to the furniture of the house, and
turning a handle, the sewing-machine can be worked by electricity, or
lathes turned; and any business operations, such as lifting by cranes,
etc., can be easily carried on.
The Swan electric lamps, which, with Mr. Edison’s, were exhibited in
Paris, and will be found at Sydenham, give about twelve candle-power
light. Edison’s lamps are made in two sizes, and vary accordingly.
The Swan lamps give a very soft light, and are as easily manipulated
as Edison’s. The Siemens system of lighting was also well seen in
Paris, and the Faure storage system enables our trains to be lighted
instantaneously by simply turning a handle. A full description of the
_Faure_ battery was given in the _Times_ by Sir William Thomson, and
in his address to the British Association at York in September last.
He pointed out that in the accumulators of M. Faure,—which can be seen
at 446, West Strand, London,—by means of a large battery it is quite
easy to draw off electricity and to apply it as Edison proposed to do,
in lighting our houses and do any little service. The electricity thus
stored would be always ready for use, and would be supplied and paid
for. It can be applied to any purpose, and locomotion by its means
will ere long become more general. In Paris Dr. Siemens exhibited
his electric tramway. This was an improvement upon the first Berlin
tramway, for in it the horses frequently received shocks which they
resented. In the later application the current comes from the generator
by metal rods carried above the heads of the passengers alongside the
line. Little rollers upon these are united with an electric machine in
the tram-car. The current is sent along the wires, and reconverted into
mechanical energy in the second machine, turns the wheels of the cars.
In this way, as the car proceeds, the rollers overhead or alongside the
track are kept moving by the car, and the connection is never broken.
But this is a digression. The electric light as applied to lighthouses
was also exhibited, and any reader desirous to obtain full information
upon the subject of lights and lighthouses will find it in a very
pleasantly-written work by Mr. Thomas Stevenson, in which the various
systems of lighting by electricity and otherwise are fully recounted,
the conclusion being in favour of electricity, which is employed and
has been used for years in France and in some English beacons. If its
penetrative power can be finally established,—for some authorities
maintain that the electric is more easily absorbed by fog than other
light,—there is no doubt about its being universally adopted.
It is very interesting to watch the uses to which the electric light
is being put. The latest experiment has been made by an Austrian,
Doctor Mikerliez. Almost incredible as it may seem, the interior of the
human stomach can now be illuminated by means of a wonderful little
instrument called the Gastroscope, which is said to be actually in
use and to have been favourably reported upon by the medical faculty
of Vienna. There is at the end of a jointed flexible tube (which can
be passed down the gullet) a miniature lamp, far more marvellous and
mysterious than that of Aladdin, in which a strip of platinum is fixed
and connected with fine wires conducting the electricity from a small
battery. When contact is made, and the “light turned on,” the cavernous
interior of the stomach is lit up. Still more extraordinary is the
fact that the tube can be made to revolve, and the light reflected
from the walls of the stomach and directed to the eye of the observer.
There is necessarily a bend in the instrument, so that the light has
literally to turn a corner before it reaches the surgeon’s eye; here
the inventor’s skill and thorough knowledge of the laws of optics are
brought into requisition. The reflected rays of light fall upon a sort
of window situated a little above the lantern, and by means of prisms
and a series of lenses, the light is twisted and turned about until it
arrives at the eye-piece. No sensation of heat is to be feared, the
little lamp being kept constantly cool by a reservoir of water.
Several contrivances have been invented within the last few years for
examining the interior of the body, but they are very costly; the
Gastroscope is likely to render great service to medical science.
The term “magneto-electric machine” is given to a collection of parts
of mechanism intended to create or gather together induced electric
currents. The invention of the magneto-electric machine was by no
means a sudden inspiration, but the gradual result of a series of
experiments and discoveries, the first of which, dating from 1820,
may be said to be Œrsted’s observation, that a magnetised needle is
deflected by the approach of an electric current as well as by that
of a magnet, clearly proving that magnetism and electricity have some
relation to one another. In the same year Arago discovered that a coil
of insulated wire wound round a core of soft iron, converts it into
a powerful magnet (_i.e._, an electro-magnet) when a current passes
through the coil. It was in 1830, however, that our countryman Faraday
proved the creation of a current by the action of a magnet on a coil of
wire, and his experiment proved shortly as follows:—If a coil of wire
be wound on a hollow core, and a permanent bar magnet be introduced
into the hollow core, whilst introducing it a current may be proved
(by a galvanometer), to be induced in the coil flowing in a certain
direction, A B, which ceases as soon as the magnet is at rest in the
centre of coil. On the withdrawal of the magnet a second current is
induced flowing in the opposite direction, B A. Therefore it is clear
that if a magnet be incessantly approached to and withdrawn from a coil
of wire a constant succession of currents will be produced, and if a
charged coil (_i.e._, a coil connected with the poles of a voltaic
battery) take the place of the magnet a precisely similar result
will be obtained. Now it will have been noticed that two opposite
currents are constantly being formed, and as the object is to obtain a
continuous flow of electricity in _one given direction_, or, in fact,
divert or reverse the current instantly on its formation to make it
practically the same current, for this purpose a commutator is used,
and as for most purposes a commutator is one of the essentials of a
magneto-electric machine, we will here give a description thereof.
(_See_ fig. 275.) The machine is composed of a cylinder, consisting of
two metallic conducting halves, separated by a non-conducting layer.
Whilst it is at rest the alternating currents, from being connected
with the halves by the current, will pass to the two contact springs,
and thence through the circuit. Now if (as is the case) the current
is constantly changing, as has been noticed, the inverse current will
at the first change pass through the same channels, but in another
direction; but if at the instant of the reversal of the current the
cylinder be revolved, the current flowing the reverse way will be
guided through other channels respectively, instead of the original
channels, and the direction of the current being changed at the same
moment as the current itself, the two inversions neutralize themselves,
and one constant current is produced. In a magneto-electric machine
the commutator revolves identically with the magnet or armature, and
the point at which sparks are being constantly produced is where
the contact is being continually broken and made by the passage of
the friction springs from over the non-conducting layer. The first
machine formed on the basis of Faraday’s experiments was Pixii’s. It
was composed of two uprights and a cross bar, to which is attached,
hanging poles downwards, an electro-magnet; underneath this, the
poles upwards, revolves a magnet. The commutator is fixed on the same
axle and revolves with the permanent magnet. Saxton, and subsequently
Clarke, made the obvious improvement of making the magnet less cumbrous
and fixed, and causing the bobbins of the electro-magnet to revolve
before or rather beside its poles; the commutator was fixed at the end
of the axle on which the revolving bobbins (or armature) are fixed.
Niaudat formed a compound Clarke machine, by setting two horse-shoe
magnets a short distance apart. The armature revolves between them,
and consists of twelve coils set between two plates; the coils are set
alternately and connected,—_i.e._, the poles of the electro-magnets
are set beside one another,—N. to S., S. to N., and so on, so that the
N. pole receding produces a current; _but_ the N. pole receding makes
the S. pole approach, and produces another current, A B; in fact, a
continuation of the same, for the _approach_ of a N. pole naturally
produces the same current as the recession of a S. pole; then as the
S. pole in turn recedes it produces an inverse current, B A, which is
in turn kept up by the approach of the next N. pole, and so on. Each
coil is attached to a radiating metal bar, which conveys the current to
be redirected to the commutator, which is affixed to the axle of the
revolving armature as in Clarke’s machine. In 1854 Siemens completed
his machine, the chief peculiarity of which was its cylindrical bobbin;
the core is grooved deeply, parallel with its axis, and the wire is
wound on cylindrically and covered with plates of brass; one end of
the coil is fixed to the metal axis, the other to an insulated ferule
at the end of the axis, where is also situate the commutator. This
armature revolves between the poles by which it is closely embraced.
One of the most celebrated of the magneto-electrical machines is that
known as the “Alliance,” invented by Nollet, and perfected by Van
Malderen. It is composed of four or six bronze discs, revolving on an
axle, round the external circumference of each of which are set sixteen
bobbins. This rotating compound armature revolves between four to six
sets of horse-shoe magnets, which, being fixed radially to the centre,
present in each set sixteen poles to the sixteen bobbins. It will be
readily understood that this immense quantity of poles and bobbins
produces a highly concentrated current, the ends of which proceed from
the axle and an insulated ferule at its extremity.
[Illustration: Fig. 275.—The Wallace Machine.]
In 1869 Mr. Holmes perfected his machine, which differs from all
previous ones (except Pixii’s), in that the electro-magnets revolve in
front of the coils instead of _vice versâ_; and besides magnetising
his electro-magnets with part of the self-produced electricity, his
bobbins are so disposed as to be able to keep several independent
lights going at once. The Wylde machine consists, as it were, of two
Siemens machines, one on the top of the other, the lower and larger
of which is worked by an electro-magnet, which is magnetised by the
action of the upper or smaller one, consisting in the ordinary way of
a permanent magnet apparatus, which is termed “the exciting machine.”
The longitudinal bobbin revolved between these permanent poles produces
alternating currents, which are commutated (or redirected), and
pass to work the larger and lower electro-magnet, which is composed
of two large sheets of iron connected by a plate (on which stands
the exciter). Its poles are two masses of iron separated by a layer
of copper, and in this armature revolves the larger longitudinal
bobbin. This lower machine is called the generator. Both bobbins
are simultaneously revolved, and an intense current of electricity
is thereby generated. Almost simultaneously with this one Mr. Ladd
invented his machine, which is distinguished from all hitherto
described by being composed of two parallel _bar_ electro-magnets,
between the extremities of which are placed two Siemens armatures, one
smaller than the other; both being revolved, the smaller excites the
electro-magnets, and the larger generates the electricity required.
The wire is wound round the magnets so that the N. and S. poles face
each other at each end. The chief advantage of the Ladd machine is the
conversion of dynamic force into electricity, there always being just
sufficient magnetism in an iron bar (by induction from terrestrial
magnetism and other causes) to produce a very feeble current in the
Siemens bobbin, and the bobbin taking it up and returning it to the
electro-magnet, and the electro-magnet at once giving it back to the
bobbin, the current gradually increases till the maximum is reached.
And when we take into consideration this modicum of utilisable
terrestrial magnetism, we may truly say in the words of M. Hippolyte
Fontaine, “The mind is lost in contemplation of the succession of
discoveries completing one another, and showing that with apparatus of
small dimensions an infinite source of electricity could be produced
if matter could withstand infinite velocities.” The Lontin machine,
which supplied the current for the electric light which used to make
night bright outside the Gaiety, is also composed of two parts, one
dividing, the other generating the electricity produced. The principle
of the dividing machine is somewhat similar to the alliance, excepting
that a number of electro-magnets arranged radially round a core,
revolve close to a corresponding number of bobbins fixed inside an iron
cylinder, outside which is the collecting and dividing apparatus. The
Maxim machine is constructed on the principle of sets of coils rotating
between powerful electro-magnets. The Wallace machine was invented by
the inventor of the Wallace-Farmer lamp. It consists of two horse-shoe
electro-magnets placed side by side, the opposing poles facing each
other. Each magnet has a rotating armature of twenty-five bobbins,
on which the wire is wound quadruply, and the current generated by
these coils is conducted away, passing through and exciting the
electro-magnets, thus utilizing the residual and terrestrial magnetism
before mentioned in connection with the Ladd machine; otherwise it
partakes of the nature of the Niaudet machine.
[Illustration: Fig. 276.—The Gramme Machine.]
We now come to what is perhaps the most perfect magneto-electric
machine, which was first constructed by M. Gramme, a Parisian, in 1872,
and differs in principle and construction from all those hitherto
noticed. Its essential characteristic is a soft iron ring, round which
is coiled one single continuous wire (_i.e._, the two ends are joined).
Round the exterior surface of the wire coil a band is bared, and on
this bared part two friction springs act. If the ring and coil be
placed before the poles of a magnet, the ring will have two poles, S.
and N., induced opposite the opposing poles N. and S. of the magnet;
and if the ring revolve the poles will remain stationary, and as the
coil revolves each coil of the wire will pass this induced pole, and
as naturally half the coil will be inducted with one current, the
other half (acted on by the other pole) will be charged with another
or opposite current, which two kinds of electricity are carried away
by the friction springs before mentioned. In the machine, as actually
constructed, the soft iron ring is composed like the magnet or wire
bundle of an induction coil, and the coils are set upon it side by
side. Inside the ring are radially set insulated pieces, to each of
which is attached the issuing end of one and the entering end of
another bobbin; these answer the same purpose as the denudation of the
external layer of wire. These pieces are bent so as to come out of the
centre of the ring at right angles, and lay side by side (insulated)
round a small cylinder. These, as they revolve, are touched by friction
springs, which draw off the electricity induced in the coils in one
continuous current. No sparks are produced at the contact of the
friction springs, and there is no tendency to become heated. To obviate
the inconvenience of the secondary or inverted current produced by
the stopping of the machine, the inventor has contrived a circuit
breaker on the principle of the electro-magnet, the magnets holding
the circuit breaker in contact so long as the machine is working; but
the decrease of velocity lessening the attractive power of the magnet,
the circuit breaker opens by its own weight (or a counter-weight), and
all danger of a reverse current is obviated. Experimental machines are
manufactured by Bréquet & Cie (Paris), composed of Jamin’s magnets, and
turned with a handle, and produce a force of eight Bunsen cells.
A great revolution, or rather the beginning of a new era in the history
of electricity, may be said to have commenced with the perfection of M.
Faure’s accumulators. These are troughs containing eleven lead plates,
each coated with oxide of lead and wrapped in felt, the fluid being
dilute sulphuric acid. The application of them to the electric light is
one of their most valuable features; at the depôt in the Strand, where
they may be seen at work, there are thirty such elements, each weighing
about 50 lbs. It takes a two-horse-power engine working an Edison or
Gramme machine six to eight hours to charge them, and when charged they
will keep almost any number of lamps of sixteen-candle-power going some
eight hours. They are used on the Brighton and South Coast Railway,
and seem peculiarly adapted to lighting by incandescence, by Swan, or
Edison’s lamp. The elements fully charged may be carried any distance
without losing their electric power. And the stored force may be used
for charging the accumulators themselves afresh from the machine. These
accumulators may be seen any day at 446, Strand, and are well worth a
visit.
The Gyroscope, though now an instrument common and familiar to all
students, is none the less the subject of a problem, the solution
of which is still to seek. It has indeed been entitled the paradox
of mechanics; for though it depends on gravitation, gravitation yet
appears indifferent to it. In order to render the operation of the
Gyroscope as continuous as possible, so as to facilitate the profound
study of its working, and also to unite another influence with those of
the ordinary Gyroscope, producing phenomena of which this instrument
affords us the spectacle, a learned American has employed electricity
as a motive power.
[Illustration: Fig. 277.—The Gyroscope.]
The Gyroscope, shown in fig. 277, has a large, heavy pedestal, with a
pointed column, which supports the instrument itself. The frame, of
which the electro-magnets form a part, is connected with a rod, having
at one end a hollow cavity which rests on the point of the vertical
column. One of the extremities of the magnetic spool is attached to
this cavity, the other end communicating with the bar which unites
the two magnets. Over this bar is a spring which breaks the current,
supported by an insulator in hard india-rubber; it is adjusted so that
it touches a small cylinder on the axis of the wheel twice during every
rotation of the latter. The wheel’s plane of rotation is at right
angles with the magnets, and it carries an armature of soft iron, which
rotates close to the magnet without touching it. The armature is so
placed in relation to the surface of contact with the cylinder that
breaks the current, that twice during each rotation, as the armature
approaches the magnets, it is attracted; but immediately afterwards,
as the armature comes directly in front of the magnets, the current
is broken, and the acquired impulsion is sufficient to move the wheel
until the armature comes again under the influence of the magnet. The
spring which interrupts the current is connected with a thin copper
wire, which stretches back as far as the point of the column, entwining
it several times to render it flexible, finally bending down and
plunging into some mercury enclosed in a round vulcanite cup placed on
the column near the pedestal. The pedestal also bears two small stakes
for receiving the wires of the battery, one connected with the column,
and the other communicating by a small wire with the mercury contained
in the vulcanite vessel. The magnets, the wheel, and all the connected
parts can move in any direction round the point of the column. When
two large Bunsen cells, or four small ones, are connected with the
Gyroscope, the wheel turns with great rapidity, and allowing the
magnets to operate, it not only sustains itself, but also the magnets
and the other objects which are between it and the point of the column
in opposition to the laws of gravitation. The wheel, besides turning
rapidly round its axis, also effects a slow rotation round the column
in the direction of the movement experienced by the _lower part_ of the
wheel. By placing the arm and the counterpoise of the machine as shown
in fig. 277, so that the wheel and the magnets balance exactly on the
pointed column, the whole machine rests stationary; but if we give the
preponderance to the wheel and the magnets, the apparatus begins to
rotate in a direction contrary to or following that of the _upper part_
of the wheel.
The Gyroscope exemplifies very clearly the persistence with which a
body undergoing a movement of rotation maintains itself in the plane of
its rotation in spite of gravitation. It shows also the result of the
combined action of two forces tending to produce rotations round two
axes which are separate, but situated in the same plane. The rotation
of the wheel round its axis, produced, in the present instance, by
the electro-magnet, and the tendency of the wheel to fall or turn in
a vertical plane, parallel to its axis, produce, as a result, the
rotation of the entire instrument round a new axis which coincides with
the column.
PEIFFER’S ELECTROPHORUS.
It will now perhaps interest our readers to describe a charming little
plaything which is a great favourite with children, and which has the
incontestable merit of early initiating them into all the principal
phenomena of the statics of electricity, and teaching them the science
of physics in an amusing form.
It is a small electrophorus invented by M. J. Peiffer, and reduced
to such a point of simplicity, that it consists merely of a thin
plate of ebonite, about the size of a large sheet of letter paper.
The tinned wooden disc of the electrophorus which is found described
in most treatises on physics, is replaced by a small sheet of tin,
about the size of a playing-card, fastened on to the surface of the
ebonite. The ebonite electrophorus produces electricity with remarkable
facility. It must be placed flat on a wooden table, and thoroughly
rubbed with the hand; if it is then lifted, and the sheet of tin
lightly touched, a spark is elicited from ¼ inch to ½ inch in
length. The electrophorus is completed by the addition of a number
of small accessories in the shape of small dolls made of elder-wood,
which exhibit in a very amusing manner the phenomena of attraction and
repulsion. After the board has been charged with electricity, place the
three little figures on the sheet of tin, and lift up the apparatus,
so as to isolate it from any support. You will then see one little doll
extending its arms, another with its silky hair standing on end, and a
third, lighter than the others, leaping like a clown, and displacing as
he does so the two small balls of elder-wood which have been placed on
each side of him. We have given an illustration of the three figures
performing at once (fig. 278), but they are generally used separately.
M. Peiffer has indeed collected every known accessory for an electric
machine, such as Geissler’s tube, the electric carillon, etc. These
different experiments are all reduced to their simplest form, and, with
their appliances, are all contained in a cardboard box. They are placed
beside the electrophorus, which thus takes the place of an unwieldy
electric machine. M. J. Peiffer accompanies this little portable
cabinet with an exhaustive pamphlet, which is a valuable guide to the
young physicist in studying the first principles of electricity.
[Illustration: Fig. 278.—M. J. Peiffer’s electrophorus with dolls.]
“It is easy to discover in the education of children,” says M.
Peiffer in his preface, “how to turn their budding faculties to the
best account. Would you utilize them in a satisfactory manner?—Then
put in their hands playthings which, in an attractive form, serve to
familiarize them at an early age with those sciences, a knowledge of
which will be at a later period absolutely indispensable to them; and
they will be much more amused than with ordinary commonplace toys.”
These are sensible words, in which we heartily concur. Yes! Science
properly taught, and properly understood, can indeed be brought within
the range of children; it should give a lasting interest to all
amusements, and form a part of the culture of the youthful mind, as at
a later period it will contribute to the perfect development of the
grown man.
MAGIC FISH.
An ingenious physicist, M. de. Combettes, who is a civil engineer at
Paris, has devoted himself to constructing a number of playthings and
scientific appliances for young people, among which we will describe
the very curious one represented in fig. 279. A jar is filled with
water, holding in suspension some fish made of tin, similar to those
which children put in water and attract with a magnet. In this case,
however, the mechanism is hidden, and the operator can turn the fish
first in one direction and then in the other at pleasure. The secret of
this experiment is easily explained by examining the illustration (fig.
279). In the wooden stand which supports the jar there is concealed
a small electro-magnet which acts on the soft iron contained in the
floating fish. When the current passes the small magnet turns round and
attracts the little fish swimming in the water. This gyratory movement
can be changed at pleasure by means of a regulator.
[Illustration: Fig. 279.—Experiment of magic fish set in motion by
electricity.]
[Illustration: Fig. 280.—An electric toy.]
We will give an illustration of a few electric toys which M. Trouvé
has found for us. In the picture (fig. 281) we see three different
objects,—a rabbit beating a small bell, a representation of a bird with
outstretched wings, and a pin surmounted by a skull. All these are
capable of having movement imparted to them by means of electricity,
although made and intended for ordinary use in the form of scarf-pin or
other ornament.
Let us take the “death’s head” pin first. It is in gold, and enamelled
with diamond eyes and articulated jaws. The rabbit is also gold, and
carries two small drumsticks, with which he can play a tiny bell. This
device also can be worn as a scarf-pin.
[Illustration: Fig. 281.—Magic toys.]
A conducting wire leads from the pin into the waistcoat pocket, where a
small “pile,” about the size of a cigarette, is hidden away. If any one
particularly admires the scarf-pin, all you have to do is to insinuate
your fingers into your pocket, and you will, by contact, cause the
electric current to act upon the pin in your scarf. The death’s head
will at once begin to roll its eyes and grind its teeth, while the
rabbit, under similar circumstances, will begin to play its bell with
the greatest energy.
The handsome diamond bird represented in the centre of the illustration
belongs to Madame de Metternich. When any lady wears it in her hair,
she can, by the concealed wire, make it flap its jewelled wings, and by
so doing cause much surprise amongst the spectators.
We will now endeavour to give a description of the manner in which
these toys play their parts in company with the “hermetic-pile” which
M. Trouvé has applied to many specialities that he has supplied to
doctors, who use them largely.
This pile is formed by a “couple” of carbon and zinc hermetically
enclosed in an ebonite box. The carbon and zinc only occupy one-half
of the case. The liquid occupies the other. The sketch (fig. 280) on
preceding page will explain the apparatus.
So long as the case is in its normal position the elements are not
immersed in the solution, and consequently no electricity is developed.
But as soon as the figure is placed in a horizontal or leaning position
the force is generated; on readjusting the box the electric current is
cut off, and all development ceases. Many curious electrical toys can
be seen in Paris. Dolls are made to talk, and many other wonders for
children can be easily procured.
ANIMAL AND ATMOSPHERIC ELECTRICITY.
Before concluding the subject of electricity we must devote a few pages
to the consideration of the electric influence possessed by certain
fishes, and to some of the phenomena of the atmosphere, especially
thunderstorms. We have seen how Galvani experimented upon the limbs of
frogs, and maintained that they possessed electricity; he attributed
the current in the muscles to that cause. This theory Volta denied,
but subsequently Nobili, in 1827, proved the existence of a current
in the frog by means of a Galvanometer. This was conclusive, and the
experiment was performed in the following manner:—He filled two vessels
with salt and water, and into one he dipped the crucal muscles of a
frog, and in the other the lumbar nerves were immersed. By putting
these vessels in communication with his improved Galvanometer, which
was extremely sensitive, he perceived a current passing from the feet
towards the head of the animal.
It is, however, to Matteucci and Du Bois Reymond that the investigation
of the phenomena of the _courant propre_ are due. The former formed a
“pile” of the thighs of frogs, and by placing the interior and exterior
muscles in contact he formed a current from the inside to the outside
muscles. This current is supposed to be occasioned by certain chemical
changes which are continually taking place, and it continues longer in
the case of a cold-blooded animal than in a warm-blooded one. There
are many interesting papers on this subject included amongst the
“Philosophical Transactions”; and the “Physical Phenomena of Living
Beings” is fully treated in Matteucci’s lectures on that subject. In
the “Transactions” for 1848 and subsequent years, other experiments
may be perused, but space will not permit us to dilate upon them. The
fact has been established, and we are told that muscles and nerves,
as well as certain glands of the body, possess certain electrical
properties.
The electricity of fishes, and the power possessed by the torpedo—whose
name is now chiefly known in connection with warlike appliances—and the
gymnotus, have been known for a very long time. This fish, popularly
known as the electric eel, inhabits the warm fresh-water lakes of
Africa, Asia, and America. A specimen was exhibited at the Polytechnic
some years ago. This was the fish experimented on at the Adelaide
Gallery by Professor Faraday, who clearly demonstrated the fact that
the electricity of the animal and the common electricity are identical.
Numerous experiments were made, and the circuit shock and even sparks
were obtained from the gymnotus. In fact, the gymnotus is a natural
electric machine. The force of the shock given by the electric eel is
very great, for Faraday has put on record that a single discharge of
the eel is equal to fifteen Leyden jars charged as highly as possible.
Its power does not even end there, for having shocked people to that
extent, it was capable of a second and occasionally of a third shock of
less violence.
[Illustration: Fig. 282.—Electric eel.]
The manner in which the gymnotus acts is from a regular battery in
the head, the sides of which are filled with a fluid. These cells are
something like a honeycomb in appearance. The shock is quite voluntary
on the part of the fish. Sometimes it will kill its prey, on other
occasions it is merely numbed. Professor Faraday on one occasion placed
a live fish in the tub with the gymnotus, which curled itself so as to
enclose the unsuspecting one. In a second the prey was struck dead, and
floated on the water. The gymnotus immediately devoured it, and went in
quest of more. Another, but an injured fish, was then introduced, but
the electric eel took no trouble about this one. It did not trouble to
give it a shock, seeing it was disabled, it merely swallowed without
killing it. It is also on record that on one occasion an electric eel
had stunned a fish which, before he began to eat it, gave signs of
returning animation; the eel immediately gave it another shock and
killed it.
[Illustration: Fig. 283.—Large gymnotus.]
There were some other curious peculiarities connected with the electric
eel. It appears to be quite capable of discriminating between animate
and inanimate touch. For instance, when touched with a glass rod it
at first gave signs of electricity, and discharged a shock at the
attacking party. But on subsequent occasions, when touched with metal
rods or glass, the fish declined to “shock”; nevertheless the Professor
succeeded the moment he touched the animal with his hands.
The torpedo is something like the well-known skate; it is sometimes
called the electric ray, and is common enough in the Bay of Biscay and
in the Mediterranean Sea. It sometimes pays England a visit, or is
caught by fishermen and brought in. We have seen one at Plymouth, and a
very ugly-looking fish it was. Its electric power is considerable.
[Illustration: Fig. 284.—Ray torpedo.
_c_, brain; _m_ _e_, spinal chord; _o_, eye; _e_, electric organs; _b_,
gills; _np_, _nl_, nerves; _n_, spinal nerve.]
There is yet another fish known as the malapterurus; one species is
called the thunder-fish. Professor Wilson has written a paper upon the
electric fishes as applied to the remedy of disease, and considers them
the “earliest electric machines ever known.”
Humboldt relates that the South-American Indians capture the gymnotus
by driving horses into ponds which the electric eels are known to
inhabit. The result is that the fish deliver shock after shock upon the
unfortunate quadrupeds. Mules and horses have frequently been killed by
these powerful eels, and even Faraday experienced a very great shock
when he touched the head and tail of the captive gymnotus with either
hand.
The malapterurus to which we have referred is an inhabitant of the
African rivers, chiefly in the Nile and Senegal. Such a fish has been
known with others for some hundreds of years; its electrical powers are
not great. There are one or two other species of fish which possess
electrical qualities, but none apparently to the same extent as the
torpedo and the gymnotus.
[Illustration: Fig. 285.—The Malapterurus.]
The electricity of plants also is in some cases very marked. Flashes
have been seen to come from some flowers in hot and dry weather.
Currents of electricity have been detected, and Wartmann investigated
the subject closely. He says the currents in flowers are feeble, but in
succulent fruits and some kinds of grain they are very marked. These
currents depend upon the season, and are greatest in the spring,
when the plant is bathed in sap. These experiences were confirmed
by Bequerel in 1850, and he concludes that the rank vegetation in
some parts of the world must exercise considerable influence on the
electric phenomena of the atmosphere. M. Buff has more recently made
experiments in this direction, and he examined plants and trees, and
even mushrooms. M. de la Rive, after carefully summing up the various
theories, comes to the conclusion that it is to chemical reactions that
the traces of electricity are due.
The subject of atmospherical electricity properly belongs to
meteorology, and under that heading we will treat of it more fully. But
lightning is so identified with electricity, and being the most common
form observable to all, we will say something about thunderstorms and
the electric discharges accompanying them.
[Illustration: Fig. 286.—Benjamin Franklin.]
Before Franklin’s ever-memorable experiment with his kite established
the identity of lightning and electricity, the resemblance between the
two discharges had been frequently noticed. The Etruscans pretended
to bring down lightning from heaven, and Tullus Hostilius, when
experimenting or performing certain “ceremonies,” was killed by the
electric discharge he desired to attract. But after all, we cannot
attribute any knowledge of electric science to the ancients, although
they were, of course, familiar with electric phenomena.
It is to Dr. Wall that testimony points as the first person who
remarked the analogy between the electric spark and lightning. This was
in 1708. Grey and other philosophers supported the theory, but could
not establish it. To Franklin, who in June 1752 actually brought down
the lightning by his kite and a key, is the actual discovery due. We
have already detailed the circumstances (page 206) and need not repeat
the account of the experiment.
Of course the American philosopher found numerous imitators, not always
with impunity. Professor Richmann was killed by the spirit he was
invoking; Lemounier and Beccaria confirmed the theory that the air was
full of electricity; while Du Saussure, from his investigations on the
Alps, and Volta from the invention of the pile, are most famous in the
history of electricity. They applied themselves with much success to
the investigation of the electric condition of the atmosphere, of which
the disturbances called thunderstorms are the result.
The amount of electricity varies in the atmosphere at different times
in the day and night. Towards midday and midnight the development is
generally greatest, and this fact will account for the prevalence of
storms during our hours of rest. Again, different kinds of clouds have
different degrees of electricity, and of different kinds. Under certain
conditions these clouds will give forth lightning, and a storm will
begin. The more clouds the more globules, and therefore in summer,
while there is more production of vapour from solutions of salts, etc.,
we are more likely to have the storms. We are most of us familiar
with the mass of the “thunder cloud” rising in the distance, light at
the upper part, very dark below, and throwing out tentacles like the
octopus, coming up sometimes—frequently, indeed—“against the wind,”
impelled by an upper current, or following the course of a river,
which is not unusual. Below, there is perhaps an army of thin dark
clouds. The nature and height of clouds have also a great deal to do
with the phenomena displayed. In general, storm-clouds are positively
electrified.
[Illustration: Fig. 287.—Cirrus cloud.]
Clouds are good conductors of electricity, and yet they may be so
insulated by the dry air surrounding them that they will accumulate
it; and when thus charged, if they encounter other clouds charged with
opposite electricity, the opposing masses will attract each other until
a discharge takes place. This is what we term lightning, and under
such conditions electricity, though very dazzling, is harmless. It is
when the cloud comes near to the earth, and a discharge is released,
that lightning is so dangerous to persons who remain in the fields.
Sometimes the discharge comes from the earth to meet that from the
cloud. Sheep are frequently killed by ground lightning, and once, at
Malvern, we had an escape from an upward stroke. The back-stroke from
a cloud is also dangerous. It may happen that the cloud has discharged
itself upon the earth many miles away, but a return discharge takes
place at the other end, and if that end be near the earth the
consequences may be serious. As a rule, the return stroke is not so
violent as the first discharge.
The colour of lightning varies very much. We have the white, the blue,
the violet, and red. The colour depends upon the distance and intensity
of the lightning, and the more there is of it the whiter the light. We
can illustrate the varied hues of the electric “fluid” by passing a
spark through the receiver of an air-pump. If the air be rarefied, or
there be a vacuum, we shall perceive a blue or violet light. Therefore
we may conclude that the blue and violet flashes have birth in high
strata of the atmosphere.
[Illustration: Fig. 288.—Cumulus cloud.]
We have all heard how dangerous it is to stand under a tree during a
thunderstorm, or rather, we should say, when the storm is approaching
us nearly. The tree is a conductor, and the lightning having no better
one at hand will pass through the tree on its way to the earth, and if
we are standing against the tree we shall be included in the course,
and die from the shock to the nerves while the lightning is passing
through us. The best position in a thunderstorm, if we are in the
neighbourhood of trees, is to sit or lie down on the ground some little
distance from the base of the nearest tree. If the tree be sixty feet
high suppose, and we sit fifty feet or less from the trunk, we shall
be pretty safe, because the lightning will reach the tree top before
it can reach us. We are protected by it as by a conductor, bad though
it be. Standing up in a boat during a storm is not wise. Lightning has
an affinity for water, and besides, if no higher objects are near, our
body will act as a conductor. Bed is the safest place, as blankets are
non-conductors. Cellars are not the safest by any means. Lightning may,
and it frequently does, strike the house and descend to the basement.
If the air be very full of electricity, and a flash be near, a person
running away may conduct the lightning to himself by creating a vacuum
into which the flash may dart.
[Illustration: Fig. 289.—Nimbus, or rain cloud.]
Arago classified lightning into three kinds: zig-zag, globular, and
sheet. The first we call forked lightning, and frequently this kind
branches out at the end, so that although there may be only one flash,
it may strike out in two or three directions at the same time. This may
be accounted for by the unequal power of the air strata to conduct the
electricity. The forked flashes are of very great length, extending
frequently for miles, and the bifurcations also are often miles apart.
The duration of the flash is less than the thousandth part of a second;
so instantaneous is it that no motion can be perceived even in a most
rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes
fancy that the flash lasts longer, but the impression received by the
eye quite accounts for the apparently prolonged sight of the lightning.
Sheet lightning, the faint flashes frequently seen upon the horizon,
are quite harmless. Sheet lightning is that which is seen reflected
behind clouds or from far-distant storms. It is sometimes very
beautiful. Ball, or globular lightning, is dangerous, and globes of
fire have been seen to descend, and striking the ground, bound onwards
for some distance. The descent of these forms of electric discharge
has given rise to the popular notion of “thunderbolts.” The “Mariner’s
Lights,” or St. Elmo’s fire, is frequently observed in ships. It is
usually regarded as a fortunate occurrence. It was noticed by Columbus.
One voyager describes the phenomena as follows:—“The sky was suddenly
covered with thick clouds.... There were more than thirty of St. Elmo’s
fires on the ship. One of them occupied the vane of the mainmast. I
sent a sailor to fetch it. When he was aloft he heard a noise like that
which is made when moist gunpowder is burned. I ordered him to take off
the vane. He had scarcely executed this order, when the fire quitted
it and placed itself at the apex of the mainmast, whence it could not
possibly be removed.”
[Illustration: Fig. 290.—Thunderstorm.]
There have been occasions when the manes and tails of horses, and
even the ears of human beings, have shown a phosphorescent light
which emitted a hissing noise. Alpine travellers have noticed similar
phenomena; and Professor Forbes, when crossing the Theodule Pass
into Italy, heard the hissing sound in his alpenstock. The tips of
rocks and grass points were all hissing too. The party were in the
midst of an electric cloud. When the Professor turned the point of
his alpenstock upwards, a vivid flash was emitted, but no thunder
followed. They descended as quickly as possible from such a dangerous
neighbourhood.
[Illustration: Fig. 291.—St. Elmo’s fire.]
It is observable that the properties of lightning and of the electric
spark are identical—the faint crackle of the latter being magnified
into the loud rolling of the thunder. The disturbance of the atmosphere
is the cause of the loud reverberations, and echoes produced from
clouds tend to intensify and prolong the peal. The sound rises and
falls, and varies accordingly as the cloud is near or far. A smart
sharp report or rattle denotes the nearness of the lightning, while the
gradual swelling and subsidence, followed, mayhap, by an increasing
volume of sound, which in its turn dies away, tells us that the danger
is not imminent. The cause o£ this loud rolling, unless it proceeds
from echoes from different clouds, has not been satisfactorily
explained. Sound travels less quickly than light, and therefore we
only hear the thunder some seconds after we have perceived the flash.
It is therefore conceivable that we may hear the last reverberations
and its echoes first, and the sound of the first disturbance with its
echoes last of all. Thus there will be distinct sounds. Firstly, the
actual noise we call thunder from the air strata _nearest_ to us;
secondly, the echoes of that disturbance from the clouds, of course
fainter; then we hear the sound caused by the tearing asunder of the
air particles farthest off, and again the echoes of that disturbance.
This theory will, we think, account for the swelling peals of thunder,
and the successive loud and fainter reverberations. At any rate, in the
absence of any other theory, we submit it for consideration. The sound
of thunder is seldom or never heard at a distance greater than fifteen
miles.
Lightning conductors are such every-day objects that no description is
necessary; but the reason the lightning runs along it harmlessly is
because the galvanized iron rod is the best conductor in the immediate
neighbourhood. Where there is not a good conductor lightning will
accept the next best, and so on, any conductor being better than none.
The point of the rod cannot contain any electricity, there being no
room for it, and the “fluid,” as it is termed, runs down to the ground,
to terminate, when possible, in water or charcoal. A great deal of
electricity is no doubt carried away from the air by the numerous
conductors without any spark passing. Until Sir W. Snow Harris brought
forward his lightning conductors for ships, the loss was great at sea.
But now we rarely hear of any vessel being disabled by lightning. We
owe to Franklin the idea of the lightning conductor.
According to observations made by Mr. Crosse, the following statement
shows the tendency of the atmosphere, in certain conditions, to
thunderstorms. We may accept the deduction of M. Peltier that grey and
slate-colour clouds are charged with negative, and white, rose-colour,
and orange clouds with positive electricity. The order of arrangement
in the following table places the most likely source of thunderstorms
first, and the least likely source at the end, with regular rotation of
intermediate probabilities intervening:—
1. Regular thunder clouds.
2. Driving fog with small rain.
3. Fall of snow, or hailstorm.
4. Smart shower on a hot day.
5. Smart shower on a cold day.
6. Hot weather after wet days.
7. Wet weather after dry days.
8. Clear frosty weather.
9. Clear warm weather.
10. Cloudy days.
11. “Mackerel” sky.
12. Sultry weather and hazy clouds.
13. Cold damp night.
14. Cold, dry north-east winds.
We have thus briefly touched upon some of the atmospherical phenomena
directly attributable to electricity. In our articles upon Meteorology
we will consider the aurora and many other interesting facts concerning
the atmosphere, and the effects of sound, heat, and light upon the air.
[Illustration: Fig. 292.—Lightning conductor.]
FOOTNOTES:
[18] We are indebted for many facts respecting Mr. Edison’s light in
this chapter to a paper by Mr. Upton.