amusing and curious illusions. This property in itself is no new
discovery, for its presence and effects must have been noticed ages ago.
The property in question is illustrated when we twirl round a stick or
cord, burning with a red glow at the end. We seem to trace a _circle_ of
fire; but as the glowing spark cannot be in more than one point of the
circle at once, it is plain that the impression produced on the eye must
remain until the spark has completed its journey round the circle, and
reaching each point successively renews the luminous impression. Like
other subjects relating to vision, this phenomenon has been carefully
examined in recent times, and its laws accurately determined.
The fact which is obvious from such an experiment, may be thus stated:
Visual impressions repeated with sufficient rapidity produce the effect
of objects continually present. This persistence of the visual
impressions is easily made the subject of experiment by means of rapidly
rotating discs; and in the common toy called a “colour top” we have a
ready means of verifying some of the conclusions of science on this
subject. Some very interesting results may be obtained by an apparatus
as simple as this, regarding the laws of the phenomenon we are
considering, and the effects of various mixtures of tints and colours.
The well-known toy, the thaumatrope, depends on the same principle. In
this a piece of cardboard is painted on one side, with a bird, for
example, and on the other side with a cage: when the cardboard is
twirled round very rapidly by means of a cord fixed at opposite points
of its length, both bird and cage become visible at once, and the bird
appears in the cage.
[Illustration:
FIG. 249.
]
[Illustration:
FIG. 250.
]
[Illustration:
FIG. 251.
]
A still more ingenious application of this principle we owe to Plateau,
who described it in 1833, under the title of the _phenakistiscope_; and
also to Stampfer, who independently devised the same arrangement about
the same time, and named it the _stroboscopic disc_. The reader may, at
almost any toy-shop, purchase one of them, provided with a number of
amusing figures; or he may easily construct for himself one which will
exemplify the principle. He requires no other materials than a piece of
cardboard, and his only tools may be a sharp penknife, a pair of
compasses, and a flat ruler. Let him draw on his cardboard a circle of 8
in. diameter, and divide its circumference by eight equidistant points.
From these radii should be drawn with the point of the compasses, and
equal distances from the centre marked off upon them, to fix the centres
of the small circles, which must all have exactly the same size (say, 1
in. in diameter) and be marked by a distinct line. In these are to be
marked the hand of a clock-face in the positions shown in Fig. 249; and
finally, in the direction of the radii, narrow slips are to be cut out
of the cardboard as shown. If a pin be put through the centre of the
disc, attaching it thus to the flat end of a cork, so that it can freely
rotate in its own plane, and the disc be turned rapidly round, as in
Fig. 250, in front of a looking-glass, while the spectator looks through
the slits, he will see the hand on the little dial apparently turning
round, with rather a jerky movement it is true, somewhat like the
dead-beat seconds-hand that is sometimes seen on clocks. The illusion is
best when the slits are so narrow that only one of the several images is
visible by reflection, namely, that which is adjacent to the slit. Thus,
as the disc rotates, each little circle is visible for an instant as the
slit passes in front of the spectator’s eye; and if the rotation be
sufficiently rapid, the impression of the disc is permanent, as it is
constantly being renewed by the successive circles, while, on the
contrary, the hands, having different positions, produce images in
different positions, giving the appearance of a jerky rotation. The
instruments sold in the shops have sometimes a thin metallic disc with
the slits in it, and a series of designs printed in smaller paper discs.
The paper discs may be screwed on the other disc as required, and a
button on a pulley with an endless band is provided for producing the
rotation more conveniently. Fig. 251 shows one of the pictures for a
disc with twelve slits, and the effect produced by it is that of a
dancing figure.
Another arrangement for showing the same illusion has lately become a
very popular toy, and quite deservedly so, for it has the advantages of
requiring no looking-glass, and of making the effect visible to a number
of persons at the same time. This apparatus, which has been termed the
Zoetrope, consists simply of a cylindrical box, like a drum with the
upper end cut off. It is mounted on a pivot, which permits its revolving
rapidly about its vertical axis when touched by the finger. The cylinder
has a number of equidistant vertical slits round the upper part of its
circumference. The figures which produce the illusion are printed on a
slip of paper, which is placed in the lower part of the drum, and when
this is in rapid rotation, and the figures are viewed through the slits,
the illusion is produced in exactly the same manner as in the revolving
disc.
[Illustration:
FIG. 251_a_.—_Edison’s Kinetographic Theatre._
]
At the end of the article on the phonograph in a subsequent page, the
reader will find a remark as to the effect that might be produced by a
combination of that instrument with instantaneous and simultaneous
photographs of some famous speaker. This combination has now been
accomplished by the great inventive genius to whom we are indebted for
the phonograph. Mr. Edison has done this so effectively that he may be
said to have given life to the _zoetrope_ by the perfection in which the
ocular illusion is produced together with the audible manifestations
that keep time with it. The amount of thought and ingenuity expended on
this new contrivance, which Edison has called the _kinetoscope_, will
scarcely be appreciated by anyone who has not given some consideration
to the many practical difficulties that have been overcome. No wonder
that the announcement made at the beginning of 1892 should have been
received with incredulity, for it was to the effect that Edison had
contrived some happy combination of photography and electricity by which
a man (presumably one who could afford to pay for luxuries) might sit in
his own room and see the moving forms of the actors in an opera
projected on a screen before his eyes, while at the same time he would
hear their voices singing. Every movement, every change of expression,
every glance of the eye, and, in fact, all that was visible to the
spectator in front of the stage would appear on the screen, while not a
note of vocalist, or chord of orchestra, would fail to reach the ear.
And all this was to be evoked at will, and repeated as often as desired,
not, therefore, of course, as a presentation of what was taking place at
the time, but as a reproduction of some previous performance. This
wonderful result has virtually been attained by the application of
delicate and ingenious machinery designed to make the phonograph and the
camera work synchronously. The first part of the problem was the
production of a succession of so-called instantaneous photographs at an
extremely rapid rate. In the actual apparatus forty-six photographs are
taken every second, a feat which would beforehand be thought
impracticable. This is accomplished by making use of a band of sensitive
celluloid film, which alone admits of being moved and stopped with the
desired rapidity. The movement is imparted by an electric motor, and the
arrangement is such that for each exposure the film is held stationary
for 9/10ths of 1/46th of a second, during which the lens is uncovered,
then for the remaining ⅒th it is covered, while at the same time the
film is jerked forward so as to expose a fresh surface to receive a new
impression. Obviously the mass moved and stopped with this rapidity
(which without the stoppages is at the rate of 26 miles an hour) must be
small, and it is found that photographs about 1 in. in diameter cannot
be much exceeded in view of this condition. The lens has to be entirely
stopped or screened during the tenth of the short interval (1/460th of a
second) in which the onward movement of the film is taking place, and it
has to be practically open during the remaining 9/10ths of the interval
(9/460ths of one second) in which the film is held stationary in order
to receive the photographic image. These alternations of movement and
stoppage must take place with the utmost regularity, and Edison has used
a beautifully regulated electro-motor as the active power, which also
simultaneously moves a phonograph so that sights and sounds shall
proceed in step, for it is thus they have to be reproduced. This is done
by developing the band of film, and from it printing photographic
_positives_ on a similar band, whose images are successively projected
on a screen by means of a lantern with a step by step movement, exactly
the same as that by which the original photographs are taken, while the
phonographic cylinder is so timed as to give off to a loud-speaking
instrument the sounds that accompanied the photographs. A description of
the ingenious mechanism by which all this is accomplished is not
suitable for these pages, for it is the result, rather than the details
of apparatuses, that interest the general reader. In a simpler form of
kinetoscope the positive images on the band of film are viewed directly
by single observers, each looking through magnifying glasses; in this a
disc with 46 slits revolves, and in its passage, as each slit
momentarily permits a view of the image, an electric flash
simultaneously lights it up. The same principle is, of course, used in
the screen projections. From the very great number of impressions made
on the eye in one second, there is none of that jerky movement that is
observable in the older appliances. Mr. Edison has found it necessary to
provide a special stage, or rather small theatre, in which the actors of
the little dramas may be photographed with every advantage in the way of
lighting, &c. Fig. 251_a_ shows this kinetographic theatre with the
electric camera in action. The subjects reproducible in the kinetoscope
include the most rapid movements, such as quick dances, blacksmiths
hammering on an anvil, &c., or incidents of ordinary life involving much
gesture and change of facial expression, and nothing can be more amusing
than to see all these shown to the life by the images on the screen, or
by the pictures viewed through the lens, especially if at the same time
the phonograph is made to emit the corresponding sounds.
[Illustration:
FIG. 252.—_Portrait of Sir W. Thomson._[5]
]
Footnote 5:
Now Lord Kelvin.
ELECTRICITY.
About sixty years ago a popular book was published having for its theme
the advantages which would flow from the general diffusion of scientific
knowledge. Great prominence was, of course, given to the utility of
science in its direct application to useful arts, and many scientific
inventions conducing to the general well-being of society were duly
enumerated. Under the head of electricity, however, the writer of that
book mentioned but few cases in which this mysterious agent aided in the
accomplishment of any useful end. The meagre list he gives of the
instances in which he says “_even_ electricity and galvanism might be
rendered subservient to the operations of art,” comprises only orreries
and models of cornmills and pumps turned by electricity, the designed
splitting of a stone by lightning, and the suggestion of Davy that the
upper sheathing of ships should be fastened with copper instead of iron
nails, with a hint that the same principle might be extended in its
application. At the present day the applications of electricity are so
numerous and important, that even a brief account of them would more
than fill the present volume. Electricity is the moving power of the
most remarkable and distinguishing invention of the age—the telegraph;
it is the energy employed for ingeniously measuring small intervals of
time in chronoscopes, for controlling time-pieces, and for firing mines
and torpedoes; it is the handmaid of art in electro-plating and in the
reproduction of engraved plates, blocks, letterpress, and metal work; it
is the familiar spirit invoked by the chemist to effect marvellous
transformations, combinations, and decompositions; it is a therapeutic
agent of the greatest value in the hands of the skilful physician. Such
an extension of the practical applications of electricity as we have
indicated implies a corresponding development of the science itself;
and, indeed, the history of electricity during the present century is a
continuous record of brilliant discoveries made by men of rare and
commanding genius—such as Davy, Ampère, and Faraday. To give a complete
account of these discoveries would be to write a treatise on the
science; and although the subject is extremely attractive, we must pass
over many discoveries which have a high scientific interest, and present
to the reader so much of this recently developed science as will enable
him to comprehend the principles of a few of its more striking
applications.
The science of electricity presents some features which mark it with
special characters as distinguished from other branches of knowledge. In
mechanics and pneumatics and acoustics we have little difficulty in
picturing in our minds the nature of the actions which are concerned in
the phenomena. We can also extend ideas derived from ordinary experience
to embrace the more recondite operations to which heat and light may be
due, and, by conceptions of vibrating particles and undulatory ether,
obtain a mental grasp of these subtile agents. But with regard to
electricity no such conceptions have yet been framed—no hypothesis has
yet been advanced which satisfactorily explains the inner nature of
electrical action, or gives us a mental picture of any pulsations,
rotations, or other motions of particles, material or ethereal, that may
represent all the phenomena. Incapable as we are of framing a distinct
conception of the real nature of electricity, there are few natural
agents with whose ways we are so well acquainted as electricity. The
_laws_ of its action are as well known as those of gravitation, and they
are far better known than those which govern chemical phenomena or the
still more complex processes of organic life.
Definite as are the laws of electricity, there is no branch of natural
or physical science on which the ideas of people in general are so
vague. Spectators of the effects of this wonderful energy—as seen
violently and destructively in the thunderstorm, and silently and
harmlessly in the Aurora—knowing vaguely something of its powers in
traversing the densest materials, in giving convulsive shocks, and in
affecting substances of all kinds—the multitude regard electricity with
a certain awe, and are always ready to attribute to its agency any
effect which appears mysterious or inexplicable. The popular ignorance
on this subject is largely taken advantage of by impostors and
charlatans of every kind. Electric and magnetic nostrums of every form,
electric elixirs, galvanic hair-washes, magnetized flannels, polarized
tooth-brushes, and voltaic nightcaps appear to find a ready sale, which
speaks unmistakably of the less than half-knowledge which is possessed
by the public concerning even the elements of electrical science.
Electricity has also a special position with regard to its intimate
connection with almost every other form of natural energy. Evolved by
mechanical actions, by heat, by movements of magnets, and by chemical
actions, it is capable in its turn of reproducing any of these. It plays
an important, but as yet an undefined, part in the physiological actions
constantly going on in the organized body, and is, in fact,
all-pervading in its influence over all matter, organic and inorganic—a
secret power strangely but universally concerned in all the operations
of nature. We are compelled to regard electricity not as a kind of force
acting upon otherwise inert matter, but rather as an affection or
condition of which every kind of matter is capable, although we are
still unable to form a conjecture of the precise nature of the action.
We have now to address ourselves to the task of unfolding so much of the
science as will enable the reader to understand the leading principles
of such important applications as electro-plating, illumination, and the
telegraph; and this will necessarily include an account of the grand
discovery of the identity, or at least intimate connection, of magnetism
and electricity.
_ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY._
The distinctive property of a magnet is, as everybody knows, to attract
pieces of iron, and this property having been observed by the ancients
in a certain ore of iron which was found near the city of Magnesia, in
Asia Minor, the property itself came to be called Magnetism. A bar of
steel, if rubbed with the natural magnet or loadstone, acquires the same
property, and if the bar be suspended horizontally or poised on a pivot,
it will settle only in one definite direction, which in this country is
nearly north and south. If a narrow magnetized bar be plunged into iron
filings, it will be found that these are attracted chiefly by the ends
of the bar, and not at all by the centre. It appears as if the magnetic
power were concentrated in the extremities of the bar, and these are
termed its poles, the pole at the end of the bar which points to the
north is called the _north pole_ of the magnet, and the other is named
the _south pole_. If a north pole of one magnet be presented to the
north pole of another, they will repel each other, and the same
repulsion will take place between the south poles, whereas the north
pole of one magnet attracts the south pole of another. In other words,
poles of the same name repel each other, but poles of opposite names
attract each other, or still more concisely, _like poles repel, unlike
poles attract each other_.
Magnetism acts through intervening non-magnetic matter with undiminished
energy. Thus, the attractions and repulsions of magnetic poles manifest
themselves just as strongly when the poles are separated by a stratum of
wood or stone as when merely air intervenes, and the attraction of small
pieces of iron by a magnet takes place through the interposed palm of
one’s hand without diminution. A delicately suspended needle in even a
remote apartment of a large building moves whenever a cart passes in the
street. It is almost too well known to require mention here, that iron
and steel are the only common substances which are capable of plainly
exhibiting magnetic forces, and, indeed, there are no known substances
capable of so powerful a magnetization as these. But the difference in
the magnetic behaviour of iron and steel is not so well understood, and
it is a point of importance for our subject, and connected with a
fundamental law which governs all magnetic manifestations. A piece of
pure iron is very readily cut with a file, whereas a piece of steel may
be so hard that the file makes no impression upon it whatever; and hence
a piece of pure iron, or rather iron holding no carbon in combination,
and possessed of no steely quality, is often spoken of as _soft iron_.
When a piece of soft iron is placed near the pole of a magnet, the iron
becomes, for the time, a magnet. If iron filings be sprinkled over it,
they will arrange themselves about the parts of the iron respectively
nearest and farthest from the magnet, thus showing that the piece of
soft iron has acquired magnetic poles. It will be found on examining
these poles that the one nearest the magnet is of the contrary name to
the pole of the magnet, and the farthest is of the same name. The
conversion of the soft iron into a magnet by the influence of a magnetic
pole is termed _induction_. It need hardly be said that the inductive
effect is more powerful in proportion to the shortness of the distance
separating the piece of soft iron from the magnetic pole, and, of
course, the effect is at its maximum when there is actual contact.
Induction thus explains, by aid of the law of the poles, the attraction
which a magnet exercises over pieces of iron, for it is plain that the
inductive influence is accompanied by attraction between the two
contiguous oppositely-named poles of the magnet, and of the piece of
iron. But attraction is not the only force, for the pole developed at
the farthest portion of the piece of iron being of the same name as the
inducing pole, these will be mutually repulsive. The attractive force
will, however, be more powerful on account of the shorter distance at
which it is exerted, and will predominate over the repulsive force,
particularly at short distances, because then the difference will be
relatively greater. At distances from the inducing pole relatively great
to the distance between the two poles of the piece of iron, the
difference may be so small that its effect in attracting the piece of
soft iron will be imperceptible, and then the piece of iron acted on by
two (nearly) equal parallel forces, will be subject to what is termed in
mechanics a _couple_, the only effect of which is to turn the body into
such a position that the opposing forces act along the same line. The
definite direction assumed by a freely suspended needle may be explained
by supposing that the earth itself is a magnet having a _south_ pole in
the _northern_ hemisphere, and a _north_ pole in the _southern_
hemisphere, the line joining these poles being shorter than the axis of
the earth, and not quite coinciding with it in position; and the fact of
the needle being turned round but not bodily attracted is then easily
accounted for, the attractive and repulsive forces being reduced to a
_couple_ in the manner just explained.
If the attempt be made to turn a piece of steel into a magnet, by the
induction of a magnetic pole, the same results will be obtained as in
the case of soft iron, but in a much feebler degree, and with this
difference: the piece of steel does not lose its magnetism when the
inducing magnet is withdrawn, whereas in the case of the soft iron every
trace of magnetism vanishes the instant the inducing pole is removed.
And if the pole of the magnet be not only put in contact with one end of
the piece of steel, but rubbed on it, the piece will acquire permanent
and powerful magnetism. Hence it will be noticed that a piece of soft
iron can by the mere approximation of a magnetic pole be converted in an
instant into a magnet, and by the removal of the magnet can as instantly
be deprived of its magnetism, and made to revert into its ordinary
condition; while steel is not so readily magnetized, but retains its
magnetism permanently.
[Illustration:
FIG. 253.—_A simple Electroscope._
]