THE MAGIC TOP—THE GYROSCOPE AND SCIENTIFIC GAMES
We will not do our readers the injustice to suppose that they are not
familiar with the ordinary top,—the delight of all school-boys and
young people,—of which, therefore, we forbear giving any description;
but we now desire to give some details of the construction of the
wonderful magic top. It is composed of a large disc, with an axis
turning on two pivots connected with a circle of iron. When in repose,
this plaything exhibits nothing of a remarkable character; it is
completely inert, obeying, like all other bodies, the laws of gravity.
But when we come to give the disc a movement of rapid rotation, this
inert instrument seems to assume a vitality of its own if we attempt to
move it; it resists, and seems to thrust back the hand, and executes
movements even in a contrary direction. Besides this, it appears to
be freed, in a certain measure, from the laws of gravity; if we place
it on its pivot, instead of falling, as it would when the disc is
motionless, it preserves the upright or inclined position in which
we place it, the upper extremity of the axis slowly describing a
horizontal circle round the fulcrum of the other extremity.
Few persons are sufficiently familiar with the theory of mechanics
to understand these phenomena, and it often happens that such a top
purchased to amuse a child becomes an object of wonder and interest
to his seniors. We do not pretend here to explain mathematically the
reason of the facts before us, but the mechanical principle on which
this top is constructed is of such great scientific importance, that
we will, in a few words, explain it to our readers. It is sufficient
to have a little knowledge of mechanics to be aware that a body in
motion, subjected to the action of a force tending to give it a
directly contrary motion, will follow a movement in a third direction,
which is termed the resultant of the two others; and this resultant
approaches nearer to one of the original directions, in proportion as
the corresponding movement is more rapid in relation to the other.
If, for example, you strike a billiard ball, which is rolling past
you, in such a manner that you drive it regularly along in the same
direction, it appears only to obey a part of the given impulsion, and
continues its course in an oblique direction, the speed with which it
commenced rolling combining with the impulsion to produce a resultant
movement. If it is rolling very quickly, and you strike it gently, it
will scarcely turn out of its course. If, on the contrary, it is moving
slowly, and receives a violent shock, it will run off almost exactly in
the direction in which it has been struck.
[Illustration: Fig. 871.—The magic top.]
Now that which occurs in this example of a body tending to two
movements at the same time, is also produced when it is a question of
movements of rotation, so that if a force acts upon a body in rotation
in such a manner as to give it a movement of the same kind round
another axis, a third movement will be originated round a third axis,
the direction of which will be nearest to that in which the rotation
is most rapid. Let us apply this very simple principle to our top,
and we shall see that magic has nothing whatever to do with these
movements, which at first glance appears so extraordinary. Having
set it in motion, we rest it on its pivot, its axis in a horizontal
position; we then find that we have two movements before us; first,
that which we gave the top ourselves, and secondly, the movement of
rotation which occurs round a second axis equally horizontal passing
through the fulcrum and perpendicular line to the first. A movement of
rotation therefore originates round a third axis placed between the
two first, but whilst the real axis of the top, obeying this resultant
movement, takes up its new position, the law of gravity continuing to
act, displaces and moves it a little further, so that in endeavouring
to reach its centre of gravity, it turns round its fulcrum (fig. 871).
From this explanation, it will be easily seen that the more rapid the
movement given to the top,—that due to gravity remaining constant,—the
nearer will be the axis of the resultant movement to its real axis,
and consequently the slower will be the movement of rotation of the
whole round the pivot. Thus this apparently incomprehensible phenomenon
is easily explained by gravity, vertical force producing a movement
of rotation in a horizontal plane. One can also explain by analogous
reasoning, and calculation of passive resistance, why the axis of
the top gradually inclines in proportion as the speed of the latter
diminishes, and the speed of rotation round the fulcrum increases; why
it falls immediately if an obstacle is opposed to the latter movement,
and finally, why it produces on the hand which holds it, movements
which astonish persons so intensely who behold it for the first time.
The principle we have just described is often enunciated, by saying
that every body in rapid rotation rests in its plane, and can only
be driven out by a considerable force; this, however, is a defective
formula. The principle should be stated in the following manner. A
body in rapid rotation tends to remain in its plane; that is, its
axis rests parallel with itself, and instead of obeying any force
tending to divert its direction, it produces in consequence of the
combination of two simultaneous movements, a displacement of the axis,
generally much feebler and of a different kind from that which this
force exercises on the same body in repose. One of the most charming
applications of this theory is due to M. Foucault. The _Gyroscope_,
which bears his name, is a heavy disc, the axis of which is supported
by a “_Cardan_” balance, so that, whatever is the position of the
contrivance, it is possible to preserve it in a constant direction.
Therefore if the disc is, by means of special mechanism, put in rapid
rotation, we may give it all kinds of possible displacement without
changing the plane in which the gyroscope moves. Supposing then that
its connection with the suspension is fixed in a relatively immovable
manner, but attracted by a movement towards the ground, the plane of
rotation of the disc will not entirely participate in this movement.
It is true, it will be carried into the movement of general removal,
but it will remain constantly parallel with itself, and only appears
displaced in comparison with the surrounding objects, which obey more
completely than itself the movement of the globe’s rotation round its
poles. Thus can we demonstrate the movement of our planet. In virtue of
the same principle, we see every day passing before our eyes a crowd
of phenomena with which we are so familiar that they do not excite our
attention. Thus it is because the hoop tends to remain in its plane of
rotation that it rolls on without falling or deviating, and for the
same reason that tops rotate vertically on their points, or when they
are running down, describe a series of concentric circles; and for the
same reason again, a juggler is able so easily to hold on the point
of a stick a plate which he puts in rapid rotation, etc. It is also
owing to this property of rotating bodies that we have been enabled
to make use of cylindrical or conical projectiles in artillery. The
coiled riflings of the cannon causing the projectiles to turn round
very rapidly, their axis preserves an invariable direction during
their whole course, until they finally strike the object at which
they are aimed. Without this rotation they would pirouette in an
irregular manner in the air, and besides any precision in firing being
impossible, the resistance of the air would diminish their range to an
enormous extent.
The Gyroscope, an instrument now familiar to most scientific persons,
is still a problem of which the solution has not yet been found. It
may be called the paradox of mechanics, for although it depends on
gravitation, it appears to be entirely indifferent to it.
An American scientist has applied electricity to the gyroscope, so
as to make its movements as continuous as possible, and to enable us
to study it more at leisure and with better results. The gyroscope
is mounted on a pedestal which tapers to a point, and supports the
instrument. The bar of the gyroscope on which the electro-magnets are
fixed rests upon the top of the pedestal. One of the extremities of the
bobbin is fixed to the cavity, when the bar and support join, the other
extremity communicates with the bar which joins the nuts of the magnets.
An insulator of hardened caoutchouc is so placed that it just touches
the axis of the wheel twice in every revolution of that wheel. Its
plane of rotation is at right angles to the magnets, and carries an
armature of soft iron which turns very close to the magnet without
touching it. The armature is put _en rapport_ with the surface of
contact of the cylinder, so that when the armature approaches it is
attracted; but immediately afterwards, as it reaches the opposite side,
the current is interrupted, and the impulse acquired is sufficient to
move the wheel to the spot where the armature can again come under the
influence of the magnet.
The magnets, the wheel, and all the parts of the instrument together
can move around in any direction. When two or four Bunsen cells are
put in connection with the gyroscope, the wheel turns with tremendous
rapidity, and by permitting the magnets to work (an operation which
requires some little dexterity), the wheel not only sustains itself,
but also the magnets and the other subjects which are between it and
the extremity of the pedestal—in opposition to the laws of gravitation.
The wheel, besides turning rapidly around its axis; revolves slowly
around the point of the column in the same direction taken by the lower
part of the wheel.
When attaching the arms and counter-poise of the machine, so that
the wheel and the magnets may balance themselves exactly on the
pointed pedestal, the machine remains stationary. But if we give any
preponderance to the wheel and magnets the rotation of the apparatus
is in a direction opposite to that which would result from turning the
upper part of the wheel.
The gyroscope illustrates the persistence with which a body that
submits to rotation maintains itself in the plane of its rotation,
notwithstanding the force of gravitation. It also shows the result of
the combined action of two forces tending to produce rotation around
two separate axes, which are, however, situated in the same plane.
The rotation of the wheel round its axis is produced in the present
case by the electro-magnet; and the tendency of the wheel to fall,
or to turn in a vertical plane parallel to its axis, results in the
rotation of the entire instrument upon a new axis, which coincides with
the pointed pedestal.
THE AMERICAN MONEY-BOX.
During a recent visit to London, as I was one day walking in the
Crystal Palace, my attention was attracted by a curious money-box,
surmounted by a pictorial representation of one of the London streets.
The carriages, horses, and pedestrians were represented by figures
cut out of cardboard, arranged in a groove. A large placard bore this
inscription: “_Notice to visitors: Throw a penny in the money-box; and
the figures will perform_.”
I at once responded to this invitation, and immediately beheld the
little _tableaux_ become moving and life-like; the cabs rolled along,
and the passers-by walked up and down the street. A number of visitors
followed my example, and there is no doubt the money-box was full at
the end of the day. This ingenious contrivance for obtaining money in
so easy a manner, and without having recourse to a “show-man,” appeared
to me worthy of investigation and description.
The _Scientific American_ (New York) has recently given an explanation
of this curious contrivance, and we will here quote what has been
published on the subject.
“Among the inventions intended to obtain contributions of money from
the visitors at the Philadelphia Exhibition,” says the American writer,
“we will describe the singular money-boxes placed in the _salons_ of
the principal hotels and the galleries of the exhibition, etc. These
contrivances all consisted of a case or box, with a glass front,
through which can be seen a landscape in miniature, with trees, houses,
figures, etc., all cut out of cardboard, and painted with great nicety.
On the box was a label requesting the visitor to drop a coin into it
and await the result of the contribution. When the penny has fallen in
it puts in motion some hidden machinery, and then we see the people in
the miniature landscape all in motion, riding or walking or hunting, as
the case may be.”
Another box is even more successful, for it places in the hands of
the contributor a photograph of some celebrated person. But to obtain
the photograph we must contribute six pennies. The _carte_ will not
come out if we do not put in the proper coins, and the apparatus is
perfectly fair and honest.
The illustration, fig. 872, shows the apparatus, which is very
simple. On the left the ordinary box is seen, on the right there is a
longitudinal section of it.
At the top of the lower portion, where the money is received in, is
a hollow support, A, which sustains the box in which the photographs
are placed upon an inclined plane, and resting against the glass.
The pieces of money, in falling, strike the extremity of a vertical
balance, which immediately turns a toothed wheel, C. This wheel has
as many teeth as there are pieces of money necessary to purchase
the photograph or _carte de visite_. Upon the escapement wheel is a
ratchet arrangement, D, the shaft being moved by a cord rolled around
it and attached to a spring, E. A bolt, F, moved by a spring, is kept
constantly pressed against the “snail,” D.
[Illustration: Fig. 872.—American money-box.]
Thus at each revolution, as the parts of the machinery are animated
by the same movement, the bolt is withdrawn sufficiently to permit a
_carte_ to fall, and then the card next following will be ready resting
upon the bolt. The photographs being placed upon an inclined plane, are
pushed forward by a movable frame, G, which has a roller at the base.
So as one card falls out another is immediately replaced close to the
glass.
We have remarked that the wheel has six teeth, so that as one piece of
money dropped in moves it one-sixth of its revolution, six pieces will
be necessary to produce the card. Of course wheels can be made with one
or more teeth, and the payment may be varied for valuable objects at
the desire of the possessor.
The invention is not only a plaything. It can be made useful in the
distribution of pamphlets, or newspapers, which can be introduced
into the box folded uniformly. They can also be used in omnibuses or
tram-cars, and tickets may be given by the machine on payment of the
proper sum of money.
We will close this chapter with an illustration of a spiral bottle,
which can be done in the manner now to be described, so that the bottle
will actually become a glass spring.
Take a mixture of 180 grammes of lampblack, 60 grammes of gum arabic,
23 grammes of adraganth, and 23 grammes of benzoin. Make these
ingredients into a paste by the addition of water, and fashion a pencil
of the charcoal thus obtained. This pencil, when heated, will cut the
glass wherever it is applied.
The process is commenced by scraping the bottle with a file and
following the instrument with the red-hot pencil. Wherever the hot
pencil is applied, the glass will be cut as shown in the illustration
herewith. It will be necessary to blow upon the heated pencil to
maintain the incandescence as long as possible. The bottle as cut and
representations of the instruments are given in the cut (fig. 873).
[Illustration: Fig. 873. Spiral bottle.]