[Illustration: FIG. 1.]
We have in former articles described the highly interesting series
of experimental researches of Dr. C. A. Bjerknes, Professor of
Mathematics in the University of Christiania, which formed so
attractive a feature in the Electrical Exhibition of Paris in 1881,
and which constituted the practical development of a theoretical
research which had extended over a previous period of more than twenty
years. The experiments which we described in those articles were,
as our readers will remember, upon the influence of pulsating and
rectilinear vibrating bodies upon one another and upon bodies in their
neighborhood, as well as upon the medium in which they are
immersed. This medium, in the majority of Professor Bjerknes earlier
experiments, was water, although he demonstrated mathematically, and
to a small extent experimentally, that the phenomena, which bear so
striking an analogy to those of magnetism, may be produced in air.
Our readers will recollect that in the spring of 1882 Mr. Stroh, by
means of some very delicate and beautifully designed apparatus,
was able to demonstrate a large number of the same phenomena in
atmospheric air of the ordinary density; and about the same time
Professor Bjerknes, in Christiania, was extending his researches to
phenomena produced by a different class of vibrations, namely, those
of bodies moving in oscillations of a circular character, such, for
example, as a cylinder vibrating about its own axis or a sphere
around one of its diameters; some of these experiments were brought
by Professor Bjerknes before the Physical Society of London in the
following June. Since that time, however, Professor Bjerknes, with the
very important assistance of his son, Mr. Vilhelm Bjerknes, has been
extending these experimental researches in the same direction, and
with the results which it is the object of the present series of
articles to describe.
[Illustration: FIG. 2.]
The especial feature of interest in all Professor Bjerknes experiments
has been the remarkably close analogy which exists between the
phenomena exhibited in his mechanical experiments in water and other
media and those of magnetism and of electricity, and it may be of some
interest if we here recapitulate some of the more striking of these
analogies.
(1.) In the first place, the vibrating or pulsating bodies, by setting
the water or other medium in which they are immersed into vibration,
set up in their immediate neighborhood a field of mechanical force
very closely analogous to the field of magnetic force with which
magnetized bodies are surrounded. The lines of vibration have
precisely the same directions and form the same figures, while at the
same time the decrease of the intensity of vibration by an increase of
distance obeys precisely the same law as does that of magnetic
intensity at increasing distances from a magnetic body.
(2.) When two or more vibrating bodies are immersed in a fluid, they
set up around them fields of vibration, and act and react upon one
another in a manner closely analogous to the action and reaction of
magnets upon one another, producing the phenomena of attraction
and repulsion. In this respect, however, the analogy appears to be
inverse, repulsion being produced where, from the magnetic analogy,
one would expect to find attraction, and _vice versa_.
(3.) If a neutral body, that is to say a body having no vibration of
its own, be immersed in the fluid and within the field of vibration,
phenomena are produced exactly analogous to the magnetic and
diamagnetic phenomena produced by the action of a magnet upon soft
iron or bismuth, its apparently magnetic or diamagnetic properties
being determined by the specific gravity of the neutral body as
compared to that of the medium in which it is immersed. If the neutral
body be lighter than the medium, it exhibits the magnetic induction of
iron with respect to polarity, but is nevertheless repelled; while
if it be heavier than the medium, its direction is similar to that of
diamagnetic bodies such as bismuth, but on the other hand exhibits the
phenomena of attraction.
In this way Professor Bjerknes has been able to reproduce analogues of
all the phenomena of magnetism and diamagnetism, those phenomena which
may be classed as effects of induction being directly reproduced,
while those which may be classed as effects of mechanical action, and
resulting in change of place, are analogous inversely. This fact has
been so much misunderstood both in this country and on the Continent
that it will be well, before describing the experiments, to enter more
fully into an explanation of these most interesting and instructive
phenomena.
For the sake of clearness we will speak of magnetic induction as that
property of a magnet by which it is surrounded by a field of force,
and by which pieces of iron, within that field, are converted into
magnets, and pieces of bismuth into diamagnets, and we will speak of
magnetic action as the property of a magnet by which it attracts or
repels another magnet, or by which it attacks or repels a piece of
iron or bismuth magnetized by magnetic induction.
[Illustration: FIG. 3.]
The corresponding hydrodynamic phenomena may be regarded in a similar
manner; thus, when a vibrating or pulsating body immersed in a
liquid surrounds itself with a field of vibrations, or communicates
vibrations to other immersed bodies within that vibratory field, the
phenomena so produced may be looked upon as phenomena of hydrodynamic
induction, while on the other hand, when a vibrating or pulsating body
attracts or repels another pulsating or vibratory body (whether
such vibrations be produced by outside mechanical agency or by
hydrodynamical induction), then the phenomena so produced are those of
hydrodynamical action, and it is in this way that we shall treat the
phenomena throughout this article, using the words _induction_ and
_direct action_ in these somewhat restricted meanings.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
In the hydrodynamical experiments of Professor Bjerknes all the
phenomena of magnetic induction can be reproduced directly and
perfectly, but the phenomena of magnetic action are not so exactly
reproduced, that is to say, they are subject to a sort of inversion.
Thus when two bodies are pulsating together and in the same phase
(i. e., both expanding and both contracting at the same time), they
mutually attract each other: but if they are pulsating in opposite
phases, repulsion is the result. From this one experiment taken by
itself we might be led to infer that bodies pulsating in similar
phases are the hydrodynamic analogues of magnets having their opposite
poles presented to one another, and that bodies pulsating in opposite
phases are analogous to a presentation of similar magnetic poles; but
it will be seen at once that this cannot be the case if three magnetic
poles or three pulsating bodies be considered instead of only two. It
is clear, on the one hand, that three similar magnet poles will all
repel one another, while, on the other, of three pulsating bodies,
two of them must always attract one another, while a third would be
repelled; and, moreover, two similarly pulsating bodies set up around
them the same lines of force as two similar magnetic poles, and two
oppositely pulsating bodies produce lines of force identically the
same as those set up by two magnets of opposite polarity. Thus it
will be seen that there is a break in the analogy between the
hydrodynamical and the magnetic phenomena (if a uniform inversion
of the effects can be called a break, for it is, as far as Professor
Bjerknes' experiments go, without an exception); and if by any means
this inversion could be reinverted, all the phenomena of magnetism and
diamagnetism could be exactly reproduced by hydrodynamical analogues;
there would thus be grounds for forming a theory of magnetism on the
basis of mechanical phenomena, and a very important link in the chain
of the correlation of the physical forces would be supplied.
While the experiments of Professor Bjerknes upon pulsating and
rectilinearly vibrating bodies and their influence upon one another
illustrate by very close analogies the phenomena of magnetism, those
upon circularly vibrating bodies and their mutual influences bear a
remarkable analogy to electrical phenomena; and it is a significant
fact that exactly as in the case of magnetic illustration, the
analogies are direct as regards the phenomena of induction, and
inverse in their illustration of direct electrical action.
If we examine the figure produced by the field of force surrounding a
conductor through which a current of electricity is being transmitted
(see Fig. 1), we see that iron filings within that field arrange
themselves in more or less concentric circles around the conductor
conveying the current. From this fact Professor Bjerknes and his
son, reasoning that, to produce a similar field of energy around a
vibrating body, the vibrations of that body must partake of a
circular or rotary character, constructed apparatus for producing
the hydrodynamic analogue of electric currents, in which a conductor
transmitting a current of electricity is represented by a cylinder to
which oscillations in circles around its axis are given by suitable
mechanical means, so as to cause the enveloping medium to follow its
motion and make similar rotative vibrations. In some of the earlier
experiments in this direction, cylinders carrying radial veins (A,
Fig. 2) or fluted longitudinally around their surfaces (B, Fig. 2)
were employed with the object of giving the vibrating cylinder a
greater hold of the liquid in which they were immersed; but it was
found that these vanes or flutings had but little or no effect upon
water or liquids of similar viscosity, and Professor Bjerkes was led
to adopt highly viscous fluids, such as Glycerin or maize sirup, both
of which substances are well adapted for the experiments, being at the
same time both highly viscous and perfectly transparent and colorless.
In seeking, for the purpose of this research, a fluid medium which
shall possess analogous properties to the luminiferous ether, or
whatever may be the medium whose vibrations render manifest certain
physical phenomena, it might be considered at first sight that
substances so dense as glycerin and sirup could have but little in
common with the ether, and that an analogy between experiments made
within it and phenomena associated with ethereal vibrations would be
of a very feeble description: but Professor Bjerknes has shown that
the chief requisite in such a medium is that its viscosity should be
great, not absolutely, but large only in proportion to its density,
and if the density be small, the necessary viscosity may be small
also. Neither is it necessary for the fluid medium to possess great
internal friction, but what is necessary to the experiments is that
the medium shall be one which is readily set into vibration by the
action of the circularly vibrating cylinder; this property appears to
be possessed exclusively by the more viscous fluids, and is, moreover,
in complete accord with what is known of the luminiferous ether
according to the theory of light.
The property is rather a kind of elasticity, which ordinary fluids
do not possess, but which facilitates the propagation of transverse
vibrations.
One form of apparatus for the propagation of rotative oscillations is
shown to the left of Fig. 3, and consists of a cylinder, A, mounted on
a tubular spindle, and which is set into circular oscillations around
its axis by the little vibrating membrane, C, which is attached to
the axis of the cylinder by a little crank and connecting rod shown
in detail in Fig. 4. This membrane is set into vibration by a rapidly
pulsating column of air contained in a flexible tube M, by which
apparatus is connected to the pulsation pump which was employed by
Professor Bjerknes in his earlier experiments. In Fig. 5, a somewhat
similar apparatus for producing horizontal vibrations is shown, and
marked N H C, the only difference between them being one of mechanical
detail necessitated by the change in the position of axis of vibration
from the vertical to the horizontal.
If circularly vibrating cylinders, such as we have described, be
immersed in a viscous fluid and set into action, the following
phenomena may be observed: 1. The effect upon the fluid itself,
setting up therein a field of vibration, and corresponding by analogy
with the production of a field of force around a wire conveying an
electric current. 2. The effect upon other circularly vibrating bodies
within that field of force corresponding to the action and reaction
of electric currents upon one another. 3. The effect on pulsating and
oscillating bodies similarly immersed, illustrating the mutual effects
upon one another of magnets and electric currents. The first of these
effects is one of induction, and, from what has been said from an
earlier part of this article, it will be understood that the analogy
between the hydrodynamic and the electric phenomena is direct and
complete. The effects classified under the second and third heads,
being phenomena of direct action (in the restricted use of the word),
are uniformly analogous to the magnetic and electric phenomena which
they illustrate.
(_To be continued._)
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