From a scientific and purely theoretical point of view
there is no
object in the whole of the Electrical Exhibition at Paris of greater
interest than the remarkable collection of apparatus exhibited by R.
C[arl] A. Bjerknes of Christiana, and intended to show the fundamental
phenomena of electricity and magnetism by the analgous ones of
hydrodynamics. I will try to give a clear account of these experiments
and the apparatus employed; but no description can convey any idea of
the wonderful beauty of the actual experiments, whilst the mechanism
itself is also of most exquisite construction. Every result which is
thus shown by experiment had been previously predicted by Professor
Bjerknes as the result of his mathematical investigations.
It has long been known that if a tuning fork be struck and held near to
a light object like a balloon it attracts it. This is an old
experiment, and the theory of it has been worked out more than once.
Among others Sir William Thomson gave the theory in the Philosophical
Magazine in 1867. In general words the explanation is that the air in
the neighborhood of the tuning fork is rarefied by the agitation which
it experiences. Consequently the pressure of the air is greater as the
distance from the tuning fork increases. Thus the pressure on the far
side of the balloon is greater than that on the near side, and the
balloon is attracted.
Dr. Bjerknes has followed out the thory of this action until he has
succeeded in illustrating most of the fundamental phenomena of
electricity and magnetism. He causes vibrations to take place in a
trough of water about six inches deep. He uses a pair of cylinders
fitted with pistons which are moved in and out by a gearing which
regulates the length of stroke and also gives great rapidity. These
cylinders simply act alternately as air compressors and expanders, and
they can be arranged so that both compress and both expand the air
simultaneously, or in such a way that the one expands while the other
compresses the air, and vice versa. The cylinders are connected by thin
india rubber tubing, and fine metal pipes to the various instruments. A
very simple experiment consists in communicating pulsations to a pair
of tambours, and observing their mutual actions. They consist of a ring
of metal faced on both sides with india rubber and connected by a tube
with the air cylinders. One of them is held in the hand; the other is
mounted in the water in a manner which leaves it free to move. It is
then found that if the pulsations are of the same kind, i.e., if both
expand and both contract simultaneously, there is attraction. But if
one expands while the other contracts, and vice versa, there is
repulsion. In fact the phenomenon is the opposite of magnetical and
electrical phenomena, for here like poles attract and unlike poles
repel.
Law of Attraction
"Juxtaposed coherent aggregates vibrating in unison, or harmonic ratio,
are mutually attracted." Keely
Law of Repulsion
"Juxtaposed coherent aggregates vibrating in discord are mutually
repelled." Keely
Instead of having the pulsations of a drum we may use the oscillations
of a sphere; and Dr. Bjerknes has mounted a beautiful piece of
apparatus by which the compressions and expansions of air are used to
cause a sphere to oscillate in the water. But in this case it must be
noticed that opposite sides of the sphere are in opposite phases. In
fact the sphere might be expected to act like a magnet; and so it does.
If two oscillating spheres be brought near each other, then, if they
are both moving to and from each other at the same time, there is
attraction; but if one of the spheres be turned round, so that both
spheres move in the same direction in their oscillations, then there is
repulsion. If one of these spheres be mounted so as to be free to move
about a vertical axis, it is found that when a second oscillating
sphere is brought near to it, the one which is free turns round its
axis and sets itself so that both spheres in their oscillations are
approaching each other or receding simultaneously. Two oscillating
spheres, mounted at the extremities of an arm, with freedom to move,
behave with respect to another oscillating sphere exactly like a magnet
in the neighborhood of another magnetic pole. I believe that these
directive effects are perfectly new, both theoretically and
experimentally. The professor mounts his rod with a sphere at each end
in two ways: (1) so that the oscillations are along the arm, and (2) so
that they are perpendicular. In all cases they behave as if each sphere
was a little magnet with its axis lying along the direction of
oscillation.
Dr. Bjerknes looks upon the water in his trough as being the analogue
of Faraday's medium; and he looks upon these attractions and repulsions
as being due, not to the action of one body on the other, but to the
mutual action of one body and the water in contact with it. Viewed in
this light, his first experiment is equivalent to saying that if a
vibrating or oscillating body have its motions in the same direction as
the water, the body moves away from the centre of disturbance, but if
in the oppposite direction, towards it. This idea gives us the analogy
of dia- and para-magnetism. If, in the neighborhood of a vibrating
drum, we have a cork ball, retained under water by a thread, the
oscillations of the cork are greater than those of the water in contact
with it, owing to its small mass, and are consequently relatively in
the same direction. Accordingly we have repulsion, corresponding to
diamagnetism. If, on the other hand, we hang in the water a ball which
is heavier than water, its oscillations are not so great as that of the
water in its vicinity, owing to its mass, and consequently the
oscillations of the ball relatively to the water are in the opposite
direction to those of the water itself, and there is attraction,
corresponding to paramagnetism. A rod of cork and another of metal are
suspended horizontally by threads in the trough. A vibrating drum is
brought near to them; the cork rod sets itself equatorially, and the
metal rod axially.
If a pellet of iron be floated by a cork on water and two similar poles
(e.g. both north) be brought to its vicinity, one above and the other
below the pellet, the latter cannot remain exactly in the centre, but
will be repelled to a certain distance, beyond which however there is
the usual attraction. The reason is that when the pellet is nearly in
the line joining the two poles the north pole of the pellet (according
to our supposition) is further from this line than the south one. The
angle of action is less; so that although the north pole is further
away, the horizontal component of the north pole repulsion may be
greater than that of the south pole attraction. Dr. Bjerknes reproduces
this experiment causing two drums to pulsate in concord, the one above
the other. A pellet fixed to a wire, which is attached by threads to
two pieces of cork, is brought between the drums, and it is found
impossible to cause it to remain in the centre.
Dr. Bjerknes conceived further the beautiful idea of tracing out the
conditions of the vibrations of the water when acted on by pulsating
drums. For this purpose he mounted a sphere or cylinder on a thin
spring and fixed as fine paint brush to the top of it. This is put into
the water. The vibrations are in most cases so small that they could
not be detected, but by regulating the pulsations so as to be
isochronous with the vibrations of the spring, a powerful vibration can
be set up. When this is done a glass plate mounted on four springs is
lowered so as to touch the paint brush, and the direction of a
hydrodynamic line of force depicted. Thus the whole field is explored
and different diagrams are obtained according to the nature of the
pulsations. Using two drums pulsating concordantly, we get a figure of
two similar magnetic poles. If the pulsations are discordant it is like
the figure with two dissimilar poles. Three pulsating drums give a
figure identical with that produced by three magnetic poles. The
professor had previously calculated that the effects ought to be
identical, and I think the same might have been gathered from the
formulae in Sir William Thomson's "Mathematical Theory of Magnetism,"
but this only enhances the beauty of the experimental confirmation.
Physicists have been in the habit of looking upon magnetism as some
kind of molecular rotation. According to the present view it is a
rectilinear motion. Physicists have been accustomed to look upon the
conception of an isolated magnetic pole as an impossibility, but here,
while the oscillating sphere represents a magnetic molecule with north
and south poles, the pulsating drum represents an isolated pole. These
are new conceptions to the physicist, let us see whither they lead us.
The professor shows that if a rectilinear oscillation constitutes
magnetism, a circular oscillation must signify an electric current, the
axis of oscillation being the direction of the current. According to
this view what would be the action of a ring through which a current is
passing? If the ring were horizontal the inner parts of the ring would
all rise together and all fall together, they would vibrate and produce
the same effect as the rectilinear vibrations of a magnet. This is the
analogue of the Amperian currents.
To Illustrate the condition of the magnetic field in the neighborhood
of the electric currents, Dr. Bjerknes mounted two wooden cylinders on
vertical axis, connecting them by link-work, which enabled him to
vibrate them in the same or opposite ways. To produce enough friction
he was forced to employ syrup in place of water. The figures which are
produced on the glass plate are in every case the same as those of
electric currents, including the case of currents in parallel and in
opposite directions.
The theory is carried out a step further to explain the attraction and
subsequent repulsion after contact of an electrified and a neutral
substance and the passage of a spark. But it is extremely speculative,
and is not as yet experimentally illustrated, and I think that at
present it is better to pass it by.
I believe that the professor will exhibit his experiments and give some
account of his mathematical investigations, which have occupied his
time for five years, to the Academié des Sciences this afternoon. His
results have not been published before.
George Forbes, Paris, August 15, 1881