Popular Science Monthly/Volume 21/June 1882/Hydrodynamics and Electricity
VISITORS at the recent Electrical Exposition in Paris were much interested in an apparatus exhibited by Dr. C. A. Bjerknes, of the University of Christiania, Norway, for the illustration of certain properties in hydrodynamics analogous to some of the manifestations of electricity and magnetism. Professor Bjerknes has been carrying on his investigations in this line for nearly twenty years, having published his first paper, "On the Internal Condition of an Incompressible Fluid in which a Sphere of Variable Volume is moving," in 1863, and having followed it up with numerous other papers relating to similar problems. The results of experiments in every case corresponded with those which had previously been indicated to him by mathematical calculations. The experiments had in view the study of molecular movements by reproducing mechanically, but in the inverse sense, as the results proved, the simple and fundamental electrical and magnetic phenomena.
Pulsating and scillating bodies are applied so as to produce vibrations in a trough of water about six inches deep. By pulsating bodies are meant those which undergo alternate changes of volume, marked by two distinct phases, one of swelling and one of contraction. The pulsations of the, two bodies are spoken of as synchronous when the similar phases occur simultaneously in them. The oscillating bodies are constant in volume, but undergo alternate changes of place, from right to left and from left to right, or in a vertical direction.
Pulsations are communicated by means of little tambours or drums made of hollow cylinders of metal, over the ends of which are stretched flexible plates or membranes. These drums are made to swell and contract by means of pumps, with which they are connected by India-rubber tubing, that compress the air within them. Two drums are usually employed in the experiments, each connected with a separate
pump, so that the rhythm of the pulsations may be regulated at will. Thus both drums may be caused to pulsate synchronously, or with an opposite rhythm. In the simplest pulsator (Fig. 1,1) the two drum-heads
beat synchronously, or suffer dilatation and collapse together, as the pump is worked. In another disposition the drum-heads are separated by a rigid partition dividing the instrument into two chambers, each having its separate connection by a distinct tube, with a different pump, making it practicable to produce either synchronous or unsynchronous pulsations. A more common disposition is to use two simple pulsators in connection with the two pumps, one of which is held in the hand, while the other is mounted in the water, so as to be left free to move.
The two phases of pulsation are regarded by Professor Bjerknes as analogous to the poles of the electrode or magnet. The phase of dilatation may thus be likened to the north pole, that of collapse to the south pole. If, now, having one of the drums mounted in the water, and the other held in the hand, we bring them near each other while both are dilating or both are contracting that is, while both are in the same phase of pulsation—attraction will take place between them. The mounted drum will assume the direction of approach toward the one held in the hand, and of following it when it is removed; but if they are in opposite phases—if one is swelling while the other is contracting—they will be repelled. Like poles attract, unlike ones repel. The phenomena are the inverse of what are observed in ordinary electricity and magnetism, where unlike poles attract and like ones repel. The pulsating drum in these experiments represents an isolated pole, a conception which physicists have not hitherto regarded as possible.
Spheres of invariable volume, but adjusted so as to oscillate in either an horizontal or vertical direction, maybe used instead of the pulsating drums, when the phenomena assume a modified shape. The oscillators used by Professor Bjerknes are mounted as in the figure (Fig. 1, 3), where the sphere on the left is arranged so as to oscillate horizontally, and the one on the right to oscillate vertically, the alternate movements of oscillation being produced, like the pulsations of the drums, by alternately forcing in and withdrawing the air. The opposite sides of the sphere assume opposite phases, and the sphere acts like a magnet. If a sphere is brought near a pulsator, so that its oscillating movement shall be toward the drum while that is dilating, attraction takes place; but, if it be turned in the opposite direction, so as to be moving away from the drum while the same is swelling, repulsion will be manifested.
If two oscillating spheres be brought near each other, attraction takes place in case they are both moving to or from each other; repulsion, in case they are both moving in the same direction: and the change can be effected at once, as before, by turning one of the spheres around.
Professor Bjerknes has a considerable variety of apparatus for modifying the aspects of the phenomena by changing the relative situations of the bodies to each other, in all of which manifestations of an inverse character to those of ordinary magnetism are developed. If one of the 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 that is free turns round its axis, and sets itself so that both spheres shall be simultaneously approaching or receding from each other. 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. These directive effects are believed by Professor George Forbes to be perfectly new, both theoretically and experimentally.
The phenomena of attraction and repulsion, described above, are supposed by Professor Bjerknes and Professor Forbes to be 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, the water being regarded as the analogue of Faraday's medium. "Viewed in this light," says Professor Forbes, "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 center of disturbance; but, if in the opposite direction, toward it. This idea gives us the analogy of diaand para-magnetism. If, in the neighborhood of the vibrating drum, we have a cork ball, retained under the 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 those 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 para-magnetism. A rod of cork and another of metal are suspended horizontally by threads in the trough, and a vibrating drum is brought near them: the cork rod sets itself equatorially, and the metal rod axially."
From these and other experiments Professor Bjerknes has concluded that the motion of a vibrating agent in the water produces a real magnetic field, with its lines of forces presenting, but always in an inverse sense, identical phenomena of diamagnetism and para-magnetism, magnetic interference, etc. He has been able to trace out the direction of the lines of force produced in the liquid with the apparatus represented in Fig. 2. A light sphere or cylinder is mounted in the midst of the liquid upon an elastic rod, so that it shall partake of every movement of the surrounding water; a brush is attached to it, and arranged in such a manner as to paint, on the glass plate above, the line of every vibration of the fluid important enough to move it. If two drums are used pulsating concordantly, a figure is obtained precisely like that produced by iron filings in a field of two similar magnetic poles. If the pulsations are discordant, the figure is like that obtained with two dissimilar poles. Three pulsating drums give a figure identical with that produced by three magnetic poles.
A number of interesting conclusions may be drawn from these experiments concerning the nature of electric and magnetic vibrations, but they need to be further confirmed before a positive announcement of them can be justified.