404 HYDRODYNAMIC ANALOGIES TO ELECTEIC1TY AND MAGNETISM. 



which assembled for the purpose of creating 

 ill-feeling between different nationalities or 

 creeds would be prohibited. 



HYDRODYNAMIO ANALOGIES TO 

 ELECTRICITY AND MAGNETISM. Though 

 in our terminology the conception of electricity 

 is still retained as an imponderable fluid per- 

 vading the interstices of matter, physicists have 

 long since ceased to hold such a view. Like 

 light, electricity is now regarded by them as 

 due to the molecular vibrations of the bodies 

 exhibiting the phenomena, and many refined 

 researches have in late years been directed 

 toward determining the form and character- 

 istics of these vibrations in the electrically- 

 excited body and in the intervening medium. 

 Very considerable light has, during the year, 

 been thrown upon this problem of electrical ac- 

 tion by the remarkable experiments of Dr. 0. 

 A. Bjerknes, of Christiania, Norway, who, by 

 simple mechanical means, has succeeded in pro- 

 ducing most of the fundamental phenomena of 

 electricity and magnetism. The experiments 

 are the more significant, from their being the out- 

 come of mathematical analysis instead of due 

 to accident. As has long been known, a vibrat- 

 ing tuning-fork, brought into the neighborhood 

 of a light object, such as a small balloon, will 

 attract it. This has been explained as due to 

 the unequal air-pressure upon the opposite sides 

 of the balloon, the air on the side toward the 

 fork being rarefied by its excursion. It is this 

 idea which Dr. Bjerknes has carried out in his 

 experiments using, however, a liquid as the 

 medium of his vibrations, instead of air. The 

 motions produced were of two kinds, pulsa- 



FIQ. 1. 



tions and oscillations, the former being made 

 by small drums with flexible heads, and the 

 latter by flexible spheres, such as are repre- 

 sented in Fig. 1. These are immersed in wa- 



ter and thrown into vibration by means of air 

 alternately forced in and withdrawn. As the 

 apparatus supplying the air can be readily ad- 

 justed to varying conditions, vibrations of any 

 rate can be produced at will. Operating with 

 simple drums, in which the two flexible heads 

 perform like motions at the same time, Dr. 

 Bjerknes found that when the motions of two 

 drums were of the same phase that is, when 

 they were inflated and collapsed at the same 

 time attraction ensued; but when the phases 

 were discordant that is, when one drum was 

 inflated while the other was collapsed there 

 was repulsion. By fixing one of the, drums 

 upon a light arm so as to allow it to revolve, 

 these effects were shown in a very marked 

 manner, the free drum approaching the other 

 during attraction and moving away during re- 

 pulsion. This effect illustrates the action be- 

 tween two magnets, though here the attrac- 

 tion occurs with like phases, and repulsion 

 with unlike ones, while with magnets the re- 

 verse is true. Since both heads of the drum 

 possess the same phase, it is analogous to an 

 isolated pole of a magnet, or to a magnet hav- 

 ing a succeeding point in the middle. In order 

 to have two poles, it is necessary to use a 

 double drum that is, one divided into two 

 compartments by a stiff partition. Each of 

 these chambers must then be connected with a 

 separate air-pump in both drums, which greatly 

 complicates the apparatus. The desired effect 

 can, however, be produced without this com- 

 plication by using oscillating spheres, which 

 present the two phases at the same time, one 

 side advancing while the other recedes. Re- 

 pulsion or attraction of the free drum can 

 therefore be obtained simply by presenting to 

 it one side or the other of the sphere. For 

 the free drum a second sphere may be substi- 

 tuted, when similar phenomena will result. In 

 the arrangement shown in Fig. 2, two spheres 

 are carried on the ends of a light cross-bar free 

 to move. If a third vibrating sphere be pre- 

 sented to either of these, rotation occurs, its 

 direction depending upon whether there is at- 

 traction or repulsion. The- effect is analogous 

 to that of two short magnets mounted upon a 

 cross-bar and free to revolve. If for the two 

 spheres a single one be substituted, as shown 

 in the dotted line, an effect is obtained similar 

 to that of a short magnet hung on a pivot like 

 a compass-needle. 



Dr. Bjerknes considers the water as analo- 

 gous to Faraday's medium, and therefore at- 

 tributes the effects, not to the mutual action 

 of the bodies upon each other, but to the ac- 

 tion ' v of each body upon the liquid. In this 

 view, when a vibrating body and the liquid 

 have their motions in] the same direction, the 

 body moves away from the center of disturb- 

 ance, and toward it when these motions are in 

 opposite directions. If a vibrating sphere be 

 brought near a ball of cork, kept in the liquid 

 by a thread, repulsion results, the phenomenon 

 corresponding to diamagnetism. If, however, 



