492 



SCIENCE. 



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- 

 compressers and expanders, and they can be arranged so 

 that both compress and both expand the air simulta- 

 neouslv, or in such a way that .the one expands while 

 the other compresses the air, and vice versa. These c\l- 

 inders 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 each of a ring of metal faced at 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 bo'h expand and both contract simul- 

 taneously, 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. 



Instead of having the pulsation of a drum we may use 

 the oscillation of a sphere ; and Dr. Bjerknes has mounted 

 a beautiful piece of appara'us 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 ths sphere are in opposite phases. 

 In fact the sphere might be expected to act like a mag- 

 net ; and so it does. If two oscilla ing spheres be 

 brought near each other, then, if they are both mov- 

 ing to and from each other at the same time, there is at- 

 traction ; but if - one of them be turned round, so that 

 both spheres move in the same direction in their oscilla- 

 tions, 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 neighbor- 

 hood of another magnetic pole. I believe that these direc- 

 tive effects are perfectly new, both theoretically and experi- 

 mentally. The professor mounts his rod with a sphere 

 at each end in two ways : (i) 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 bt'le 

 magnet with its axis lying along the direction of oscilla- 

 Iation. 



Dr. Bjerknes looks upon the wa*er 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 

 opposite direction, towards it. This idea gives us the 

 analogy of dia- and piramagnetism. If, in the neigh- 

 borhood of a 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. It, 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 oscilla- 

 tions of the ball relatively to the water are in the opposite 



direction to those of the water itself, and there is attrac- 

 tion, 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 equatoriaily, and 

 the metal rod axially. 



If a pellet of iron be floated by a cork on water and two 

 similar poles (e. both north) be brought to its vicin- 

 ity, 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 

 ol 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 hori- 

 zontal component of the north pole repulsion may be 

 greater than that of the south pole attraction. Dr. 

 Bjerknes reproduces this experiment by 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. Bjerkes conceived further the beautiful idea of 

 tracing out the conditions of the vibrations of the water 

 when acted upon by- pulsating drums. For this purpose 

 he mounted a sphere or cylinder on a thin spring and 

 fixed a 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 pulsa- 

 tions 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 hydro- 

 dynamic line of force is depicted. Thus the whole field 

 is explored and different diagrams are obtained according 

 to the nature of the pulsations. Using two drums pul- 

 sating concordantly, we get a figure exactly like that pro- 

 duced by iron filings in a field of two similar magnetic 

 poles. If the pulsations are discordant it is like the fig- 

 ure with two dissimilar poles. Three pulsating drums 

 give a figure identical with that produced by three mag- 

 netic 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 Magne- 

 tism," but this only enhances the beauty of the experi- 

 mental confirmation. 



Physicists have been in the habit of looking upon mag- 

 netism 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 phy- 

 sicists, let us see whither they lead us. The professor 

 shows that if a tectilinear oscillation constitutes magnet- 

 ism, 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 pro- 

 duce the same effect as the rectilinear vibrations of a mag- 

 net. This is the analogue of the Amperian currents. 



To illustrate the condition of the magnetic field in the 

 neighborhood of electric currents, Dr. Bjerknes mounted 

 two wooden cylinders on vertical axes, 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 fig- 

 ures which are produced on the glass plate are in every 

 case the same as those which are produced by iron filings 

 in the neighborhood of electric currents, including the 



