June 19, 1890] 



NATURE 



^n 



smaller vibrating system consisted of two short brass 

 cylinders terminating in gilt brass balls of the same size, 

 and arranged in the same way as the smaller system de- 

 scribed by Prof. Hertz in Wiedemann s Annalen, March 

 1889. This latter system was placed in the focal line of 

 a cylindrical parabolic mirror of thin zinc plate, such as 

 that described by Prof. Hertz in this paper. 



These generators of electro-magnetic oscillations may 

 be called electric oscillators, as the electric charge oscil- 

 lates from end to end. A circle of wire, or a coil in 

 which an alternating current ran, or, if such a thing were 

 attainable, a magnet alternating in polarity, might be 

 called a magnetic oscillator. A ring magnet with a 

 closed magnetic circuit is essentially an electric oscillator, 

 while a ring of ring magnets would be essentially a mag- 

 netic oscillator again. The elementary theory of a mag- 

 netic oscillator can be derived from that of an electric 

 oscillator by simply interchanging electric and magnetic 

 force. Electricity and magnetism would be essentially 

 interchangeable if such a thing existed as magnetic con- 

 duction. The only magnetic currents we know are mag- 

 netic displacement currents and convection currents, such 

 as are used in unipolar and some other dynamos. It is 

 in this difference that we must look for the difference 

 between electricity and magnetism. 



In order to observe the existence of these electro-mag- 

 netic oscillations we can employ the principle of reson- 

 ance to generate oscillations in a system whose free 

 period of oscillation is the same. A magnetic receiver 

 may be employed, consisting of a single incomplete circle 

 of wire broken by a very minute spark-gap, across which 

 a spark leaps when the oscillations in the wire become 

 sufficiently intense. In order that a large audience may 

 observe the occurrence of sparks, the terminals of a gal- 

 vanometer circuit were connected, one with one side of 

 the spark-gap, and the other with a fine point which could 

 be approached very close to the other side of the spark- 

 gap. It was observed that, when a spark occurred in the 

 gap, a spark could also be arranged to occur into the 

 galvanometer circuit, and, with a delicate long-coil gal- 

 vanometer (that used had 40,000 ohms resistance), a very 

 marked deflection can be produced whenever a spark 

 ■occurs. This arrangement we have only succeeded in 

 working comparatively close to the generator, because 

 the delicacy required in adjusting the two spark-gaps is 

 so great. It can, however, be employed to show that the 

 sparks produced in this magnetic resonant circuit are due 

 to resonance by removing this receiver from the generator 

 to such a distance that sparks only just occur, and then 

 substituting for the single circuit a double circuit, which, 

 except for resonance, should have a greater action than 

 the single one, but which stops the sparking altogether. 

 An electric receiver was also used, which was identical 

 with the generator, and had a corresponding, only much 

 smaller, spark-gap between the two plates. When the 

 plates are connected with the terminals of the galvano- 

 meter, upon the occurrence of each spark the galvano- 

 meter is deflected. It is not so easy to obtain sparks 

 when the plates are connected with the galvanometer as 

 when they are insulated, and it is this that has limited 

 the use of this method of observation. By making the 

 first metre or so of the wires to the galvanometer of 

 extremely fine wire, so as to reduce their capacity, we 

 have found that the difficulty of getting sparks is less 

 than with thick wires. We have not observed any effect 

 ^lue to the thickness of the wires after a short distance 

 from the receiver. 



In the case of the small oscillator, a receiver exactly 

 like the one described by Prof Hertz in his second paper 

 already quoted was placed in the focal line of a cylindrical 

 parabolic mirror, and its receiving wires were connected 

 with the wires leading to the galvanometer by some very 

 fine brass wire. With the large-sized generator and re- 

 ceiver, which were placed about 3 metres apart, it was 



NO. 1077, VOL. 42] 



shown that the sparking was stopped by placing a thin 

 zinc sheet so as to reflect the radiations from a point close 

 behind the receiver. By means of a long india-rubber 

 tube hung from the ceiUng, it was shown how, when 

 waves are propagated to a point whence they are reflected, 

 the direct and reflected waves interfering produce a 

 system of loops and nodes, with a node at the reflecting 

 point. It was explained that these nodes, though places 

 of zero displacement, were places of maximum rotation, 

 and that the axis of rotation was at right angles to the 

 direction of displacement. It was explained that an 

 analogous state of affairs existed in the electro-magnetic 

 vibrations. If the electric force be taken as analogous to 

 the displacement of the rope, the magnetic may be taken 

 as analogous to its rotation, and the two are at right 

 angles to one another. In the ether the electric node is 

 a magnetic loop, and vice versd. Though the two are 

 separated in loops and nodes, they exist simultaneously 

 in a simple wave propagation, just as in a rope when 

 propagating waves in one direction the crest of maximum 

 displacement is also that of maximum rotation. It was 

 explained that by placing the reflector at a quarter of a 

 wave-length from the receiver this would be at an electric 

 loop, and have its sparking increased. It may thus be 

 shown that there are a series of loops and nodes pro- 

 duced by reflection of these electromagnetic forces, like 

 those produced in any other case of reflected wave- 

 propagation. This was Hertz's fundamental experiment, 

 by which he proved that electro-magnetic actions are pro- 

 pagated in time, and by some approximate calculations 

 he verified Maxwell's theory that the rate of propagation 

 is the same as that of light. It follows that the lumini- 

 ferous ether is experimentally shown to be the medium to 

 which electric and magnetic actions are.due, and that the 

 electro-magnetic waves we have been studying are really 

 only very long light waves. 



A rather interesting deduction from Maxwell's theory is 

 that light incident on any body that absorbs or reflects it 

 should press upon it and tend to move it away from the 

 source of light. Illustrating this, an experiment was 

 shown with an alternating current passing through an 

 electro-magnet, in front of which a good conducting plate of 

 silver was suspended. When the alternating current was 

 turned on the silver was repelled. It was explained that 

 as the silver could only be affected by what was going on 

 in its own neighbourhood, and that if sufficiently powerful 

 radiations from a distant source were falling on the 

 silver, it would be acted on by alternating magnetic 

 forces, this experiment was in effect an experiment on 

 the repulsion of light, which was too small to have been 

 yet observed, even in the case of concentrated sunshine. 

 These slow vibrations are not stopped by a sheet of 

 zinc, though much reduced by a magnetic sheet like tin- 

 plate, though the rapid ones are quite stopped by either — 

 thus showing that wave-propagation in a conductor is of 

 the nature of a diffusion. 



In all cases of diffusion where we consider the limits of 

 the problem, terms involving the momentum of the parts 

 of the body must be introduced. It appears from ele- 

 mentary theories of diffusion as if it were propagated 

 instantaneously, but no action can be propagated from 

 molecule to molecule, in air, for instance, faster than 

 the molecules move, i.e. at a rate comparable with that 

 of sound. In electro-magnetic theory corresponding 

 terms come in by introducing displacement currents in 

 conductors, and it seems impossible but that some such 

 terms should be introduced, as otherwise electro-magnetic 

 action would be propagated instantaneously in conductors. 

 The propagation of light through electrolytes, and the 

 too great transparency of gold leaf, point in the same 

 direction. 



The constitution of these waves was then considered, 

 and it was explained that if magnetic forces are analogous 

 to the rotation of the elements of a wave, then an ordinary 



