472 



NATURE 



\_March 14, 1889 



Increase either of these, and the rate of oscillation is dimin- 

 ished. Increasing the static capacity corresponds to lengthening 

 the spring ; increasing the self-induction corresponds to loading it. 

 Now the static capacity is increased simply by using a larger 

 jar, or by combining a number of jars into a battery in the very 

 old-established way. Increase in the self-induction is attained 

 by giving the discharge more space to magnetize, or by making 

 it magnetize a given space more strongly. For electro-magnetic 

 inertia is wholly due to the magnetization of the space surround- 

 ing a current, and this space may be increased or its magnetiza- 

 tion intensified as much as we please. 



To increase the space we have only to make the discharge take 

 a long circuit instead of a short one. Thus we may send it by a 

 wire all round the room, or by a telegraph wire all round a town, 

 and all the space inside it and some of that outside will be more 

 or less magnetized. More or less, I say, and it is a case of less 

 rather than more. Practically very little effect is felt except close 

 to the conductor, and accordingly the self-induction increases 

 very nearly proportionally to the length of the wire, and not in 

 proportion to the area inclosed : provided also the going and 

 return wires are kept a reasonable distance apart, so as not to 

 encroach upon each other's appreciably magnetized regions. 



But it is just as effective, and more compact, to intensify the 

 magnetization of a given space by sending the current hundreds 

 of times round it instead of only once ; and this is done by 

 inserting a coil of wire into the discharge circuit. 



Yet a third way there is of increasing the magnetization of a 

 given space, and that is to fill it with some very magnetizable 

 substance such as iron. This, indeed, is a most powerful method 

 under many circumstances, it being possible to increase the 

 magnetization and therefore the self-induction or inertia of the 

 current some 5000 times by the use of iron. 



But in the case of the discharge of a Leyden jar iron is of no 

 advantage. The current oscillates so quickly that any iron in- 

 troduced into its circuit, however subdivided into thin wires it 

 maybe, is protected from magnetism by inverse currents induced 

 in its outer skin, as your Professor of Natural Philosophy ^ has 

 shown, and accordingly it does not get magnetized ; and so far 

 from increasing the inductance of the discharge circuit it 

 positively diminishes it by the reaction effect of these induced 

 currents : it acts, in fact, much as a mass of -copper might be 

 expected to do. 



The conditions determining rate of oscillation being under- 

 stood, we have next to consider what regulates the damping out 

 of the vibrations, i.e. the total duration of the discharge. 



Resistance is one thing. To check the oscillations of a vibrating 

 spring you apply to it friction, or make it move in a viscous 

 medium, and its vibrations are speedily damped out. The 

 friction may hie made so great that oscillations are entirely pre- 

 vented, the motion being a mere dead-beat return to the position 

 of equilibrium ; or, again, it maybe greater still, and the motio'i 

 may correspond to a mere leak or slow sliding back, taking 

 hours or days for its accomplishment. With very large con- 

 densers, such as are used in telegraphy, this kind of discharge is 

 frequent, but in the case of a Leyden jar discharge it is entirely 

 exceptional. It can be caused by including in the circuit a wet 

 string, or a capillary tube full of distilled water, or a slab of wood, 

 or other atrociously bad conductor of that sort ; but the conditions 

 ordinarily associated with the discharge of a Leyden jar, whether 

 it discharge through a long or a short wire, or simply through its 

 tongs, or whether it overHow its edge or puncture its glass, are 

 such as correspond to oscillations, and not to leak. [Discharge 

 jar first through wire and next through wood.] 



When the jar is made to leak through wood or water the dis- 

 charge is found to be still not steady : it is not oscillatory indeed, 

 but it is intermittent. It occurs in a series of little jerks, as 

 when a thing is made to slide over a re-ined surface. The 

 reason of this is that the terminals discharge faster than the 

 circuit can supply the electricity, and so the flow is continually 

 stopped and begun again. 



Such a discharge as this, consisting really of a succession of 

 small sparks, may readily appeal to the eye as a single flash, but 

 it lacks the noise and violence of the ordinary discharge ; and 

 any kind of moving mirror will easily analyze it into its con- 

 stituents and show it to be intermittent. [Shake a mirror, or 

 waggle head or opera-glass.] 



It is pretty safe to say, then, that whenever ajar discharge is 

 not oscillatory it is intermittent, and when not intermittent is 

 oscillatory. There is an intermediate case when it is really dead- 

 * Lord R.-^y'eigh, 



beat, but it could only be hit upon with special care, while its 

 occurrence by accident must be rare. *••/-• 



So far I have only mentioned resistance or friction as the 

 cause of the dying out of the vibrations ; but there is another 

 cause, and that a most exciting one. 



The vibrations of a reed are damped partly indeed by friction 

 and imperfect elasticity, but partly also by the energy transferred 

 to the surrounding medium and consumed in the production of 

 sound. It is the formation and propagation of sound-waves 

 which largely damp out the vibrations of any musical instrument. 

 So it is also in electricity. The oscillatory discharge of a Leyden 

 jar disturbs the medium surrounding it, carves it into waves 

 which travel away from it into space : travel with a velocity of 

 185,000 miles a second : travel precisely with the velocity of 

 light. [Tuning-fork.] 



The second cause, then, which damps out the oscillations in 

 a discharge circuit is radiation : electrical radiation if you like 

 so to distinguish it, but it differs in no respect from ordinary 

 radiation (or "radiant heat " as it has so often been called in 

 this place) ; it differs in no respect from Light except in the 

 physiological fact that the retinal mechanism, whatever it may 

 be, responds only to waves of a particular, and that a very 

 small, size, while radiation in general may have waves whic]> 

 range from 10,000 miles to a millionth of an inch in length. 



The seeds of this great discovery of the nature of light werr- 

 sown in this place : it is all the outcome of Faraday's magneto- 

 electric and electrostatic induction : the development of them 

 into a rich and full-blown theory was the greatest part of the 

 life-work of Clerk-Maxwell : the harvest of experimental verifi- 

 cation is now being reaped by a German. But by no ordinary 

 German. Dr. Hertz, now Professor in the Polytechnicum of 

 Karlsruhe, is a young investigator of the highest type. Trained 

 in the school of Helmholtz, and endowed with both mathematical 

 knowledge and great experimental skill, he has immortalized 

 himself by a brilliant series of investiyrations which have cut 

 right into the ripe corn of scientific opinion in these islands, 

 and by the same strokes as have harvested the grain have opened 

 up wide and many branching avenues to other investigators. 



At one time I had thought of addressing you this evening on 

 the subject of these researches of Hertz, but the experiments are- 

 not yet reproducible on a scale suited to a large audience, and I 

 have been so closely occupied with some not wholly dissimilar, but 

 independently conducted, researches of my own — researches led 

 up to through the unlikely avenue of lightning-conddctors — that I 

 have had as yet no time to do more than verify some of them for 

 my own edification. 



In this work of repetition and verification Prof. Fitzgerald 

 has, as related in a recent number of Nature (February 21, 

 P- 39i)> probably gone further ; and if I may venture a suggestion 

 to your Honorary Secretary, I feel sure that a discourse on 

 Hertz's researches from Prof. Fitzgerald next year would be not 

 only acceptable to you, but would be highly conducive to the 

 progress of science. 



I have wandered a little from my Leyden jar, and I must 

 return to it and its oscillations. Let me very briefly run over 

 the history of our knowledge of the oscillatory character of a 

 I>eyden jar discharge. It was first clearly realized and distinctly 

 stated by that excellent experimentalist, Joseph Henry, of 

 Washington, a man not wholly unlike Faraday in his mode of 

 work, though doubtless possessing to a less degress that astonish- 

 ing insight into intricate and obscure phenomena ; wanting also 

 in Faraday's circumstantial advantages. 



This great man arrived at a conviction that the Leyden jar 

 discharge was oscillatory by studying the singular phenomena 

 attending the magnetization of steel needles by a Leyden jar 

 discharge, first observed in i824by Savary. Fine needles, when 

 taken out of the magnetizing helices^ were found to he not 

 always magnetized in the right direction, and the subject is 

 referred to in German books as anomalous magnetization. It is 

 not the magnetization which is anomalous, but the currents which 

 have no simple direction ; and we find in a memoir published 

 by Henry in 1842, the following words : — 



"This anomaly, which has remained so long unexplained, 

 and which, at first sight, appears at variance with all our 

 theoretical ideas of the connection of electricity and magnetism^ 

 was, after considerable study, satisfactorily referred by the author 

 to an action of the discharge of the Leyden jar which had never 

 before been recognized. The discharge, whatever may be its^ 

 nature, is not correctly represented (employing for simplicity the 

 theory of Franklin) by the single transfer of an imponderable fluid! 



