March 14, 1S89] 



NATURE 



473 



from one side of the jar to the other ; the phenomenon requires 

 us to admit the existence of a principal discharge in one direction 

 and then several reflex actions baclrward and for^vard, each more 

 feeble than the preceding, until the equilibrium is obtained. All the 

 facts are shown to be in accordance with this hypothesis, and a 

 ready explanation is afforded by it of a number of phenomena, 

 which are to be found in the older works on electricity, but 

 which have until this time remained unexplained. "^ 



The italics are Henry's. Now if this were an isolated passage 

 it might be nothing more than a lucky guess. But it is not. 

 The conclusion is one at which he arrives after a laborious repe- 

 tition and serious study of the facts, and he keeps the idea con- 

 stantly before him when once grasped, and uses it in all the rest 

 of his re'^earches on the subject. The facts studied by Henry do 

 in my opinion support his conclusion, and if I am right in this 

 it follows that he is the original discoverer of the oscillatory 

 character of a spark, although he does not attempt to state its 

 theory. That was first done, and completely done, in 1853, by 

 Sir William Thomson ; and the progress of experiment by 

 Feddersen, Helmholtz, Schiller, and others has done nothing 

 but substantiate it. 



The writings of Henry have been only quite recently collected 

 and published by the Smithsonian Institution of Washington in 

 accessible form, and accordingly they have been far too much 

 ignored. The two volumes contain a wealth of beautiful ex- 

 periments clearly recorded, and well repay perusal. 



The discovery of the oscillatory character of a Leyden jar dis- 

 charge may seem a small matter, but it is not. One has only to 

 recall the fact that the oscillators of Hertz are essentially Leyden 

 jars — one has only to use the phrase "electro-magnetic theory 

 of light " — to have some of the momentous issues of this discovery 

 flash before one. 



One more extract I must make from that same memoir by 

 Henry," and it is a most interesting one ; it shows how near he 

 was, or might have been, to obtaining some of the results of 

 Hertz ; though, if he had obtained them, neither he nor any 

 other experimentalist could possibly have divined their real 

 significance. 



It is, after all, the genius of Maxwell and of a few other great 

 theoretical physicists \Ahose names are on everyone's lips^ which 

 endows the simple induction experiments of Hertz and others 

 with such stupendous importance. 



Here is the quotation : — 



" In extending the researches relative to this part of the in- 

 vestigations, a remarkable result was obtained in regard to the 

 distance at which induction effects are produced by a very small 

 quantity of electricity ; a single spark from the prime conductor 

 of a machine, of about an inch long, thrown on to the end of a 

 circuit of wire in an upper room, produced an induction suf- 

 ficiently powerful to magnetize needles in a parallel circuit of 

 iron placed in the cellar beneath, at a perpendicular distance of 

 30 feet, with two floors and ceilings, each 14 inches thick, inter- 

 vening. The author is disp.sed to adopt the hypothesis of 

 an electrical plenum [in other words, of an ether], and from 

 the foregoing experiment it would appear that a single spark is 

 sufficient to disturb perceptibly the electricity of space through- 

 out at least a cube of 400,000 feet of capacity ; and when it is 

 considered that the magnetism of the needle is the result of the 

 difference of two actions, it may be further inferred that the dif- 

 fusion of motion in this case is almost comparable with that of a 

 spark from a flint and steel in the case of light." 



Comparable it is, indeed, for we now know it to be the 

 self-same process. 



One immediate consequence and easy proof of the oscilla- 

 tory character of a Leyden jar discharge is the occurrence of 

 phenomena of sympathetic resonance. 



Everyone knows that one tuning-fork can excite another at a 

 reasonable di-tance if both are tuned to the same note. Every- 

 one knows, also, that a fork can throw a stretched string at- 

 tached to it into sympathetic vibration if the two are tuned to 

 unison or to some simple harmonic. Both these facts have their 

 electrical analogue. I have not time to go fully into the matter 



' " Scientific Writinsts of Joseph Henry," vol. i. p. 201. Published by 

 the Smithsonian Institution, Washington, 1886. 



'^ Loc. cit. , p. 204. 



3 And of one whose name is nst yet en everybody's lips, but *hose pro- 

 found researches into electro-magnetic waves have penetrated further than 

 anybody yet understands into the depths of the subject, and whose papers 

 have very likely contributed largely to the theoretical inspiration of Hertz — 

 I mean that powerful mathematical physicist, Mr. Oliver Heavbide. 



to-night, but I may just mention the two cases which I have 

 myself specially noticed. 



A Leyden jar discharge can so excite a similarly-timed neigh- 

 bouring Leyden jar circuit as to cause the latter to burst its 

 dielectric if thin and weak enough. The well-timed impulses 

 accumulate in the neighbouring circuit till they break through a 

 quite perceptible thickness of air. 



Put the circuits out of unison by varying the capacity or by in- 

 cluding a longer wire in one of them ; then, although the added 

 wire be a coil of several turns, well adapted to assist mutual in- 

 duction as ordinarily understood, the effect will no longer occur. 



That is one case, and it is the electrical analogue of one 

 tuning-fork exciting another. It is too small at present to show 

 here satisfactorily, for I only recently observed it, but it is 

 exhibited in the library at the back. 



The other case, analogous to the excitation of a stretched string 

 of proper length by a tuning-fork, I published last year under 

 the name of the experiment of the recoil kick, where a Leyden 

 jar circuit sends waves along a wire connected by one end with 

 it, which waves splash off at the far end with an electric brush or 

 long spark. 



I will show merely one phase of it to-night, and that is the 

 reaction of the impulse accumulated in the wire upon the jar 

 itself, causing it to either overflow or burst. [Sparks of gallon 

 or pint jar made to overflow by wire round room.'] 



The early observations by Franklin on the bursting of 

 Leyden jars, and the extraordinary complexity or multiplicity of 

 the fracture that often results, are most interesting. 



His electric experiments as well as Henry's well repay perusal, 

 though of course they belong to the infancy of the subject. 



He notes the striking fact that the bursting of a jar is an extra 

 occurrence, it does not replace the ordinary discharge in the 

 proper place, it accompanies it ; and we now know that it is 

 precipitated by it, that the spark occurring properly between the 

 knobs sets up such violent surgings that the jar is far more violently 

 strained than by the static charge or mere difference of potentials 

 between its coatings ; and if the surgings are at all even roughly 

 properly timed, the jar is bound to either overflow or burst. 



Hence a jar should always be made without a lid, and with a 

 lip protruding a carefully considered distance above its coatings : 

 not so far as to fail to act as a safety valve, but far enough to 

 prevent overflow under ordinary and easy circumstances. 



And now we come to what is after all the main subject of my 

 discourse this evening, viz. the optical and audible demonstration 

 of the oscillations occurring in the Leyden jar spark. Such a 

 demonstration has, so far as I know, never before been attempted, 

 but if nothing goes wrong we shall easily accomplish it. 



And first I will do it audibly. To this end the oscillations 

 must be brought down from their extraordinary frequency of a 

 million or hundred thousand a second to a rale within the 

 limits of human audition. One does it exactly as in the case of 

 the spring — one first increases the flexibility and then one loads 

 it. [Spark from battery of jars and varying sound of same.] 



Using the largest battery of jars at our disposal, I take the 

 spark between these two knobs — not a long spark, \ inch will be 

 quite sufficient. Notwithstanding the great capacity, the rate of 

 vibration is still far above the limit of audibility, and nothing 

 but the customary crack is heard. I next add inertia to the circuit 

 by including a great coil of wire, and at once the spark changes 

 character, becoming a very shrill but an unmistakable whistle, 

 of a quality approximating to the cry of a bat. Add another 

 coil, and down cjmes the pace once more, to something like 

 5000 per second, or about the highest note of a piancT. Again 

 and again I load the circuit with magnetizability, and at last the 

 spark has only 500 vibrations a second, giving the octave, or 

 perhaps the double octave, above the middle C. 



' Durng the course of this experiment, the gilt paper on the wall was 

 observed by the audience to be sparkling, every g.lt patch oyer a certain 

 area discharging into the next, after the manner of a spangled jar. It was 

 pr.ibably due to some kind of sympathetic resonance. Electricity splashes 

 about in conductors in a surprising way everywhere in the neighbourhood of 

 a discharge. For instance, a telescope in the hand of one of the audience 

 was reported afterwards to be giving off little sparks at every discharge of 

 the jar. Everything which happens to have a period of electric oscillation 

 corresponiling to some harmonic of the main oscillation of a discharge is 

 liable to behave in this way. When light falls on an opaque surface it is 

 quenched. What the audience saw was probably the result of waves of 

 electrical radiation being quenched by the walls of the room, and generating 

 electrical currents in the act. It is these electric surgings which render 

 such severe caution necessary in the erection of lightning-conductors. This 

 explanation is merely tentative. I have had no time to investigate the matter 

 locally. 



