1889.] 071 the Discharge of a Leijden Jar. 419 



The two volumes contain a wealth of beautiful experiments 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 experi- 

 mentalist could possibly have divined their real significance. 



It is, after all, the genius of Maxwell and of a few other gi'eat 

 theoretical j)hysicists whose names are on everyone's lipsl 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 j)art of the investi- 

 gations, 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, jDroduced an induction sufficiently powerful to magne- 

 tise 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, intervening. The author is disposed to adopt 

 the hypothesis of an electrical jAenum [in other words, of an etherj, 

 and from the foregoing experiment it would appear that a single 

 spark is sufficient to disturb perceptibly the electricity of space 

 throughout 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 diffusion 

 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 oscillatory 

 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 



* Loc. cit., p. 204:. 



t And of one whose name is not yet on everybody's lips, but whose profound 

 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 Heaviside. 



