1.^4 



NATURE 



[June 7, 1894 



Tbe simplest way will be for me hastily to summarise oui 

 knowledge of the subject before the era of Hertz. 



Just as a pebble thrown into a pond excites surface ripples, 

 which can heave up and down floaiing straws under which they 

 pass, so a struck bell or tuning-fork emits energy into the air 

 in the form of what ate called sound waves ; and this radiant 

 energy is able to set up vibrations in other suitable elastic 

 bodies. 



If the body receiving them has its natural or free vibrations 

 violently damped, so that when left to itself it speedily returns 

 to rest, then it can respond feebly to notes of almost any pitch. 

 This is the case with your ears and the tones of my voice. 

 Tones must be exceedingly shrill before they cease to excite 

 the ear at all. 



If, on the other hand, the receiving body has a persistent 

 period of vibration, continuing in motion long after it is left 

 to itself, like another tuning-fork or bell for instance, then 

 lar more facility of response exists, but great accuracy of tuning 

 is necessar)' if it is to be fully called out ; for if the receiver is 

 not thus accurately syntonised with the source, it fails more or 

 less completely to resound. 



Conversely, if the source is a persistent vibrator, correct 

 tuning is es^ential, or it will destroy at one moment motion 

 which it originated the previous moment. Whereas if it is a 

 dead beat or strongly-damped excitor, almost anything will 

 respond equally well or equally ill to it. 



What 1 have said of sounding bodies is true of all vibrators 

 in a medium competent to transmit waves. Now a sending 

 telephone or a microphone, when spoken to, emits waves into 

 the ether, and this radiant energy is likewise able to set up 

 vibration in suitable bodies. But we have no delicate means 

 of dirtctly detecting these electrical or elherial waves, and if 

 they are to produce a perceptible effect at a distance they must 

 be conhned, as by a spe.iking tube, prevented from spreading, 

 and concentrated on tne distant receiver. 



This is the (unciion of the telegraph wire; it is to the ether 

 what a speaking-tube is to air. A metal wire in air {in function, 

 not in details ol analogy) is like a long hollow cavity surrounded 

 by nearly rigid but slightly elastic walls. 



Sphere- charged from Eleclrophcrus. 



Furthermore, any conductor electrically charged or dis- 

 charged with sufficient suddenness must emit electrical waves 

 into the ether, because the charge given to it will not settle 

 down instantly, but will surge to and fro several times fust ; and 

 these surgings or electric oscillations must, according to Max- 

 well, start waves in the ether, because at the end of each half 

 swing they cause electrostatic, and at the middle of each half 

 wings tbeycause electromagnetic effects, and the rapid alternation 

 (roni one of these modes of energy to the other constitutes 

 etherial waves.' If a wire is handy they will run along it, and 

 may be felt a long way off. If no wire exists they will spread 

 out like sound from a bell, or light from a spark, and their in- 

 tensity will decrease according to the inverse square of the 

 distance. 



Maxwell and his followers well knew that there would be 

 such waves ; they knew the rate at which they would go, they 

 knew that they would go slower in glass and water than in air, 

 they knew that they would curl round sharp edges, that they 

 would t>c partly al>soibcd but mainly reflected by conductors, 

 that if turned back upon themselves they would produce the 

 phenomena of stationary waves, or interlerence, or nodes and 

 loops ; It was known how to calculate the length of such wave-, 

 and even how to pro'hice them of any required or predetermined 

 wavelength from looo miles to a loot. Other things were 

 known aijout them which would take too long to enumerate : 

 any homogeneous insulator would transmit them, would refract 

 or concentiate them if it were of suitable shape, would reflect 

 none ol a particular mode of vibration at a certain angle, and so 

 on, and so on. 



All this was "known," 1 say, known with varying degrees of 

 confidence, but by some known with as great confidence as, per- 

 haps even more confidence than, is legiiimate before the actuality 

 of experimental verification. 



1 Sfriclljr fpcalcioK. in tfle waves tfiem^tdvc^ tfierc t» no lag or dilTcrcnce 

 cf \\xx<r r*t*'_'n if,rrt.;.(r; I.I1.I if..- i;,.i.:ii. tk vil.i.iti -n^. the dilTcrencc 



.••liiiim. f nlc 

 r-' 'ii£tt> from tlic 



^ fgti'ic of pluM 



u obiitri-i!'. 



NO. 1284, VOL. 50] 



Hertz supplied the verification. He inserted suitable con- 

 ductors in the path of such waves, conductors adapted for the 

 occurrence in them of induced electric oscillations, and to the 

 surprise of everyone, himself doubtless included, he found that 

 the secondary electric surgings thus excited were strong enough 

 to display themselves by minute electric sparks. 



Syntonic Leyden Jars. 



I shall show this in a form which requires great precision 

 of tuning or syntony, both emitter and receiver being persist- 

 ently vibrating things giving some thirty or forty swings before 

 damping has a serious effect. I take two Leyden jars with cir- 

 cuits about a yard in diameter, and situated about two yards 

 apart. 1 charge and discharge one jar, and observe that the 

 surgings set up in the other can cause it to overflow if it is 

 syntonised with the first.' 



A closed circuit such as this is a feeble radiator and a feeble 

 absorber, so it is not adapted for action at a distance. In fact, 

 I doubt whether it will visibly act at a range beyond the 1^ at 

 which true radiation of broken off energy occurs. If the coat- 

 ings of the jar are separated to a greater distance, so that the 

 dielectric is more exposed, it radiates better ; because in true 

 radiation the electrostatic and the m.agnetic energies are equal, 

 whereas in a ring circuit the magnetic energy gre.atly pre- 

 dominates. By separating the coals of the jar .is far as possible 

 we get a typical Hertz oscillator, whose dielectric extends out 

 into the room, and this radiates very powerfully. 



Ordinary size Hertz \'ibrcttor. 



In consequence of its radiation of energy its vibrations are 

 rapidly damped, and it only gives some three or four good 

 strong swings. Hence it follows that it h.is a wide lange of 

 excitation, i.<.'. it can excite sparks in conductors barely at all 

 in tune with 11. 



The two conditions, conspicuous energy of radiation and 

 persistent vibration electrically produced, are at present in- 

 compatible. Whenever these two conditions coexist, consider- 

 able power or activity will of course be necessary in the source 

 of energy. .'\t present they only coexist in the sun and other 

 stars, in the electric arc, and in furnaces. 



T-vo Circular Vibrators sparking in sympathy. 



The receiver Hertz used w.is chiefly a circaLir resonator, not 

 a good absorber but a persistent vibrator, well adapted for 

 picking up disturbances 01 precise and measurable wave-length. 

 1 find that the circular resonators can act as senders too ; here 

 is one exciting quite long sparks in a second one. 



Electric ."iynlimy — that was his discovery, but he did not 

 stop there. He at once proceeded to apply his discovery to the 

 verification of what had already been predicted about the 

 waves, and by laborious and difficult interference experiments 

 he ascertained that the previously calculated length of the waves 

 was thoroughly borne out by f.ict. These inteilerence experi- 

 ments in free space are his greatest achievement. 



He worked out every detail of the theory splendidly, sepa- 

 rately analysing the electric and the magnetic oscillation — using 

 language not always such as we should use now, but himself 

 growing in theoretic insight through the medium of what would 

 have been to most physicists a conlusing maze of tioublesome 

 facts, and disentangling all their main relations most har- 

 moniously. 



Iloltz Machine, .'I and B Sparks ; Glass and Quart: Fanes in 

 .Screen, 



While Hertz was observing sparks such as these, the primary 

 or exciting spark and the secondary or excited one, he observed 

 as a bye issue ilia', the secondary spark occurreil more easily if 

 the light from the primary fell upon its knobs, fie examined 

 this new influence of light in many ways, and showed thai 

 although spark light and electric brush light were peculiarly 

 effective, any source of light that gave very ultra-violet rays 

 produced ihe same result. - 



Wiedemann and Eberl, and a number of expeiimenters, have 

 repealed and extended this discovery, proving that it is the 

 cathode knob on which illumination takes effect ; and llall- 



' Sec Naturi., vjI. <i, p. 368; or J. J. '1 liomson, " Rtc^nt Researches" 



' 1 he experiment vhouo in I'ne lecture wa-i on the lines of tho-ie described 

 in my book, '' Lijthtning Conductors," pp. J14 and 340; the connections 

 being much as on p. 285, or as depicted in /'/-ft. Key. ■StK., vol. So, P- 4- 



