WEEKLY EVENING MEETING, Friday, June 1, 1894. LUDWIG MOND, Esq. F.R.S. Vice-President, in the chair. PROFESSOR OLIVER LODGE, D.Sc. LL.D. F.R.S. The Work of Hertz.* THE untimely end of a young and brilliant career cannot fail to strike a note of sadness and awaken a chord of sympathy in the hearts of his friends and fellow-workers. Of men thus cut down in the early prime of their powers there will occur to us here the names of Fresnel, of Carnot, of Clifford, and now of Hertz. His was a strenuous and favoured youth; he was surrounded from his birth with all the influences that go to make an accomplished man of science-accomplished both on the experimental and on the mathematical side. The front rank of scientific workers is weaker by his death, which occurred on January 1, 1894, the thirty-seventh year of his life. Yet did he not go till he had effected an achievement which will hand his name down to posterity as the founder of an epoch in experimental physics. In mathematical and speculative physics others had sown the seed. It was sown by Faraday, it was sown by Thomson and by Stokes, by Weber also doubtless, and by Helmholtz; but in this particular department it was sown by none more fruitfully and plentifully than by Clerk Maxwell. Of the seed thus sown Hertz reaped the fruits. Through his experimental discovery, Germany awoke to the truth of Clerk Maxwell's theory of light, of light and electricity combined, and the able army of workers in that country (not forgetting some in Switzerland, France and Ireland) have done most of the gleaning after Hertz. This is the work of Hertz which is best known, the work which brought him immediate fame. It is not always that public notice is so well justified. The popular instinct is generous and trustful, and it is apt to be misled. The scientific eminence accorded to a few energetic persons by popular estimate is more or less amusing to those working on the same lines. In the case of Hertz no such mistake has been made. His name is not over well known, and his work is The illustrations in this abstract appeared, after the delivery of the discourse, in a little book called "The Work of Hertz and some of his Successors," by Professor Lodge, and are inserted here by the kind permission of the proprietors of the Electrician. VOL. XIV. (No. 88.) Ꮓ immensely greater in every way than that of several who have made more noise. In closing these introductory and personal remarks, I should like to say that the enthusiastic admiration for Hertz's spirit and character, felt and expressed by students and workers who came into contact with him, is not easily to be exaggerated. Never was a man more painfully anxious to avoid wounding the susceptibilities of others; and he was accustomed to deprecate the prominence given to him by speakers and writers in this country, lest it might seem to exalt him unduly above other and older workers among his own sensitive countrymen. Speaking of the other great workers in physics in Germany, it is not out of place to record the sorrow with which we have heard of the recent death of Dr. August Kundt, Professor in the University of Berlin, successor to Von Helmholtz in that capacity. When I consented to discourse on the work of Hertz, my intention was to repeat some of his actual experiments, and especially to demonstrate his less known discoveries and observations. But the fascination exerted upon me by electric oscillation experiments, when I, too, was independently working at them in the spring of 1888,* resumed its hold, and my lecture will accordingly consist of experimental demonstrations of the outcome of Hertz's work rather than any precise repetition of portions of that work itself. In case a minority of my audience are in the predicament of not knowing anything about the subject, a five minutes' explanatory prelude may be permitted, though time at present is very far from being "infinitely long." The simplest way will be for me hastily to summarise our 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 floating straws under which they pass, so a struck bell or tuning-fork emits energy into the air in the form of what are 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, Fig. 1, then it can respond fully 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, Fig. 2, like another tuning fork or bell, for instance, then far more facility of response exists, but great accuracy of tuning is necessary 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. *Phil. Mag., xxvi. pp. 229, 230, August 1888; or "Lightning Conductors and Lightning Guards” (Whittaker), pp. 104, 105; also Proc. Roy. Soc., vol. 50, p. 27. Conversely, if the source is a persistent vibrator, correct tuning is essential, or it will destroy at one moment, Fig. 3, motion which it originated the previous moment. Whereas, if it is a dead-beat or strongly-damped exciter, almost anything will respond equally well or equally ill to it. What I have said of sounding bodies is true of all vibrators in a medium competent to transmit waves. Now a sending telephone or a FIG. 1. FIG. 2. Oscillations of Dumb-bell Hertz Oscillation of Ring-shaped Hertz Resonator excited by the syntonic Vibrator which gave the curve Fig. 1 (after Bjerknes). 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 directly detecting these electrical or ethereal waves; and if they are to produce a perceptible effect at a distance, they must be confined, as by a speaking-tube, prevented from spreading, and concentrated on the distant receiver, FIG. 3. Oscillation of Ring Resonator similarly excited but not quite syntonic with Radiator. (For method of obtaining these curves see Fig. 14.) This is the function 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 of analogy) is like a long hollow cavity surrounded by nearly rigid but slightly elastic walls. Furthermore, any conductor electrically charged or discharged 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 first; and these surgings or electric oscillations must, according to Maxwell, start waves in the ether, because at the end of each half-swing they cause electrostatic, and at the middle of each half-swing they cause electromagnetic effects, and the rapid alternation from one of these modes of energy to the other constitutes ethereal 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 intensity 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 be partly absorbed but mainly reflected by conductors, that if turned back upon themselves they would produce the phenomena of stationary waves, or interference, or nodes and loops; it was known how to calculate the length of such waves, and even how to produce them of any required or predetermined wave-length from 1000 miles to a foot. Other things were known about them which would take too long to enumerate; any homogeneous insulator would transmit them, would refract or concentrate them if it were of suitable shape, would reflect none of a particular mode of vibration at a certain angle, and so on, and so on. All this was known, I say, known with varying degrees of confidence; but by some known with as great confidence as, perhaps even more confidence than, is legitimate before the actuality of experimental verification. Hertz supplied the verification. He inserted suitable conductors in the path of such waves, conductors adapted for the occurrence in them of induced electric oscillations, and to the surprise of every one, himself, doubtless, included, he found that the secondary electric surgings thus excited were strong enough to display themselves by minute electric sparks. I shall show this in a form which requires great precision of tuning, or syntony, both emitter and receiver being persistently vibrating things giving some 30 or 40 swings before damping has a serious effect. I take two Leyden jars with circuits about a yard in diameter, and situated about two yards apart, Fig. 4. I 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 Strictly speaking, in the waves themselves there is no lag or difference of phase between the electric aud the magnetic vibrations; the difference exists in emitter or absorber, but not in the transmitting medium. True radiation of energy does not begin till about a quarter wave-length from the source, and within that distance the initial quarter period difference of phase is obliterated. + See Nature,' vol. xli. p. 368; or J. J. Thomson, Recent Researches,' p. 395. 1 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 A at which true radiation of broken-off energy occurs. If the coatings of the jar are separated to a greater distance, so that the dielectric is more FIG. 4. Overflow Path and Air Gap. Slider for Tuning Experiment with syntonic Leyden Jars. exposed, it radiates better; because in true radiation the electrostatic and the magnetic energies are equal, whereas in a ring circuit the magnetic energy greatly predominates. By separating the coats of the jar о FIG. 5. C Standard Hertz Radiator. as far as possible we get a typical Hertz vibrator, Fig. 5, whose dielectric extends out into the room, and this radiates very powerfully. In consequence of its radiation of energy, its vibrations are rapidly damped, and it only gives some three or four good strong Machine. |