'Über die Ausbreitungsgeschwindigkeit der elektrodynamischen Wirkungen' [On the finite velocity of propagation of electromagnetic actions], pp. 197-210 in: Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin, VI. VII. 10 February 1888

Berlin: Königlichen Akademie der Wissenschaften, 1888. First edition. FIRST PROOF THAT ELECTROMAGNETIC EFFECTS PROPAGATE AT THE SPEED OF LIGHT.

First edition, complete journal issue in original printed wrappers, of one of the two most important papers of Hertz's on electromagnetic waves, in which he demonstrates for the first time that electromagnetic effects propagate at the speed of light (it was only a month later that he first produced and detected electric waves). "In his Treatise on Electricity and Magnetism (1873) [Maxwell] gave no theory of oscillatory circuits or of the connection between currents and electromagnetic waves. The possibility of producing electromagnetic waves in air was inherent in his theory, but it was by no means obvious and was nowhere spelled out. Hertz's proof of such waves was in part owing to his theoretical penetration into Maxwell's thought" (DSB). "Experimental proof by Hertz of the Faraday-Maxwell hypothesis that electrical waves can be projected through space was begun in 1887, eight years after Maxwell's death ... The experiments were reported periodically from 1887 onward in Annalen der Physik und Chemie" (PMM). "This discovery [of electromagnetic waves] and its demonstration led directly to radio communication, television and radar" (Dibner). "In the early 1890s the young inventor Guglielmo Marconi read of Hertz's electric wave experiments in an Italian electrical journal and began considering the possibility of communication by wireless waves. Hertz's work initiated a technological development as momentous as its physical counterpart" (DSB). This paper was first published in the Sitzungsberichte (offered here) and then three months later in Annalen der Physik (Bd. 270, Heft 7, pp. 551-569). ABPC/RBH list one copy of the offprint (Swann 1994) and none of the journal issue.

Hertz's work on electric waves began with Helmholtz's proposal in 1879 of a prize problem connected with the behaviour of unclosed circuits in Maxwell's theory. "Central to Maxwell's theory was the assumption that changes in dielectric polarization yield electromagnetic effects in precisely the same manner as conduction currents do. Helmholtz wanted an experimental test of the existence of these effects or, conversely, of the electromagnetic production of dielectric polarization. Although at the time Hertz declined to try the Berlin Academy problem because the oscillations of Leyden jars and open induction coils which he was familiar with did not seem capable of producing observable effects, he kept the problem constantly in mind; and in 1886 shortly after arriving in Karlsruhe, he found that the Riess or Knochenhauer induction coils he was using in lecture demonstrations were precisely the means he needed for undertaking Helmholtz' test of Maxwell's theory ...

"He produced electric waves with an unclosed circuit connected to an induction coil, and he detected them with a simple unclosed loop of wire. He regarded his detection device as his most original stroke, since no amount of theory could have predicted that it would work. Across the darkened Karlsruhe lecture hall he could see faint sparks in the air gap of the detector. By moving it to different parts of the hall he measured the length of the electric waves; with this value and the calculated frequency of the oscillator he calculated the velocity of the waves. For Hertz his determination at the end of 1887 of the velocity - equal to the enormous velocity of light - was the most exciting moment in the entire sequence of experiments. He and others saw its significance as the first demonstration of the finite propagation of a supposed action at a distance" (DSB).

Hertz himself gives an account of the present paper in the introduction to Electric Waves (pp. 7-8), which contains English translations of his most important papers on the subject. "A scheme was conceived which was carried out as described in the research 'On the finite velocity of propagation of electromagnetic actions.' The first step that had to be taken was easy. In straight stretched wires surprisingly distinct stationary waves were produced with nodes and anti-nodes, and by means of these it was possible to determine the wavelength and the change of phase along the wire. Nor was there any greater difficulty in producing interference between the action which had travelled along the wire and that which had travelled through the air, and thus in comparing their phases. Now if both actions were propagated, as I expected, with one and the same finite velocity, they must at all distances interfere with the same phase. A simple qualitative experiment which, with the experience I had now gained, could be finished within an hour, must decide this question and lead at once to the goal. But when I had carefully set up the apparatus and carried out the experiment, I found that the phase of the interference was obviously different at different distances, and that the alternation was such as would correspond to an infinite rate of propagation in air. Disheartened, I gave up experimenting. Some weeks passed before I began again. I reflected that it would be quite as important to find out that electric force was propagated with an infinite velocity, and that Maxwell's theory was false, as it would be, on the other hand, to prove that this theory was correct, provided only that the result arrived that should be definite and certain. I therefore confirmed with the greatest care, and without heeding what the outcome might be, the phenomena observed: the conclusions are given in the paper. When I then proceeded to consider more closely these results, I saw that the sequence of the interferences could not be harmonised with the assumption of an infinite rate of propagation; that it was necessary to assume that the velocity [in air] was finite but greater than that in the wire. As shown in the paper, I endeavoured to bring into harmony the various possibilities; and although the differences in the velocities appeared to me to be somewhat improbable, I could see no reason for mistrusting the experiments."

In the introduction to the translation of the present paper, Hertz goes on to write, "The experiments ... have shown that the inductive action is undoubtedly propagated [in air] with a finite velocity. This velocity is greater than the velocity of propagation of electric waves in wires. According to the experiments made up to the present time, the ratio of these velocities is about 45 : 28. From this it follows that the absolute value of the first of these is of the same order as the velocity of light" (Electric Waves, p. 108). At the end of the paper, Hertz concludes: "There are already many reasons for believing that the transversal waves of light are electromagnetic waves; a firm foundation for this hypothesis is furnished by showing the actual existence in free space of electromagnetic transversal waves which are propagated with a velocity akin to that of light. And a method presents itself by which this important view may finally be confirmed or disproved. For it now appears possible to study experimentally the properties of electromagnetic transversal waves, and to compare these with the properties of light waves" (ibid., p. 123).

Hertz went on to do just this, in his paper 'Ueber elektrodynamische Wellen im Luftraume und deren Reflexion' [On electrodynamic waves in air and their reflection] (Annalen der Physik, Bd. 34 (1888), pp. 609-623), which was published on 20 May 1888. It was in this paper that Hertz first demonstrated the existence of electromagnetic waves in air.

The sequence of Hertz's researches on electromagnetic waves, and the details of their publication, are complex, the most complete account being that of Buchwald (The Creation of Scientific Effects, 1994). Some were published in the Sitzungsberichte, and then later (sometimes in modified form) in the Annalen, and others were published only in one of the two journals. Jungnickel & McCormmach (p. 87) state: "As quickly as Hertz completed his experiments, he wrote them up, usually sending them to Helmholtz for publication with the Prussian Academy. On Wiedmann's request, he published papers in the Annalen der Physik too. Early in 1888 he reported in the Annalen an indication of a 'finite velocity of propagation of electric distance actions,' which gave him renewed confidence in his work. In subsequent papers, Hertz elaborated experimentally and theoretically on the fact of finite propagation." The paper published "early in 1888" in the Annalen to which Jungnickel & McCormmach refer was 'Ueber die Einwirkung einer geradlinigen electrischen Schwingung auf eine benachbarte Strombahn' [The action of a rectilinear electric oscillation on a neighbouring circuit], Annalen der Physik, Bd. 34 (1888), pp. 155-170 (the comment about finite propagation is on p. 93 of Electric Waves). This article was published in the 15 March 1888 issue of the Annalen, one month after the offered paper.

Hertz, Electric Waves, 1893. Jungnickel & McCormmach, Intellectual Mastery of Nature. Theoretical Physics from Ohm to Einstein, Vol. 2, 1986.

Large 8vo, pp. 195-211. Original orange printed wrappers (strip of matching paper pasted onto lower blank margin of front wrapper).