Abstract
Among the most influential and well-known experiments of the 19th century was the generation and detection of electromagnetic radiation by Heinrich Hertz in 1887–1888, work that bears favorable comparison for experimental ingenuity and influence with that by Michael Faraday in the 1830s and 1840s. In what follows, we pursue issues raised by what Hertz did in his experimental space to produce and to detect what proved to be an extraordinarily subtle effect. Though he did provide evidence for the existence of such radiation that other investigators found compelling, nevertheless Hertz’s data and the conclusions he drew from it ran counter to the claim of Maxwell’s electrodynamics that electric waves in air and wires travel at the same speed. Since subsequent experiments eventually suggested otherwise, the question arises of just what took place in Hertz’s. The difficulties attendant on designing, deploying, and interpreting novel apparatus go far in explaining his results, which were nevertheless sufficiently convincing that other investigators, and Hertz himself, soon took up the challenge of further investigation based on his initial designs.
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06 February 2021
A Correction to this paper has been published: https://doi.org/10.1007/s00407-020-00267-8
Notes
For short accounts of Maxwell’s work and its consequences among his followers see, respectively, Siegel (2014) and Yeang (2014). Details are in Buchwald (1985), while Buchwald (2001) discusses Hertz’s early work with Helmholtz’s system. Darrigol (2000) provides an excellent overview of 19th-century electrodynamics.
There was another potential issue, which Hertz would certainly have known though he did not mention it. Namely, Helmholtz’s theory requires both transverse and (non-dispersive) longitudinal waves in a non-conducting medium with finite susceptibility, with the longitudinal waves travelling faster than the transverse ones at \( \sqrt[{}]{{1 + 4\pi \chi_{\text{e}} }}/\left( {A\sqrt[{}]{{4\pi \chi_{\text{e}} }}} \right) \). Later in 1888 Hertz examined the polarization of the air wave, obviating any possibility that he had detected a longitudinal air wave: “From the mode in which our ray was produced we can have no doubt that it consists of transverse vibrations and is plane-polarized in the optical sense” (Hertz 1893, 177). For the structure of Helmholtz’s theory and its application to wire waves see Buchwald (1994, 375–388).
Hertz (1893, 29) used a definition of period different by half of what became common later, writing that the period of a circuit with capacitance C and induction P is \( \pi \sqrt[{}]{PC/A} \) where A is the speed of light. Hertz’s units set capacitance in meters and induction in seconds.
The amplitude of the discharger’s radiation would of course decrease with distance, but Hertz always adjusted the size of his spark gap so that the wire wave did not overwhelm the direct action since he observed only the interference change on rotating the resonator, not the magnitudes of the waves at different distances.
Hertz apparently never used more than one resonator at a time, probably to avoid differences in the responses of different detectors even if constructed as similarly as possible.
See, for one such example, Doncel (1991, 22).
For full details of Hertz’s results in the discovery experiments based on his notes see Buchwald (1994, 269–298).
Hertz (1893, 120–21) again provided no formula, but it’s easy to see what he had in mind. Suppose that adding some amount b to the backlength of the wire reproduces the same interference at position z + \( \delta z \) that it had at position z. Then any change to the phase of the interference must vanish, in which case \( \delta z \) must equal \( \left( {\frac{b}{{v_{\text{m}} }}} \right)/\left( {1/v_{\text{a}} - 1/v_{\text{m}} } \right) \), where \( v_{\text{m}} ,\; v_{\text{a}} \) are, respectively, the wire and air speeds. If the air speed is greater than the wire speed then the phase position shifts back toward the discharger.
Curtis Forbes, then a PhD student at the Institute for the History and Philosophy of Science and Technology, University of Toronto, offered a venue for the carpenter work and assisted the construction.
We used a 12 V and 12 AH lead battery to power the coil. Hertz’s power source comprised “six large Bunsen cells,” a form of electrochemical battery with a zinc anode in sulfuric acid and a carbon cathode in nitric acid, with each such cell producing approximately 1.9 V. Our source was accordingly close in magnitude to Hertz’s.
Hertz’s coil was 52 cm long and 20 cm in diameter, while ours was only 38.1 cm by 22.86 cm. If the number of winds per unit length were similar, then ours would have been much less effective than Hertz’s.
This was done at Buchwald’s suggestion, who, with Naum Kipnis, had essayed a qualitative version of the air-wave experiment on June 3–4, 1993 in Minnesota (Buchwald 1994, 286–288).
We were enabled to do so through the assistance of Bruno Korst and Professors Sean Hum and Willy Wong at Toronto.
One obvious difference distinguishes our experiment from Hertz’s. He had to judge the point at which the resonator’s sparking stopped on expanding the micrometric gap. This was certainly not very precise given sparking vagaries, but it did at least obviate having to judge the differences visually between stronger, weaker, and no change. Visual perception however varies logarithmically with optical intensity, whereas the latter is directly proportional to spark strength, and we did judge according to the visual strength of the bulb’s light. Nevertheless the bulb responded quite sensitively to the current across it, and so it was not hard to perceive such qualitative changes in its luminosity despite the eye’s logarithmic responsiveness.
We accordingly ignore propagation along the dipole proper. Strictly, the burst at the dipole spark gap should be modeled as a damped wave using a Heaviside step-function for a given time and then the phase at a given point along the extended dipole calculated via retardation. Moreover, the large plates of the discharger are hardly point loci. In what follows we have nevertheless reduced the plates to points. And since the discharger is sufficiently short in comparison with most of Hertz’s observation loci we can reasonably assume the phase at each of its loci at a given time to be effectively the same.
In our model we consider only the E field at a point (i.e. the resonator’s spark-gap or a point directly opposite it) and do not integrate the field around the loop directly or, equivalently, by computing the changing magnetic induction through it. Calculation indicates no significant observable differences between two such methods given the dimensions of Hertz’s apparatus.
We have used Hertz’s parameters to the extent that he provided them: discharger length of 80 cm, total wire length of 300 cm, wire height of 30 cm, spark-gap height (estimated) of 25 cm, and wavelength of 5.6 m.
Hertz’s experimental space was not a simple wave guide because of the presence within the guide space of his discharger, in effect placing a radiating antenna within the guide proper. We have not attempted to model this situation given the complexity introduced by the presence of Hertz’s capacitance-providing plates. However, a configuration involving an antenna placed within a cylindrical guide can be arranged so that “the waveguide field is nulled near the antenna” (Onofrei and Albanese 2017, 93). In this case, the antenna consists of a dipole array arranged around the cylinder’s axis, with the dipoles parallel to the axis and occupying a significant cross-section of the guide. Hertz’s radiator is much different, but given the large dimensions of his experimental space in comparison with the size of his discharger, it seems feasible that a direct antenna field might dominate near the discharger with guide modes coming into play further out.
Ironically, wave guidance and related processes were of central interest to British Maxwellians like J.J. Thomson and George Fitzgerald and accounts in part for their conviction that artificially-produced electromagnetic radiation would likely forever escape detection (Buchwald 1994, 333–339).
The publication sequence and its implications in connection with the laboratory notebook were uncovered in Doncel (1991), which provides careful details.
For details see Buchwald (1994, 251–254).
This is in contrast to the two air modes that would have arisen had the discharger been oriented horizontally—but even in such a case intermodulation would not have affected Hertz’s reflection measurements because the effective wavelength due to it would have been 18.43 m.
Unlike Hertz’s, both in Toronto and Florence our spaces were nearly devoid of disturbing metal barriers and were much smaller in length and width than his.
Maxwell’s theory drew a sharp distinction between electric and magnetic fields, linking the two through the Ampère and Faraday laws. Helmholtz’s, whether in the Maxwell limit of infinite (electric) polarizability or not, does not work with fields but with forces—and throughout his early papers Hertz did not use the term ‘field’. In Helmholtz’s system both electric and magnetic forces arise from a potential function, electric by time variation, magnetic by space variation. In an electrically and magnetically polarizable substance both the electric and magnetic forces will as a result obey a wave equation, with the two forces differing in phase by 90°. Only in the limit of infinite polarizability however do Helmholtz’s equations become fully equivalent to Maxwell’s. For details see Buchwald (1994, 375–388) and especially Darrigol (2000, 412–419).
The reciprocity theorem allows the interchange of transmitting and receiving antennae in calculating field results. If, namely, a volume distribution J1 of current produces a field E1, while a current distribution J2 produces a field E2, then the integral of the inner product of E1 and J2 over the volume equals that of the inner product of E2 and J1.
For details see Buchwald (1994, 251–254).
Hertz himself estimated the circumference of his resonator loop at roughly half the radiation wavelength. Antenna theory indicates that for a half-wavelength loop or dipole, the current is distributed in a form similar to a single-lump standing-wave pattern, in which the current amplitude is minimum at the open-circuit points and maximum at a point opposite to the center between them. Our representation, strictly speaking, does not precisely capture the distribution around the gap since the current in that region does not vanish but is closer to the excitation current introduced to the antenna through its terminals. Nevertheless, our purpose here is not to assess the details of the distribution but rather to estimate the overall effect of the receiving antenna on the incident signal intensity. For that purpose it is important to focus on the region of the loop that makes the most significant contribution, in which case inaccuracies around the gap proper will not substantially affect our claim (which is consistent with Hertz’s own, and his was based in part on observation), that the region opposite the gap principally governs the loop emf.
Presumably this follows from Hertz’s graph because the direct and standing wave exchange positions on either side of the node, thereby reversing the direction of the driving force at symmetric positions on either side of the node (C on his graph) and so keeping the emf across the gap running always in the same direction around the loop. In which case either the emf never changes at all whatever the position of the gap, or if it does vanish at some orientation that places the gap on the side of the resonator toward the wall, then it must also vanish at a symmetric position on the side facing away.
Instead of looking for a node with its attendant observational difficulties, Hertz might instead have looked for a second S0 null point, which would presumptively have occurred a half-wavelength further on. This would have given him a different way to measure the wavelength (S01–S02). Why did he not do so? In the absence of his laboratory notes we cannot offer a definitive answer. However, to work in such a manner would not have been convincing to his intended audience because the positions of the gap nulls do not relate in an obvious manner to the characteristics of the standing wave, as we can see from Hertz’s discussion in which he carefully described what happens to a null point as the loop moves along. To be convincing he needed the distance between a node and an antinode.
References
Buchwald, J. 1985. From Maxwell to Microphysics. Aspects of Electromagnetic Theory in the Last Quarter of the Nineteenth Century. Chicago: The University of Chicago Press.
Buchwald, J. 1989. The Rise of the Wave Theory of Light. Chicago: The University of Chicago Press.
Buchwald, J. 1994. The Creation of Scientific Effects. Heinrich Hertz and Electric Waves. Chicago: The University of Chicago Press.
Buchwald, J. 2001. A Potential Disagreement Between Helmholtz and Hertz. Archive for History of Exact Sciences 55: 365–393.
Darrigol, O. 2000. Electrodynamics from Ampère to Einstein. Oxford: Oxford University Press.
Doncel, M.G. 1991. On the Process of Hertz’ Conversion to Hertzian Waves. Archive for History of Exact Sciences 43: 1–27.
Doncel, M.G. 1995. Heinrich Hertz’s Laboratory Notes of 1887. Archive for History of Exact Sciences 49: 197–270.
Hertz, H. 1893. Electric Waves. London: Macmillan and Co.
Hertz, J., C. Susskind, and M. Hertz. 1977. Heinrich Hertz: Memoirs, Letters, Diaries. San Francisco: San Francisco Press.
Holloway, C., P. Perini, R. Delyser, and K. Allen. 1995. A Study of the Electromagnetic Properties of Concrete Block Walls for Short Path Propagation Modeling. NTIA Report 96-326. U.S. Department of Commerce.
Hong, S. 2001. Wireless: From Marconi’s Black-Box to the Audion. Cambridge, MA: MIT Press.
MacDonald, H.M. 1902. Electric Waves. Cambridge: Cambridge University Press.
Marcuvitz, N. 1986. Waveguide Handbook. London: IEE.
Onofrei, D., and R. Albanese. 2017. Active Manipulation of EM Fields in Open Waveguides. Wave Motion 69: 91–107.
Poincaré, H. 1894. Les Oscillations Électriques. Leçons Professées Pendant Le Premier Trimestre 1892–1893. Paris: Georges Carré.
Sarasin, E., and L. de la Rive. 1893. Interférences Des Ondulations Électriques Par Égalité Des Vitesses de Propagation Dans l’air et Le Long Des Fils Conducteurs. Archives Des Sciences Physiques et Naturelles 29: 358–393.
Siegel, D. 2014. Maxwell’s Contribution to Electricity and Magnetism. In Flood, R., McCartney, M., and Whitaker, A. (eds.) James Clerk Maxwell: Perspective on His Life and Work, 204–222. Oxford: Oxford University Press.
Simpson, T. 2018. Revisiting Heinrich Hertz’s 1888 Laboratory. IEEE Antennas and Propagation Magazine 60: 132–140.
Smith, Glenn S. 2016. Analysis of Hertz’s Experimentum Crucis on Electric Waves. IEEE Antennas and Propagation Magazine 58: 96–108.
Thorne, K. 2020. One Hundred Years of Relativity. From the Big Bang to Black Holes and Gravitational Waves”. In Buchwald, J., (ed.) Einstein Was Right. The Science and History of Gravitational Waves, 19–46. Princeton: Princeton University Press.
Trouton, F. 1889a. Experiments on Electro-Magnetic Radiation, Including Some on the Phase of Secondary Waves. Nature 40: 398–400.
Trouton, F. 1889b. Repetition of Hertz’s Experiments, and Determination of the Direction of the Vibration of Light. Nature 39: 391–393.
Wittje, R. 1996. Die Frühen Experimente von Heinrich Hertz Zur Ausbreitung Der ‘Elektrischen Kraft:’ Entstehung, Entwicklung Und Replikation Eines Experiments. University of Oldenburg.
Yeang, Chen-Pang. 2013. Probing the Sky with Radio Waves: From Wireless Technology to the Development of Atmospheric Science. Chicago: The University of Chicago Press.
Yeang, Chen-Pang. 2014. The Maxwellians: The Reception and Further Development of Maxwell’s Electromagnetic Theory. In Flood, R., McCartney, M., and Whitaker, A. (eds.) James Clerk Maxwell: Perspective on His Life and Work, 204–222. Oxford: Oxford University Press.
Acknowledgements
The project received financial support from the Canadian Social Sciences and Humanities Research Council, grant no. 435-2014-0385, with Yeang as principal investigator in Canada. Buchwald received support from the Division of Humanities and Social Sciences at Caltech. Stemeroff, Barton, and Harrington performed the replication of Hertz’s wave experiment at Toronto under Yeang’s direction, participating at all stages of design, construction, data recording, and analysis. Stemeroff participated with Brenni, Buchwald and Yeang in the replication at Florence. We especially thank Dr. Paolo Brenni and Anna Giatti for their extraordinary work in producing the apparatus for the Florence reproduction. We also thank Dr. Curtis Forbes for his help with the construction of the experimental apparatus in Toronto.
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Buchwald, J., Yeang, CP., Stemeroff, N. et al. What Heinrich Hertz discovered about electric waves in 1887–1888. Arch. Hist. Exact Sci. 75, 125–171 (2021). https://doi.org/10.1007/s00407-020-00260-1
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DOI: https://doi.org/10.1007/s00407-020-00260-1