Thursday, February 22, 2007

A Tribute to Heinrich Hertz

Cell phones, radio, television, satellite communication, WLAN - it is hard for us today to imagine a world without the technology of electromagnetic waves. And yet, the man who first created, detected and studied them was born just 150 year ago today, on February 22, 1857.

Heinrich Rudolf Hertz (February 22, 1857 - January 1, 1894),
the discoverer of electromagnetic waves.

Heinrich Hertz grew up in Hamburg, Germany, where his father was a lawyer and senator of the city of Hamburg. After school, he prepared to be a civil engineer, but he was fascinated by mathematics and physics, and after his first year at university, he changed from engineering to physics. He studied in Munich and Berlin, got his PhD at the age of 22 with Kirchhoff and Helmholtz - very young even at that time - became a postdoc at Berlin and Kiel and professor in Karlsruhe and, finally, Bonn - quite a typical meandering path for a German scientist in the 19th century.

In Berlin, Helmholtz got him interested in the status of electromagnetic theory. At that time, Maxwell had formulated his equations already 20 years before, but in Germany, the action-at-distance theories of Weber and Neumann still hold strong. In these theories, electromagnetic phenomena were described by augmented versions of the Coulomb electrostatic law, with velocity-dependent terms added. Helmholtz had intensively studied all these different theories, including Maxwell's, and proposed a unified formulation - the only problem was that all of them were completely compatible with the then-known experimental facts. So, he suggested that his student Hertz may have a closer look at this issue, and maybe find an experimental clue.

The experimental apparatus used by Heinrich Hertz to produce and detect electromagnetic waves. The coil in the background on the left produces a high voltage, which feeds an antenna, the two rods with the big spheres at the end, via a spark discharge between the two small spheres at the centre of the rods. This antenna is the prototype of what we call today a Hertzian dipole (Source: Deutsches Museum, Munich).

Indeed, in a series of experiments in 1887, building on work with resonant circuits fed by spark discharges, Heinrich Hertz managed to create electromagnetic waves. He could detect them with a small wire loop, he could determine the wavelength and calculate from the known frequency that they propagate with the speed of light, he could demonstrate interference and polarisation. After these experiments, it was clear that Maxwell had formulated not only a beautiful, but also a true theory, and that light is indeed an electromagnetic wave, as are the "Hertzian" waves we know today as radio waves.

Creating electromagnetic oscillations of a sufficiently high frequency was an essential prerequesite for the detection of electromagnetic waves. Hertz' initial apparatus produced oscillations of about 500 million cyles per second, corresponding to a wavelength of 60 cm. To commemorate the discovery of electromagnetic waves, the official SI unit for frequency in cycles per second is now the Hertz.

In his work with spark discharges to create high-frequency oscillations, Hertz remarked that electric charges could get "lost" from his apparatus when illuminated by the bright flashes of the spark. As a by-product of his work on electromagnetic waves, he discovered the photoelectric effect!

The configuration of the electromagnetic field around a dipole, as calculated by Heinrich Hertz. This plot from his 1889 paper "The forces of Electric Oscillations, treated according to Maxwell's theory" features in every text on electrodynamics. It was drawn by his wife Elisabeth. (From the collection "Electrical waves".)

Hertz was not only an ingenious experimentalist, but also an excellent theorist. For example, he gave Maxwell's equations a form similar to that we know today (well, not too modern: no vectors, no forms), and made them popular and well-understood in Germany. He applied the theory to calculate the field configuration of electromagnetic waves around a dipole, the "Hertzian dipole". You can speculate what he could still have achieved, had he not died young, at age not even 37.

It is sometimes said that Hertz did not think that the electromagnetic waves he had discovered would have any practical consequence. This is probably a misinterpretation of a mail exchange he has had with a civil engineer named Huber in 1889, who had asked him if he thought it possible to transmit acoustic oscillations via electromagnetic waves. Hertz replied that this would not work since the wavelengths corresponding to such low frequencies are just to big and not possible to handle. Indeed, it still took more than 30 years until the first transmission of voice and sound via radio waves was achieved.

Whatever Hertz may have thought about the later use of "his" waves: If we listen to the radio today, or talk to someone via cellphone, or watch TV, it is a good idea to toast to Heinrich Hertz, and to follow Albert Einstein's suggestion made in a speech at the opening of the 7. Deutsche Funkausstellung in Berlin in 1930, broadcast via radio:

Ladies and Gentlemen who are present and who are not!

When you hear the radio think also about the fact how people have come to possess such a wonderful tool of communication. The origin of all technical achievements is the divine curiosity and the play instinct of the working and thinking researcher as well as the constructive fantasy of the technical inventor. [...] Think also of Maxwell who showed us the existence of electric waves by using a mathematical way, of Hertz who as the first person generated them with the help of a spark and thus proved them. [...] And everybody should be ashamed who uses the wonders of science and engineering without thinking and having mentally realised not more of it than a cow realises of the botany of the plants which it eats with pleasure.

  • A huge compilation of references about Hertz can be found here. However, there seems to be not too much literature easily available online.

  • While his experiments leading to the discovery of electromagnetic waves, Heinrich Hertz has kept a detailed notebook. Moreover, there are many letters he (or his wife) has sent to his parents during this period, so that historians know a lot about how the discovery has happened. A detailed account is given in Jed Buchwald: "The creation of scientific effects - Heinrich Hertz and Electric waves". General background on Electrodynamics in the 19th century can be found in Oliver Darrigol: "Electrodynamics from Ampère to Einstein".

  • Differences between Weber and Maxwell electrodynamics are discussed, e.g., in this paper by A K T Assis and H Torres Silve, "Comparison between Weber's electrodynamics and classical electrodynamics", Pramana 55 (PDF)

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  1. I love the history of science and it's progressions. I have studied different scenarios to get a feel of the "race for technological advancement."

    When I read about the link to the first radio broadcast, I immediately thought of the debate between Marconi and Reginald Fessenden?

    Maybe it is the "differences between the Canadian and American "versions of technological development" that was troubling then, but since then, the borders have been removed from the "race of ingenuity to development." The "creative aspect of learning." How you apply that science.

    The "first transistor" radios?

    Studying the history of Silicon valley is always quite enlightening as well in this regard.

    I am sorry I missed the earlier post of yours in December of 24.

    Imagine going from large rooms of magnetic tapes to the computers we have today? A large boom box on the shoulder, to what is now the Ipod?

    Moore's law?

  2. Nice post, Stefan...I'll drink some wine at dinner tonight and I'll be sure to make a toast to Hertz!


  3. Dear Stefan,

    Thanks for this interesting post! It occurred to me that Hertz' experiments roughly mark the time at which physics began to withdraw from our direct sensory experience. A broad spectrum of electromagnetic waves isn't visible to our eyes (and if I imagine what amount of EM waves is in the air around me right now I'm glad I don't see other people's emails flying by). And in the century after Hertz, experiments had to get more and more sophisticated to convince nature to share her secrets with us.

    I'm just reflecting on this. As I've written before somewhere, to me it's not such a big surprise that progress in physics has slowed down. It takes more time, more preparation, more effort to reach out to the frontiers of our knowledge now.



  4. Very interesting post! It really is amazing how much technology has 'exploited' (not a good word but all I can think of at the moment) electromagnetic energy.

  5. Thank you a lot for posting this. Just so you know, this is the best source I've had for my report so far!!

  6. Hi Anonymous,

    thanks for kind words, and great that the post was helpful for you! I hope you have found still a few more useful sources, though.

    Best, Stefan


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