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Heinrich Hertz
The Discovery of Radio Waves
Hands On Activity: Repeat Hertz’s Experiments that Confirmed the Existence of Electromagnetic Radiation

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  • The Discovery of Electromagnetic Radiation

    Heinrich Hertz
    Heinrich Hertz (1857-1894) on a German stamp featuring 100 years since his birth
    The most dramatic prediction of Maxwell's theory of electromagnetism, published in 1865, was the existence of electromagnetic waves moving at the speed of light, and the conclusion that light itself was just such a wave. This challenged experimentalists to generate and detect electromagnetic radiation using some form of electrical apparatus.

    The first clearly successful attempt was made by Heinrich Hertz in 1886. For his radio wave transmitter he used a high voltage induction coil, a condenser (capacitor, Leyden jar) and a spark gap - whose poles on either side are formed by spheres of 2 cm radius - to cause a spark discharge between the spark gap’s poles oscillating at a frequency determined by the values of the capacitor and the induction coil.

    This first radio waves transmitter is basically, what we call today, an LC oscillator. For an animated explanation of this device click here. More information about this subject could be found in basic electronics text books.

    To prove there really was radiation emitted, it had to be detected. Hertz used a piece of copper wire, 1 mm thick, bent into a circle of a diameter of 7.5 cm, with a small brass sphere on one end, and the other end of the wire was pointed, with the point near the sphere. He added a screw mechanism so that the point could be moved very close to the sphere in a controlled fashion. This "receiver" was designed so that current oscillating back and forth in the wire would have a natural period close to that of the "transmitter" described above. The presence of oscillating charge in the receiver would be signaled by sparks across the (tiny) gap between the point and the sphere (typically, this gap was hundredths of a millimeter).

    Hertz Spark Gape Transmitter and Detector
    Conceptual Schematic of Hertz's Experiment

    In this experiment Hertz confirmed Maxwell’s theories about the existence of electromagnetic radiation.

    In more advanced experiments, Hertz measured the velocity of electromagnetic radiation and found it to be the same as the light’s velocity. He also showed that the nature of radio waves’ reflection and refraction was the same as those of light, and established beyond any doubt that light is a form of electromagnetic radiation obeying the Maxwell equations.

    Summing up Hertz's importance: his experiments would soon trigger the invention of the wireless telegraph and radio by Marconi and others and TV.

    In recognition of his work, the unit of frequency - one cycle per second - is named the “hertz”, in honor of Heinrich Hertz.

    Repeat Hertz’s Experiments

    Warning: experiments with electricity should be performed under the supervision of teachers or adults familiar with electricity safety procedures. Especially, take in account that experiments with induction coils and capacitors can produce high voltage shocks.

    Hertz first experiment – creating, sending and detecting radio waves – is relatively simple, not beyond the abilities of middle school students. In order to begin, read carefully the experiment links and ensure that you understand the basic principals. Brows further the web and consult your local library, your teacher and other knowledgeable adults and experts.

    Hertz’s more advanced experiments, mentioned above, require some extra ability and knowledge, and in order to perform these experiments successfully the students are also required to be able to read and understand a few books by Hertz or about Hertz listed in the resource section.

    Resources

    Books
    Buchwald, Z. The Creation of Scientific Effects : Heinrich Hertz and Electric Waves. The University of Chicago Press, 1994.

    Heinrich Hertz, The Principles of Mechanics Presented in a New Form. Dover Phoenix Editions

    Heinrich Hertz, Electric waves: Being researches on the propagation of electric action with finite velocity through space.

    Heinrich Hertz, Miscellaneous Papers, 1896

    Links
    Hertzian Waves
    Heinrich Rudolph Hertz - Corrosion Doctors
    Heinrich Hertz's Wireless Experiment - Harvard College
    Heinrich Rudolph Hertz - Michael W. Davidson
    Hertz Finds Maxwell's Waves: and Something Else - Michael Fowler
    The Discovery of Radio Waves - Sparkmuseum
    The 1901 Marconi's Transatlantic Radio Experiment (Part 1) - Henry M. Bradford
    The 1901 Marconi's Transatlantic Radio Experiment (Part 2) - Henry M. Bradford
    Radio & Wireless Science Fair Projects and Experiments






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