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In his proof of the existence of radio waves Hertz used the setup depicted in the following figure:

Hertz Oszillator

A sparc is created at regular intervals when the induction coil (2) creates a potential difference high enough to cause the sparc.

Wikipedia describes the process as follows:

Hertz's first radio transmitter: a dipole resonator consisting of a pair of one meter copper wires ending in 30 cm zinc spheres. When an induction coil applied a high voltage between the two sides, sparks across the center spark gap created standing waves of radio frequency current in the wires, which radiated radio waves. The frequency of the waves was roughly 50 MHz, about that used in modern television transmitters.

How does the process work by which radio waves are created when a sparc passes the gap?

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When the spark occurs between those two spheres, air gets ionized due to the high value of electric field, due to this electrons travel from an area of low potential (-ve side of battery) to a region of high potential ( +ve side of battery ) but they travel through air, and when they do that they are accelerated for a very short time, and due to this acceleration electric flux in the region surrounding it changes continuously and this induces a magnetic field, and then due to that changing magnetic field, an electric field is created, and both changes travel in the form of an electromagnetic wave, in this case a radio wave because Hertz arranged his voltage parameters in such a way that the frequency of that oscillation matched with that of the radio waves.

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  • $\begingroup$ The short discharge makes a really wide frequency range burst. The arc does not have a narrow frequency band. This is one reason that arc or spark gap transmitters were rapidly phased out when radio took off - one person transmitting would mean nobody else could across a huge swath of the spectrum. $\endgroup$ – Jon Custer Feb 22 '17 at 17:49
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    $\begingroup$ In detail every of the accelerated electrons lose kinetic energy by the emission of photons. Since this electrons get accelerated nearly synchronously the emitted photons are nearly in phase to each other. So the oscillating electric and magnetic field components of each photon produce a common EM radiation. This radiation periodically has maximum and minimum in dependence from the frequency of the varying current of the electric source. $\endgroup$ – HolgerFiedler Feb 22 '17 at 19:22

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