Also see CuriousMarc's companion video, which shows how the spark gap works with a coherer for primitive radio transmissions. <a href="https://www.youtube.com/watch?v=9zG_DlxyugQ" rel="nofollow">https://www.youtube.com/watch?v=9zG_DlxyugQ</a>
reply
Excerpts:<p>"So where is the spark gap in this unit? It turns out to be the ceramic cylinder. I opened up the cylinder and found a stack of eight metal disks with (maybe) carbon electrodes in the center. The disks are separated by mica washers to leave 0.33 mm gaps between each pair. This forms a series of 7 tiny spark gaps.<p>This type of spark gap is known as a "quenched spark gap". Spark gap transmitters were the first form of radio transmitter, used from 1887 to 1920. They used a spark to transmit Morse code via radio waves (details). The quenched spark gap was one type of spark gap used in these transmitters, as shown in the diagram below.<p><i>By combining multiple small gaps, the quenched spark gap could cool off efficiently.</i>"<p>[...]<p>"<i>Spark gaps generate radio waves across a wide spectrum</i>; inventor David Hughes first noticed this interference in 1878."
A similar circuit is in every plasma cutter.[1] For the same reason; you need a high voltage pulse to ionize the working gas (usually air) and start the arc.<p>[1] <a href="https://guelphweddingshop.blogspot.com/" rel="nofollow">https://guelphweddingshop.blogspot.com/</a>
That schematic of a Tesla coil is the same as a spark-gap transmitter, except of course the latter is designed to emit as much as possible and has an antenna connected where the output torus would be; see page 375 of this excellent book: <a href="https://archive.org/details/principlesunderl00unit" rel="nofollow">https://archive.org/details/principlesunderl00unit</a><p>(Although very old, that book is great at explaining the basics of what radio actually is.)