Part of a <a href="https://en.wikipedia.org/wiki/spark-gap_radio_transmitter" class="extiw" title="w:spark-gap radio transmitter">spark radio transmitter</a> from the early 1900s. This was the earliest type of <a href="https://en.wikipedia.org/wiki/radio_transmitter" class="extiw" title="w:radio transmitter">radio transmitter</a>, used for transmitting information by <a href="https://en.wikipedia.org/wiki/Morse_code" class="extiw" title="w:Morse code">Morse code</a> <a href="https://en.wikipedia.org/wiki/radiotelegraphy" class="extiw" title="w:radiotelegraphy">radiotelegraphy</a>.  It consists of a bank of <a href="https://en.wikipedia.org/wiki/leyden_jar" class="extiw" title="w:leyden jar">leyden jar</a> <a href="https://en.wikipedia.org/wiki/capacitor" class="extiw" title="w:capacitor">capacitors</a> which store electric charge from a high voltage transformer <i>(not shown)</i>.  To the right is an <a href="https://en.wikipedia.org/wiki/inductor" class="extiw" title="w:inductor">inductor</a> consisting of a coil of wire.  Above the coil is a <a href="https://en.wikipedia.org/wiki/spark_gap" class="extiw" title="w:spark gap">spark gap</a>.  When the transformer charges up the capacitors to a high enough voltage, a spark occurs across the spark gap, discharging the capacitors through the coil, creating <a href="https://en.wikipedia.org/wiki/radio_frequency" class="extiw" title="w:radio frequency">radio frequency</a> oscillating currents in the coil.  The coil is attached to a suspended wire <a href="https://en.wikipedia.org/wiki/antenna_(radio)" class="extiw" title="w:antenna (radio)">antenna</a>, which radiates the energy in the currents as <a href="https://en.wikipedia.org/wiki/radio_wave" class="extiw" title="w:radio wave">radio waves</a>.
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Hans Orstadius / Sjöhistoriska museet