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English: British Post Office engineers inspect Guglielmo Marconi's wireless telegraphy (radio) equipment, during a demonstration on Flat Holm island, 13 May 1897. This was the world's first demonstration of the transmission of radio signals over open sea, between Lavernock Point and Flat Holm Island, a distance of 3 miles.

In the background is the spark-gap transmitter, an induction coil (right) that generates high voltage pulses that create sparks between the balls of the Righi spark gap (left), which excites oscillating currents in a wire antenna suspended aloft by the pole seen in the center, ratiating radio waves. Information is transmitted by switching the transmitter on and off rapidly using a switch called a telegraph key (not visible), spelling out text messages in Morse code.

In the foreground is the receiver. When receiving from the Lavernock station, the oscillating voltage from the antenna is applied to a coherer; a primitive radio wave detector consisting of a small tube containing two electrodes with metal filings between them. When a radio wave from a distant transmitter strikes the antenna and is applied to the coherer, the filings clump together and conduct electricity. A second circuit is attached to the coherer consisting of a battery which operates a relay (cylindrical objects at sides), which in turn sends a pulse of current to a Morse paper tape recorder (center). When a radio wave turns on the coherer, it sends a pulse to the recorder, which makes a mark on a paper tape. The Morse code message from the remote transmitter can be read from the tape, as the seated man is doing. The relays are contained within cylindrical metal shields to prevent their sparks from interfering with the sensitive coherer. Information from Retrospective:Radio waves across the water,]

Source Cardiff Council Flat Holm Project
Author Cardiff Council Flat Holm Project
(Reusing this file)
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