For historic allocation purposes the
electronic age started in 1837 with the invention of the telegraphy
Morse in the USA. Through
his experience with eletrical sparks
the German physicist Heinrich Hertz noticed in 1887 the phenomenon
of producing alternate currents in high frequenies. He named
this phenomenon as electromagnetic waves later known as Hertzian waves.
|The carborundum radio
wave detector made
in England circa 1904
In the same time the French researcher E. Branley invented a device
known as coherer, made of a small glass tube filled with loose iron
filings. In this device, when the electromagnetic waves or signals
struck the metallic part they cohered - thus its name - and so occuring
Guglielmo Marconi, an Italian scientist, in 1896 transmitted electromagnetic
waves at a distance of 3600 meters by using a spark transmitter similar
the one used by Hertz, but now provided with an aerial. In the receiver
he used a coherer as a wave detector.
In 1906, H.A. Dundwoody and G.W. Pickard working simultaneously to
get a more reliable electromagnetic wave detector discovered the property
that same crystals like the galena and the quartz; they could detect
Hertezian waves and thus developing the famous crystal radio receiver.