Evolution through radio circuits
This communication system was used in both World Wars.
The first receivers were asymptotic: every signal that received the antenna was transmitted and listened to by people, This resulted very little selective radio (you could not select a single channel exclusion of others), but also very expensive.
In 1864 Maxwell broke new ground as the laws of reflection of electromagnetic radiation, that is, the laws that regulate the behavior of the waves, which were used especially in the use of radar.
Subsequently the American scientist H.C. Dunwoody, in 1906, patented a crystal detector, the base of the crystal radio, which spread in the 20s: it operates according to the grinding properties of the crystal, They do not have a high price and then they were within the reach of more people.
In 1883 Edilson invented the valve radios, or radio stations that replaced the galena radios. It exploits the properties of free electrons:
if you bring a piece of metal to the incandescence, it emits free electrons, that at a sufficiently high temperature they exceed the potential barrier that holds them in the metal, a phenomenon called "thermionic emission" and discovered by Edilson.
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