Dr. DeForest em 1905
O Audion de Lee De Forest de 1907, apenas com uma placa (single win) e com filamento de tântalo.
In 1907, U.S.  patent nš 841.387, entitled "Device for amplifying Feeble Electric Currents", was issued to the inventor Lee De Forest by adding a third element to the Fleming valve, or the grid.  This all importante paper in the amplifier's evolution describes its fundamental property: the output current is controlled by an electrode that does not draw appreciable current itself. Since scientific and technologial discoveries are great events, but generally they pass a period of inaction, in spite of the enormous potential of the grid audion, its application was not immediate.
Inittialy it was used as wireless detector rather than an amplifyer. This latter capability was first employed only a few years later, based on the pioneer research work of several scientists and engineers.
The De Forest experiment:
1º - The original idea of audion in the conductivity of gases. Two electrodes in a flame was the first audion detector.The origin of the word thermionics.
2º - The first audion valve consisted of two hot electrodes in a gaseous medium.
3º - The first audion valve Audion. - Radio News