The development of the audion, or triode vacuum tube, has been called one of the most significant technological inventions of all time. While Lee De Forest is generally credited with this invention, his work actually was closely linked to the earlier invention of a diode tube by John Ambrose Fleming and the later creation of a feedback circuit by Edwin H. Armstrong--although De Forest claimed to have been solely responsible for adapting Fleming's diode and for indirectly creating the feedback circuit. Interested in improving receivers for wireless telegraphy, De Forest began experimenting with improvements on Fleming's diode almost immediately after Fleming received his U.S. patent in 1905. In just a little more than a year, De Forest applied for his own patent, claiming his device was notably different from Fleming's and a significant advance in receiving telegraph signals via sound waves sent from locations miles away from his receiver.

Technically, De Forest's audion worked on the principle of radio waves' ability to affect electrical current. The first audion was a diode tube quite similar to Fleming's. It consisted of a gas-filled glass cylinder containing a filament, an arrangement similar to an incandescent light bulb, into which a second metal plate was inserted. The positive terminal of a 22-volt battery was connected to the metal plate, and a pair of headphones added to the circuit; the negative terminal was connected to the lamp filament. Almost immediately after De Forest obtained his first patent for the audion in 1906, he began making modifications to his device. The most significant alternation consisted of the insertion of a thin metal wire, bent in the shape of a gridiron, between the filament and the plate. Doing so allowed him to regulate the flow of electrical current being generated by the action of the radio waves, permitting a continuous flow of electricity and making it possible to "tune" the receptor to achieve greater audibility. De Forest did not progress further with his invention at the time, satisfied that he could receive and detect telegraphic signals with sufficient accuracy to make the audion commercially successful as a receiver for wireless telegraphy. He did take steps to demonstrate how the audion could be used to transmit voice and music, "broadcasting" programs as early as 1907.

Soon, a method for achieving sufficient amplification was available, attained by linking a number of audions in sequence. The original headphones were soon replaced by speakers, and the radio industry was born.

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