Career
He was employed at the Western Electric Company in Chicago. Initially he was in the dynamo department though he eventually also began to devise telephone equipment. During this time he also began to conduct his own experiments during his leisure time and developed an electrolytic detector of Hertzian waves.
In 1902, he founded his own business, the De Forest Wireless Telegraph Company which sold radio equipment that he had developed. He had hoped to make profits on his inventions by starting his own company but he was cheated by his own business partners and the company became insolvent by 1906.
He developed an electronic amplifying vacuum tube, the Audion, in 1906. It was the first electronic device that could amplify a weak sound into a stronger signal. At that time however he had no idea about the potential his great invention held.
He designed a diode vacuum tube detector, a two-electrode device which was a variant of the Fleming valve which had been invented two years earlier. He then developed a three-electrode device which was a better detector of electromagnetic waves. This invention was granted a patent in 1908.
After the failure of his first company he started another company called the De Forest Radio Telephone Company. By 1909 this company too began to fail, again because of his cheating partners. He was even indicted in 1912 but was later acquitted of the charges.
In 1916, he broadcasted the first radio advertisements for his own products from the experimental radio station 2XG in New York City. The Presidential election report for Charles Evans Hughes and Woodrow Wilson was also broadcast by him.
He developed a sound-on film technique for which he filed a patent in 1919. It was called the De Forest Phonofilm process. Over the next few years he studied the other sound film systems and established his De Forest Phonofilm Company in 1922 and demonstrated Phonofilm to the press in 1923.
The Phonofilm process synchronized sound directly onto film and was used to record stage performances, speeches and music concerts. By the late 1920s Hollywood too began to use sound-on film systems.