(The above text is a modified version of introductory material in Koenigsberg’s Patent History of the Phonograph 1877–1912 [1990], used by permission. Copyright 1989 by Allen Koenigsberg. All rights reserved.)
A number of significant patents were granted after 1912. Electrical recording systems were made possible by the prior invention of the Audion—British patent #1427, issued in 1908—and the single-stage amplifier—U.S. #841,387; 1907—by Lee De Forest. Among the great innovations of the 1920s was...