GB application 439457, Heil, Oskar, "Improvements in or relating to electrical amplifiers and other control arrangements and devices", published 1935-12-06, issued 1934-03-02 European Patent Office, filed in Great Britain 1934-03-02, (originally filed in Germany 1934-03-02).
Lilienfeld, Julius Edgar, "Method and apparatus for controlling electric current" アメリカ合衆国特許第 1,745,175号 1930-01-28 (filed in Canada 1925-10-22, in US 1926-10-08).