
De Forest audion 1906-1915
1906-1915
1906-1915
1839 (original); circa 1988 (replica)
1903
1907
1904
1910
1903
1915
1914-1920
1912
1906
1902
1910
1910
1914
after 1903
1910
1904
1908
1909
circa 1905
1912
1918
1915
1905
1904-1914
1917
1902
1906
1907-1917
1907-1917
1907-1917
1920
1907
1913
1914-1918
1914-1918
1872 (original); 1909 (model)
1919
1915
1915
1911
1920
1910-1920
c.1912
1910
1907-1920
1915-1917
1916
1921