reflecting astatic moving iron galvanometer
circa 1920
circa 1920
1955
circa 1932
1920
1885
1883
1905
1945
1920
1930
1925
1945
1950
1904
1905
1910
1920
1930
1930
1890
1910
1890
1910
1930
1905
1915
1896
1905
1941
1907
1905
1901
1940
1920
1940
1940
1904
1941
1940
1935
1910
1945
1910
1910