Cambridge portable potentiometer
1957
1957
1930-1960
1955-1964
1910
1954-1974
1940-1959
1960-1969
1946
1931
1940
1928
1900-1918
1893
1900-1920
1971
1956-1966
circa 1947
1889
circa 1940s-circa 1950s
circa 1940s-circa 1950s
1890-1900
1897
1950-1959
1963
1920
circa 1925
1966
circa 1953
1900
1953