
Caesium Atomic Clock, 1955.
1955
1955
1960
1959
1963
1957
1964
1959
circa 1956
circa 1956-1959
1961-1962
1955
1955
1960
1830 (original); 1967 (model)
1839 (original); circa 1988 (replica)
1961
1960
1963
1955
1955
1957
1962
circa 1960
1957
1961
1955
1961
circa 1955
1963
1960
1962
1960
1960
1960
1960
1964
1957
1955
1955; 1830
1955
1959
2000-100 BCE
1955
1957
1957
1962
1957
1963
1960
1958