Example to illustrate use of charcoal to produce high vacuum
1917-1927
1917-1927
1931
1906-1914
1877-1900
before 1875
1990
1959
2020
circa 1930
2020
2020
1929
2020
2020
1959
2020
1959
1937
1959
1959
1974
circa 1943
1930-1950
circa 1943
1902