Properties of the hydrogen maser and details of the apparatus are discussed. Experimental work is described which yields a new value for the hyperfine separation of hydrogen in its ground state. The result is
Δν = 1,420,405,762 ± 4 cps.
This result is based on a value of the hyperfine separation in Cs133 which is taken to be
Δν(Cs133) = 9,192,631,840 cps.
Daniel Kleppner, H. Mark Goldenberg, and Norman F. Ramsey, "Properties of the Hydrogen Maser," Appl. Opt. 1, 55-60 (1962)