Lombardi, M. A.; Heavner, T.P.; Jefferts, S. R. (2007). "NIST Primary Frequency Standards and the Realization of the SI Second". NCSLI Measure. 2 (4). NCSL International: 74–89. doi:10.1080/19315775.2007.11721402. S2CID114607028.
Lombardi, M. A.; Heavner, T.P.; Jefferts, S. R. (2007). "NIST Primary Frequency Standards and the Realization of the SI Second". NCSLI Measure. 2 (4). NCSL International: 74–89. doi:10.1080/19315775.2007.11721402. S2CID114607028.