Comparison of an Ammonia Maser with a Cæsium Atomic Frequency Standard

SEVERAL atomic frequency standards of different types are now in use for comparisons with astronomically determined time. As the adopted frequencies of these atomic standards have been derived from the smoothed universal time U.T. 2, which is itself variable in time1, they disagree by several parts in 109. In order to obtain a uniform base for the atomic times determined by the different standards, their frequencies have to be compared directly. This has been done between caesium resonators at the National Physical Laboratory in Teddington and the Naval Observatory in Washington using the GBR 19.6 kc./s. transmissions. Later the same comparison was repeated between similar caesium resonators at the National Physical Laboratory2.