The precision of the Ramsey method is inversely proportional to the wait time T of the cloud. The use of an atomic fountain with a cooled atomic cloud allows for wait times on the order of one second, which is vastly greater than what can be achieved by performing the Ramsey method on a hot atomic beam, which may have interaction times on the order of tens of microseconds. This is one reason why NIST-F1, a caesium fountain clock, with a fractional instability of 2 × × 10 − − 16 can keep time more precisely than atomic clocks that use interrogate atomic beams, for example the NIST-7 caesium beam clock, with a fractional instability of 1 × × 10 − − 14 .