Before revision: This is one reason why NIST-F1, a caesium fountain clock, with a fractional precision of can keep time more precisely than NIST-7, a caesium beam clock .
After revision: This is one reason why NIST-F1, a caesium fountain clock, with a fractional instability of can keep time more precisely than atomic clocks that use interrogate atomic beams, for example the NIST-7, with a fractional instability of caesium beam clock .