Comparisons of the PTB primary clocks with TAI in 1999

The primary clocks CSI, CS2 and CS3 of the Physikalisch-Technische Bundesan stalt (PTB) have contributed to the realization of International Atomic Tim e, TAI (Temps Atomique International), throughout 1999. Their operation, th eir performance and the uncertainty with which they realized the Internatio nal System of Units (SI) second are described in some detail, together with the results and associated uncertainties of comparisons of the realized se cond with the TAI scale unit for each clock.