text: These time scales are denoted in the form UTC(NPL) in the UTC form, where NPL here identifies the National Physical Laboratory, UK.The TAI form may be denoted TAI(NPL).The latter is not to be confused with TA(NPL), which denotes an independent atomic time scale, not synchronised to TAI or to anything else.The clocks at different institutions are regularly compared against each other.The International Bureau of Weights and Measures (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible.This combined time scale is published monthly in "Circular T", and is the canonical TAI.This time scale is expressed in the form of tables of differences UTC − UTC(k) (equal to TAI − TAI(k)) for each participating institution k.The same circular also gives tables of TAI − TA(k), for the various unsynchronised atomic time scales.Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T.Aside from this, once published in Circular T, the TAI scale is not revised.In hindsight, it is possible to discover errors in TAI and to make better estimates of the true proper time scale.Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT).History Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously.Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL).It was used as a basis for calibrating the quartz clocks at the Royal Greenwich Observatory and to establish a time scale, called Greenwich Atomic (GA).The United States Naval Observatory began the A.