Definition
The CS2 primary atomic clock of PTB emits the signals marking the seconds intervals of legal time (CET or CEST), by which all radio clocks in Germany are controlled via the long-wave transmitter located at Mainflingen near Frankfurt.
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
Realization
The definition already points to the method by which the seconds can be realized: In an atomic clock, microwave radiation is used to make the caesium atoms change from one energy level to another. This process works best at a specific frequency (corresponding to a specific duration of a period) of the radiation. The second is obtained with high accuracy by counting the correct number of periods (cf. the above number). The second is the SI base unit that can be realized with the highest accuracy: With PTB's best primary atomic clock, the caesium fountain clock CSF1, one would have to wait 30 years before it has deviated from the "ideal" second by a mere millionth of a second!
The time scale is composed of the sequence of individual seconds. The time scale realized at PTB is referred to as Coordinated Universal Time, with the acronym PTB added: UTC(PTB). UTC(PTB) plus one hour is PTB's realization of Central European Time (CET), the legal time in Germany. Unless, of course, when it's daylight savings time (Central European Summer Time, CEST): then legal time is given by UTC(PTB) plus two hours.
Additional information
PTB working group "Unit of Time"© Physikalisch-Technische Bundesanstalt, last update: 2011-01-20, Volker Großmann
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