In order to determine an exact time, caesium atoms are measured in different energetic states:
The Cs atoms are vaporized in a furnace and magnetically sorted, so that only atoms of a low energetic state reach the so-called cavity resonator. In this cavity the atoms are animated to change their state by irradiation with a microwave field. The atoms that have changed their state are now collected in a special chamber. Since the amount collected is greatest at a certain frequency of the microwave field, the frequency is held and counted.
After exactly 9 192 631 770 periods, one second has finally elapsed.
In the course of a year, the deviation of this clock is a maximum of 25-12 billionths of a second relative to an ideal clock. For a quartz watch, the deviation is a few seconds per month; mechanical wristwatches have significantly larger deviations. There is a special so-called chronometer test for mechanical watches which is passed only with a maximum average deviation of -2 to +4 minutes per month.