Physikalisch-Technische Bundesanstalt

The well-known time check telling us the time can be traced back to the Physikalisch-Technische Bundesanstalt.

Put your quartz watch to the time announced and you will have at your disposal the most precise measuring instrument encountered in everyday life. Through the time signal transmitter DCF77 the PTB disseminates legal time which is realized by the atomic clock CS2 which is the PTB's best clock. Its error is of the order of 1 to 2 billionths of a second per day (for comparison: for the same period of time, your wristwatch has an error of a few seconds). At the PTB's Time Laboratory, four such atomic clocks measure the passing of the seconds by reference to the change of state of the cesium atoms. The frequency of this microwave field which optimally initiates these changes of state is the guiding principle proper, and as a fundamental quantity it is so constant that is can be used to check the variations of the earth's rotation which in comparison are almost imperceptibly greater. In fact, in terms of time, the earth does not rotate precisely!