From IDW / Physikalisch-Technische Bundesanstalt:
The "atomic second" turns 50
The "atomic second" was the beginning of a revolutionary era: it was born as early as 1955, when the first cesium atomic clock was put into operation. In the fall of 1967, it was included in the International System of Units. This was the beginning of a development which will, in all likelihood, come to an end in the fall of 2018 when the 26th General Conference on Weights and Measures (CGPM) decides that the entire International System of Units (SI) is to be based on invariable properties of nature – on fundamental constants. In this development the second came next to the meter, but in the race for accuracy it has an outstanding role: no other unit can be realized with such accuracy. Today's cesium atomic clocks – such as the four primary clocks of the Physikalisch-Technische Bundesanstalt (PTB), which are responsible for realizing and disseminating legal time in Germany –provide the time unit with the unimaginable accuracy up to 16 decimal places!
"You are giving us a beautiful topic to meditate about: measuring the trajectory of the stars in the infinite depth of space based on the oscillation of an infinitesimally small atom." This is how poetically the then French foreign minister, Couve de Murville, expressed what was about to happen in Paris. In 1967, the scientists and politicians gathered in Paris for the 13th General Conference on Weights and Measures (CGPM) decided to re-define the second. The decision fell on 13 October 1967: "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 cesium ¹³³ atom."
Now, you can purchase used Cesium clocks on eBay for a few hundred bucks.
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