At 05:48 PM 2/23/2010, Poul-Henning Kamp wrote...
A Cs clock is primary because when you turn it on, it latches onto
the physical phenomenon of a known and invariant frequency subject
to no systematic errors.
The reason the small Rb's do not qualify as primary is that each
unit has a slightly different frequency, due to vapour pressure,
isotopemix and other physical details, and thus you cannot know the
frequency of a particular unit, until you have measured it relative
a primary clock.
There is no difference between Cs and Rb in that regard, except perhaps
scale.
The frequency of a Cs is subject to gravitational and electromagnetic
effects. Which is why the definition of the second was clarified in
1997 by the CIPM to refer "to a caesium atom at rest at a temperature
of 0 K."
Since absolute rest and absolute zero are impossible conditions for a
real world clock, Cs clocks do not have a "known and invariant
frequency." If they did, why would they have a C-Field knob to twiddle,
and why would TAI be a weighted average of multiple Cs clocks? I'd
guess that Cs clocks can also be thrown off by trace gasses in the
tubes and numerous other effects, impossible to completely remove.
Even if one could obtain absolute zero acceleration and temperature,
Heisenberg would still be heard.
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