[time-nuts] Primary Time Standards

Hal Murray hmurray at megapathdsl.net
Thu Jul 14 07:08:12 UTC 2011


> Primary Standards are ones which don't have to be calibrated against others.
> My understanding is that Caesium and Hydrogen masers are Primary Standards
> (in our field). 

I don't think it quite works that way.

Cesium is the primary standard because it's the definition of a second.  
Hydrogen and Rubidium and everything else are secondary standards as a result 
of that definition.

Even cesium clocks have lots of quirks.  As technology gets better some of 
the quirks get sorted out and we can measure time more accurately.  (The one 
that I did a double take on was the blue shift correction for the differences 
in gravity due to the differences in elevation of various national standards 
labs.)

> But why is it that Caesium Clocks and Hydrogen Masers have an adjustment
> facility?

For Cesium, it's to correct for those quirks.  The idea is not to set the 
time so this clock matches some better clock, but to set some parameter to 
some known value.  (Sorry, I don't have a good example handy.)

--------

If someday, somebody builds a superwonderful clock out of xxxx that is more 
stable than Cesium clocks, the standards bodies will probably redefine the 
second to use it.

Rubidium is interesting because it's long term stability is better than 
quartz and it's much less expensive than Cesium.

Hydrogen Masers have very good short term stability.  Unfortunately, they 
have a big price tag.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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