[time-nuts] Zeeman effect and tuning cesium clocks

Tom Van Baak tvb at leapsecond.com
Sun Jan 23 01:02:56 UTC 2005


> The discussion of cesium (or Caesium) beam clocks has shaken
> my feeling that there had to be an absolute clock in physics
> somewhere. I guess that's why NIST at Boulder is still coming
> up with new oscillators.

It turns out there are quite a few factors which affect
the center frequency of a cesium clock. But as long
as the designers of the clock understand the physics
and can measure, correct, or compensate for the
offsets, you end up with a great clock. The fact that
there are a host of subtitle issues and the fact that
modern clocks account for *all* the important ones
should not shake your faith in atomic clocks; it should
strengthen it!

There is no perfect clock; it's all a matter of how
accurate you want. Any old vintage Cs clock will
give you 10 or 11 digits of accuracy by just plugging
it in. If you get a nice 5061A or B you get 11 or 12
within an hour. If you degauss it and do a Zeeman
calibration you get 12 or 13 digits. If you happen to
have a 5071A a host of good physics and clever
firmware gets you 14 digits with no adjustments.

> Now I'm wondering how they ever got two portable cesium beam
> clocks of the seventies to show relativistic affects when their
> accuracy depends on external magnetic fields.

The magnetic effect is not a big issue. It's all part of
the design and the tubes shielded just enough to meet
the accuracy spec of the instrument. On the other
hand I've heard that unshielded atomic clocks are
specifically used to measure magnetic fields!

Inthe 70's they flew fast and high enough and so the
relativistic effects were in the tens of microseconds.

> What does Boulder use for secondary standards to transfer the
> frequency of the near-perfect oscillator to Fort Collins?

I believe Boulder uses common-view GPS to sync
their sites.

By the way, one of the things Boulder corrects for
is altitude. Their clocks run about 5e-13 fast due to
General Relativistic effects. I'm at 1000 ft here in
Bellevue so mine run only 1e-13 fast.

> 
> Maybe I should go back to looking for the perfect pendulum, but
> I fear gravity will prove to be inconstant.

I spoke at a Pendulum conference a few months ago.
You would not believe how many factors affect the
performance of a pendulum! But they hit a performance
floor just below 0.1 ppm because things like the moon
are constantly changing g on you. There are a couple
of precision pendulums where tidal effects are clearly
visible in the phase plots. No free lunch.

/tvb







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