[time-nuts] Primary Time Standards

[Poul-Henning Kamp]

In message <CALH-g5YuKCDiyGxDf4s-MhHtK=dS7Qr8z2st17jrqiLwVR0XoA at mail.gmail.com>
, Jim Palfreyman writes:

>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).
>
>Secondary Standards are calibrated against the Primary Standards. My
>understanding is that Rubidium is an example of a Secondary Standard.

Almost, but not quite.

Primary standards run at an intrinsic freqyency which is determined
by the atomic/physical principle used.

Secondary standards run at a frequency which is different from unit
to unit due to side-effects from the physical principle used.

Rule of thumb:

If the timekeeping atoms are not in vaccuum, it's not a primary std.

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

Because there are external effects that need to be cancelled out,
mainly the magnetic field from the earth.

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