[time-nuts] Pendulums & Atomic Clocks & Gravity

Tom Van Baak tvb at LeapSecond.com
Sat May 26 22:54:21 EDT 2007


> Even fused silica is unstable (see attachment).
> Single crystal materials should be significantly better.
> Ageing Invar doesn't do much for its dimensional instability.
> 
> Bruce

Nice plot. Thanks Bruce. Where'd you find it? Someday
I want to visit your library! You are just amazing.

Yes, I presume everything is unstable, depending on how
close you look, or how long you watch. This is true for
quartz crystals, pendulum rods, rubidium vapor cells, or
hydrogen masers. The art of making high-end, low-drift
frequency standards is to create (through sound engineering),
or to batch select (through dumb luck), those ingredients
with the least instability. Try to order a hydrogen maser or
a BVA oscillator and you'll see what I mean.

I have heard, but can't provide reference, that some of the
US or UK invar made in the early 1900's (think Shortt), or
the Russian invar made in the 1950's (think Fedchenko) is
far superior to the commercial invar made today. Makes me
wonder if it's like famous violins: they don't (or can't) make
'em like they used to.

Back to the plot. Unlike the electronic frequency standards
that we time-nuts play with, the timescale real men use for
pendulum clocks is years, not days. Too bad the plot doesn't
show how super invar, zerodur, and fused silica do over N
years instead of N days. Do you know if zerodur is available
in long rods (as in pendulums), or just blocks (telescopes)?

I know zerodur was mentioned by some of the pendulum
guys at the latest NAWCC pendulum clock conference. On
the other hand, the best modern pendulum clock to date
was made by Hall, and he used a vintage invar rod, I think.

/tvb




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