[time-nuts] Pendulums & Atomic Clocks & Gravity

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 27 03:47:34 UTC 2007


Tom Van Baak wrote:
>> 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
>>     
>
>   
Tom
> Nice plot. Thanks Bruce. Where'd you find it? Someday
> I want to visit your library! You are just amazing.
>
>   
I scanned it from a text on Opto-Mechanical design.
Original data taken from Berthod, J.W., Jacobs, S.F., and Norton M.A.,
Dimensional stability of fused silica, Invar and several ultralow 
thermal expansion materials,
Appl. Opt. 15, 1898, 1976.

A more comprehensive study with greater sensitivity using the material 
under test as spacers in Fabry Perot resonators:
Dimensional stability of Materials Useful in Optical Engineering,
 S.F Jacobs Applied Optics and Optical Engineering Vol X pp71-107.
Edited by Shannon and Wyant. Academic Press, 1987, ISBN 0-12-408610-1
This study also examines thermal expansivity gradients within a sample.
> 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.
>   
Modern super Invar is quite a complex alloy of Iron, Nickel, Cobalt, 
Carbon, Silicon and Manganese and these are just the intentional 
constituents.
LR35 invar is principally 35% Nickel with 65% iron.
Less complex alloys with lower impurity content may be more stable.

It shouldn't be too difficult to repeat the Fabry Perot resonator tests 
using ancient invar samples if you can obtain them.
> 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 guess that DARPA didn't want to fund a study of dimensional 
instability that might span several decades.
Zerodur is available in rod form (<4m length) from Schott:
http://www.us.schott.com/optics_devices/english/products/zerodur/index.html?highlighted_text=zerodur
You can even order it online. However it is relatively expensive as is 
most optical quality glass in these days of Lead, Arsenic, and Thorium 
free varieties.
However It comes in several varieties some of which are more stable than 
others.
It is best at constant temperature or at least stay below 130C for the 
standard variety.
It makes nice ring laser gyros - widely used, at least before the advent 
of fibre ring laser gyros.
> 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
>   

Bruce



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