[time-nuts] A different timenuts interest

[jimlux]

Eugen Leitl wrote:
> On Wed, Jul 21, 2010 at 10:40:48PM -0400, Bob Bownes wrote:
>> Silly me, I just realized you need to compensate for the change in
>> length with temperature.
> 
> You could use an Invar wire.
>  
Some insight from a friend (a proto-timenut) who was thinking about 
building a 1ppm free pendulum in air for a fancy grandfather clock.

Invar (aside from being expensive) isn't appropriate here, depending on 
the design. Its low CTE properties depend on not being mechanically 
stressed.

A better scheme is the traditional bimetal pendulum compensation 
approach of steel rod and brass bob that can slide along the rod. You 
pick the dimensions so that as the steel gets longer, the bob expands at 
a different rate (pushing the CG back up), so that the net movement in 
CG position is zero.  You change the relative diameters of the two metal 
parts to get the CTEs and movement to balance out.