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.
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