Newsgroups: rec.crafts.metalworking
Subject: Re: Synchronome Company? (Invar for pendulum)
From: whit@carson.u.washington.edu (John Whitmore)
Date: 24 May 1993 21:47:42 GMT

In article <1993May19.092356.634@news.uwyo.edu> jimkirk@news.uwyo.edu (Jim Kirkpatrick) writes:
 
>Along the same line, does anybody know a source for things like
>Invar rod?  Or perhaps even a modern composite that has an
>essentially zero temperature coefficient
 
	Various invar alloys have zero temperature coefficient at
reasonable temperatures, BUT ONLY AT ONE TEMPERATURE.  There is
a phase transition involved (from magnetic to nonmagnetic) and
it is magnetostriction that counters the (normal) thermal expansion
over some small temperature range.
 
	It is common to make composite pendulums that have zero
net thermal expansion, by using a combination of well-characterized
materials (steel and brass, typically).
 
       (pivot)
          |
          |
xxxxxxxxxxxxxxxxxxxxx
S                   S
S  xxxxxxxxxxxxxx   S
S  B      S     B   S
S  B      S     B   S
S  B      S     B   S
S  B      S     B   S
S  B      S     B   S
xxxx      |     xxxxx
        (mass)
 
something like this (when heated, the brass expands more than the steel,
so the net effect is that the distance from the mass to the pivot point
remains constant).  I think it was done with three sets of steel rods
DOWN and two sets of brass rods UP, but you can look up the handbook
values and figure a similar scheme for any well-characterized
alloys.
 
	Since there is no phase transition involved, this sort of
scheme might work well over a wide range of temperatures.
 
 
	John Whitmore