Dear N_Cook:

On Apr 11, 7:10*am, "N_Cook" wrote:
Another old piece of kit this week. Steel case with a
bakelite closure that was a devil to separate. Metal
not rusted and I'm assuming steel does not shrink
over decades.

Plastics can absorb moisture (the universal solvent) over time.
Additionally, steel does in fact shrink with a shrinkage rate of about
1 part in 10^8 per year. Invar expands with about the same magnitude,
which is why such alloy rods were abandoned as length standards in the
1960s.

http://nvl.nist.gov/pub/nistpubs/jre...tml/j43bee.htm
.... what a job... measuring the same rod four times a year for
decades.

David A. Smith