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Richard Pisko, after dipping into Richard J Matthys' book "Accurate Clock Pendulums", wrote:

"For example, I had not realised there was such a long delay between the
rod reaching its new length (say to an arbitrary 99 percent), and the
greater mass of the brass weight coming to the new equilibrium temperature
by the same percentage.  This leads to a step in accumulated time error,
even if the temperature compensation is perfect at both temperatures.
 From what I read in his book, an Invar tube inside the lower half of the
weight, and a brass tube of the proper length below the weight (both over
the Invar rod) would seem to produce the smallest step.  Does this seem
reasonable to you?"

It certainly seems reasonable, but most people are not sufficiently obsessed 
by accurate mechanical time keeping any more to do it. I did once contemplate 
making a super-clock using a fused quartz pendulum rod with invar 
compensation and bought a length on a visit to Texas. Unfortunately, baggage 
handlers throughout the world took the  "Fragile" stickers to be a challenge 
and it arrived home in many pieces. It will be a little while before I try to 
repeat the exercise. Texas in summer seems to me to be a good preparation for 
an eternity spent in Hell...

"I was also very surprised by the difference in pendulum rate caused by the
tightening of the suspension nuts."

I solder mine.

"I was also intrigued by the "zero friction" crossed spring suspension,
although it seems sensitive to the total weight of the pendulum system,
 from the way I read that page or two."

I think this crossed strip hinge may have originated in the National Physical 
Laboratory in Teddington. It's the sort of thing that sounds as though the 
ingenious FH Rolt ought to have invented it.

Bill Morris






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