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Richard Pisko asks:
"Where is the pendulum weight fastened to the invar rod; and how?

I was wondering . . .  if the rod were to have male threads at a certain
pitch, say 20 threads per inch on a 1/2 inch diameter rod, and the brass
cylinder were to have female threads of say 3/4 by 20 tpi, there is room
for a short sleeve about 1/8 inch thick to hold the rod and pendulum
weight together.  The sleeve would be threaded inside and out at the same
20 tpi to fit the rod and weight, and a couple of notches at the lower end
would allow the sleeve to be screwed up or down by a hollow shaft
screwdriver.  The relative location of the brass weight and the rod would
remain the same, but the amount of temperature compensation would change,
I think.  For example, as the sleeve is screwed downward, the top end of
the brass weight would remain in the same position relative to the rod
initially, but would move upward more as the temperature increases.  The
change in position of the sleeve (lowered) would have to be compensated by
adding a weight in your tray, I believe, or by screwing the brass weight
upward a bit on the sleeve and rod.  Is this what you did?  If the Invar
rod length were actually invariable in a moderate temperature range, I
think placing the sleeve in the mid point of the brass cylinder would be a
good start experimentally, but where would theory suggest?"

It's an interesting suggestion. The pendulum bob is a lead-filled brass 
cyclinder with a sleeve up the middle for the pendulum rod, which latter is 
of Invar 9.5 mm diameter (3/8 inch in American). The end of the pendulum rod 
is threaded for an adjusting nut. Sitting on top of the nut is a brass 
compensating sleeve that slides freely in the bob and over the rod. The top 
of the sleeve carries the weight of the bob at the bob's mid-point, so that 
the expansion of the bob has no effect on compensation. As the invar expands 
a little downwards with increase in temperature, the short brass compensating 
sleeve expands up a lot, relatively, lifting the bob. If the length of the 
sleeve is correct, the centre of mass of the pendulum stays where it was. If 
I have undertood Richard correctly his suggestion would not work with my bob. 
No changes in the relative lengths of the pendulum rod and compensating 
sleeve would take place.

The Synchronome clock, a variation of which gave us Greenwich mean time for 
some years, used a solid pendulum of type metal (an alloy of lead, tin and 
antimony)sitting directly on the adjusting nut, with an Invar pendulum rod. 
The upward expansion of the bob itself took care of compensation. There are 
several other  systems of compensation, but none that relies on screw threads 
carrying the weight of the bob for adjustment is satisfactory for precision 
clocks because .02 mm axial movement results in about 1 second a day change 
in timekeeping for a 2 second pendulum: even a fine thread does not give 
enough sensitivity. A further problem with a load bearing thread is that 
machining irregularities prevent smooth adjustment (and it is difficult to 
get a fine finish with standard Invar, a very tough nickel-iron alloy).

But there is much more to precision time keeping than pendulum compensation. 
For example, variations in the driving force that keeps the pendulum going 
will result in changes in the length of its arc, as will changes over time in 
the oil that lubricates the escapement. Variations in atmospheric pressure 
and air density also set limits to what is achievable. Richard J Matthys' 
book "Acurate Clock Pendulums" will tell you more than most people will wish 
to know about the subject.

Bill




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