Re: Fused quartz pendulum - temperature coef


Yes Bepi - Matthus  reports the results of what would have been a very lengthy series of tests in his book. He used a FQ pendulum with a brass bob and did a number of tests with different compensators and different bob support positions. He heated the entire clock by wrapping with an electric blanket, waiting a day or 2 then removing the heat and measuring the step change in rate with a set change of temperature.
A few months ago I attempted to calculate the TC of my entire cast iron bob, invar pendulum and phosphor bronze dual suspension, and compensate with an aluminum spacer, but it was difficult to do in the middle of summer with the daily high ambient temperatures in my shop. I have since then dismantled the clock and purchased several lengths of fused quartz tube ( the type used to make heater elements). I'm right now in the process of building a new suspension with soldered in place chops. I may do a You-tube video if the motivation strikes.  I will support the bob ( cast iron) in the middle, use a glass spacer and set the compensator outside of the bob.
 Matthus also makes the same comment as you about supporting a cast iron bob at the bottom, and not needing any further compensation, but of course the suspension has as much if not more TC that the entire pendulum rod, so getting it right is easier said than done.

On 8/04/2020 02:00 am, Bepi wrote:
On the subject of temperature sensitivity when I changed over to a lead bob I noticed that the original gray cast iron bob had, in principle, just the right coefficient of expansion (1.08 10-5 ºC-1) and height to compensate for an invar rod (1.21 10-6 ºC-1) when the two are rigidly connected at the bottom of the bob. Does anybody know if this design feature is typical of all synchronomes?
In principle for a given bob weight, one can always compensate exactly for the pendulum temperature coefficient with the right choice of materials and bob aspect ratio, so that the problem changes from finding the smallest coefficient material to the best known coefficient material. I would also be curious to know if anybody has attempted to measure thermal expansion experimentally with decent precision other than from period vs natural ambient temperature fluctuations.

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