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The first photo shows the components of the compensated pendulum rod. It is composed of a solid steel inner rod, a zinc tube that slides over the steel rod and another, larger steel tube around the prior two assemblies. The differing coefficients of expansion between the steel and zinc metals along with the way these components are linked together to suspend the 200 lb. pendulum bob, result in minimal changes to the overall length of the pendulum rod in response to changes in temperature. The holes drilled into the zinc and outer steel tubes are to allow the changes in atmospheric temperature to effect the system quickly and evenly. An interesting manufacturing feature is shown in the second photo. One would normally expect that the zinc tube would be a solid piece of metal. However in this case it is composed of a rolled sheet. Not only that, but the sheet was composed of five smaller strips soldered together to make the larger sheet before rolling. Zinc sheet was commonly used in roofing at the time and the Seth Thomas factory chose to use the available (and presumably cheaper) off the shelf material rather than manufacture a specially machined tube. The end of the zinc roll is carefully contoured to fit into the steel plug shown to avoid sideways slippage. There is very little space between the separate tubes and these all move past each other to perform the compensation. Using rolled zinc allowed the maker to cheaply make a long tube with precise inner and outer diameters.  The last photo shows the upper suspension assembly. Fine rating adjustments are made by turning the large faucet-shaped handle (which is not easy to do!). Notice the use of the safety holes on either side of the large pin that connects the rod to the suspension spring. In the event of the suspension spring breaking, the pendulum assembly would be caught by the pin being trapped in the holes. Considering that the entire assembly weighs over 250 lb. and is in quite a lively motion under normal operating conditions, one wonders how much damage would be sustained by the pin and holes. However, it is better than this system crashing to (or though) the floor below!

The first one to make a compensated pendulum rod using different metals; specially pieced together was John Harrison in 1726 though his invention of the brass and steel 'gridiron' pendulum. It was visually beautiful as well as practical. The first use of different metals to compensate an entire pendulum was by George Graham in 1721. He used a mercury jar for the bob attached to a regular uncompensated rod.

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