Putting on the Accurate Operating Flat

The pendulum having been mounted, when actually embarking upon an experimental run, it is first necessary to fit the flat which you will use in practice, i.e. a metrologically accurate flat whose top looks like a mirror. You have already epoxied the first non-accurate flat into the depression in the support arm, and this is intended to provide a planar surface of reasonable quality which can serve as a support for the accurate flat. So, after cleaning the parts, you just put a couple of little drops of superglue on the top of the first rough flat, and take the second accurate flat and place it down firmly on top. It will adhere instantly with a very thin film of superglue remaining between the two surfaces. When you do this a certain amount of superglue will inevitably squish out from the sides, so that it is mandatory to wear a latex glove - or else you'll get glued to the pendulum!

The advantage of this mounting system is that, when the time comes, it is very easy to change the accurate flat for a new one - you just give it a good solid tap with a hammer and fine chisel, and the superglue will crack away. There is no risk of breaking the epoxy that holds the first rough flat, because it is much much stronger than the superglue. However, the superglue is very rigid, and holds the accurate flat perfectly adequately without wobbling. Rigidity (which is essential) should not be confused with strength (which is not). [I think that was a blind spot of Allais's, shown up by the construction of his support.] The next shot shows the accurate flat securely mounted on top of the rough flat. (It was not possible to show shots of the actual process of gluing on the second flat, because of the awkwardness and lack of space inherent in our overhead support configuration).

This next photo shows the parking arrangements for the pendulum. The pendulum bob typically has a sharp pointer screwed into its bottom, the end of which reaches down to only a couple of millimeters above some registration device like the one to be described shortly. Often you want to park the pendulum off the support flat - for example, to clean or change the flat, or to oil it at the end of an experimental run. So you must consider what happens when you lift the whole pendulum up a couple of centimeters, remove the ball, and then move the ring along the support bar and let it down.

(A) The ring should not clonk about against the support bar;

(B) The pointer should not dig into whatever is below it.

Requirement (a) dictates that a pad should be provided which is somewhat soft; requirement (b) dictates that the top surface of this pad should be higher than the top surface of the accurate flat plus the diameter of the ball. We used wooden pads with bits of vinyl floor covering glued on:

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