Paraconical Pendulum Amplitude Decay
with a Horizontal Lenticular Bob


First, for comparison, we should consider what rate of decay Allais obtained with his pendulum. He clearly states:

"During a 14 minute experiment the amplitude decreased from 11 cm to about 9 cm."

so we have a target to match ourselves against. And in this graph that Allais gives in Anisotropie and in the NASA report elsewhere on this website:

the three small graphs along the bottom unambiguously show a rate of decay from about 11 cm to about 9 cm over 16 minutes. With a paraconical pendulum there is almost no friction in the suspension, so we suppose that virtually all the decay was due to air friction, almost all of it on the bob.




We set up the paraconical pendulum and positioned a camera to take photographs of the indicator needle below the bob. This image shows the position before release, at about 231 mm. (This picture is a good illustration of why a conical needle is NO GOOD.) In fact the other end of the swing was at about 10 mm, so our initial amplitude was almost exactly the same as Allais's, i.e. 11 cm.



Then we started the pendulum by burning the retaining line. The period of this pendulum was almost exactly 2 seconds (so-called "seconds pendulum"). We set the camera to take repeated 1/2 second time exposures, as fast as it could, so that about one shot in five, at random, showed the pendulum approaching its limit of travel and going back again. This first image was obtained virtually at the start, and it shows the limit as being 231 mm. (The time was recorded on the file and has been overprinted afterwards):



Nine minutes later, the limit of swing is now 225 mm:



At 16.5 minutes, the limit is now 220 mm:



At 21.2 minutes, the limit is now 217 mm:



At 24 minutes, it's now 215 mm:



And finally, at 33.5 minutes, the swing amplitude is 210 mm:



Anyway, simply by comparing the third photograph with the first, and noting that (due to the angle of the camera) there is a certain parallax effect which we have not yet allowed for, it is clear that over 16 minutes our pendulum lost about 10 mm in swing amplitude, so that our decay rate is about half of that of Allais's pendulum. That's pretty good: it implies that our pendulum is also only half as sensitive to side breezes, and that irregular wobbles due to air turbulence behind the bob are also greatly reduced.

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