Our Paraconical Pendulum Launch System

First have a look at an overall photo showing the entire contraption (Please ignore the red clamp, which was a temporary expedient):

What are you seeing here? Well, the pendulum bob is on the right, pulled out about 95 mm from its bottom dead center point and ready to be launched. The vertical brass plate in the middle carries the positioning/adjusting screw, which is pierced along its axis with a thin through hole. And on the left is the pulling-out device - a modified fishing reel. This should be mounted on a very solid and heavy support, so that it doesn't lurch about, even to the tiniest extent, when the pendulum is released and the strain comes off. Here is a diagrammatic view of the arrangements:

First we discuss the fishing reel. We started with an Abu Garcia "Five-Star" fishing reel - the smallest good one available at the time - which cost about US$65 in Kuching - and we made the following modification. At its lower front there is a little device that slides back and forth, driven to and fro sideways by a small rotating shaft with two spirals cut in it, for distributing the line evenly as it is wound upon the reel. (Fisherman will know instantly what I am talking about). We removed the slider and the shaft, and substituted a new shaft, without spirals, but with a 2 mm transverse through hole. For better access, we also cut away most of a tube surrounding this shaft, leaving just a bit at the bottom, which we reinforced with a bar and epoxy metal filler (this part was done a bit untidily).

During normal winding-up operation of the reel, this small shaft can only turn in one direction, due to the operation of the main reel ratchet. So, if one end of a piece of fishing line is tied to something (the pendulum) which you want to pull on, you poke the other end of the line through the hole in the shaft, and, while keeping some tension on that far end by hand, you wind up the fishing reel, and the line will wind up on the small shaft like this in a smooth and controlled manner:

As soon as you have wound up a few turns the hand tension becomes unnecessary, and you can keep winding as much as required. This photo shows how the line gets wound around the shaft (it was taken after the pendulum had been released so that the line went slack):

Next we view the positioning/adjusting screw with its locking nut. It is about 60 mm long, and it can be screwed through any of three holes, bored side by side through the vertical brass plate. This allows for a freedom in adjusting the horizontal position of its end which faces towards the pendulum, in both X and Y directions, at least as great as the extent of the flat at the top of the pendulum (which is 35 mm x 25 mm). This is because A PARACONICAL PENDULUM HAS NO CENTER! If for example you have decided to release the pendulum in a NW azimuth with an amplitude of 95 mm (which is about right), then (if you set the adjusting screw up right) you can do just that, whatever be the position of the ball upon the flat. And if you decide to use a new place upon the flat, you can alter the setting of the adjusting screw, and the hole into which it is screwed, to compensate for that change. Well, maybe our Y adjustment transverse to the plane of swing is a bit crude... arranging for the brass plate to be slid to and fro horizontally in its own plane by a traveling screw would allow for infinitely fine adjustment, and that would really be better than our three-hole system which only allows three separate Y positions... indeed, traveling screws could be used for both X and Y adjustments...

So, of course after having stopped the pendulum, you take the fusible link (described earlier) in hand, you pass the free end of the line through the positioning screw, lead this free end to pass through the little hole in the shaft on the fishing reel, pull it through, hook the ring of the terminal over the pin on the underneath of the pendulum, and then pull the line gently taut. This photo shows the pendulum and positioning screw in this state, before much sideways pulling has been performed:

While maintaining a gentle tension on the far end of the line, you wind up the fishing reel, and Hey Presto! the pendulum is smoothly pulled sideways (leftwards in the images) as the line winds up on the small shaft of the reel. This photo shows an intermediate situation:

When the stopper on the line comes into contact with the positioning screw, you turn the fishing reel winder through about 120o more, so that the portion of the line from the reel to the positioning screw is tensioned quite a lot more, thus pulling the stopper tightly against the end of the screw. This is a very important detail. As a consequence, looking at the photo below, the portion of the line coming out leftwards from the positioning screw is, finally, under much more tension than is the small right side portion between the stopper and the terminal which is hooked to the pendulum. In fact this small right side portion - about 2 mm long - is only under the tension necessary to hold back the pendulum from swinging down towards its bottom dead center, which is about 15 newtons or so. And, because this portion of the line is so short, very little energy (momentum) is stored in it.

The pendulum is now set up ready to be launched. If you have done everything correctly, you should be able to twang the line between the fishing reel and the screw, without causing any disturbance whatsoever to the pendulum; this shows you have put enough extra tension into this run of the line. Now you should wait for two or three minutes in order to allow any residual wobbling motion of the pendulum to die away. After this pause, you are ready to release the pendulum, which is done by burning the fusible link. You should bring the flame down from above onto the 2 mm line portion:

The 2 mm portion of line will fuse and disappear, the pendulum will swing smoothly away, the terminal will drop down off the pin, and the stopper on the remaining run of line will not move at all where it rests still tightly pulled against the end of the positioning screw. (If any movement takes place at this time, it might cause a disturbance to the pendulum.) Thus the long portion of the line, between the fishing reel and the positioning screw, remains tensioned and sits in place just as before. Subsequently, of course, you can press the ratchet release on the reel and unwind the remnants of the line by pulling smoothly on both ends, thus rotating the small shaft backwards; then you remove the line and throw it away. (Don't attempt to reuse the line; it has been scarred by its experiences).

Back to the description of operating the English paraconical pendulums in Sarawak.

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