The Pendulum Bob

The bob shown at the Millersville web site is made from PVC pipe cap, with mass of about 100 gram. The pipe cap is threaded onto a hollow threaded rod used in lamp hardware. The pitch the threads is one thread per mm. The pipe cap can easily be moved vertically a distance as small as 0.1mm. This sensitivity is needed to match the natural pendulum frequency to the driver frequency.

The pendulum resonance is narrow, perhaps 0.0005 Hz for a frequency of 0.5 Hz. The length needs to be adjusted with a precision of a part in 10,000.

The bob described here is simple. The effective length of the pendulum is adjusted by moving small weights along the string. The bob is attached to the pivot by a cylindrical shoe string, made for hiking boots. The shoe string is epoxied to the bottom end of the guitar string, with care taken to maintain cylindrical symmetry.

The bob itself is a hollow rubber ball, sold for playing the game of racquetball. The low mass and large surface area of the bob makes it lose energy more quickly than a heavy bob, but the pumping driver provides enough to replace the loss. The high energy loss broadens the pendulum resonance, making the adjustment for length somewhat more forgiving.

The ball has a small hole cut in the top and a larger hole in the bottom. The shoe string is threaded through the top and then the bottom. A knot is tied in the string and pushed back through the bottom hole. The knot holds agains the inside top of the ball, so that the top of the ball is 1 meter from the pivot. (The design assumes a 1 second square wave driver.)

The effective length of the pendulum is adjusted with a 1cm length of solder wire, 1mm in diameter. The wire is spiral wrapped tightly around the shoe string. It is slide up and down to adjust the effective length of the pendulum.

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