We operated the pendulum in two ways: with a fixed suspension
and with a motor-driven suspension. The fixed suspension
was used to observe the decay of the pendulum’s
amplitude. The driven suspension permits observation of the
pendulum motion at constant amplitude near the resonant
frequency of the pendulum.
A glass aquarium tank is filled with the fluid in which the
pendulum bob oscillates. For the cases described here, the
working fluid is de-ionized water, with a small amount of
thymol blue dissolved therein. The pendulum is constructed
using various spherical metal balls suspended on either lightweight
magnet wire or fishing line. A brushless linear motor,
driven sinusoidally, was used for the driven suspension.
A digital video camera was used to measure the amplitude
of oscillation at any point in time. The ring radii were measured
with a digital camera. The camera setups were calibrated
by imaging a steel rule located on the focal plane.
In the decaying oscillation experiments, two pendulum
lengths were used _315 and 155 cm_. Assuming a maximum
amplitude of 10 cm, then the angle cosines would be cos
=0.9995 and 0.9979, thus preserving the small-angle approximation.
For most of the observations, the camera location
was such to limit the parallax error to less than 0.15%.
The Baker electrolytic technique is used to visualize the
vortex rings from the spherical bobs. The electrochemistry
and other physical details are described in Mazo et al. _20_.
The pendulum and resultant rings are photographed in silhouette.
The oscillation amplitude is extracted from the recorded
images with the use of an in-house-written software. Vortex
ring sizes were measured using standard software packages
_such as PHOTOSHOP_.