The pendulum bob is a small nut located directly below the pendulum ball; it is also sometimes called the pendulum adjustment nut or regulating nut. Loosening or tightening the nut changes the speed at which the pendulum swings. When the nut is adjusted, the pendulum must be held still with the other hand to prevent damage to the suspension spring. Raise the bob toward the ball to cause the pendulum to run faster or turn it away from the ball to decrease the pendulum's swing speed.
Grandfather clocks are a wise choice for experimental pendulums. They are the most widely available and familiar pendulums, which makes them easier to locate than other pendulum systems. Clocks offer the additional benefit of providing an additional source for observation in their time-keeping mechanisms. Scientists observe changes in measurement of bob distance and pendulum arc, as well as elapsed time according to the experimental clock and real time. It is imperative that a traditional wind-up system be used, rather than an electric pendulum clock, since the electricity interferes with accurate observations of the natural laws operating on a pendulum's spring system.
A simple experiment to conduct on a pendulum bob is to observe whether a pendulum's speed affects the length of its arc. This experiment will require an operational pendulum with an adjustable nut, a large writing surface, a permanent marker and a flexible tape measure. The pendulum should originally be motionless, then released and allowed to complete its normal arc path. Set the large writing surface, such as a dry erase board or large poster sheet, behind the pendulum as a wall, and draw a small mark to indicate the endpoint on both sides of the arc. Adjust the bob to make the pendulum run slower and mark the path endpoints. Tighten the regulating knob to swing the pendulum faster and draw a third set of marks. Measure the distance between each set and compare.
Teachers will incorporate earth science principles in physics lessons by conducting an experiment to measure the effects of expansion and contraction due to seasonal changes. At the beginning of the year, install a pendulum-operated clock in a prominent location, with an atomic clock, or other highly precise clock, located nearby. Set the pendulum clock to the exact time the digital clock reads. Observe for one week with the classroom temperature set near 70 degrees consistently. During the second week, run the room air conditioner at least 10 degrees cooler and instruct students to keep careful record of any changes in time. At the end of the week, reset the clock if needed. During the third week, set the temperature around 80 degrees and observe any changes. Although results will vary by location and are also dependent on outside temperatures, in general, cooler temperatures should cause the rod to shrink and the bob to drop, slowing the time, while the reverse should be true of raising the temperature.