Gridiron Pendulum

   The period of a pendulum is a function of the distance from the pivot point to the center of mass. If the clock is to keep uniform time, this distance must be kept constant. A metal pendulum rod will grow longer as the temperature increases, making the period increase and slowing the clock. The solution is to make the rod a combination of steel and brass rods, forming a gridiron. The construction can be best seem in the gridiron pendulum from Amherst College at the left, below. There are five steel rods, connected to expand downward, and four brass rods, which expand upwards. The lengths of the various rods are adjusted to give a net expansion of zero. The middle pendulum, from Union College, and the one at the right at the United States Military Academy, have three steel rods and two of brass.

   A variation of this is the mercurial pendulum, in which a container of mercury on the pendulum expands upward at the same time that the rod itself expands downward. An example of a mercury-compensated pendulum is shown at the bottom of this page.
   This astronomical clock with a mercury-compensated pendulum is in the apparatus collection of the physics department at the University of Vermont.

   The clock was made by John Stockell of New York City who was in business from 1825 to 1858. In a note from 1934 attached to the clock it was said to have come from the Hitchcock Estate, and that "one of this family had an observatory in a private house."

   The mercury was contained in a glass container through which the brass pendulum rod ran. The bottom of the container was attached to the end of the rod, so that as the rod expanded downward, the mercury expanded upward, thus keeping the center of mass of the system the same distance from the point of suspension of the pendulum system.

   The gridiron pendulum at the right is from the University of Texas, and probably dates from about 1930.

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