Sister Mary's Pendulum

"The only skyscraping girls' school in the world'' is small (600 students) Mundelein College, which is lodged near Lake Michigan in a 15-story building on Chicago's North Side. A popular teacher of physics and botany at Mundelein is Sister Mary Therese, member of the Order of the Blessed Virgin Mary, daughter of a Chicago contractor. When Mundelein moved into its skyscraper in 1930, it was seen that only two of its three elevator shafts would be needed for vertical traffic. This gave bespectacled Sister Mary, who was then in her late 205, an idea. Last week, in the idle elevator shaft, installed and ready to operate was a 120-foot Foucault pendulum, the longest in existence.

French Physicist Jean Bernard Leon Foucault (1819-68) installed the first Foucault pendulum in Paris's Pantheon in 1851. That one, since dismantled, was 200 feet long. Foucault's idea was to prove the rotation of Earth on its axis. A pendulum which is swinging freely in space keeps to the same line, whereas compass directions beneath the pendulum are constantly changing as the earth rotates. This apparent shift was duly performed by the pendulum of Jean Bernard Leon Foucault. Such demonstrations always make a great impression on students of elementary physics.

Sister Mary persuaded her superiors to provide the money for a pendulum to occupy nine stories in the unused shaft. The chromium-plated bob weighs 30 pounds, makes one back-&-forth oscillation through an arc of about six feet every twelve seconds. At the centre of the swing, the bob passes close to a waxed indicator table, and by means of a high voltage transformer an electric spark is passed from the bob to the wax, makes a mark showing the amount of rotation every hour--or oftener if desired for demonstration purposes. To start the pendulum going, without torque, it is held at one end of its arc by a string which is then burned through (see cut).

Scientists are now generally agreed that the earth rotates on its axis, and the speed of rotation has been accurately measured. Nevertheless, Sister Mary's pendulum will be something more than a mere exhibition. Since a pendulum's rate of oscillation depends on the force of gravity at the point where it operates, it will keep a constant record of the force of gravity for the Chicago area.

* Holding a match to burn the cord which releases the pendulum. The four wires from which the bob hangs are barely visible in the picture.

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