The pendulum was set in motion by experimental physicist Leon Foucault (foo-KOH). While working on a clock used with astronomical observations, he theorized that the plane of oscillation of a pendulum would remain fixed while the earth moved beneath it. After smaller scale tests, French President Louis Napoleon (later Emperor Napoleon III) requested a larger pendulum be hung in the Pantheon. This simple, almost silent demonstration was visible proof of the earth’s rotation. It ignited a worldwide phenomenon known as “pendulum mania.”
Large pendulums were set in motion all over Europe and the British Isles. In the United States, dozens of public demonstrations were held at universities, churches, public buildings, and even private homes. Two months after the Paris display, University of Alabama Prof. Frederick A. P. Barnard, assisted by Dr. J. M. Jennings of the Alabama Central Female College, suspended a 90-foot pendulum under the dome of the former State Capitol in Tuscaloosa. The Crystal Fount, a local newspaper, reported on June 13, 1851, that an iron ball was suspended from a fine wire and swung close to the floor over a space of about 25 feet. It was noted that even after one or two swings, the pendulum would “deviate to the left, and at the opposite extremity, to the right” and regardless of the direction of release, the deviation was always to the left of observer.
Foucault expressed the theoretical basis of his pendulum in the equation: Ω = 360sinφ/day. The angular speed Ω in degrees per sidereal day* is proportional to the sine of the latitude φ. Simply stated, the plane of oscillation of the pendulum is fixed at the equator, while at either pole, the plane rotates once per sidereal day. In the northern hemisphere, rotation of the plane of oscillation is clockwise; in the southern hemisphere, it is counterclockwise.
The old State Capitol sits at latitude 33.21°. Therefore, according to Foucault’s formula, the angular speed Ω is 197.18° per sidereal day or 8.22° per hour. One complete rotation (360°) would take about 43 hours, 49 minutes. A report in one local newspaper stated that results obtained by Barnard were very close to the predicted value. Generally speaking, documented results obtained at other US locations were closer to the predicted values than those in Europe.
Leon Foucault made significant contributions to early photography, optics, electro-magnetics, and astronomy. In 1862, he was the first to accurately measure the speed of light. His value was within 0.001% of the modern accepted value of 186,282.3960 miles per second—an astounding achievement considering his use of purely mechanical methods. Foucault died in 1868, and is memorialized as one of France’s greatest scientists.
Like Foucault, Barnard made contributions to early photography. He was noted for endeavors in many fields but is best remembered as an advocate for higher education. He later became Chancellor of the University of Mississippi and President of Columbia University in New York, and he was one of the first advocates of co-education for women. Barnard College was subsequently named in his honor.
Today, Foucault’s Pendulums are used to teach basic science, and are popular exhibits at museums and universities. In 2001, scientists hung a makeshift Foucault Pendulum in the stairwell of a research center at the South Pole. Although conditions were extreme (-90°F at 11,000’ elevation) they obtained results that approximated Foucault’s predicted angular speed of one rotation per sidereal day. Pendulum mania may have been a passing fad over 160 years ago, yet this simple, elegant proof that the earth does move, continues to fascinate.
*The sidereal day is used by astronomers and physicists. It is based upon the earth’s rotation relative to fixed stars as opposed to the solar day which is based upon the position of the sun in the sky. A sidereal day is 23.93447 hours or about 4 minutes less than a solar day. The difference is reconciled each leap year.