Christine Rizzuti, age 16, of Twin Falls, Idaho, for her question:
WHO DISCOVERED THE PENDULUM?
An Italian physicist named Galileo discovered the laws of the pendulum in about 1600. He noticed that a hanging church lamp would swing with a constant period, whether the arc was large or small. He believed that a pendulum could be used to regulate the movements of clocks.
The simplest pendulum consists of a small weight hanging from a string. The path traveled by the weight is called the arc. The period of vibration is the time it takes the weight to pass back and forth once over this arc.
A Dutch scientist named Christian Huygens built the first pendulum clock in 1657.
Simple pendulums, such as those used in clocks, usually consist of a rod with a heavy weight at one end and a hard bearing at the other. A screw at the end of the rod permits the weight, or bob, to be raised or lowered. When the bob is lowered, the pendulum swings slower and the clock runs more slowly. When the bob is raised, the pendulum swings faster and the clock runs faster.
The bearing on which the pendulum swings must be as nearly frictionless as possible. It is often made of a knife edge of agate set in a grooved agate plate. The weight of the pendulum tends to make the bearing slip sideways as the pendulum swings. For this reason, pendulums are made long so that they beat slowly, and swing through a small arc.
A device called an escapement is fastened to the clock's mechanism. It gives small but regular pushes to the pendulum and keeps it swinging. The escapement lets one tooth of a toothed wheel turn past it at a time. It does this each time the pendulum swings aside. This gives the familiar "tick tock" sound to the clock.
The rod in a clock pendulum tends to expand when it is warm and to shorten when it is cold. This would make a clock run slower in warm weather than in cold.
Clocks known as regulators come with so called gridiron pendulums.
In 1851, the French scientist Jean Foucault hung a large iron ball on a wire about 200 feet long to show that the earth rotates. A Foucault pendulum swings in one direction in relation to space. But the earth turns under it, so the direction of the pendulum's movement in relation to the rotating earth changes at a regular rate.
At the equator, a Foucault pendulum does not change its apparent direction at all. The change in apparent direction is fastest at the North Pole.
One remarkable type of pendulum measures minute differences in the strength of gravity. It can detect a change of one part in a million. Geologists use it to detect underground mineral deposits.
Another type of pendulum is held up by two strings instead of one. This is called a bifilar pendulum. A form of bifilar pendulum was developed by Lord Kelvin as an excellent earthquake detector. A bifilar pendulum is very sensitive to variations of the plumb line, or the line toward the center of the earth.