Julie Metz, age 12, of Merrillville, Indiana, for her question:

What runs an atomic clock?

One would expect an atomic clock to be run by atoms. And so it is. However, to perform their duties, these tiny talented timekeepers need held from the ingenious mind of man. The atom is invisibly small, yet scientists have figured out its structure, its built in energies and how various forms of outside energy cause it to change. Then, certain of these reactions were computed in precise equations and this data used to build precise atomic clocks.

Nature supplied us with many clocks to mark the passage of time. In the great dome of the sky, moving heavenly bodies point out the passing moments in step with the earth's daily rotation. This celestial clock was used in prehistoric times. Through the centuries, inventors created clocks with pendulums, wheels and a multitude of other devices to mark the passage of time. But none of these can compare with the precise time keeping of our modern atomic clocks.

In a rough and ready fashion, this principle can be compared with sound waves created by a vibrating tuning fork. Atoms in certain substances also can be made to vibrate with a precision tuning fork. Vibrations are frequencies that jog at so many cycles per second. In atomic clocks, particles of quartz with ammonia or cesium are set to vibrate at precise numbers per billionth part of a second. These frequencies are in megacycles.

The energy that runs an atomic clock is emitted by quartz crystal. When stimulated by some form of radiation, the quartz molecules in the delicate lattice crystal vibrate and also emit a weak electric current. The current is converted to certain radio frequencies and monitored inside the complicated clock.

In the first models, energy from the quartz crystal was directed into a small sealed chamber of ammonia gas. The ammonia molecule happens to be a pyramid with a triangle base of three hydrogen atoms and a nitrogen atom at the top. When the quartz beam is set at 23,870 megacycles, it stimulates the nitrogen atom to Jog madly, though precisely, back and forth between the triangle of hydrogen atoms. The clock is fitted with superfine equipment to receive these fast, accurate vibrations. The data is used to record the passage of time in split seconds with almost perfect accuracy.

The quartz ammonia clock is accurate to one part in 100 million. In later models, cesium was used with quartz instead of ammonia. Its atoms can be adjusted to vibrate with even finer precision. The quartz cesium clock is accurate to one in 10 billion parts and misplaces no more than one second in 3,000 years.

The first workable atomic clock was made by American scientists at the National Bureau of Standards and completed in 1949. At that time, this quartz ammonia clock was in a class by itself. But the natural desire to make a good thing better resulted in the quartz cesium clock of the 1960s. Atomic clocks of this sort are used for global time signals    and naturally much too costly and complicated for home use.