Joel France, age 13, of Glendale, Ariz., for his question:

WHAT IS A HERTZ?

A hertz (Hz) is the unit used to measure the frequency of waves and vibrations or their rate of occurance.

Waves exist in many forms. In human vocal chords, vibrations make sound waves. Radio transmitters send electromagnetic waves.

The musical note "A" has a frequency of 440 Hz, or 440 cycles per second. Radio waves have frequencies of many million hertz.

The hertz was adopted in 1960 by an international group of scientists at the General Conference on Weights and Measures. The unit was named for Heinrich R. Hertz, a German physicist. Hertz had opened the way of electromagnetic waves between 1886 and 1888.