Michael Conca, age 11, of Framingham, Massachusetts, for his question:

What use is a cloud chamber?

It was invented in 1912 by Charles Wilson, a British physicist, and we call it the Wilson cloud chamber. The clever invention provides visible evidence of subatomic particles. These midgets of matter, being parts of atoms, are smaller than atoms. They travel at fantastic speeds and Wilson's cloud chamber forces them to leave traces of their trails behind them. The instrument is a glass walled chamber, brightly lit and filled with fine, fine steamy mist. Fine mist tends to condense, even on subatomic particles.

Fragments from radioactive materials zoom off in all directions. If a scrap of uranium is placed in the cloudy chamber, subatomic particles zoom around, leaving trails of condensed droplets behind them. Their paths tell a great deal about these speeding fragments. For example, certain ones leave heavier and others leave thinner trails.