Tony Commisso, age 12, of Toronto, Ontario, Canada, for his question:

WHAT IS A GEIGER COUNTER?

A Geiger counter, or more accurately a Geiger Muller counter, is an instrument used to detect radioactive radiations, such as those given off in the release of atomic energy.

The counter is usually in the form of a metal cylinder enclosed in a glass tube. The metal wall serves as one electrode. A straight wire projected into the cylinder is the other electrode. The electrodes are maintained at a voltage which is just short of the breakdown potential of the air or other gas in the cylinder.

If any other ionizing influence is sent into the cylinder, the gas will ionize. This sets up a weak current which is revealed by  light signals, clicks picked up by earphones or by readings on a meter. A German physicist named Hans Geiger came up with the first model in 1908.