Michael Cunningham, age 15, of Sparks, Nev., for his question:

WHAT IS AN ELECTROSCOPE?

An electroscope is an instrument used to detect the presence of an electric charge and to tell whether the charge is positive or negative. Electroscopes can also detect X rays and other electromagnetic radiation.

One of the simplest types of electroscopes, the gold leaf electroscope, has two slender strips of gold foil hanging from a metal conductor. A nonconductor, such as a cork, holds the conductor in a stand made of glass or metal.

When the conductor has no electric charge, the foil strips hang straight down.

If something with an electric charge touches the conductor, the strips become charged. Both strips receive the same kind of charge and, because like charges repel each other, the strips spread apart in an upside down V.

To find out whether a charge is positive or negative, an activated electroscope is tested with a known charge. If a known positive charge makes the foil strips fall back together, the unknown charge is negative because opposite charges neutralize each other. If the strips spread farther apart, the unknown charge is positive.