Rich Boyce, age 14, of Miami, Fla., for his question:

WHAT IS A LEYDEN JAR?

A Leyden jar was one of the earliest forms of condensers used to store an electric charge. It received its name because it was first used in 1746 in the city of Leyden in the Netherlands.

A Leyden jar is a glass jar about the size of a quart mayonnaise jar. About half of the inside and the outside of the jar is coated with tinfoil and the opening is covered with a cork.

The tinfoil coverings are conductors of electricity. The glass between them is the nonconducting insulator, or the dielectric. A brass rod is inserted through the cord and brought in contact with the tinfoil inside the jar.

A charge is then sent down the rod from an electric source. This charges the tinfoil by conduction. Current cannot pass through the glass, but the tinfoil outside becomes charged by induction.

If the rod is connected to the outside layer of tinfoil by a conductor, the two opposite charges will cause a spark.