Jan Archibald, age 13, of Missoula, Mont., for her question:

WHAT IS A DIPPING NEEDLE?

A dipping needle is a magnetic needle suspended so that it will dip toward the earth. It points toward magnetic substances and is somewhat like a compass.

Dipping needles are used to study the earth's magnetism, to locate iron and other magnetic objects just under the earth's surface and to locate underground meter boxes and water pipes.

The north seeking end of a dipping needle would point straight down if it were held over the north magnetic pole. The needle remains horizontal at the magnetic equator. At all other places on the earth, the needle points down at an angle from the horizontal. A scale on the case shows the amount of dip from the true horizontal.

In a compass, the north seeking end of the needle will swing around horizontally until it points toward the north magnetic pole. The north seeking end of a dipping needle will swing down until it points toward the north magnetic pole.