Tyler Wolfe, age 13, of Birmingham, Ala., for his question:

WHAT IS A GALVANOMETER?

A galvanometer is an instrument that measures very small amounts of electric current.

A Danish physicist named Hans Christian Oersted made a discovery in 1819. He found that a magnetic needle, brought up to a wire conducting electricity, would swing until it was almost at right angles to the wire. This showed that a magnetic field was formed around the wire when an electric current flowed through it.

The strength of the magnetic field, Oersted found, depended on the strength of the electric current: the stronger the current, the stronger the magnetic field.

If two magnets are brought together, they try to arrange themselves with their magnetic fields lined up in the same direction. All galvanometers make use of this principle in design.

In one type of galvanometer, a coil of wire hangs between the poles of one or more permanent horeshoe magnets. The electromagnetic field is set up by a flow of electricity. The coil turns to line up with the field of the permanent magnets.

The stronger the current, the more the coil will turn. A mirror fastened to the wires reflects a small beam of light from a lamp onto a dial scale as the coil is twisted.

Another group of galvanometers uses a fixed coil of wire with a permanent magnet turning inside the coil. A pointer attached to the magnet shows the amount of the rotation.