Magnetometer is an instrument for measuring magnetic forces. It is used to record the changes in the intensity of the earth's magnetism. One form consists of a bar magnet suspended by two equal threads which would naturally hang in a vertical plane. The magnet, however, causes them to twist out of that plane, and as the intensity of the earth's magnetic force alters from time to time the magnet takes up a new position. This instrument is known as the bifilar magnetometer. It is usually provided with a small mirror, which reflects a beam of light, and is thus enabled to photograph its own movements.