Magnetometer, an instrument for measuring the strength and sometimes the direction of a magnetic field. An important use of magnetometers is in measuring the earth's magnetic field. By detecting irregularities in the earth's magnetic field, a magnetometer can indicate the location of deposits of magnetic ore, such as iron ore, or of geological formations associated with petroleum.

Another use of magnetometers is at airports to screen boarding passengers for concealed guns or other metallic weapons. In a typical system, the passenger walks through a fluctuating magnetic field, which sets up secondary magnetic fields of various strengths around metallic objects he or she may be carrying. When the magnetometer detects a secondary magnetic field characteristic of a weapon, an alarm sounds.

Magnetometers have been based on a number of different principles. Most magnetometers contain a magnetic device sensitive to an external magnetic field. Some magnetometers use a permanent magnet, others an electromagnet, and yet others make use of the magnetic properties of the nuclei of atoms.