MAGNETOMETER, in its customarily re stricted sense, an instrument employed in ob servatories to determine the intensity of ter restrial magnetism. Essentially the instru ment consists of a magnetic needle free to swing in a horizontal plane within a circular scale. The magnet may be supported on a fine vertical pivot, or by suspension on a fine un twisted silk thread. Bifilar suspension is more to be depended upon; the magnet is supported by two parallel threads and screws enable the tensions in the threads to be equalized and their distance asunder to be adjusted. For delicate readings the instrument has been highly refined, a mirror being attached to the magnet, reflect ing a beam of light upon a scale. To prevent possible oscillations due to air currents the mag net is enclosed within a box with glass sides, through which it may be observed. A tall tube above the box encloses the suspending thread.
The unifilar magnetometer is set up with its magnet lying in the direction of the magnetic meridian. The bifilar instrument is set with the magnet at right angles to that meridian —1 much more sensitive position. To fill the needs of traveling observers, portable magnetometers have been devised. These are more comprehen sive, comprising within a single unit an astro nomical telescope with a magnetometer—for the determination of both the magnetic declination and the horizontal intensity; and also a dip circle for the determination of inclination and total intensity. Consult Terrestrial Magnetism, Vol. XVI, page 1, 'Two New Types of Mag netometers'; and Vol_ XVIII, page 105, 'De scription of the CIW (Carnegie Institution, Washington) Combined Magnetometer and Earth Inductor> (September 1913). See MAG NETISM — Terrestrial.