MAG'NETOM'ETER ( from Gk. adyans, mag lies, magnet + Iherpov, ?net ron, measure). Any compass needle so mounted that its angular de flections may be measured is a magnetometer. It may be a simple small magnetic needle supported upon a point and swinging over a divided circle, or it may be a large magnet hanging upon a fibre of silk or quartz or other material, and the deflections read with a mirror on the magnet and using a telescope and scale or a reflected beam of light. In a simple magnetometer the magnet hangs in a small house or box of wood or copper and the attached mirror is visible through the little window. The magnet and mirror are sus pended upon the long silk or quartz fibre in the tube at the top of the instrument. Fig. 1 shows (q.v.) and its variations; that is to say, it will show the deviation between the magnetic north as shown by the needle, and the true north as determined by astronomical observations. When the magnet is larger and supported, for example, on two fibres, as in Fig. 2, then the instrument is so set up that the bifilar suspension holds the magnet at right angles to the magnetic meridian.
a more complicated form of magnetometer with an attached bar for supporting the magnet whose strength is to be measured, and which is placed at a definite distance from the magnetometer, In this position the magnetometer measures the strength of the horizontal force of the earth's magnetic field, because if this latter increases then the bifilar system is twisted a little more, and if the earth's field decreases then the bifilar untwists a little.
A magnet so mounted that it can swing in a vertical plane will similarly give the inclination and with its axis pointing toward the magnetom eter, or at right angles to that direction.
A magnetometer set up with its magnet in the magnetic meridian will show the declination of the earth's field, or the 'dip' and its varia tions. Spots of light reflected from mirrors on the needles of these three last forms and allowed to fall upon a moving strip of photographic paper will give an autographic record of the variations of these three components of the field of the earth's magnetism. namely the declination, inclination. and horizontal intensity. The action of such an instrument is shown by the accom panying sketch. These instruments are used at the various permanent stations of the Division of Terrestrial Magnetism of the United States Coast and Geodetic Survey. Figs. 4 and 5 show a theodolite used for the determination of the as tronomical meridian, or true north, at a certain given station, and a standard form of magnetome ter. In this the astronomical meridian is deter book, to be sung or said after the first lesson at evening prayer. It was omitted in the American prayer-book, but restored at the recent revision.