MAGNETOMETER, in its most general sense an instru ment used to measure the intensity of any magnetic field, but often used in the restricted sense adopted in this article of an instrument for measuring the horizontal component, H, of the earth's magnetic field. The earth's magnetic elements which are usually measured directly (see TERRESTRIAL MAGNETISM) are declination, horizontal force and inclination. For the last named, see INCLINOMETER the two other elements, D and H, are usually measured with the same instrument, the unifilar magnetometer. Measurement of Declination, D.—Two operations are in volved, the determination of (a) the geographical meridian and (b) the magnetic meridian, the angle between the two being D. For the former operation at an observatory use is made of a distant mark, the bearing of which has been accurately determined once for all; but in field work the geographical meridian is de duced from an observation of the pole star, or from a transit observation of the sun or ordinary star. In the case of the Kew pattern unifilar shown in fig. I, when a distant horizontal mark is available the mirror N is removed, and the distant mark is sighted through the small glass windows of the magnet box A, which must be truly plane parallel. When a transit observation is necessary, the mirror N must be carefully adjusted. The axis about which it rotates should be parallel to the surface of the mirror, i.e., hori zontal, and perpendicular to the line of collimation of the tele scope. - When adjusting the mirror the magnet box A is replaced by a long hollow tube to prevent stray light falling on the object glass of the telescope B. The mirror having been adjusted, the time of transit of, say, the sun across the vertical wire of the telescope is determined with the aid of a chronometer. The sun's azimuth can be calculated if the latitude and longitude of the place are known. Thus the readings of the verniers on the azimuth circle made when the transit was observed enable us to deduce the reading which corresponds to the geographical meridian. Before determining the magnetic meridian, the torsion must be taken out of the suspension, usually a long thread of unspun silk, with the aid of a brass plummet. The collimator
magnet, shown suspended in fig. I is a hollow steel cylinder, about ocms. long and 'cm. in diameter, fitted with a lens in the end nearest the telescope and a scale in the other end. The scale is at the principal focus of the lens, and so can be sighted by B when focussed on a distant object. The scale is illuminated by light reflected from the mirror N. During an observation the sides of the box A are closed by shutters, to protect the magnet from draughts. The upper end of the magnet's silk suspension is carried by a brass rod, which can be raised or lowered by a screw. The rod passes tightly through and can be clamped in the torsion head H, seen at the top of the suspension tube in fig. I. The magnet having been brought to a level suitable for invision, and the position of the mirror N adjusted for good illumination, the instrument is rotated about its vertical axis until the central vertical division on the scale appears to coincide with the vertical cross wire of the telescope. The two verniers on the azimuth circle having been read, the magnet is inverted, i.e., turned 18o° round its horizontal axis, and the setting is repeated. A second independent setting is generally made with the magnet inverted, and then another setting with the magnet in its original position. The mean of all the vernier readings is taken as corresponding to the magnetic meridian. The difference between this and the reading corresponding to the geographic meridian is the desired declination. The object of inverting the magnet is to eliminate error due to non-coincidence of the magnetic and optical axes. The magnet shown suspended in fig. I is intended only for H observations. To fit it for D observations it should have another shank. The most usual arrangement is to employ a single double shanked magnet, of the type M in fig. 1, which serves for both the D and the H observations. The short horizontal tube seen in fig.