238 M A G N E T I Deter mination of mag netic axis, and of the magnetic declina tion. Mirror method. Colli- mator magnet. matter here only in so far as it concerns the work of a physical laboratory, the rest belonging more properly to the subject of Terrestrial Magnetism (see METEORO LOGY). Axial Direction and Magnetic Declination. The magnet is suspended, usually by means of one or more fibres of unspun silk, so as to be free to move about a vertical axis. We shall suppose, for simplicity, that the magnetic axis is in a horizontal plane. If this is not so, instead of deter mining the axial direction, we determine a vertical plane through it. In order to obtain a fixed line of reference in the magnet, two marks may be made on it as nearly in the direction of the axis as can be guessed to begin with ; this arrangement is used with dipping needles and also for horizontal needles when no great accuracy is required. For declination needles two contrivances of greater refine ment are used. 1. A mirror is" rigidly attached to the magnet, so that the normal to its surface is nearly parallel to the magnetic axis. The image of a fixed horizontal scale in this mirror is observed by means of a fixed telescope, and the angular motion of the magnet deduced from the motion of the scale divisions over the wires of the telescope. This is called the mirror method. 1 2. A more compact arrangement is to attach to the magnet a small photographic scale and a lens, the former being placed at the principal focus of the latter, so that the line joining the middle division of the scale to the optical centre of the lens is nearly parallel to the axis of the magnet. The scale is viewed through the lens by means of a fixed telescope, and it is clear that the line just mentioned gives us a fixed direction in the magnet, and that the motion of the magnet can be followed by observing the apparent motion of the scale across the wires of the telescope. This may be called the collimator method. 2 Unifilar The apparatus usually employed in the United Kingdom for magneto- observing the magnetic declination, 3 and also for other absolute meter. magnetic measurements, is the portable unifilar magnetometer, the upper part of which is shown in figs. 28, 29. The lower part con sists simply of a tripod stand supporting three V-shaped groves, into which the points of the levelling screws attached to the fixed limb of the instrument are set. In making an observation of the declination the instrument is arranged as in fig. 28. The declina tion collimator magnet is suspended in the box A (the sides of which are removed to allow the interior to be seen) by means of the suspension fibre D, attached to the torsion head PH. The scale of the magnet is observed through the small telescope QBG. The first step is to remove the torsion as far as possible from the suspension fibre by hanging to it a brass plummet E of the same weight as the declination magnet. After this weight has come to rest, it is replaced by the declination magnet, so that the latter shall rest as nearly as possible in the magnetic meridian without intro ducing torsion of the fibre. The movable limb is now turned till some division of the magnet scale is on the cross wires of the tele scope. It is then clamped. The magnet is now inverted, and the number of the scale division on the wires again read. The mean of these readings gives the point of the scale the line from which to the centre of the collimating lens is parallel to the axis of the magnet. This point of the scale (axial point) will remain the same so long as the magnetism of the magnet does not alter, or the adjust ment of the scale and collimating lens is not interfered with. The tangent screw is now worked until the cross wires of the telescope are on the axial point of the scale. The verniers of the limb are then 1 The mirror method was first suggested by Poggendorff (Poyy. Ann., vii., 1826). It was carried out in practice by Gauss. 2 There seems to be some doubt to whom the collimator method is due. Airy, Lloyd, Lamont, and Weber all did something for it. See Lamont, Handb. d. Maynctismus, p. 154. 3 Want of space compels us to omit all but the leading points. Readers in search of full practical detail must be referred to The Admiralty Manual of Scientific Enquiry, pp. 84 sq.; Maxwell, Electricity and Magnetism, 449 sq. ; Lamont, Handbnch der Magnetismus, and Erd-Magnetismus, where references to all the authorities up to his time will be found. They should also study the classical memoirs of Gauss to be found in the fifth volume of his col lected works. read. The next step is to observe the azimuth of the sun or of some other heavenly body, by means of which we can refer the azimuth ol the magnetic meridian to the true north. For this purpose the instrument is provided with a small transit mirror NO, which has a motion in altitude so as to bring any object into the field of the telescope. To use it, the limb is undamped, and it aud the FIG. 28. Unifilar Magnetometer, arranged to indicate declination. mirror moved until the sun or star comes into the field of the telescope ; the limb is then clamped and the time noted at which the heavenly body passes the intersection of the cross wires. The verniers are again read. The differences of the readings, added to the azimuth of the heavenly body found by means of the time from the Nautical Almanac, gives the declination at the time and place of observation. There are several causes of error to be guarded against. (1) Tor sion is reduced within as small limits as possible to begin with, and if there is reason to suspect any residual error we may test the apparatus by turning the torsion head of the suspension tube first 90 one way and then 90 the other. If the deflexion of the magnet is exactly the same and oppositely directed in the two cases, then we may conclude that the torsion is zero in the azimuth of equilibrium. If not, then we may turn the torsion head so as to reduce the error still further; or we may calculate its amoimt (assuming torsion to be proportional to twist) from the two observa tions, and allow for it. (2) If the axis of the magnet is not very nearly parallel to the line of collimation of the telescope to begin with, and consequently the two scale readings far apart, an error may arise 4 from the vertical axis of suspension not being exactly reversed by the inversion. This error is reduced by repeating the observation, after adjusting the axis of the magnet and telescope so as to be more nearly parallel. (3) If mean declination for a given day be desired, correction must be made for the diurnal variation, and under certain circumstances this variation may even produce disturbances in the course of a single observation. Magnetic Moment, Horizontal Intensity of the Earth s Magneti< Force. If merely relative measurements of the magnetic moment moment K of a given magnet, or of the horizontal and hori ZOlltill 111 intensity H of the earth s force, are desired, there are two tensity. methods of obtaining them. The first is the method of vibrations. Having found the moment of inertia A of the magnet M about its vertical axis of suspension, and the
4 See Swan, Trans. Roy. Soc. Edin., vol. xxi., 1855.