Magnetograph

MAGNETOGRAPH, an instrument for continuously re cording the magnetic elements, usually declination (D), hori zontal force (H) and vertical force (V), but sometimes two rec tangular force components in place of D and H. (See TERRESTRIAL MAGNETISM.) In each case variations only are recorded, the absolute values being determined by means of absolute in struments, e.g., the unifilar magnetometer and the inclinom eter.

Declination.—A suspended magnet carries a mirror, im mediately below which is a second fixed mirror. Light passing through a vertical slit falls on the mirrors. After reflection the light passes through a plano-cylindrical lens with axis horizontal, and falls on a sheet of photographic paper, wrapped round a drum, which is rotated uniformly about a horizontal axis by clockwork. The two images due to the mirrors appear as dots of light on the paper. The light from the fixed mirror answers to a fixed direction, that from the magnet mirror to a direction which alters with the pointing of the magnet. The distance apart of the dots thus shows the changing inclination of the two mirrors, and so the changes of D. When the sheet is taken off and de veloped, we have a straight (base) line from the fixed mirror, and a curved line from the magnet mirror. The variations in the length of the ordinate measure the variations of declination.

Horizontal Force.

The suspension of the H magnet may be bifilar or unifilar, in the latter event a quartz fibre or fine metal wire. The suspension depends from a "torsion head" rest ing on the top of the suspension tube. By turning the torsion head the magnet is brought to be perpendicular to the magnetic meridian. A magnet of moment M inclined to the meridian at an angle 0 experiences a magnetic couple MHsine urging it towards the meridian ; the change in the couple for a small change dO in 0 is MHcos0c10. Thus when the magnet is per pendicular to the meridian the couple is Mil and is insensitive to small changes in 0. Balancing this is the torsional couple (d-0), where T and d are constants. If H increases, the magnet turns

in the direction diminishing 0, and small changes in the angle are proportional to the changes in H. There is a magnet mirror and a fixed mirror, as for D, and similar optical arrangements. As before, the change in ordinate is proportional to the change in angle between the two mirrors and so to the change in H. Owing to the secular change of the magnetic meridian the instru ment requires readjustment from time to time.

Vertical Force.—In what is often called the "Lloyd balance" the V magnet is mounted on a knife-edge in the Kew and Watson patterns, but on a frame standing on fine points in the Eschen hagen instrument. In either case it can turn freely about a horizontal axis, at right angles to its length. The magnet is balanced so as to be horizontal, the final adjustment being made with a horizontal screw. The magnetic moment being M, the couple VM is balanced by a gravitational couple, the centre of gravity being slightly on the opposite side of the knife edge from the dipping end of the magnet. Supposing V to alter, the magnet tilts one way or the other according as V increases or diminishes. The tilt shows itself by a change in the distance between two dots of light, one due to light reflected from a mir ror carried by the magnet, the other to light reflected from a fixed mirror closely adjacent. In this case the axis of the drum carrying the photographic paper is naturally vertical. But in the Eschenhagen pattern the vertical motion of the dot of light is converted into horizontal motion by means of a reflecting prism. This enables a single drum with horizontal axis, carrying a single sheet of paper, to record the D, H and V traces, in addition to the trace of a thermograph. In another type of V instrument, first devised by W. Watson, the magnet is carried by a stretched wire or quartz fibre, the torsion in which opposes tilting caused by changes in V.