Magnetic Work in India 1

Abstract

THIS relates to the temperature coefficients of horizontal force rnagnetographs of the Watson pattern belonging to the Indian Survey. The magnet system consists of magnetised iron wires, fixed parallel to one another, in a framework which is attached to a quartz fibre. The upper end of the fibre is attached to a torsion head, by turning which the magnet system is brought nearly perpendicular to the magnetic meridian. With increase of H (horizontal force) the magnet turns until the increred torsion balances the increased magnetic couple, and the position of the magnet is recorded photographically by means of a beam of light reflected from a mirror attached to the magnet. With rise of temperature the magnetic moment diminishes slightly, whilst the rigidity of the quartz increases, both causes tending to diminish the angle of torsion and so simulating a fall in H. Measurements made on one of the magnetographs prior to its despatch from England showed a temperature coefficient of approximately 6γ for 1° C. (1γ ≡1 × 10–5 C.G.S.); but the values obtained in India with different magnet systems and suspensions are mostly about 12.5γ for 1° C. There is, however (see footnote p. 13), no necessary contradiction between these results. The untwisting caused by a given rise of temperature varies as the total angle of torsion, and this varies as the local value of H. But H in India is nearly twice as large as in England. Thus the movement of the magnet due to the change of rigidity in the fibre caused by a rise of 1° is nearly twice as big in India as in England. The memoir discusses the temperature experiments made in India, and the difficulties arising from imperfect temperature control, defects in quartz fibres or in the method of fixing them, and from other causes. The observational data are recorded, and exhibited in the curves, with a detail which is unusual in a printed volume. The values found for the temperature coefficients in India are five times larger than those applicable in England to some magnetographs of older types with metal suspensions. Even in magnetic chambers under refined temperature control, a small temperature coefficient has advantages which can be fully appreciated only by those experienced in the reduction of magnetic data. Thus the results of the present memoir, though of limited general interest, deserve the attention of instrument makers.