The magnetic susceptibility of some binary alloys

Abstract

The object of this research has been to find a connection between the mass susceptibility and chemical composition of a number of binary alloys of several metals. The susceptibility measurements were made with the aid of a Curie and Cheneveau magnetic balance, the permanent magnet of which produced a field with an intensity of about 640 gauss. It was found early in the work that the method of attachment of the torsion wire to the balance beam and to the torsion head was not satisfactory; consequently the hook and loop arrangement was replaced by a small clamp and screw, which was attached to the beam of the balance and the torsion head. Since the torsion wire was very thin it became necessary, in order to fasten it rigidly at both ends, to face the jaws of the clamps with thin lead foil. This arrangement ensured a perfectly satisfactory attachment of the torsion wire. The torsion wires used were of platinum 0.10 mm. diameter. To obtain concordant readings it was found necessary to anneal the wires before use. This was achieved by suspending the wire under a slight stress and passing a slowly increasing current of electricity through it, over a period of 30 minutes, until the temperature reached 150°C. When this temperature was reached, the current was maintained steady for 30 minutes and then slowly reduced to zero, a further 30 minutes being taken for the reduction. During the heating the wire was seen to untwist, so that the effect of annealing appears to be the removal of a twist imparted to the wire during drawing. Annealed torsion wires gave reproducible deflections which, on the whole, were greater than those obtained with unannealed wires. The experiments to be described were made with materials of low susceptibility, and consequently the difference between the deflections for the two positions of the permanent magnet was rather small. To increase the accuracy of the readings, a method of oscillation was adopted. The magnet was placed in one of the positions of maximum deflection and the balance arm with its load allowed to oscillate freely. After a few minutes the oscillation had died down sufficiently for the turning points of a spot of light, reflected from a mirror on the balance arm, to be read easily on a millimetre scale situated 200 cms. from the mirror. Five successive turning points were taken, three on one side of the zero and two on the other side, from which the position of rest was calculated. This position was obtained five times for each position of the magnet, and the mean values for each position accepted as the deflection due to the specimen. Adoption of this practice enabled the deflections to be read to 0.05 mm. The readings obtained in this way were identical with those obtained by allowing the beam to come to rest, and they were obtained much more rapidly.