The gyro-magnetic ratio for magnetite and cobalt

Abstract

In a recent paper a null method of determining the gyro-magnetic ratio was described, and results were given for iron, nickel and the Heusler alloys. The mean value for the three metals was found to be 0·502, with an average variation of 0·004 from the mean. The present communication extends the results to cobalt and magnetite. In this method, which was described fully in the above-mentioned paper, the specimen is suspended vertically along the axis of a solenoid, and magnetised by alternating current of the same frequency as that natural to the suspended system. Apart from disturbing couples, which can be reduced to very small dimensions, the torsional oscillations produced are due only to the angular momentum resulting from the changing magnetism of the specimen. The system is brought to rest by the current, induced by the changing magnetism, in a small “induction” helix coaxial with the specimen. This current passes through a pair of Helmholtz coils in series with an adjustable resistance, and acts on small permanent magnets rigidly fixed to an aluminium wire attached to the lower end of the specimen. When the gyro-magnetic effect is completely neutralised, we have U/M = 4 πncϕ /S i where U = the angular momentum resulting from a change M of magnetic moment, n = the number of turns per centimetre on the “induction” helix, c = torsion constant of suspension, S = neutralising resistance in the circuit of the induction helix necessary to bring the system to rest, i = steady current which when passed through the neutralisation coils produces a deflection ϕ of the permanent magnets.