On the measurement of low magnetic susceptibility by an instrument of new type

It is well known in instruments of the Curie type that when the magnetising force impressed upon the substance is varied and the mechanical force is balanced against that of the torsion in a suspending fibre, the deflec­tions follow the square law and the range through which the magnetic force can be varied is restricted. In fact, it is difficult with the Curie balance to measure the susceptibility of a given specimen outside a narrow range of magnetic force of about one to five. One object in the design of the present instrument was to be able to measure susceptibility with a given specimen through a wide range of magnetic force. Another object was to make the instrument portable in the sense that no spot of light and scale are required, and yet be able to measure susceptibility through a wide range. In all the tests so far made the instrument was placed on an ordinary table. The fundamental idea is to replace the force due to torsion in a suspending fibre by either an electro-magnetic or electro-static system in which the mechanical force is due to two components—one proportional to the magnetic force impressed upon the specimen, and the other also proportional to the magnetic force in the case of constant susceptibility, but variable if the susceptibility varies. In the electro-magnetic case a moving coil can be employed, which is suspended in a magnetic field proportional in strength to the magnetic force acting upon the specimen. This is the method adopted in the present paper. It has the advantage that the absolute value of the susceptibility can be calculated from the known details of the instrument, and a considerable mechanical force can be produced. In the electro-static method we are concerned with the product of two voltages—one propor­tional to the magnetic force acting upon the specimen, and the other also proportional to this force if the susceptibility is constant, but other­wise varying with the susceptibility. This method has the advantage that the suspending fibre has only to carry a charge to the moving system, and not a definite current as in the electro-magnetic system. Also there is no question as to magnetic disturbance due to stray fields.