Determination of Susceptibility: Curie Balance Method

The specimen whose susceptibility is to be measured is kept inside porcelain bulb B. The bulb is attached to one end of a long light arm suspended by quartz fibre F from a torsion head T. The other end of the arm carries a scale pan P into which suitable weights can be put to obtain balance. A damper D is used to prevent the disturbing oscillations. A pointer P1 moving on a calibrated scale S measures the displacement produced. An inhomogeneous magnetic field is provided by the pole pieces N and S of an electromagnet. The poles are having plane ends and are kept inclined at an angle of 70o with respect to the axis so symmetry. The specimen enclosed in the bulb is kept in this magnetic field in space. When the magnetic field is switched on, the pointer gets displaced. The displacement can be measured correct to 0.001mm using a sensitive microscope. Measuring the displacement and knowing the elastic constant the suspension fibre, the force acting on the specimen can be estimated.

A force acts on the sample placed in a non-uniform magnetic field. If the specimen has a susceptibility χ₁ and in immersed in medium with susceptibility χ₂, the force
        F_x = (χ₁ – χ₂) μ0 VH_y dH_y /d_x
Where F_x is the force along X-axis, V the volume of the specimen, Hy the intensity of magnetic field perpendicular to X-axis (i.e., along Y-axis). The only appreciable component of the magnetic field is famous to be Hy and the appreciable rate of change to be dH_y /d_x.

The component Hy of the magnetic field is obtained by using a search coil and ballistic galvanometer. The value of Hy is determined for different value of x. The value of dHy/dx is found from the graph of H_y against x. Knowing F_x and V, the susceptibility χ₁ of the specimen can be estimated at any given temperature.

Curie’s method is also suitable for the measurement of the magnetic susceptibilities of paramagnetic liquids and gases.




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