A Faraday balance is a device for measuring

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

. Magnetic susceptibility is related to the force experienced by a substance in a

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

. Various practical devices are available for the measurement of susceptibility, which differ in the shape of the magnetic field and the way the force is measured. In the Faraday balance the field is homogeneous. The pole pieces of the magnet are so shaped that there is a region in which the product of the field strength and field gradient in the ''z'' direction is constant. The sample is placed in this region. The force in this case is independent of the packing of the sample and depends only on the total mass of the material present. The method is sensitive and highly reproducible and can be applied to single crystals. The force is measured as a weight change, using a

torsion balance A torsion spring is a spring that works by twisting its end along its axis; that is, a flexible elastic object that stores mechanical energy when it is twisted. When it is twisted, it exerts a torque in the opposite direction, proportional ...

. An alternative method for measuring

is the

Gouy balance The Gouy balance, invented by the French physicist Louis Georges Gouy, is a device for measuring the magnetic susceptibility of a sample. Background Amongst a wide range of interest in optics, Brownian motion, and experimental physics, Gouy als ...

. In this technique there is an inhomogeneous field in the central region between two (flat) poles of a permanent magnet, or an electromagnet. The sample, in the form of a powder in a cylindrical tube, is suspended in such a way the one end lies in the centre of the field and the other is effectively outside the magnetic field. Errors due to inefficient packing in the sample tube are difficult to eliminate.

References

Magnetic devices {{electromagnetism-stub