An alternative method for measuring magnetic susceptibility is the Gouy balance. In this technique there is an inhomogeneous magnetic field in the central region between two (flat) poles of a magnet, either a permanent magnet or an electromagnet. The sample must be in powder form and is placed in a cylindrical tube, which is then suspended in such a way that one end lies in the centre of the field and the other is effectively outside the magnetic field. However, errors caused by inefficient packing in the sample tube are difficult to eliminate.