A method of measuring magnetic susceptibility. The sample is suspended from a balance, with the bottom part of the sample between the poles of an electromagnet. When the magnetic field is switched on, the sample experiences a field gradient which causes an apparent change in weight. In particular, paramagnetic substances show an increase in weight, which, after correction for a smaller diamagnetic contribution, can be used to calculate the paramagnetic part of the susceptibility. This can be used to calculate the number of unimpaired electrons in the sample. Magnetic measurements of this type are widely used to investigate the electronic structures of metal complexes. The Evans balance is a portable version of the Gouy balance using permanent magnets and giving a direct readout. Other methods of measuring magnetic susceptibility include magnetic resonance techniques and the use of a SQUID (superconducting quantum interference device). The Gouy balance was suggested by Louis Georges Gouy (1854–1926) and subsequently used very extensively.