An Evans balance, also known as a Johnson-Matthey balance(after the most prolific producer of the Evans balance) is a device for measuring magnetic susceptibility. Magnetic susceptibility is related to the force experienced by a substance in a magnetic field. 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. The Evans balance employs a similar sample configuration, but measures the force on the magnet.