The physical basis

Abstract

A magnetic field is produced by the movement of an electrical charge. So, at the lowest level, the movement of an electron results in the creation of a magnetic field. Electrons will normally spin about their axes and also orbit their nucleus and therefore have two types of motion that can produce magnetic fields. All substances can thus be regarded as being magnetic at an atomic level and can be classified into two types. In diamagnetic substances the electron shells are full and the precession of electron orbits when placed in a magnetic field results in the creation of a magnetic field in the opposite direction to the applied field. The magnetization acquired per unit field applied, the magnetic susceptibility, is small, less than 10⁻⁵ SI. In substances in which the electron shells are incomplete, paramagnetic materials, each atom has a magnetic moment due to the uncompensated electron spins. When placed in a magnetic field, the electron orbits precess but the magnetic moment is aligned in the same direction as the applied field and is generally stronger than that of diamagnetic substances, having susceptibilities of the order of 10⁻³ to 10⁻⁵ SI.