The branch of science that describes the effects of the interactions between charges due to their motion and spin. These may appear in various forms, including electric currents and permanent magnets. The interactions are described in terms of the magnetic field, although the field hypothesis cannot be tested independently of the electrokinetic effects by which it is defined. The magnetic field complements the concept of the electrostatic field used to describe the potential energy between charges due to their relative positions. Special relativity theory relates the two, showing that magnetism is a relativistic modification of the electrostatic forces. The two together form the electromagnetic interactions which are propagated as electromagnetic waves, including light. They control the structure of materials at distances between the long-range gravitational actions and the short-range “strong” and “weak” forces most evident within the atomic nucleus.