Experiments on salts of the iron group of elements. ESR has made possible the study of such phenomena as the structural defects that give certain crystals their colour, the formation and destruction of free radicals in liquid and solid samples, the behaviour of free or conduction electrons in metals, and the properties of metastable states (excited states that are long-lived because energy transfer from them by radiation does not occur) in molecular crystals. A particle of matter that is spinning about its own axis or moving in an orbit around some external point acts like a gyroscope: it resists forces that tend to change its state of motion. The measure of this resistance is the mechanical angular momentum, which depends on the mass of the particle, its size or that of its orbit, and the angular velocity (the number of revolutions per unit time). The angular momentum is represented by a vector directed along the axis of rotation. An electric charge in such motion creates a magnetic field with strength and direction represented by a magnetic vector denoted μ. This vector, which is proportional to the magnitude of the charge (instead of the mass of a particle), measures the tendency of the charge’s axis of rotation to align itself in the direction of an external magnetic field. The motion of a particle that has both mass and charge is characterized by both of these vectors, which will be collinear but may be oppositely directed, depending on the sign of the charge.