Magnetic resonance, absorption or emission of electromagnetic radiation by electrons or atomic nuclei in response to the application of certain magnetic fields. The principles of magnetic resonance are applied in the laboratory to analyze the atomic and nuclear properties of matter. Electron-spin resonance (ESR) was first observed in 1944 by a Soviet physicist, Y.K. Zavoysky, in 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