Magnetic Resonance

Abstract

Magnetic resonance is the name of a phenomenon discovered less than sixteen years ago, which from the start has had a high theoretical importance and is now attaining a notable practical valve. Nuclear magnetic resonance occurs when a substance containing magnetic nuclei is exposed to crossed magnetic fields, one being steady and the other oscillating, and the strength of the former field and the frequency of the latter are matched in a particular way. When these are properly matched, the nuclei are turned over in the steady field, and energy is absorbed from the oscillating field. Another way of describing the effect is to say that resonance occurs when the applied frequency is equal to the frequency of precession of the nuclei in the steady field. This phenomenon illustrates very clearly some of the fundamental laws of Nature. For the purposes of nuclear physics it is used to determine the magnetic moments of nuclei and their relaxation-times in the substance that contains them. It is also used for chemical analysis, for measurement of magnetic fields, for analysis of crystal structure and for locating changes of phase of the substance containing the nuclei. Magnetic resonance of electrons is similar, but for a fundamental reason is confined almost exclusively to free atoms of certain kinds, to ferromagnetic substances and to certain strongly paramagnetic salts. For these last it serves to throw light on the fields prevailing within the crystals.