A Third of a Century of Paramagnetic Relaxation and Resonance

Abstract

The reason that I have selected the title A third of a century of paramagnetic relaxation and resonance is, as most of you know, that Gorter made the first successful experiments on paramagnetic resonance and dispersion in 1936. As an alternative title I considered A third of a century of magnetic resonance and non-resonance, for paramagnetic relaxation is not usually considered a resonance phenomenon. On the other hand, a pure mathematician might argue that it is a special case of resonance in which the resonance frequency is zero. The important thing is that the dynamic paramagnetic susceptibility is different from the static one, and Gorter’s pioneer experiments verified that this was indeed the case. Fifteen years earlier, in 1921, Gregory Breit, while on a visiting fellowship at the University of Leiden, tried unsuccessfully to find a dependence of the magnetic susceptibility on frequency, as at about that time Lenz and Ehrenfest had predicted the existence of such an effect. No doubt most of you will say that the reason Breit could not detect any relaxation phenomena was that he was a theoretical physicist trying to do experimental work, but actually the explanation was poor, insensitive apparatus.

Except for minor changes (and a slight amplification) this is the script of an after-dinner talk which I gave at the Symposium Dinner to a mixed audience. It does not pretend to be a highly technical or authoritatively documented history of the subject matter. For this reason no references to the literature are given. I have been helped greatly by conversation with Professor Purcell.