Recollections of solid state physics phenomena, but since these results depended strongly on the state of purity of the substance they seemed too complicated for a basic theoretical approach What I looked at was the Frenkel exciton, as it is now called, though I believe I had not read Frenkel's paper I was particularly puzzled by the origin of the non radiative decay of such excitations, which involved the transfer of energy to the phonons. One was accustomed to treating such situations by perturbation theory, treating anharmonic effects as small, and on that basis it seemed impossible to understand the decay, since the energy of the electronic excitation was much greater than the maximum phonon energy, so that the decay required the creation of many phonons in a single event. It took some trouble to appreciate that the forces on the excited atom would be appreciably different from those on a normal atom,even regards the equilibrium configuration, so that a perturbation expansion in terms of displacements from a common equilibrium made no sense. The situation was similar to the familiar Franck-Condon treatment for molecules, except that one was dealing with much more numerous degrees of freedom One other worry concerned the magnetoresistance in metals. This was troubled that all the mple theories, such as So gave a negligible effect. Similarly, an attempt by Bloch to invoke oic ener ots amagnetic spin reversal of some electrons, which alters the distribution of kine gave an even smaller result. The other problem was that Kapitza's experim gave, for many metals, a resistance which, over a long range, was linear in the magnetic field, whereas the usual theories always gave an A2 law. Thinking about the field dependence I noticed that the usual approximation of expanding in powers of H should break down when the Larmor period became shorter than the collision time, and that for very high fields the resistivity should probably tend to a constant limit. This argument therefore predicted a point of inflexion, which was going some way towards Kapitza's linear law. I was proud of this finding, and announced a talk on the subject at a conference in Leipzig in 1930. 9) To my embarrassment, I discovered on the eve of my lecture that, in the model I was using, the coefficient of my pleasing magnetoresistance law was zero. The presentation of my talk was thereby made rather difficult. I found later that the fault was in using an isotropic model in which, crudely bility, and that anisotr surface, for example, would cause differences in mobility, and hence a substantial magnetoresistance. 20)This had also been found by Bethe, and i do not remember clearly how far my ideas had been inspired by his paper. One still could not see why the magnetoresistance in the alkalis was of the same order as in other metals, although their Fermi surface was suspected, and has since been proved, to be very accurately spherical. I am not sure who first pointed out that the anisotropy of the phonon spectrum(which is anisotropic even for long waves in a cubic crystal) was responsible Landau had visited Zurich when his results on free-electron diamagnetism were new. According to his theory the diamagnetism would compensate one-third of theRecollections of solid state physics 35 phenomena, but since these results depended strongly on the state of purity of the substance they seemed too complicated for a basic theoretical approach. What I looked at was the Frenkel exciton, as it is now called, though I believe I had not read Frenkel's paper. I was particularly puzzled by the origin of the non￾radiative decay of such excitations, which involved the transfer of energy to the phonons. One was accustomed to treating such situations by perturbation theory, treating anharmonic effects as small, and on that basis it seemed impossible to understand the decay, since the energy of the electronic excitation was much greater than the maximum phonon energy, so that the decay required the creation of many phonons in a single event. It took some trouble to appreciate that the forces on the excited atom would be appreciably different from those on a normal atom, even as regards the equilibrium configuration, so that a perturbation expansion in terms of displacements from a common equilibrium made no sense. The situation was similar to the familiar Franck-Condon treatment for molecules, except that one was dealing with much more numerous degrees of freedom. One other worry concerned the magnetoresistance in metals. This was troubled by two difficulties. One was that all the simple theories, such as Sommerfeld's, gave a negligible effect. Similarly, an attempt by Bloch to invoke the paramagnetic spin reversal of some electrons, which alters the distribution of kinetic energies, gave an even smaller result. The other problem was that Kapitza's experiments gave, for many metals, a resistance which, over a long range, was linear in the magnetic field, whereas the usual theories always gave an H 2law. Thinking about the field dependence I noticed that the usual approximation of expanding in powers of H should break down when the Larmor period became shorter than the collision time, and that for very high fields the resistivity should probably tend to a constant limit. This argument therefore predicted a point of inflexion, which was going some way towards Kapitza's linear law. I was proud of this finding, and announced a talk on the subject at a conference in Leipzig in 1930.(19) To my embarrassment, I discovered on the eve of my lecture that, in the model I was using, the coefficient of my pleasing magnetoresistance law was zero. The presentation of my talk was thereby made rather difficult. I found later that the fault was in using an isotropic model in which, crudely speaking, all electrons had the same mobility, and that anisotropy of the Fermi surface, for example, would cause differences in mobility, and hence a substantial magnetoresistance.(20) This had also been found by Bethe, and I do not remember clearly how far my ideas had been inspired by his paper. One still could not see why the magnetoresistance in the alkalis was of the same order as in other metals, although their Fermi surface was suspected, and has since been proved, to be very accurately spherical. I am not sure who first pointed out that the anisotropy of the phonon spectrum (which is anisotropic even for long waves in a cubic crystal) was responsible. Landau had visited Zurich when his results on free-electron diamagnetism were new. According to his theory the diamagnetism would compensate one-third of the 2·2