Recollections of solid state physics inversely proportional to the velocity. Since even in a degenerate Fermi gas there is a small spread of velocities of conducting electrons around the Fermi velocity he could obtain a small spread of collision times, and hence a small magneto- resistance, several orders of magnitude less than the observed effect He followed the classical ideas by thinking of the electrons as free, except for collisions with atoms(therefore the magnitude of the mean free path required to explain the observed conductivity, even larger than in the classical picture because of the greater velocity, was another paradox ). He also ignored the mutualinteraction of the electrons. This had always been done in the classical theory, electron- electron encounters conserve the total electron momentum hence the total current and thus do not contribute to the resistance. I do not recall Sommerfeld mentioning and there was no reason why he should have taken a different pointon reatments this argument explicitly, but it is always clearly stated in the classical As a very junior theoretician I listened to Sommerfeld's exposition and was duly mpressed, but was not yet at the stage of criticizing or questioning the assumptions It was characteristic of Sommerfeld's positive attitude that one learnt more about the successful solution of difficulties than about the mysteries that remained. He was completely fair in listing the contradictions-it was just a matter of emphasis When at Easter 1928 Sommerfeld left for a sabbatical year, I joined Heisenbergs group at Leipzig, where Felix Bloch had just completed his treatment of electrons in periodic potentials(a)and his explanation of the order of magnitude and tempera ture dependence of the resistivity in terms of lattice vibrations. Bloch also did not orry about the electron-electron interaction- I do not know whether he recalled the old arguments that it should not matter, or whether he was simply content to extend the theory by taking in one more factor which had previously been ignored without necessarily including everything My first substantial research assignment in leipzig was to see whether Bloch's starting-point of independent electrons(electron orbitals we would say today was unavoidable, and how far one could get if one started from a Heitler-London model (which Heisenberg was about this time applying to ferromagnetism). Today it would be obvious to any undergraduate that there could not be any conductivity in the Heitler-London model unless it is supplemented by ionized states, in which ome atoms have more, and others fewer, than their normal complement of electrons But at the time this conclusion was not obvious to me, and evidently not to Heisen- berg. It took some struggle with exchange integrals for a many-body system before I concluded that, at least in the linear chain I was using as a model, the only way a current could be obtained was by all electrons making a quantum jump simul taneously, and for a macroscopic dimension this makes therate astronomically small In the summer of 1928 I was fairly sure about this conclusion. I spent the summer vacation in England, mostly as a tourist, but I visited Cambridge and called on Dirac, whom I had met. He introduced me to R. H. Fowler. When Fowler heard I was from Leipzig, he asked me to talk to the Kapitza Club about bloch's work. At that time neither my English nor my command of physics was really adequate forRecollections of solid state physics 29 inversely proportional to the velocity. Since even in a degenerate Fermi gas there is a small spread of velocities of conducting electrons around the Fermi velocity, he could obtain a small spread of collision times, and hence a small magneto￾resistance, several orders of magnitude less than the observed effect. He followed the classical ideas by thinking of the electrons as free, except for collisions with atoms (therefore the magnitude of the mean free path required to explain the observed conductivity, even larger than in the classical picture because of the greater velocity, was another paradox). He also ignored the mutual interaction of the electrons. This had always been done in the classical theory, since electron￾electron encounters conserve the total electron momentum, hence the total current, and thus do not contribute to theresistance. I do not recall Sommerfeld mentioning this argument explicitly, but it is always clearly stated in the classical treatments, and there was no reason why he should have taken a different point of view. As a very junior theoretician I listened to Sommerfeld's exposition and was duly impressed, but was not yet at the stage of criticizing or questioning the assumptions. It was characteristic of Sommerfeld's positive attitude that one learnt more about the successful solution of difficulties than about the mysteries that remained. He was completely fair in listing the contradictions - it was just a matter of emphasis. When at Easter 1928 Sommerfeld left for a sabbatical year, I joined Heisenberg's group at Leipzig, where Felix Bloch had just completed his treatment of electrons in periodic potentials(3) and his explanation of the order of magnitude and tempera￾ture dependence of the resistivity in terms of lattice vibrations. Bloch also did not worry about the electron-electron interaction - I do not know whether he recalled the old arguments that it should not matter, or whether he was simply content to extend the theory by taking in one more factor which had previously been ignored, without necessarily including everything. My first substantial research assignment in Leipzig was to see whether Bloch's starting-point of independent electrons (' electron orbitals' we would say today) was unavoidable, and how far one could get if one started from a Heitler-London model (which Heisenberg was about this time applying to ferromagnetism). Today it would be obvious to any undergraduate that there could not be any conductivity in the Heitler-London model unless it is supplemented by ionized states, in which some atoms have more, and others fewer, than their normal complement of electrons. But at the time this conclusion was not obvious to me, and evidently not to Heisen￾berg. It took some struggle with exchange integrals for a many-body system before I concluded that, at least in the linear chain I was using as a model, the only way a current could be obtained was by all electrons making a quantum jump simul￾taneously, and for a macroscopic dimension this makes the rate astronomically small. In the summer of 1928 I was fairly sure about this conclusion. I spent the summer vacation in England, mostly as a tourist, but I visited Cambridge and called on Dirac, whom I had met. He introduced me to R. H. Fowler. When Fowler heard I was from Leipzig, he asked me to talk to the Kapitza Club about Bloch's work. At that time neither my English nor my command of physics was really adequate for