Memories of electrons in crystals derived could be combined in a single formula which was found over a wide tem perature range to agree well with the observed data The memories that I have presented so far are connected with my first and most important contribution to solid state theory. Rather than to extend them into my later work in this field, particularly that on ferromagnetism, I shall add an epilogue concerning my unsuccessful encounter with superconductivity. It started right after I had my Ph. D and returned to Zurich as Paulis assistant for the academic year 1928-9. Pauli thought that superconductivity was the only remaining matter of some interest in the theory of metals and that I should get on with it so as to be finally done with all these 'dirt-effects'. Actually, I had already started to think bout the problem and had realized that the explanation of persistent currents required a consideration of the previously neglected interaction between electrons The idea, independently expressed by Landau, was that it should thus be possible at low temperatures to obtain a minimum of the free energy in a state of the metal with finite current. My own confidence in the idea was supported by the analogy with ferromagnetism whereby I saw a parallelism between permanent magnetiza tion below the Curie point and persistent current below the critical temperature I therefore started industriously to consider various types of interaction, regardless of their possible origin, and to look whether the Schrodinger equation would allow stationary states of the electrons with non-vanishing current and a minimum of the Once in a while i thought that i had indeed found such states but it never took Pauli long to point to some error in the calculations. While he did not object to my approach he became rather annoyed at my continued failure to come out with the desired answer to such a simple question. It finally turned out that there was a quite general reason for my lack of success. Assuming that a given state could be varied by letting the momentum of each electron increase by the same infinitesimal amount, I found that the corresponding differential change of the energy was proportional to the current with the consequence that an extremum of the former always led to a vanishing value of the latter. Now that one knows about the formation of Cooper pairs and the long-range order, so clearly manifested in flux quantization, it is easy to see that the assumption was unjustified. It took a far deeper insight into the nature of a superconductor than was available at that time, however, to understand why it is here essential not to treat the momentum in a system of macroscopic dimensions as a continuously variable quantity. Indeed I was so discouraged by my negative result that I saw no further way to progress and for a considerable time there was for me only the dubious satisfaction to see that others, without noticing it, kept on falling into the same trap. This brought me to the facetious statement that all theories of superconductivity can be dis proved, later quoted in the more radical form of ' Bloch's theorem: Super conductivity is impossible'l After the fog, which so long enveloped the phenomenon, had begun to lift many years later, I could not resist reminding Pauli that the problem was not quite as easy to solve as he thought when he gave it to me. Since that time he had become more mellow -so much more, in fact, that he agreedMemories of electrons in crystals 27 derived could be combined in a single formula which was found over a wide tem￾perature range to agree well with the observed data. The memories that I have presented so far are connected with my first and most important contribution to solid state theory. Rather than to extend them into my later work in this field, particularly that on ferromagnetism, I shall add an epilogue concerning my unsuccessful encounter with superconductivity. It started right after I had my Ph.D. and returned to Zurich as Pauli's assistant for the academic year 1928-9. Pauli thought that superconductivity was the only remaining matter of some interest in the theory of metals and that I should get on with it so as to be finally done with all these 'dirt-effects'. Actually, I had already started to think about the problem and had realized that the explanation of persistent currents required a consideration of the previously neglected interaction between electrons. The idea, independently expressed by Landau, was that it should thus be possible at low temperatures to obtain a minimum of the free energy in a state of the metal with finite current. My own confidence in the idea was supported by the analogy with ferromagnetism whereby I saw a parallelism between permanent magnetiza￾tion below the Curie point and persistent current below the critical temperature. I therefore started industriously to consider various types of interaction, regardless of their possible origin, and to look whether the Schrodinger equation would allow stationary states of the electrons with non-vanishing current and a minimum of the energy. Once in a while I thought that I had indeed found such states but it never took Pauli long to point to some error in the calculations. While he did not object to my approach he became rather annoyed at my continued failure to come out with the desired answer to such a simple question. It finally turned out that there was a quite general reason for my lack of success. Assuming that a given state could be varied by letting the momentum of each electron increase by the same infinitesimal amount, I found that the corresponding differential change of the energy was proportional to the current with the consequence that an extremum of the former always led to a vanishing value of the latter. Now that one knows about the formation of Cooper pairs and the long-range order, so clearly manifested in flux quantization, it is el1sy to see that the assumption was unjustified. It took a far deeper insight into the nature of a superconductor than was available at that time, however, to understand why it is here essential not to treat the momentum in a system of macroscopic dimensions as a continuously variable quantity. Indeed, I was so discouraged by my negative result that I saw no further way to progress and for a considerable time there was for me only the dubious satisfaction to see that others, without noticing it, kept on falling into the same trap. This brought me to the facetious statement that all theories of superconductivity can be dis￾proved, later quoted in the more radical form of 'Bloch's theorem': 'Super￾conductivity is impossible' ! After the fog, which so long enveloped the phenomenon, had begun to lift many years later, I could not resist reminding Pauli that the problem was not quite as easy to solve as he thought when he gave it to me. Since that time he had become more mellow - so much more, in fact, that he agreed