Opportunities missed and apportunities seized satisfying explanations of the electrical and thermal conductivities and the hall effect both at room temperatures and at low temperatures, it had not had the same success in dealing with the magneto-resistance effect the increase in the electrical resistance produced by a uniform magnetic field. Various theories had indeed been put forward, but they had not stood up to detailed criticism. Simplifying assump tions had to be made to permit a rudimentary start on these and other problems nd it was not easy to light upon assumptions that were sufficiently plausible It was at this stage that I first became interested in the theory of metals, largely because P. Kapitza, who was then working in Cambridge, published a number of papers on the electrical resistance of bismuth crystals in strong magnetic fields (1929). In these pioneering experiments Kapitza used magnetic fields of up to 200000G(20T)and worked at temperatures down to liquid air, and he came to the conclusion that the change in resistance was proportional to the magnetic field but that this linear behaviour was masked in weak fields by disturbances in the metal that are equivalent to that produced by an inner magnetic field. I had not got very far in tackling this problem when I was extremely fortunate in receiving the offer of a Rockefeller Travelling Fellowship, and I decided to spend the best part of a year in Leipzig, interspersed with shorter stays in Copenhagen I arrived in Leipzig in the first week in January 1931, and Heisenberg immediately pressed me into giving a colloquium on magnetic effects in metals, remarking Peierls work is undoubtedly important, but the mathematics is complex and the physical ideas not easy to disentangle. We ought to have a thorough discussion of the whole subject, and, as you have more free time than the rest of us, would you agree to give the first talk? 'I had studied Peierls's papers and had not found them too easy, and though I thought I had understood them well enough for my own purposes, to give a colloquium on them, and in German, was a formidable task. I had to find ways of shortening and simplifying the arguments. I was unsuccessful for a couple of weeks, but just at the end of January it suddenly occurred to me that the Bloch-Peierls theories could be enormously simplified and made intuitively more plausible if one assumed that quasi-free electrons, like valency electrons in single atoms, could form either open or closed shells Bloch's theory had in fact proved too much. Before his paper appeared it was difficult to understand the existence of metals. Afterwards it was the existence of insulators that required explanation. This problem had been by-passed because it had been taken for granted that insulators were merely bad conductors rather than non-conductors, the difference between metals and insulators being a quantitative and not a qualitative one. This concept I now challenged I went to see Heisenberg the next day and told him that I wanted to change and broaden the subject for the colloquium which was scheduled to take place two weeks later. I expounded my thoughts to Heisenberg who grasped the significance of them at once and fetched in Bloch from the adjoining room to take part in the discussion. Bloch was highly sceptical and stuck to the view that insulators were substances for which the overlap integral(g, h)(defined in(7)above)was negligiblyOppor tniti甜刑臼sed apportuniti seized 45 satisfying explanations of the electrical and hermal conductivities and the Hall effect both at room temperatures and at low temperatur咽， it not had the same success in dealing wi the neto -resista effect he incre in the electrical resistance produced by a uniform magnetic field. Various 仙。ories had indeed been pu forward but they not stood up to detailed criticism mplifying sump tions had to be made to pefffi rudimentary 的时也 on these a. nd 。也her problems, and no to li gh upon su mptions that were 8ufficiently plausible It was 抽出is stage hat 曲曲 became interested in the theory of metals, largely because P. Kapitza, who was hen working in Cambridge, published a number of paper on the electrical resistance of bismu crys也叫 in strong magnetic elds (1929). In these pioneering experiments Kapitza used magnetic fields of up to 200000 G (20 T) and worked "'mp atures down to uid air，创ld he came he conclu on that the change in re糊的ance was proportional to magnetic field, but that is Iinear behaviour masked in weak fields by disturbances in metal that are 呵旧valent to 也hat produced by an inner magnetic field 1 had not got very far in tackling his problem when 1 extremely fortunate in receiving the offer of a Rockefeller Travelling Fellowship, and 1 decided to spend the best of a year in Leipz 毡， 阳回persecl with shorter stays in Copenh en 1 arrived in Leipzig in he firs week in January 1931 , and Heisenberg immedia.tely pce ed me in giving a colloquium on magnetic effects in metals, remarking 'Peierls' work is undoubtedly importa. 毛， but t he mathema.tics is complex and he physical ideas not to disentangle Ne ough也 to have a thorough discu ion of whole subject, and, as you have mo free tiroe than the rest of us, would you agree to give the firs talk?' 1 had studied Peierls's papers and had no found hem oe :y and though hough ha.d understood them well enough for my own purp田田， to give a. colloquium on hem and in German, formida.ble task. 1 had to find ways of shortening a.nd simplifying the arguments. 1 unsuccessful for a. couple of weeks, bu乞 )u，也 he end of a. nu时'Y'也 suddenly occurred to me that Bloch-Peierls theories could be enormously simplified and made intuitively more plausible if one assumed that quasi-free electrons, like valency electrons in single a.toms, couJd form ei er open or closed sheJls Bloch's theory ha.d in fa.ct proved too much. Before his paper appe ed it was d; cult to understand exÎstence of metals. Af rwa. rds the existence of insulators tha requu唱d expla.nation. This problem had been by-passed because i