A major technical advance was the further development of a method of recording the

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A major technical advance was the further development of a method of recording the internal potential of a cell by pushing the very fine (0.001mm) tip of a saline-filled glass tube through the surface membrane, so that it could be used for recording short-lived action potentials as well as the steady resting potential. These experiments were mostly done in collaboration with Richard Keynes and were continued by him, establishing many features of the movements of ions into and out of nerve fibres.In the intervals between the summer seasons at Plymouth, Hodgkin did other important work at Cambridge. To my surprise, these "Hodgkin-Huxley equations" have survived with relatively little modification, though at the time I thought that they were very provisional and would soon be superseded.Shortly after the war, Hodgkin had started experiments with radioactive tracers to follow movements of ions in nerve fibres. We obtained a satisfactory fit to our observations on the basis of these "Hodgkin-Huxley equations" and calculated from them the time course and the velocity of an action potential to be expected; these agreed satisfactorily with those recorded experimentally.These results were published in five papers in 1952, and led to the award to us, jointly with Sir John Eccles, of the Nobel Prize in Physiology or Medicine in 1963. We separated the components of current carried by sodium and by potassium ions, and we fitted their time courses with equations that seemed plausible if the currents were carried by the ions diffusing through "gates" in the membrane which were opened or closed by changes of membrane potential.

He and Katz started work with it and I joined them later, obtaining results generally similar to Cole's.We improved the apparatus before the 1949 season, when Hodgkin and I did our final series of experiments, in which we recorded the effect of altering the external sodium concentration. He learnt from Cole about this experiment and in turn told Cole of the experiments with Katz on the effects of low sodium concentration. Hodgkin's voltage clamp equipment was ready for the Plymouth season of 1948. Hodgkin realised that the instability would be avoided if a wire were pushed down inside a giant nerve fibre and used to draw off the current carried inwards by sodium ions. This could be achieved with feedback amplifier, an arrangement that came to be known as voltage clamp. Cole had the same idea and was the first to have such an equipment running, in 1947, but he made only limited use of it, showing only that current through the membrane did indeed vary continuously with potential, with a region in which the relationship would be unstable if the feedback were not operating.Hodgkin visited the US again in the spring of 1948. As the internal potential was raised the permeability increase would allow entry of sodium ions whose positive charge would raise the internal potential still further, so that the situation would be unstable and the internal potential would rise explosively, causing the all-or-none character of the action potential.The instability also makes it difficult for an experimenter to control the situation so as to investigate the causation of the permeability change.

This was shown to be the correct explanation by Hodgkin, together with Bernard Katz, in the summer of 1947.The experiments before the war in which Hodgkin had seen local responses had led him to believe that the increase of permeability of the membrane was graded with the amount of the change of internal potential. The increase of permeability had been confirmed by Cole's experiment, but the overshoot remained a puzzle.With hindsight, Hodgkin and I later felt that we had been stupid not to have seen at once that it would be expected if the increase of permeability was specific for sodium ions: these would diffuse inwards because they are much more concentrated outside than inside the fibre and would carry their positive charge inwards. The action potential was thought to be caused by a sudden increase of permeability of the membrane to all kinds of ions, making a short-circuit so that the internal potential would rise towards that of the external solution but no further. Initially, he was on equipment for night fighters but was transferred to work for bombers, first for target location and later on gun control.Like most of the scientists engaged on these projects, Hodgkin was strongly opposed to the bombing of open cities, which was the principal aim of Churchill and his adviser Lord Cherwell; persuasion from Patrick Blackett and others succeeded in getting a few of the radars designed for locating ground targets diverted to antisubmarine purposes, where they had a decisive effect in reducing our shipping losses.Hodgkin returned to Cambridge and research on nerve in the autumn of 1945 and I joined him at the beginning of 1946 Our first question was the origin of the overshoot. Most of Hodgkin's work for the rest of the war was the development of airborne equipment working at 9cm and later 3cm, including much flying to test the equipment. He was already skilled in electronics but learnt much more that was valuable in his post-war research.