Blackett and Wilson had been studying... they had put a lead plate across the centre of the expansion chamber and had been investigating the large showers created by the impact of energetic particles. The chamber was automatic. A single particle going through a series of Geiger counters would trigger the whole mechanism and it was this technique which I inherited in Manchester. Now, there was... a great dispute arose because Blackett and Wilson had published a number of papers and they claimed their results did not agree with the conventional theory of the impact of high energy electrons and that the conventional theory was breaking down at high energies.

Now, I should remind you that the Bethe and Heitler had in those years developed the cascade theory in which a high energy particle from outer space would enter the atmosphere, and by a series of paired productions, by collision with atoms would create positive and negative electrons, which in turn would create photons and then the photon would collide with another electron pair. And, therefore, you had a great shower of positive and negative electrons emerging from the impact of a single particle. In the atmosphere, but also in the lead plate in the cloud chamber. So Blackett said, would I design an electron magnet. He had transported his usual electron magnet to Manchester, but he wanted me to build a smaller electron magnet to surround my cloud chamber and repeat these measurements or extend the measurements which he had been making with Blackett and Wilson. So I went to Metropolitan-Vickers, designed an electron magnet and eventually surrounded my cloud chamber with this electromagnet and happily settled down to investigating.