The story of the development of high–voltage nuclear transmutation equipment in the Cavendish Laboratory and its use in l932 in J.D. Cockcroft and E.T.S. Walton's disintegration of lithium by protons has been told many times.1 The pparatus, shown schematically in figure 1, was based on a transformer–rectifier–condenser multiplying circuit generating a steady potential of up to about 600 kV. Its success in the transmutation experiments was due largely to the penetration of the Coulomb barrier of the light target nuclei by the incident protons in accordance with the calculations of Gamow.2 Extension of the disintegration work to heavier nuclei using a Cockcroft–Walton generator required higher voltages and by 1934, when many results had been or were about to be published, there was every enthusiasm for continuing high–voltage development. The steps that were taken in this direction in the Cavendish from 1935 until the beginning of the war in 1939 have not been so fully described, although the main events have been recorded.3 The present article attempts to trace the expansion of the high–voltage laboratory over that four–year period, during which the author worked in the laboratory under M.L.E. Oliphant4 and P.I. Dee.5 It is a shortened version of a longer account which has been placed in the Churchill Archives Centre6 together with a number of relevant documents held since 1939 by the author. In writing, use has been made of the author's personal notebooks and diary kept during the period covered.