University of Wisconsin physicist Raymond George Herb testing the controls of his invention named "Long Tank" a 4 million volt Van de Graaff electrostatic particle accelerator (March 10, 1943). A month later, his machine was shipped to Los Alamos for nuclear fission experiments involving bombarding plutonium-239 with 1 MeV fast neutron beams. Invented in 1929 by R.J.Van de Graaff, the electrostatic accelerator used a very simple principle to build a large potential on a sphere. Electrons or positive ions carrying a small electric potential were fed onto a moving belt that carried them to a sphere, where the charge accumulated until several million volts were built up. The potential of the sphere was limited only by the breakdown of the insulating medium surrounding it. Ray Herb's innovation was to develop a pressurized container that could hold a non-conducting gas at high pressure around the sphere, making possible the accumulation of much higher potentials. With this refinement, the Van de Graaff generator replaced most other direct-voltage generators previously used for particle acceleration. His pressurized Van de Graaff delivered a steady parallel beam of particles, free from stray radiation, which made it an ideal source for nuclear studies in the range of energies for fast neutron research. Its unusually homogeneous beam energy could be focused on a small target and varied at will, so nuclear processes could be studied as a function of bombarding energy. The device was copied at a number of American research institutions, much of the quantitative data on the nuclear properties of elements came from his machines in the late 1930s.