H E need, much emphasized by Rutherford (1927), for copious sources of charged bodies similar in energy to the particles emitted from naturally radioactive materials remains an active requirement in nuclear physics and other fields. I n general, it is desirable not only that such particles be available in adequate amounts and with sufficient energy to penetrate the atomic nucleus, but also that they be homogeneous and steady in energy and that they emerge from the apparatus in which they are produced in a parallel beam with little accompanping stray radiation. hloreover, it should be feasible to measure the voltage of such particles with great accuracy and to vary it readily over a U ide range. I n the energy range up to 10 million volts, which is sufficient for investigating many of the fundamental phenomena of nuclei throughout the periodic table, these rather rigorous requirements for precise nuclear research can be met with directness and simplicity by the combination of an electrostatic generator and an accelerating tube. Examples of such combinations are those described by Van de Graaff (1931)) Van de Graaff, Compton and Van Atta (1933), Tuve, Hafstad and Dah1 (1935), and Herb, Parkinson and Kerst (1937. For those nuclear investigations where energies of higher ranges are required, more indirect methods of acceleration must be employed, as in the case of the cyclotron, betatron and synchrotron.T h e electrostatic generator and accelerating tube can likewise be employed with advantage in non-nuclear applications. For radiation therapy, penetrating roentgen rays characterized by high skin tolerance, and particularly effective for the irradiation of deep tumours, can be produced by compact generators operating at several million volts (Trump and Van de Graaff, 1939 ;Trump and Cloud, 1943). For industrial radiography, the intensity and penetrating nature of such radiation increases radiographic speed, definition, sensitivity, and latitude in the examination of heavy metal sections, as has been shown by Van de Graaff, Buechner, Feshbach, Sperduto, Burrill, and McIntosh (1 946). For this radiographic application, the constant voltage makes possible the concentration of the electron beam on an exceedingly small target focal spot with subsequent point-source emission, Such steady controlled sources of charged particles amenable to accurate application and measurement are indicated for many physical, biological, and photochemical investigations.