The limitations imposed by space charges on the separation of ions in the usual magnetic mass spectrograph and the possibility of trapping electrons in the ion beam are described. It is found that high voltages and intense magnetic fields are required for moderate ion currents unless these are neutralized. Calculations are given on velocity modulated or interrupted ion beams and the performance of a modulated separator is described. The theory of a radial magnetic separator is given in some detail and an experimental arrangement of such a separator proved more successful than the separator employing modulation. Some ion sources and suggested improvements are described.