1. R. V. Southwell:Relaxation Methods in Engineering Science (Oxford, 1940);Relaxation Methods in Theoretical Physics (Oxford, 1946). See also:G. Liebmann:Adv. in Electronics,2, 101 (1950).
2. G. Liebmann:Brit. Journ. Appl. Phys.,1, 92 (1950).
3. D. C. de Packh:Rev. Sci. Instr.,18, 798 (1947).
4. See for instanceV. E. Cosslett:Electron Optics (Oxford, 1950), p. 35.