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Published online by Cambridge University Press: 24 October 2008
Some general considerations regarding the design of hot cathode X-ray tubes for producing an accurately constant and repeatable beam of X-rays are first given. The construction of a tube in accordance with these considerations is then described. This tube has been used to show that except with small tube-currents the quality of the vacuum in such a tube has little influence on the steadiness of the beam if the electrical input to the tube can be satisfactorily controlled.
It is shown that the X-radiation associated with the fluorescence often observed in X-ray tubes has usually but not always a negligible effect on the steadiness of the beam.
Some factors limiting tube current when the filament is enclosed in a box containing a focussing slit are mentioned, and the mechanism of formation of the focus is discussed with reference to its size. The use of an auxiliary electrode to increase the tube current and improve the focussing is also discussed.
* Proc. Roy. Soc. 137 (1932), 199.Google Scholar
† In a forthcoming number of the Journal of Scientific Instruments.
* Siegbahn, , Spektroskopie der Röntgenstrahlen, 2e Auflage, p. 80.Google Scholar
* M is copper soda-glass. When this part of the tube was made the technique for copper pyrex seals (used for E) had not been developed. For this technique see Skinner, and Burrow, , J. Sci. Inst. 7 (1930), 290.CrossRefGoogle Scholar
* Clay, , Proc. Phys. Soc. 40 (1928), 221CrossRefGoogle Scholar, found that in a Shearer gas tube under certain conditions the size of the focus was altered by a factor of more than five times when the gas pressure was altered by less than 50%. Now a Shearer tube can be run at a pressure of 6·10−3 mm. of Hg, and I have found it possible to run a tube as a hot cathode tube at a pressure twenty times smaller. Conditions are very different in the two types of tube, but the behaviour of the gas tube suggests that an appreciable effect is not unlikely to occur in other forms of hot cathode tube.
* See Harper, loc. cit.
† To be described in a forthcoming number of the Journal of Scientific Instruments.
* Siegbahn, loc. cit.
* Phil. Mag. 10 (1930), 600.Google Scholar