A theory is proposed that accounts for the variations in the observed focal spot patterns on the anodes of x‐ray tubes caused by changes in the geometry of the cathode structures. The focusing of electrons emitted by a helical filament within a focal cup can be explained qualitatively by elementary electrostatics. Data are presented to show the variations in size and energy distribution within the focal spots when changes are made in the dimensions of the focal cup mouth, throat, filament diameter and the mounting depth of the filament in the trough of the focal cup.

Topics
Vacuum tubes, Electrostatics
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© 1937 American Institute of Physics.
1937
American Institute of Physics
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