In a Coolidge tube, the electrons are focused upon the target by a metal tube surrounding the filament. As this tube projects more and more beyond the filament the electrons come to a smaller and smaller focal spot. The magnitude of the electronic current from the filament increases slightly as the voltage is increased, but it increases very rapidly as the filament is made hotter. As the filament is heated still hotter the electronic current does not increase toward infinity but attains a maximum value. This saturation value may be as low as 40 mili-amperes.
Proceedings of the Iowa Academy of Science
©1923 Iowa Academy of Science, Inc.
Zumstein, R. V.
"Production of High Amperage in Low Voltage Coolidge Tube,"
Proceedings of the Iowa Academy of Science, 30(1), 80-80.
Available at: https://scholarworks.uni.edu/pias/vol30/iss1/23