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Re: [Phys-L] Child's Law & the Limits on Scaling Arguments & Dimensional Analysis (long)

It is tempting to treat an ionization gauge as an inside-out triode, so instead of central cathode, then grid, then  anode, we would have central anode,  grid, and external cathode. But it is better to treat the central electrode as a cation collector, and the grid as a permeable anode with the filament/cathode on the outside. i.e. a sort of diode with an ion collectorin the enclosed volume of the wire grid anode.An  anion in an ionized gas is an electron which returns to the posiitively charged wire-grid anode - by definition. A cation (" + ion") is a postively charged particle which is carried to the negative collector, so that collector current is a measure of gas density The cite from bc makes this clear.
https://mmrc.caltech.edu/Vacuum/Teledyne%20Hastings/Ion%20Gauges.pdf

Brian On Tuesday, November 30, 2021, 03:05:49 AM CST, bernard cleyet via Phys-l <phys-l@mail.phys-l.org> wrote:



On 2021/Nov/29, at 10:35, David Bowman <David_Bowman@georgetowncollege.edu <mailto:David_Bowman@georgetowncollege.edu>> wrote:

  Unfortunately, essentially nobody builds vacuum tube diodes like this. Rather, they tend to use a cylindrical geometry with the much smaller cathode on the inside and a much larger anode on the outside. 



A common exception is a vacuum gauge.  Where the directly heated cathode outside a grid (+ HV to attract the emitted electrons.  Since a wire spiral many continue on and ionize the residual gas.  The + ions are coiled by the anode wire.  The resulting current is a function of the gas pressure. 


https://en.wikipedia.org/wiki/Hot-filament_ionization_gauge <https://en.wikipedia.org/wiki/Hot-filament_ionization_gauge>   

more:  https://mmrc.caltech.edu/Vacuum/Teledyne%20Hastings/Ion%20Gauges.pdf <https://mmrc.caltech.edu/Vacuum/Teledyne%20Hastings/Ion%20Gauges.pdf> 


bc  used another type in his research, the vac-ion pump.  The fore pump two sorption pumps. 
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