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Re: IONS/vacuum pedagogy



A good comment always invites questioning. Joe wrote:

If you mean space charge limited currents in a vacuum tube,
I have generally thought that resulted from the geometry of
the tube and the large emission current. Since the filament
has a small area compared to the collector, the electron
density in the space around the cathode is much higher than
near the anode.

A diode with a flat cathode (two ribbons forming a parallel
plates capacitors) would not, according to this explanation,
have a current which grows gradually when the positive plate
potential is being increased. Its i=f(V) plot would be practically
vertical after V exceeds the value of the work function. Do
experimental data support this prediction? Vacuum diodes
with nearly vertical transitions (from zero to a saturation
current) would be desirable before semiconductors.

I wish I had facilities to check the effect of the diode geometry
on the slope of the i(V) curve. This was certainly done by many
people in the past. What were the results? Did they show that
diodes with flat cathodes have no space charges due to geometry?
Or that diodes whose anodes are thin wires (parallel to hot
ribbons) have clouds of electrons around cold wires?

Ludwik Kowalski