I would like to talk about arcs....
The two most common discharges are:
Glow discharge ( the blue glow in the partial vacuum inside a tube)
Arc discharge ( BANG!!!!)
A glow discharge takes place where the density of the atoms in the partial
vacuum is such that an
electron can gain enough kinetic energy between leaving an atom and
striking another to make the
second atom eject an electron that stands a good chance to do the same. The
electrical potential
between the atom that has provided the electron and the one that is struck
by it must exceed the
first ionization potential which is less than 15 electron volts for most
gases. The second ionization potential
is usually about 2 or 3 times that of the first.
Oddly enough there are two ways to prevent a discharge.
1. Have so few atoms or molecules in the partial vacuum that the chances of
an electron ejected from
one strikes a second before the electron gets to its destination even
though it has gained enough
energy to eject one or more electrons from the impact with second atom or
molecule.
2. Make the density of the gas such that the chances are small that the
ejected electrons have
can travel thru enough potential difference to gain the kinetic energy to
eject electrons from impacting
another atom or molecule.
So either high pressure or high vacuum can prevent discharges.
Usually an arc occurs where the electron can gain a kinetic energy greater
than the second ionization
potential. In which case the atom struck ejects 2 electrons and you get an
avalanche of electrons
flowing. Here is the problem.
It is kind of a chicken and egg problem. How does an arc start in a vacuum?
I suspect that some electron flow heats up a very small bit of metal
somewhere (hotspot?) and vaporizes it.
Then the metal vapor is the material that allows the arc to form. The first
ionization potential of most
metals is almost half that of gases. What I am calling gasses are those
elements that are in
gaseous state at atmospheric pressures and room temperature.
I have to ask some atomic/molecular physicists about this one.
See what they know..
73
Bill wa4lav
At 06:51 PM 2/11/2003 -0800, 2 wrote:
> >2 wrote:
> >>
> >>
> >>>2 wrote:
> >>>>>High enough to initiate a plate supply short through the tube?
> >>>>
> >>>>If the anode-grid path shorted, there would be an arc-mark on the grid.
> >>>>I have not seen one in a grid-fil shorted tube -- nor have I found a
> >>>>shorted tube that was gassy. I doubt that Mr. Rauch's disappearing gas
> >>>>theory is possible without direct intervention from the Fairy Godmother.
> >>>>
> >>>Since you persistently refuse to understand how a getter works, or to
> >>>accept that arcs can happen in tubes that appear perfectly good, you're
> >>>unlikely to find much evidence to change your mind.
> >>>
> >>Ian -- Please explain how a gassy, shorted 3-500Z is gettered between its
> >>removal from an amplifier and its being tested for gas with a high-pot a
> >>minute or so later?
> >
> >We keep going around this argument in cycles of a few months; and every
> >time you act as if nobody had ever explained all this before. I am only
> >explaining it this time for the sake of any new arrivals.
SNIP___-------------------------------------------------------------------------------------------------------
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