>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!!!!)
** Interesting dissertation, Bill. What is the explanation for an arc
in a near vacuum producting a stentorian Bang outside the envelope?
>
>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___-------------------------------------------------------------------
>------------------------------------
>
>
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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