Oh, dear Jon (how often does a gent get to begin a letter with "dear
Jon" eh?, our poor dear junior engineer.
Some of your questions do embarrass those of us who think highly of
college education. One would have thought the immediate and volumnous
response you got from the first time would have answered your question,
but perhaps another try is worth it.
>
>OK,
>
>Thanks to those who responded to my seemingly dumb question about why
no
>carbonization marks exist around tubes which supposedly had an arc-over
>in them. Specifically this is in regards to the bent filament helix
>phenomena in the 3-500Z's.
>
>So here's a follow up question:
>
>Several people pointed out that you wouldn't get carbon scoring from an
>arc because the tube is evacuated and therefore there is nothing that
>would cause carbonization (OK, maybe that's not the scientific
>explanation, but you know what I mean). However, if there were no gas
of
>any kind inside the tube, then how did the arc occur?
Arcs do not depend on the existence of a gas. Do you believe you could
never get an arc in outer space, for instance? Please refer, if you
will, to your sophomore physics book. In a vacuum, with adequate
potential between two conductors, the electromagnetic forces ultimately
cause particles to traverse the vacuum gap from one conductor to
another. Thus, in a vacuum, and arc causes material to transfer. This
is the physics behind sputtering, which, by the way, does not produce
rough particulate material deposition, but actually produces very
uniform film deposition, which is why sputtering is used to coat disk
drive surfaces, and put metal films on glass for LCD computer screens.
However, as you point out, in the presence of a gas, something different
occurs. You ionize the gas, and as the gas de-ionizes, it produces
energy, usually visible light, thus the traditional purple glow in a
gassy tube. Again, sophomore physics. Taking one more lesson from
sophomore physics, the source of the gas is the metal in the tube
itself. Metals are very good at absorbing gas - that's why most glass
tubes and some metal tubes have a "getter". In the presence of gas, the
ionization of the gas can cause a plasma, which reduces the ultimate
breakdown between the two conductors - thus a "gas arc" which means that
the arc occurs at lower voltage than were the gas absent.
Unfortunately, metals are also porous (Rich, I know you'll argue this
point, but it is true - just as glass is a fluid, as demonstrated by the
shape of stained glass windows 150 years old that are thicker at the
bottom - just because it's a slow fluid or a slow porosity does not mean
they're not fluid or porous), and gas can seep in after years of non-use
of a tube (heat tends to force out gas from metals). By the way, the
ionization of gas is not by any means the same as an arc. But it does
provide a path making an arc easier to create. And yes, to get
carbonization in an arc, one must have carbon. I know of no elemental
gasses (those are what a metal can absorb) which contain carbon. And,
yes, powdered metal tends to be black, but usually because it is
oxidized, but sometimes merely because the tiny particles don't reflect
light well, not being polished. But again, in a vacuum or in a very
low-pressure gas, an arc causes sputtering, which transfers metal in a
film-like manner, not as tiny particles.
>Again, it may sound like I am asking old or dumb questions, but I am
>relatively new to this group (couple months) and haven't heard all the
>answers before. And as I pointed out before, I am not an expert on
tubes.
Being new to the group may explain lack of expertise on tubes, but it
does not explain absence of knowledge of undergraduate physics.
Sorry to be so rough, Jon, but ... I've spent a good bit of my career
attempting to prove to the powers that be that our higher educational
system is going to the dogs and...well...fifteen years ago no junior
could have asked the questions you ask, so I believe my point is
demonstrated, although not proven. So I've an emotional stake here, but
not attached to tubes, merely to the evident failures of higher
education.
>
>73,
>
>Jon
>KE9NA
>
>
>-------------------------------------
>Jon Ogden
>KE9NA
>
>http://www.qsl.net/ke9na
>
>
>"A life lived in fear is a life half lived."
>
Not learning what you're exposed to creates ignorance. Nearly all fear
comes from ignorance. Please Jon, continue to learn, but please also do
not take anything on here as either true or even evidence. Insist on
textual references, especially the fundamental physics parts.
Arlen
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