I’m fact checking (maybe I should call it a sanity check.)
[list=1][li] A high vacuum (something in the e-9 Torr range) will have a lower resistance than free space, right? I’m pumping from air for an electron gun, if that matters.[/li]
Is Torr short for anything? For some reason, I keep thinking of Torrington, but that’s a type of bearing.[/list=1]
As to (1), does “free space” mean the average interstellar or intergalactic space, or some other level? I’m just not familiar with the terms you’re using, so probably couldn’t help anyway.
For (2), torr refers to Evangelista(?) Torricelli, a scientist who experimented and discovered things about air pressure and therefore got something named after him. One Torr = 1 mm Hg
Free space is a perfect vacuum, so a high vacuum will have lower resistance than free space; just imagine charged particles flying around and producing a convection current. The word Torr, is just the short hand name for Torricelli, the guy who measured atmospheric pressure with a column of mercury. Each millimeter of height for the mercury column corresponds to one torr. Since one atmospheric pressure pushed the mercury up by 760 millimeters, it was defined to be equal to 760 torrs of pressure. Torr is not a metric unit of pressure, so there has been some effort to replace it with mbars instead, whose value is very close to torr.
To elaborate…
Since free space has no particles to move charge around, it will have infinite resistance. Air at 10^-9 torr, though, isn’t much different since all the things floating around are neutral. You need charged things (ions, electrons) to carry a current. It’s true that there will be a teeny tiny number of molecules that are ionized at any one time, but if it’s just plain, room temperature air this number is exceedingly small. This nitpick is mainly to emphasize that the stuff much be charged. You could envision a chamber with cold, completely un-ionized helium atoms at low pressure. There are things floating around, but they will carry no current because they are all neutral, so the resistance will be infinite.
All of this is ohmic, V=IR style resistance. That is, if you put 5 volts across the chamber, you’d get no current. As soon as you get to really high voltages, though (like the ones in your electron gun), you have to start worrying about breaking down the air. (Perhaps this is what you were asking about?) This is not really a resistance since a given voltage does not produce a given current and since it is a chaotic and unstable phenomenon. The electric field due to the big voltage is high enough to ionize the molecules, resulting in a sudden disharge through the medium – it arcs. The less stuff there is to ionize, the harder it is to arc. (Of course, arcing can happen in a vacuum, too. The electric field just needs to be high enough to rip electons right off the negative electrode; there are no handy molecules to ionize and carry the current for you.)
-P
Just want to make sure - you are not running high voltages while you pump down, are you? Air at 1-torr range has surprisingly low ‘resistance’ - I’d hesitate to use anything over 50V. Once you reach the 10[sup]-9[/sup] torr range I wouldn’t worry about using thousands of volts, unless the voltage appears over an extremely narrow gap.
[This article](One source of information I found useful) and references within may be helpful.
Sorry, this is the link I meant to post.
Thanks to all. I know I heard the name Torricelli before. I promise this time to remember it as long as this link is archived. :rolleyes:
FWIW, I’m fact checking a SEM manual, parts of which were quite frankly hacked together (ok, I’m tearing it apart.) One of its many errors had that a partial vacuum had a lower resistance than a complete vacuum. It sounded like complete bullshit to me, so much so that I wanted a second opinion before I said as much. Of course, the vacuum isn’t really for resistance. You just don’t want anything obstructing or shorting the electron beam or ruining the source. The closer you get to a complete vacuum, the better (I hope I’m not coming across as a wise guy; I’ve been reading and mulling this over all night.)
FWIW (for anyone wondering), here’s a good basic overview of SEM operation. I stumbled across it this morning.
Scr4, the system is interlocked to keep people from running it before the vacuum is at least reasonably ok. Of course, what Mark Twain said about making things foolproof applies here as well.
Quoth Pasta:
Unless, of course, you count the vacuum polarization effect… I think that that could support a slight current.
Chronos said:
I would think that vacuum polarization would only effect the vacuum’s dielectric strength, not its conductivity, since the induced particle/hole pairs are still stuck together.