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Re: Thoraited Tungsten Rods



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 02:44 PM 4/15/2007, Tesla list wrote:
Original poster: William Beaty <billb@xxxxxxxxxx>

On Thu, 12 Apr 2007, Tesla list wrote:

> The thoriated rods produce ionization in the air around the
> rod.

But is this a common myth?

No, it's not a myth.. the thorium emits particles which ionize (a very few) air molecules near the electrode. It doesn't change the breakdown voltage, per se, but makes the time required for breakdown more consistent (e.g. you don't have to wait for a stray cosmic ray to ionize that first atom in the gap to trigger the avalanche). If your gap voltage is anything other than a stiff DC supply, the reduction in timing jitter will be manifested as a reduction in breakdown voltage variability.

They do this in highly precise triggering systems as well (e.g. the Krytron has some radioactive Nickel in it for this purpose)

Then, there's the thoria/lanthia incandescence effect. All of these rare earth oxides have "nice" emission properties when they get hot.. Again, the idea in welding is to make sure there's plenty of ions hanging around in the gap when you apply the voltage (from the HF/HV source in the welder) the arc starts, right now.

They put cerium oxide in carbon arc electrodes and thorium oxide (thoria) in lantern mantles for this reason.. they are refractory and glow very white when hot.



   Do non-radioactive materials work almost as
well when put in tungsten?

yes from the incandescence standpoint, no from the ionizing radiation to stabilize the arc standpoint

   Don't forget that vacuum tube filaments
usually have an oxide coating to lower the Work Function at the surface,
which lets the surface spew out lots more electrons.   If that oxide
coating is not used, then the tube filament must be run at incandescent
yellow-orange temperature in order to produce enough electrons.

> This will cause the spark to jump easier.

Google sources say that many oxides do the same.

Perhaps the radioactivity has an effect.   But just the oxide alone
apparently has a huge effect.

(((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))
William J. Beaty                http://staff.washington.edu/wbeaty/
beaty@xxxxxxxxxxxxxxxxxxx       Research Engineer
billb@xxxxxxxxxx                UW Chem Dept,  Bagley Hall RM74
206-543-6195                    Box 351700, Seattle, WA 98195-1700