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In-air thermionic emission?

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Adam Munich

Thu Jun 16 2011, 06:03PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

If you watch this video closely...

...You'll notice that when the razor blade is cold, the arc starts at about 2cm (0:07). However, once the razor is glowing orange hot the arc starts much farther, perhaps 4cm. (0:40)

Could this be the result of in-air thermionic emission? If it's not then I have no idea what could be causing it.

Sulaiman

Thu Jun 16 2011, 06:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I think it's something in the air (ions etc.) more than the blade temperature
that's why forced-air is good for sgtc etc.

Inducktion

Thu Jun 16 2011, 06:31PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Isn't it because hot air conducts electricity a lot better than cold air? If the blade is hot, then the surrounding air should be hot too.

Adam Munich

Thu Jun 16 2011, 06:37PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Sulaiman wrote ...

I think it's something in the air (ions etc.) more than the blade temperature
that's why forced-air is good for sgtc etc.

That doesn't really explain why the arcs start from farther away when the blade s hot.

@induckion

Air doesn't conduct anything until it's ionized, so that's not the case.

LD LANGER

Thu Jun 16 2011, 07:51PM

Registered Member #3824 Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54

This may have useful info in it. Or not. Regardless, quite interesting if you can understand what exactly he's talking about.

Mattski

Thu Jun 16 2011, 08:13PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

I think the thermionic emission idea is viable. The idea behind air breakdown is that you have a high enough electric field that a free electron will accelerate so quickly that by the time it collides with an air molecule it has so much energy that it releases at least two additional electrons. So the number of free electrons increases with times and it's called an avalanche breakdown. But you need electrons in the air to start with or it doesn't work. So there are three mechanisms I'm familiar with which can cause these free electrons: cosmic rays (seriously, people who study fluorescent bulbs talk about them), field emission (electric field overcomes the work function of the metal in air), or thermionic emission (heat overcomes the work function of the metal in air).

If it is indeed thermionic emission which enables the wider spacing to strike the arc then you have a diode of sorts, just like a vacuum diode but in air. With AC it won't matter but if you rectify your transformer output then with the hot blade at positive potential you should get the arc struck at 4cm and if you switch it so the hot blade is negative you would go back to 2cm.

Marko

Fri Jun 17 2011, 06:01AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hot air does ionize much more easily than cold air, which I suspect is due to lower rate of recombination.
A candle flame is normally not conductive but if put into a spark gap it can increase the breakdown distance for twice or more.
I suspect having one electrode glowing hot helps the same way.

Even putting the flame much under the spark gap or using the hot air gun can yield observable difference.

Thermionic emission probably isn't the best explanation for this since liberated electrons can't travel very long in air (as they could in vacuum). And once air is ionized this doesn't matter anymore.

Marko

Bored Chemist

Fri Jun 17 2011, 10:22PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"A candle flame is normally not conductive"
Yes it is.

I think many people fail to realise that ions in air are fairly stable, you can blow then around for a few feet with a fan and they take up to a few seconds to recombine.

Proud Mary

Fri Jun 17 2011, 11:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here is a colour chart from which you can estimate the temperature of the glowing blade tip.

The sparks of steel burning in air which we see in the video suggest local temperatures of 1350Â°C or so.

Nah

Sat Jun 18 2011, 02:43PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Yes, BC is right.

This man here made a triode using a flame.

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