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Re: [TCML] Arc Temperature




-----Original Message-----
>From: Tim Meehan <btmeehan@xxxxxxxxx>
>Sent: Dec 3, 2007 10:46 PM
>To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>Subject: Re: [TCML] Arc Temperature
>
>I would imagine that the temperature is hard to measure.  How accurately do
>you need to know the temperature?  If you don't really need the temperature
>exactly, you could make the (poor) assumption that it is a "blackbody"
>radiator, bluish-white is about 7000k (probably much too high).  For
>comparison, the melting point of steel is around 1600k.
>
>If you get a reference on how it is actually measured - I'd be curious to
>know ...

It's measured spectroscopically...

Two ways.. one is the black body emitter method..

But, a more accurate method is to measure the spreading of a spectral line emitted by one of the atoms (e.g. Nitrogen).  The higher the temperature, the higher the speed of the atoms. The higher the speed of the atoms, the more doppler shift there is. Since not all the atoms are going in the same direction, you get a distribution of doppler shifts ranging from atoms moving right towards the observer  to those moving away.  There's a relationship that tells you for X broadening of the line, the temperature must be Y.

This is how they measure the temperature within lightning strokes, for instance.

You can also probe the plasma column with a laser, but I'm less certain how that method works.

Yet another technique is to do a thermal balance (you know that the radiative transfer goes as the fourth power of temperature, etc.)

The little book by Somerville, "The Electric Arc" talks about this in some detail, but is hard to get a hold of.

Uman's book on lightning is highly recommended, and describes a lot of spark/arc physics.  The edition from Dover is cheap (<$20) and worth every penny.

Cobine's book "Gaseous Conductors" is a good reference if you can get it. It was published as an inexpensive Dover edition, but I think it's out of print.  Cobine has everything that was in Somerville.


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