Re: Tesla Coil RF interference (humidity)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 05:22 PM 5/26/2004 -0600, you wrote:
>Original poster: Brett Miller <brmtesla2-at-yahoo-dot-com>
>David,
>
>Yes, I have noticed the same consistency in arc
>lenghth over different heat and humidity levels.
>If there is an effect on tesla coils, it is one that
>is difficult to measure, and certainly not noticable
>with the naked eye.  So as far as the effect of
>humidity on the arcs we love to see, I would say
>almost significant.
>
>-Brett


Given that the breakdown field for air doesn't change a whole lot with 
humidity, I wouldn't expect to see a big change.  Terry made some 
measurements showing that the Q (loss) of the secondary might change with 
humidity (particularly with cardboard tubes that happen to have some sort 
of salt in them), and that might change the output power of the coil.

There might also be a change due to hot air being less dense (and, for that 
matter, humid air is less dense than dry air), since breakdown voltage is 
mostly a function of density.

Spark propagation is weird though, particularly in a Tesla coil.  Imagine 
that you are running your coil in a gas with half the breakdown strength of 
air.  You've got several conflicting, interacting effects. The initial 
spark starts easier, but that drains charge off the topload faster, which 
reduces the peak voltage, which reduces the overall electric field trying 
to push the leader out.

I think that spark extinction time (i.e. the spark channel cooling enough 
to become nonconductive) probably doesn't vary signficantly (in a spark 
propagation sense) with small changes in air temperature. You're talking 
about something cooling from 7000K  or so, and whether it's cooling by 
conduction to something at 40 degrees C or 0 degrees C won't make a heck of 
a lot of difference.  The primary cooling mechanism at that temperature is 
going to be radiation anyway (that T^4 thing...).  And even if it did cool, 
it's cooling from the outside in, so it just makes the channel get 
skinnier, but still conductive.