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Re: Air insulation

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Hi Richard and All,
With the discussions about insulators (oil fill and such) I was going to
mention that adding a dielectric does not always reduce the arcing when
high voltage is present. I had an overcoupled coil that I tried to
'insulate' the primary from the secondary with a roll of polypropylene. It
seemed to make the situation worse. I did this back before joining this
discussion and know little about the nature of what was going on. It
appears that the electric field will concentrate in materials with higher
dielectric constants (K). Like adding magnetic materials to increase
magnetic coupling, dielectrics add to electric coupling or concentration.
If it is the jump in K then maybe we need intermediate K values to allow a
gradual change in K. Like the coating on a camera lens. Maybe this would
work in capacitors as well.
Here I go again, thinking aloud. 8?)
Dave
----------
> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject: Air insulation
> Date: Thursday, October 17, 1996 12:05 AM
> 
> >From hullr-at-whitlock-dot-comWed Oct 16 22:37:18 1996
> Date: Wed, 16 Oct 1996 11:08:54 -0700
> From: Richard Hull <hullr-at-whitlock-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Air insulation
> 
> All,
> 
> Air is a pretty good and a pretty bad insulator.  Air is free and 
> naturally present so to use it, we only need design our materials so 
> there is an adequate distance separating the two high voltage points in 
> question.  All other insulators must be secured through purchase and cut 
> to fit an application.
> 
> Air is a gas or, more appropriately, a mixture of elemental gases.  The 
> bulk of our air is nitrogen with a small amount of oxygen thrown it to 
> help us breathe.  Argon is the only ohter gas worth noting and is presnet

> to about 1%.  This mixture is genrally assumed to have a stand- off 
> voltage of 50 volts per mil.(.001").  This is grossly affected by 
> moisture (we do have water vapor in the air also.)  As most water vapor 
> is a distilled form and hard, it might be considered for the most part 
> non-ioninc and therefore non-conducting (pure water is a superb 
> insulator).  The snag comes when the water accumulates on a surface and 
> becomes macroscopic (liquid) it is just itchin' to load up with ions!
> Thi is when water becomes a detrement to HV insulation.  Air transports 
> harmless water which condenses on solids to become an ionic medium.
> 
> Gases have a varying voltage breakdowns based on pressure and this is of 
> little concern to us for insulating purposes, for when we use air, it is 
> fixed at a normal, but slightly varying pressure of around 760mm of 
> mercury.
> 
> The really neat thing about air is it has a dielectric constant of about 
> 1 (unity) or virually equivalent to the vacuum lof outerspace!!  This is 
> nice and is airs one great saving grace as an insulator.
> 
> In the study of dielectrics, high voltage, and the like, it is quickly 
> realized that some of the best solid insulators have rather higher 
> dielectric constants than air. (k= 2-6).  It is also quickly learned that

> with a given field voltage, the field intensity grows to horrendous 
> levels at the interfaces of dielectrics of varying dielectric constant.  
> Have you ever seen sparks crawl along a sheet of mylar in air?  I have!  
> the contact of two dielectrics of differing values is a collection point 
> for intense fields!
> 
> The lower the dielectic constant the less the field gradient becomes for 
> a given voltage.  Thus, avoid close quartered high differential 
> dielectric constant materials and you won't see sparks seeming to prefer 
> to zip along this corridor, rather than leap out into air.  Why should 
> they?!  They are happy staying where the fields are intense.  Therefore, 
> the best way to use air insulation is to allow for large radius of 
> curavature metallic charged surfaces to contact air directly without 
> other insulation.
> 
> Richard Hull, TCBOR