Re: mini coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jim Lux" <jimlux@xxxxxxxxxxxxx>


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, December 30, 2004 10:02 AM
Subject: Re: mini coil


> Original poster: Finn Hammer <f-h@xxxx>
>
> My thinking on this subject:
>
> Plastic is heated by RF, when it is subjected to an electrical field, not
> to a magnetic field. This means when it is used as a dielectric to
separate
> potentials, not just because it is sitting inside a coil.

Where there is an alternating electric field there is an alternating
magnetic field, and vice versa.

However, there's a difference between dielectric loss (an E field effect,
predominantly) and breakdown (also an E field effect).

>
> Look at the secondary former: The electrical field is lengthwise on the
> coil, means, one end grounded, the other alternating at extreme potential.
> So: what we really ought to worry about is a short coil, since it will be
> stressed more than a long one, potentials equal. However, the field
> strength is orders of magnitude lower than the breakdown field strength,
> measured along the length of the coil. Radially outwards, from coil center
> to surroundings, the field is even, so there is no dielectric action in
> that direction.
> A couple of numbers:
> Dielectric strength of PVC: ~300V/mill -> 300.000 V/Inch -> a 20 inch slab
> of PVC is good for 6MV, but a 20 inch long tesla coil probably only
> produces about 300-400kV.

True enough, if you had a big block of dielectric. For most situations, the
voltage breakdown along the surface is much lower than the breakdown in
bulk, and is mostly determined by the breakdown properties of the
surrounding medium (air, in this case).  So, you should really be looking at
air's breakdown at 30kV/cm (71 kV/inch).. A good rule of thumb is to assume
that the breakdown along a surface is 1/3 that of free space, so really, the
breakdown voltage along a 20 inch PVC tube is probably around 400 kV.