Re: additional transformers [rolled caps]

From: 	richard hull[SMTP:rhull-at-richmond.infi-dot-net]
Sent: 	Thursday, July 31, 1997 8:30 PM
To: 	Tesla List
Subject: 	Re: additional transformers [rolled caps]

At 08:27 AM 7/31/97 -0500, you wrote:
>From: 	Peter Electric[SMTP:elekessy-at-macquarie.matra-dot-com.au]
>Reply To: 	elekessy-at-macquarie.matra-dot-com.au
>Sent: 	Thursday, July 31, 1997 6:15 AM
>To: 	Tesla List
>Subject: 	Re: additional transformers [rolled caps]
>Tesla List wrote:
>> From:   richard hull[SMTP:rhull-at-richmond.infi-dot-net]
>> Sent:   Wednesday, July 30, 1997 1:27 PM
>> To:     Tesla List
>> Subject:        Re: additional transformers
>> >Just though I'd throw in my two bobs worth after having blown up a few
>> >rolled poly caps!
>> >
>> >All of mine failed at the edges (even being under oil) so I reckon the
>> >secret is to put big overlaps between the plastic and the Al plates i.e.
>> >at least an inch or more. Also here the thick poly sheet is bloody
>> >expensive! so use multiple sheets of 8 mil concreters plastic,
>> >preferably the clear stuff but if all else fails use black plastic with
>> >engine oil! It ran ok on my coil for more than 3 months.
>> >
>> >Cheers,
>> >
>> >Peter E.
>> >
>> >Peter,
>> I recommended leaving a full 2" edge separation metal to edge of dielectric
>> in all rolled capacitors!
>> Richard Hull, TCBOR
>Thanks for the info on that, I will go 2" on my future overlaps. It
>seems strange to me that sparks seem to travel much further along the
>surface of an insulator, no matter how clean it seems to be. Maybe this
>is due to microscopic impurities on the surface of the plastic?
>Another little trick that you probably have already figured out is to
>insulate the input connections to the plates by rolling a few layers of
>poly around them.
>Cheers, Peter E.

The effect you note is an old one and is purely elecrostatic in nature.
Current will readily flow along differing dielectric boundry edges in the
presnce of rapidly changing voltage gradients.  The sharper and more angular
they are the more likeily a flashover might occur.  The classic example is
the plane, flat, dielectric surface with a knife edge lying flat on it
(plate and insulator sheet)  Amazing arc lengths can be supported at
relatively low voltages provided the dv/dt is steep enough.

Richard Hull, TCBOR