secondary forms

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

Original poster: "Mr Gregory Peters by way of Terry Fritz <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

I think all you people interested in secondary form materials should 
read Richard Quick's coilbld.txt available at www.pupman-dot-com. It 
describes coil forms in good detail. 

My first coil was a 4" diameter coil wound on 2/3" think cardboard. It 
worked fine up to about 2' of spark, but once I upped the kW, it burned 
down the centre as it was not sealed at either end. The replacement 
coil was made from 1/8" acrylic tube and produced far better sparks at 
the same power. 

Something else you may want to consider is the possibility that the 
conductive carbon particles present in black plastics may set up minute 
eddy currents. Though these would probably be almost insignificant, 
they may lose you a few tenths of a percent spark length for all you 
purists out there :)  

My 4" coil made following Richard's instructions has produced sparks 
almost 3 times the length on the secondary winding (36" sparks) without 
any breakdown. 

I really don't think the insulation properties of the form should be 
obsessed over, but I wouldn't want to use a semi-conductive former like 
black plastic or cardboard. Don't waste money buying acrylic forms 
though. PVC is fine if treated properly. It should be dried before use 
as it will definitely retain water if it is ever "wetted". This is 
simply because it is very porous, I once made a float for a pump from 
pvc pipes, with sealed end caps. These rapidly filled with water - 
through the walls of the pvc. 

As for coil coatings, I tested my good 6" coil with no coating 
whatsoever. It worked fine and has never broken down. I only coated it 
later to protect the windings from damage and to hold the wire in place.

Anyway, thats my two cents.

Cheers,
Greg Peters
Department of Earth Sciences,
University of Queensland, Australia
Phone: 0402 841 677
http://www.geocities-dot-com/gregjpeters