Re: Capping secondary coil forms

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Greg,
          I don't bother to cap coils either although only several
I've built produce sparks exceeding the winding length to a moderate
degree. If the instantaneous top voltage is high enough, sparks are
most likely to first track down the windings, then to reach over the
top and track around the edges of the coilform. As it happens,
repetitive hammering of the discharge channel allows output
discharges to greatly exceed the length of the coil with peak
voltages below those required to cause arcs tracking over the former
or windings. For this reason, it is impossible to gauge output
voltage by spark length in a repetitively firing coil (there is a
sparklength/breakrate dependency which bears no relation to peak
output voltage). If the coil is short and producing voltages which do
threaten to track over its surfaces, capping and/or insulating disks
placed along the windings to increase creepage distance can be very
useful. My approach is not to bother with such extras and simply go
for longer coils.

Regards,
Malcolm

On 17 Dec 2002, at 13:00, Tesla list wrote:

 > Original poster: "Gregory Peters by way of Terry Fritz 
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
 >
 > Hello all,
 >
 > All these years I have been capping my secondary forms with
 > plexiglas/acyrlic or lexan/polycarbonate discs. This apparently
 > prevents arcs down the centre of the secondary. I have never
 > questioned this theory until now. I just made a new secondary and
 > really couldn't be bothered capping it, though I eventually did
 > anyway. But I really think that if the toroid is large enough, arcing
 > inside the form wouldn't happen anyway. I've noticed that arcs never
 > seem to leave the centre of the toroid, supporting my theory. Is this
 > just another theory that stems from the days of doorknob discharge
 > terminals? Anyone out there running uncapped coils with outputs that
 > vastly exceed the winding length?
 >
 > Cheers,
 >
 > Greg.
 >
 >
 >