Re: Very High Mutal inductance

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

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Erik, All!

>If so the secondary could be immersed into a large bath of oil.  This
would
>prevent arcing along the secondary and allow higher coupling before
>dielectric breakdown.


I know that many, many years ago Tesla coils were indeed operated in
oil baths, but I'm not aware of anyone repeating this in recent years.
I think the main purpose was purely for insulation's sake, given that
the insulators of the day back then were things like gutta percha and
shellac-soaked cotton etc which were maybe not so effective as modern
"enamel".  If you did try it, it would be interesting to have a modern
view of how well this works.  Probably very good, but a bit messy and
inconvenient.

However, increasing the coupling beyond a certain point (around
15-20%, YMMV) is unavoidably going to increase the logarithmic
decrement and broaden the spectrum of the "emissions" which suggests a
high probability of decreased efficiency, because the secondary
current will drop.  However, before things get that bad, the frequency
spectrum splits into two humps (it will with any significant coupling)
and with a bit of juggling you can detune primary and secondary to
reinforce one hump at the expense of the other, and this provides a
current maximum.  I have scanned graphs from an old book which show
this effect:

http://home.freeuk-dot-net/dunckx/wireless/inductive/inductive.html

The relevant bit is maybe two thirds the way down the page.

As ever, there are exceptions to this, I think Antonio posted details
of a magnifier configuration where very much closer coupling (60%+)
can be used to good effect and with results essentially as you have
said in your post, but then magnifiers often use tight coupling
between primary and coupling coil.  The blurb above refers to the
conventional two coil setup.  An oil bath for a magnifier might be a
good possibility, though a bit heavy.

Dunckx
Geek#1113 (G-1)