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Re: Temporary electrodes for rotary

Jeffery:
	I've used plain brass acorn nuts, plain copper, nickel plated
and chrome plated acorn nuts in my quest for "reasonable" discharge
points, and I have to say that brass were the cleanest of all four.

They all worked, but the nickel and chrome nuts quickly coated
the inside walls of the polycarbonate box with a residue that
I have to clean off from time to time in fear of creating a
conductive path (unsightly too).

The plain copper gave the most variable performance depending
on the amount of erosion and pitting with about the least amount
of "box deposition".  The points corroded green pretty quickly
after a few uses though so I won't use pure copper anymore.

The brass nuts were by far the least expensive and in general the
most predictable in performance regardless of the level of
erosion (even to pretty extreme), but will eventually create
the box deposition layer. It seems to take a while (maybe just not
as dark a layer).  They were reasonable to clean up or just toss
depending on my mood at the moment.  They may take a moment to
get initial sparkover when corroded, but almost immediately settle
in and run ok.

The nickel and chrome nuts take the longest to show significant
erosion, but the surface pitting and corrosion deposits directly
on the nuts are the hardest to clean up.  The chrome nuts are
outrageously expensive though, and I've taken to using them only
for "showcase" demonstrations since I know they seem to give the
most predictable short-term performance.

Also, in my case, the rotary spark gap points (on the disk) are
always the same material regardless of the fixed discharge points.

I've been using hardened steel dowel wrist pins that are available
in bulk from almost any machinist supply house.  They are very
inexpensive (under $0.50 ea) and come in (almost) perfect uniform
lengths .. no cutting or polishing or anything, just stick them in
or toss them if they get too worn.

The reality is that I am still using the exact same dowel set for
more than a year now, and I've run more than 100 hours during that
time at power levels ranging from 500W up to 2KW peaks with the
average around 1.2KW.  There are very minimal signs of erosion so
far.

I've repolished or replaced the brass acorn nuts three times but
have been using chrome nuts for the last 20 hours or so. They appear
to be ok at the moment (but I need to clean the box again!).

The very cleanest operation were with brass fixed points and brass
rotating points, but the disk balance was too hard to maintain when
I changed the nuts on the disk so I gave brass up (on the disk).

I have some tungsten rod to test out, but just haven't had time
to put together a fixture to "give them a spin" so to speak.
The tungsten is very expensive and hard to cut to uniform
lengths, but do-able with a carbide cutter and a little sanding.

I expect that these will make "lifetime" points, but the biggest
problem (I feel) is the tungsten dowels are not really large
enough in diameter (0.100") for the instantaneous currents.

DC driven TC's can have large peak currents, upwards of 4000A from
these particular mica impulse caps I'm using.  I suspect that I will
have to contend with excessive heating, which will not be a problem
for the tungsten, but the polycarbonate disk I use won't be happy.

Also, the small diameter rod will change the "sparkover" profile,
perhaps improving the quenching (not a problem now) and
substantially decreasing the dwell time (which is also not really
a problem either, although at low speeds (a few RPS) there is a
tendency for there to be multiple jumps during alignment as if
I were using a static gap.. Probably happen with the tungsten gaps
too).  I can't say whether the changes will be good or bad, but the
peak current transfer is a critical problem beyond all other
potential dubious improvements.

The smaller rod size would allow me to increase the number of points
from 8 to 16, but I have to convince myself that the added heat
distribution will compensate for the higher point temperature (and
potential undesirable current limiting).

I hope this helps!

ko

----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Temporary electrodes for rotory
> Date: Saturday, March 04, 2000 11:03 AM
> 
> > Original Poster: "Jeffery Pederson" <kd4lyh-at-webtv-dot-net> 
> >  
> >  Which is better nickle plated brass or just brass acorn nuts for
> >  temporary electrodes in a rotory.
> >  Regards
> >