Re: Voltage determination

["Finn Hammer" <f-hammer-at-post5.tele.dk> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Rscopper, all:

The description below, describing your method of controlling the rotary,
with increasing gap length, including extra static gaps, prompts me to
write this little piece, to sum up my own experience with
SRSG`s
I relate this to my own experience, based on the "Milennium Lace" gap
driving my own 6 kW coil with a 20 kV transformer and Kurt Schraner`s
"Black and White" 10 kW coil with ~15 kV transformers.

In both instances, the electrodes are set as close as I feel safe,
around 1 mm apart, (40 thousands of an inch), and there are 2 gaps in
series, each 8mm in diametre, (5/16 "). 300 BPS at home, 200 BPS at
Kurt`s

I can only guess at the reason why you have to set the electrodes
further apart, and add static gaps too, but my gap has hemispherical
ended electrodes, and I think this is the best shape, since it will
create a more uniform field, and provide a higher breakdown voltage, in
combination with the presumably higly turbulent airflow between the
electrodes.

I have uploaded a picture of one fixed electrode from this gap here:

http://home5.inet.tele.dk/f-hammer/SRSGrod.jpg

I get a couple of usefull clues from this picture:
8mm rod is more than ample at this power level, the arc channel only
leaves a 3mm diametre track on the electrodes.
Mechanical dwell is of no consequence: the gap has quenced _well before_
the electrodes even _allign_. (And this is at a lazy 5th. notch quench).

One important thing to watch is that in a multible bang per half wawe
setup, the electrodes must be unequally spaced apart, in order to get
equal bangsize. To verify this condition, it is necessary to monitor the
primary voltage with a scope, this can be done with Terry`s fiberprobe
or other equipment. The setting of the electrodes must, as far as I am
aware, be determined experimentally, and a very time saving method to do
this is to model the coil in Microsim, and do trial and error, untill
the bangsize is equal, then transfer the results to the gap , for
example using the method that I have described at this adress (the
bottom link), where the gap, along with the rest of the coil is also
described:

http://home5.inet.tele.dk/f-hammer/tesla/tesla.htm

To sum up my beliefs:

Use big hemispheric ended electrodes (higher breakdown voltage for
shorter, lower resistance, arc; mechanical dwell of no relevance)

Thorium 2-4% tungsten rod may improve the efficiency some, due to the
superior ability of thorium to emit electrones. When the gap has broken
down it gones into emission mode.

Spend some time to learn Microsim. This has been the single best
investment in time spent/insight gained that I have done in my coiling
hobby.


Equal bangsize with consistent firing will create a sound that leaves
the impression on the innocent bystander that he/she is witnessing a
performance by someone that knows what he is doing, it may even fool
yourself to think that you really know what you are doing. :-)

This piece would not be complete, without a description on how to make
these electrodes, hopefully denouncing a couple of myths along the way.

The hobbyist can cut the tungsten _metal_ with an ordinary hacksaw, with
fine tooth blades in the 30+ teeth/inch variety. I stress the hobbyist,
because nobody in their right mind would want to do this very often, but
the 10 electrodes for this gap were cut this way. Just apply a pressure
high enough, so that the blade does not skid along on the metal, and go
slow, 1 stroke per second is fine, using the full length of the blade.
This is slow cutting, and as long as you don`t rush it, you`l be fine.
Get impatient and try to cut it faster and the blade gets dull in a
second.

The hemisphere is made this way: Mount the electrodes in a hand held
drilling machine/power driver. Start to grind a 45 deg. chamfer on a
bench grinder with alluminum oxide wheels (the grey ones) The electrode
should be rotated with the power driver rotating opposite the grinding
wheel. Again go slow, don`t force things. By the time all the electrodes
are finished, the impression in the grinding wheel will have a nice
round shape, producing the desired hemisphere.

Use goggles and mask, preferably also dust extraction. 

Cheers, Finn Hammer


Tesla list wrote:
> 
> Original poster: Rscopper-at-aol-dot-com

snip

> -The old transformer operated at 4KV - the new one is 14.4KV, 

so I had to
> open the gaps on the RSG and add two series gaps.  I set them as far as I
> could without the firings becoming erratic.
>
snip
> 
> R. Scott Coppersmith