What next for longer sparks?

From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Thursday, July 02, 1998 6:56 PM
To:  Tesla List
Subject:  Re: What next for longer sparks?

Hi Marco,
        I think you need a bigger low loss top terminal.  The small toroid
is probably going into high corona loss too early.  At only 12 pF any loss
will drain it quickly.  With a 5.7 inch large radius and 1.6 inch small
radius it is probably breaking out far too soon.  Your coil should do about
500kV but it appears to be only getting around 180kV. Get a pair of big
smooth salid bowls or some other high capacity high voltage terminal and see
what happens.  I think with higher top capacitance and lower loss the
voltage will go much higher.  Your coil is a little small (20 inches) for
real high voltage outputs so be careful of arcs to the primary and such.
The primary system should be fine if it is working properly.

BTW If you think a top terminal can be "too" big check out Gary Weaver's 38"
stainless steel terminal at:


Picture at:

He gets 116 inch sparks but he needs a small point added to the sphere to
get the sparks to breakout!

 "I can get a continious 84 inch long spark to a target. Maximum spark so
far is 116 inches. I can not get any sparks to break out on the sphere
without a break out point. I put a 2" long nail on the side of the sphere
and get lots off sparks from the nail."

I think we have much to learn form Gary's terminal! 

Hope this helps,

        Terry Fritz


At 11:51 PM 7/1/98 -0500, you wrote:
>-----Original Message-----
>From: Marco Denicolai <marco-at-vistacom.fi>
>To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
>Date: Wednesday, July 01, 1998 12:29 AM
>Subject: What next for longer sparks?
>After 3 hours of running my TC I got a strike from toroid to the primary
>killed one of my doubler diodes. I had no strike rail that time. I replaced
>now the single diodes with two diode pairs (two diode in series). In
>parallel with each of the 9000KV 350mA diodes a 33Mohm 3.5KV resistor
>and a series of two 1.5nF 3KV capacitors. Guys, that was the best I could
>get from the local store! I also grounded back my strike rail.
>In a few days I'll be testing this new setup.
>Anyway, after initial excitement, I was able to precisely measure strike
>length to a grounded rod and that was 20" (not 1 meter as I wrote same time
>ago). That means that the primary-secondary rises my tank HV roughly by a
>factor of 60 (from 6.5 KV to 400 KV).
>The voltage drop in my tank (from 8KV to 6.5KV) is due to the third MOT
>works as a reactor in series with the MOT primary windings. Without that I
>always blow my 10A fuse.
>My questions are:
>- is 20" the best I can get for my tank and my secondary size?
>- what should I do to increase the streamer length?
>I ask this because I feel (primary capacitor size, MOT max current, etc.) I
>have got plenty of power to get longer strikes. What is the best way to go:
>- bigger toroid?
>- bigger secondary?
>- more coupling (I didn't go lower than 1.5", I didn't reach overcoupling
>- different spark gap?
>My coil data:
>Secondary 4.33" diameter, 19.7" wind with AWG 26 enamel wire, 7 layers of
>epoxy paint. Toroid 3.15" diameter tube, 11.4" outer diameter, 12.7 pF.
>Mounted 1.6" over the secondary top. The pair resonates at 240 KHz.
>Primary 7 turns of 3/16" copper tube, inner diameter 5.3", turn spacing
>0.3". Flat pancake. Clearance from inner turn to secondary 0.5".
>Secondary capacitor 0.05uF 50KV. Set to resonate also at 240 KHz.
>Secondary first turns 1.5" higher than primary first turn.
>Tank supply made of two MOTs and two voltage doublers, output 6.5 KVAC,
>almost 0.5A (I believe). A third MOT primary (with shorted secondary)
>is in series with the 2 MOT primaries on the 220V side to work as a shunt.
>Spark gap is a RQ static gap with blower and two series of 2 of 0.275" gaps
>connected in parallel.
>Marco Denicolai                   Vista Communication Instruments, Inc.
>Hardware Development Manager      www.vistacom.fi
>marco-at-vistacom.fi                 Kaisaniemenkatu 13 A
>fax:    +358-9-622-5610           SF-00100 HELSINKI
>phone:  +358-9-622-623-15         Finland
>   Remember, Murphy was an optimist! I am not...