Re: Voltage determination

["Mark Broker" <broker-at-uwplatt.edu> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Finn,

<snip>

> 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.

An easier way (it works with 5/32" diameter and smaller, but I don't know about
anything larger) is to grind a small notch into the rod, and snap it using a
pair of vise grips, or a bench vise and a hammer.  It takes all of about 30
seconds per "cut."  The down side is that accuracy is not that great - maybe 15
mils if you are really careful and use a grinding wheel with a sharp corner
(read "like new").


> 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.

To add a really nice finish, mount the rod in a relatively high speed drill,
and use some emory cloth on the hemishperical end.  Finsh with 320 grit wetted
with a little oil (10W 30 if you have nothing better).


I completely agree with your statements.  My SRSG uses 4x 5/32" thoriated
tungsten rods spaced about 30 mils from the stationaries (again, 5/32"
thoriated tungsten).  With the caps disconnected, the motor off, and the
electrodes aligned, I have to crank the NST variac to about 65V before it'll
fire!  After adding the caps, with the motor of and electrodes aligned, the
knob raches about 100V!  Starting the motor makes no difference.  I made these
measurements on a day with ~60% humidity with a temp of 75F.  I have not taken
voltage measurements with the caps hooked up, as the only HV probe around has
been fried (ground wire got too close to an NST HV terminal... :-b  )

I guess this goes to show that the ~1kV/mm "rule of thumb" isn't always
accurate.


Mark B.