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In normal Tesla Coil operation, the setting of the spark gap is not so critical that minute changes effect the output dramatically...but in operating coils with small currents, and espescially lower voltages, small changes often do have dramatic effects.  The recent unusual cold weather and lack of humidity here in South Florida has opened some new doors to the study of Phantom Streams.  When the weather was in the 40s, the streams would change polarity, shape, and form with adjustments of the spark gap hardly perceptible to the operator.  I recognized many changes in the output from Kinraide's photographs, and was surprised that the weather alone opened so many new doors of interest.

Some phenomena seen recognized are shown here:
http://www.electrotherapymuseum.com/2010/PhantomAnalysis1/index.htm

I had previously worked on spark gaps of various designs to employ fine adjustment, but found with these latest experiments that 1/1000th of an inch accuracy was often required and in some instances not good enough.




Construction details of this gap can be found here:
http://www.electrotherapymuseum.com/2010/10000Gap/index.htm

Jeff

Jeff
The photos in the link are fantastic! What type of camera and set up are you using?
Jim Sheen

Jim,
Its a simple Sony Cyber-shot.  It cost $125 or so a year or two back.  I had a slightly nicer model, but Mejla, our female bulldog bit it in half - along with my mobile phone.  The same afternoon she also managed to write on the sofa with a black Sharpie after biting that in half too.  I was famous that night when my wife got home...she's outgrown those tasks a bit since then...

The newer cheaper Sony does take nice pictures still.

Jeff

Jim -
PS -

I just realised something.  I forgot there were two links in that posting.  The first link shows photographs that were made in the 1890s by direct contact exposure to coil discharges.  The original negatives are 8 x 10 and 11 x 14" glass plates.  I've developed them on photographic paper and scanned them.  I tinted the photos a purplish hue in photoshop to somewhat match the colors actually seen.  These phenomena are too faint and too quickly-moving to capture by digital camera.  You get only a faint blur of a small fraction of them.  Direct exposure to photographic emulsions is the best way I've found...but unfortauntely, I cannot get supplies locally any more and can't really afford a dark room again and buying online.  Most photographic chemicals from Ebay are expired stock...maybe in the future things will improve and I can carry on this experimentation.  During the darkroom days here I wasn't able to produce this phenomena to the extent I can now. 

Kinraide originally balanced the glass plates on the spheres on top of his coils.  We found the coils nearby the negatives in some hidden passages beneath his former home in Jamaica Plain, MA  a few years ago.  The blotchy parts of the pictures are where the emulsion was washed from the plates.  Unfortunately all of the plates were damaged in some way from being stored improperly for over a century...they were made from 1897-1899.  Still though, the images present are magical.  Until I reproduced some of the experiments, I wouldn't have believed the images possible, or perhaps would have written it off as ancient emulsion formulas/etc. showing a different representation than what is really seen.   I can assure you, this isn't the case...
Jeff

...in continuing this experimentation, I've found even finer increments are necessary for the spark gap.  Have started using gaps where the adjustability is divisible by 3.  This simply makes drilling holes easier on a rotary table or dividing head.  Using the same principle of a dividing head, here is a spark gap I designed that can move in fixed increments down to 1 / 14400th of an inch, or .000069444444444444..."


http://www.electrotherapymuseum.com/2010/11March10/index.htm

The need for this will be posted soon, in what might be my last Tesla-related project for some years to come.

Jeff