Re: Graphite spark gaps (fwd)

---------- Forwarded message ----------
Date: Wed, 29 Apr 1998 12:33:56 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Graphite spark gaps (fwd)

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Graphite spark gaps (fwd)
> Date: Wednesday, April 29, 1998 11:26 AM
> ---------- Forwarded message ----------
> Date: Wed, 29 Apr 1998 09:22:02 -0400
> From: Aric C Rothman <Aric_C_Rothman-at-email.whirlpool-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Graphite spark gaps
>      Has anyone had success using graphite electrodes in a static spark 
>      gap?
>      I've built one using hexagonal pieces of artist's graphite (approx. 
>      1/2 inch across, 6B grade (alot of C, little binder)).  I tried it
>      by connecting it across the series-connected secondaries of two 
>      microwave transformers.  It produced a fierce, bright spark. 

It might be bright from the incandescent carbon in the cathode spot. This
is how carbon arc lamps work.

> The spark was so hot it ignited the wood base of the spark gap assembly. 

By putting some energy into making a bright light, you've made a more
effective heat radiator. A spark by itself doesn't radiate particularly
well, but that incandescent cathode spot does.

>      I've read that graphite has a negative temperature coefficient (as
>      gets hotter, its resistance goes down).  This property would appear
>      make it a great choice for static spark gap material.  Other than
>      mechanical problems associated with using graphite (fragile, can't
>      soldered or welded), are there any reasons why graphite would be a
>      choice for Tesla coil use?

The graphite will oxidize (it is hot, it is in air, it is carbon, it WILL
oxidize) and go away, changing your gap length
>      BTW.  When I placed a very strong magnet under the gap, the steady, 
>      sizzling spark mentioned above became a very loud staccato(sp?)
>      I assume this is caused by destruction of the spark by its
>      once current starts flowing through it.  Once the spark is broken
>      current stops flowing, the magnetic field has no effect, leading to 
>      spark formation, and...the cycle continues.
Yep. This technique is used in big circuit breakers. The magnetic field
stretches the arc to the point where it will cool to non-conductivity
(often aided by some refractory material slots or panels in the arc path).

>      BTW BTW.  I connected the same gap/magnet assembly to a NST with a 
>      series resistor to limit current, and observed a swirling haze of 
>      purple (plasma?) in a disk pattern.  The plane of the disk was
>      to the lines of force of the magnet.  Shifting the magnet around 
>      (horseshoe type), caused the disk to tilt.  Neat.  Dr. Dan, my
>      prof., was right after all!

I love it when Maxwell's equations actually work in a visible way, rather
than by implication. I think that is why sparks are so much fun: you can
actually almost see the electricity moving, rather than just knowing it is
flowing through the wire.