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
> 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.