Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
Hi Jim,
Yes, I suppose I agree that the object will reach equilibrium, but
air time is a factor that should be considered. The slow rise to
temp for a large mass in comparison to a small one (tube) is greatly
hampered by external cooling. The gap stays cool longer and can
remain cool enough indefinitely if enough air is supplied (the same
goes for a tube). I've let it run about 30 minutes non-stopped
without any noticeable diminishing of spark length or behavior with
a 12/60 NST.
I may expand upon this brass gap idea. The brass rod stock I
captured off of eBay is 5/8" diameter. It might be fun to try and
drill a series of holes in the brass just to increase surface area
or maybe experiment with adding fins 90 deg out on each side away
from the electrode arc face. I'm more or less simply curious if a
relatively high current static gap can be easily built off of the idea.
The coiler who started this post was talking about a static gap for
a 120mA NST. 120mA is high enough where a RSG would be a smarter
choice (IMHO) to maximize gap stability.
Take care,
Bart
Tesla list wrote:
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 05:22 AM 1/21/2006, you wrote:
Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
Hi Jim,
Surface area is certainly important for cooling, but mass is also
a major factor. A tube has little mass and as a result, transfers
heat very quickly (heats up quickly, cools quickly). It also has
the added surface area inside the tube for heat dissipation. A
solid round stock will heat much slower and only has the outer
surface to dissipate heat, but, the mass of the object has more
"mass" to transfer within itself. This lowers the overall heat the
object will need to dissipate and will require less cooling on the
outer surface.
Until the solid bar reaches equilibrium. Then, you've got half the
radiating/conduction surface to get heat out of (assuming air can
move through the inside of the tube).
Of course, something else to think about is that in either case,
you're nowhere near melting the copper. If, at steady state, the
temperature (relative to the air) is double, you'll reject twice as
much heat to the air. Since we're talking about a few hundred
degrees here, that's pretty likely.
I have to say, I just favor the tubes because they are cheaper and
use less material.
There probably is a minimum wall thickness that will work well. At
some point, the wall gets too thin to carry the heat away from the
point where the spark is to somewhere else.
Optimizing it is another one of those "solving Poisson's equation"
problems. Another finite element analysis problem using diffusion
of heat instead of charge. There's probably an analytical solution
from M. Fourier of transform fame. The problem's not much
different from heat dissipation in cannon barrels.