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Re: spark gap

Original poster: "Qndre Qndre" <qndre_encrypt@xxxxxxxxxxx>

Hey Barton,
the reason for using thick wire if you have much current passing through them is the fact that I2R losses are not only influenced by current but also by resistance. If you use thick wire (wire with a large cross section) you effectively lower the wire's resistance resulting in lower losses along the conductor resulting in less heat being generated along it.
The heat generated in a tank circuit's spark gap is not only due to losses along the electrodes but also due to losses in the arc. Of course larger thermal capacitance of the electrodes means extended run time but it also means longer time to cool down. This has to be considered. 

Regards, Q.

----Original Message Follows----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: spark gap
Date: Sat, 21 Jan 2006 20:24:03 -0700
Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Q,
Yes, but there "is" external cooling which you did not address. Your analogy is valid, and I am aware of that. But the analogy does not consider an external cooling to the solid electrodes (which are brass). Mass and surface area are intertwined. Consider wire size and current and how that relates to wire heating. It isn't all surface area that does the trick to keep the wire cool, it is also the mass of the wire that is integral to the heating. Surface area is also integral to the heating. My point is that both play a significant role as does the time domain where external cooling plays a significant role (if you see where I'm going with that).
Many coilers run their coils for maybe 30 second bursts, and a few let them run for much longer periods. Regardless of electrode type discussed, a terrible or excellent gap can be designed. This is just personal experience of which I've built both types (many types actually). I once built a rotary copper tube gap way back when. It was horrible! But hey, you have to experiment to find missing pieces in theory. Without experimenting, well, it's like the red-necks say, "it's like wiping before you poop, it just don't make sense!" 

Take care,
Bart

Tesla list wrote:


Original poster: "Qndre Qndre" <qndre_encrypt@xxxxxxxxxxx>

Hey, Barton.
If your spark gap operates, electric energy is drawn from the circuit due to "losses" in the gap. This energy is actually converted into thermal energy which is then stored in the copper of the electrodes. You can compare the thermal behavior of the gap with an electrical circuit:
If the electrodes are huge and solid, they act like a capacitor being charged by electrical current. It's just that it's not charge which is stored in them but heat. The longer you operate the gap, the hotter they get. The longer you charge a capacitor, the more charge is stored in it. If the electrodes get too hot, they vaporize. If a capacitor is overcharged, dielectric breakdown occures. If you choose a cap with a large capacitance, it will take longer till dielectric breaks down but if you continue charging, it WILL break down. If you use solid copper, it will take longer till the electrodes vaporize but they WILL vaporize if you continue operating your coil. If you add a load resistance to your capacitor, it will discharge and breakdown might never occur. If you make the surface area larger, more heat will be radiated and the gap might never vaporize no matter how long it is run. 

Regards, Q.

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From: "Tesla list" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: spark gap
Date: Sat, 21 Jan 2006 06:22:30 -0700
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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.
For our spark gap coils, we can certainly build gaps with tubes or solid stock and achieve stable running temperatures. In either case, it's a matter of just enough air to keep the gaps at a stable temperature, and in both cases, this can be achieved.
BTW, polished brass electrodes are very cool! After I drill and tap a hole in the end of the solid round brass stock, I simply thread in a 2" long threaded stock and attach it to a high speed hand drill. Then place a wad of steel wool in my hand and spin the electrode in the steel wool. The result is a polished electrode.
Here's a pic. The image is old and the connection terminals have since been changed, but it does show the electrodes. 
http://www.classictesla.com/temp/bba_brass_v2.jpg
The inside (white) tube which the electrodes are mounted to will slide out of the main gap tube attached to the fan. Thus, another set of electrodes can easily just be inserted (I ran across some larger solid brass stock on ebay for cheap and will probably make another insert). Haven't had the motivation yet to make the electrodes. I don't have machining tools, so everything I do is always with a vice and a hand drill. The baffle on the front is just another simple gizmo which directs airflow across only the electrodes. 

Take care,
Bart


Tesla list wrote:


Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
When it comes to heat transfer (which is what you're doing here), surface area is the thing. The tube has area on both the inside and the outside through which air can flow to carry the heat away.
Increased mass may keep it cooler for a time, but it's like a big capacitor, it just delays the inevitable.