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Re: [TCML] "Means for increasing the intensity of electricaloscillations" The Tesla Superconductor of 1901



I agree--I got to wondering where I got that statement, and looked in Uman & Rakov, and found a conductivity of 10E4 S/m, which is 10E-4 ohm-meters, about that of carbon, way less than copper. With a spark of 1 mm square and 5 mm long (my TC), we get a resistance of one half ohm, which agrees with your figure. Cathode drops are of the order of 20 volts, not a big deal. This is still small compared to the 100 ohm impedance of the resonant circuit, and my cap has a series resistance of a fraction of an ohm, so the unloaded Q of the primary circuit should still be quite high. That's an easy measurement so I'll do it one of these days. I would still say that the top corona is still the major source of loading in a TC. In the future I will hopefully check the facts before I spout out something that I've known for most of my life.

---Carl





On 4/12/11 8:29 AM, Carl Noggle wrote:
All true, except that I would disagree that the spark gap resistance is a major factor. The current in the primary of a moderate sized TC (mine, at 1600 watts, for example) is of the order of 100 amps. Going through a spark channel that is a fraction of a mm in dia gives a very high current density, not too different from lightning. A plasma at these current densities has a resistivity that is lower than any metal. A spark gap is a remarkably good switch for high currents.

Uhh.. not really. It's a good switch (and fast), but not as good as metal. There's the cathode drop of several tens of volts plus the IR loss in the spark channel. A lot of research says that the spark/arc is about 7000K, so you can go look out the bulk resistivity. .

the lightning channel (per Uman) is on the order of 1cm diameter, carrying some tens of kA. If our spark gap is 1sq mm, that's 100 times smaller, corresponding to some hundreds of Amps. (I don't know that TC primary currents actually get that high... The L is too big)

Most authors (e.g. Uman, Rakov, etc.) give energy dissipated in a lightning stroke on the order of 3-10 kJ/meter over a few tens of microseconds: that is, 5kJ/20E-6 = 250 MW. So can figure it out.. 250E6 = 20E3^2 * R => 250E6/400E6 or around an ohm/meter.

say it's 1square mm and 1 mm long.. That implies about 100 ohms/meter for the spark in the gap, or 0.1 ohm for a 1mm long gap. 1 sq mm = 0.0015 sq inch ( that's roughly AWG 17)

A foot of AWG10 wire has a resistance of about 0.001 ohm. AWG 16 has 4 times the resistance, and is fairly close to the diameter of the spark, so 0.004 ohm for 300 mm, call it 0.004 milliohms...

Orders of magnitude less than the resistance of the spark.



The main source of resistance in the pri circuit is the coil wire, due to the very shallow skin depth. The bigger the better, stranded might help a little, the best would be a lot of small insulated wires bundled into Litz wire, but who wants to go to that much work?

A fair amount of research (in the archives) has shown that for round numbers, about half the primary power is dissipated in the sparkgap.

I think the Litz wire approach has also been discussed.


An interesting thing to be gleaned from the table is that the RF resistance of any reasonable wire used for the secondary will have hardly any increase of R at TC frequencies, so the ohmmeter resistance of the secondary is the RF resistance. The secondary coil Q should be high too. It seems that losses in the TC are small, and almost all the energy should be going into the coronas. If you look though them in the daytime, you can see quite a bit of heat distortion, even though it's hard to see the sparks themselves.


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