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Re: [TCML] Voltage - Flame



Jim makes an excellent point; as long as the air has enough energy to stay ionized, the arc will exist.  If one supplies more energy than necessary for arc maintenance, the arc length can increased. In elaboration,  the secondary capacitance should be large so that a large amount of current can be pumped through the plasma channel and the arc's length can increase further.  The arc of a Tesla coil actually pulses in accordance with the oscillations of the secondary. Between capacitive peaks (where the energy is stored in the inductor itself and not in the capacitive part of the coil) the arc ceases to exist because there is simply not enough current going through the arc at that time.  The air, however, is still hot and, when the energy transitions back to the capacitve part of the circuit, it is more readily ionized.  Whatever current is not used then to re-ionize the air is used to ionize even more air and extend the arc.  This insinuates
 that the break rate should be high (however, clean breaks must still be achieved) so that there isn't much time where the energy in the secondary is far under its maximum. This is exactly the case.

Furthermore, when energy is stored in the capacitive part of the secondary, it is done so in two (main) locations: the terminal capacitance and the coil capacitance.  Now, the energy stored in the coil capacitance CANNOT be tapped into via the terminal and ground end of the coil (I can vouch for this); therefore the majority of the capacitance should be in the top load.  That is, since energy stored in a capacitor is directly proportional to the capacitance of the same, C(terminal):C(coil) determines, capacitively speaking, the resonator efficiency.  This brings about a consideration of Medhurst's empirical formula or whatever other formula's there are for determining coil capacitance.  In order to minimize coil capacitance one might look at the table of constants and assume, since capacitance is directly proportional to the constant and the constant is determined by the physical aspect ratio of the coil, that the coil capacitance is the lowest
 when its aspect ratio is near 1:1.  And he would be wrong (sorry Eric).  This is only true when the coil diameter is kept constant; however, as the height of the coil changes for constant diameter, the surface area changes - the surface area is determined by the length of the wire and the physical diameter of the same! Therefore, as the height changes, the length of the wire changes!  Rather than simply assuming a constant diameter, one must assume a constant surface area; that is, what are the characteristics of the coil that will ensure the lowest possible coil capacitance for a given length of wire.  I've done all the math, a constant aspect ratio emerges: ~0.66641:1 (h:d).

Mattison S. Siri
Mattison Electric

P.S. My apologies for getting a little of topic.


________________________________
 From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: tesla@xxxxxxxxxx 
Sent: Saturday, January 28, 2012 7:43 PM
Subject: Re: [TCML] Voltage - Flame
 
On 1/28/12 7:27 AM, James Zimmerschied wrote:
> 
> That being said, the discussions did bring up some interesting
> points:1) there does not appear to be a reliable way to determine the
> voltage a coil has to produce to create a given spark length. If
> there was someone could have pointed to the data.

That's because there isn't a particular "voltage" that's needed to create a given spark length.  The Jacob's Ladder example is a good one, because you can create a very long spark with fairly small initial voltage.  That's the classic "drawn arc" kind of situation.

ALl you can really say is that the voltage has to be high enough to start the spark growing, and that just means that the voltage has to be high enough that the field locally exceeds 3MV/meter.

That said, I think that there is a practical relationship:  Say you had a 2 cm diameter top load.  You could start breakout at 30kV (or less, if it had a sharp point on it), but such a terminal doesn't have the charge storage capability to keep the spark growing.



2) the theoretical
> voltage on the top terminal is stated to be dependent on the radius
> of curvature of the top load  - however spherical top loads do not
> produce the longest sparks.

Precisely so.


3) Other types of coils use a breakout
> point (DRSSTC, VTTC). The DRSSTC seems able to create very long
> sparks with this small radius of curvature point at the top load. I
> think it is a good idea to find a reasonable and accurate way to
> measure the voltage at the top load of a Tesla coil and then if
> enough folks make the measurement with different coils it may be
> possible to say that "on average" these type coils produce x voltage
> at the top terminal when producing y spark length.

I think it's more complex than that. It's about supplying charge to a growing spark channel, and that depends on the inductance & capacitance of the terminal, more than the actual voltage.

because there is a coupling between amount of charge and voltage & capacitance, and the latter is related to size, that's why one can probably say

Bigger sparks require higher voltages on the TC topload

but you also have to say

But spark length depends on a lot of other factors.


You certainly can't boil it down to a distance per volt sort of metric.

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