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RE: getting started

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Godfrey, all,

On 14 May 01, at 21:02, Tesla list wrote:

> Original poster: "Loudner, Godfrey by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <gloudner-at-SINTE.EDU>
> Hi Matt an All
> Well you certainly pinned me to your wrestling mat with my
> quarter-wave length advice. I cannot recall the exact statement, but
> someone had commented that Tesla had introduced the quarter-wave
> length idea as a convenience, not a fundamental principle, for the
> kind of or size of coils he was building. That readers of Tesla had
> misconceived his statements about the quarter-wave length as a
> fundamental principle for all coils. 
> Godfrey Loudner

I think the picture in Tesla's mind was of an electrical disturbance 
racing along the wire (i.e. round and round the coil) and spiralling 
to the top, hitting a discontinuity and then racing back down. Not a 
bad analogy is the oft demonstrated principle of producing sinusoidal 
motion from a rotating pin on a disk. Basically Tesla's idea is 
correct. If you view the coil from side on, you see a wave travelling 
towards the top of the resonator in the same time it would take the 
wavefront to travel to the end of a long wire which was a 1/4 of a 
wavelength long at the frequency of interest. In effect, the 
wavefront is slowed up along the coil axis.
    Where calculating resonant frequency based on wirelength comes 
unstuck is that in coiling the longwire up, mutual inductance between 
sections of the wire comes into play which modifies the wire's 
propagation characteristics. Additionally, there is mutual 
capacitance between wire sections and in reducing the resonator 
length to that of a coil, the capacitance to other objects drops. The 
result is that a bare coil always resonates at a frequency higher 
than it would if the wire was stretched out into a 1/4 wave monopole. 
The wire in the coil is "apparently" too long for the frequency of 
oscillation. So a popular notion is to add a terminal capacitance 
such that the resonator frequency drops to the point where the 
oscillation frequency matches that of the longwire monopole. Note 
that overall coupling (i.e. top to bottom or vice versa) is << 1 so 
lumped descriptions of operation cannot apply. The current 
distribution in the resonator is not uniform as it would be in a 
lumped coil which itself is a somewhat idealized notion.
     As far as making sparks are concerned, more top C is in general 
always better as it provides a reservoir of charge available to 
produce high spark currents. I haven't seen a single bit of 
operational evidence which suggests that that particular terminal 
capacitance is optimum. In my experience, more is always better as 
long as enough power is fed in to get it to breakout voltages and the 
voltages don't have to become so high that the resonator is then too 
short to sustain the voltage (i.e. the winding starts to flashover 
its length).