Re: Tesla and Measurements.

Hello everyone,
                Jim Fosse responded to my query....

MW>        I badly need some information from someone who has access
MW> to the Colorado Springs Notes. I am hoping to get a copy later
MW> this year but I need the info now.
MW>    Questions : (a) Did Tesla measure the Q's of his primary
MW>coils in isolation (including spark gaps)? (b) If so, what
MW> technique did he use to measure them (I imagine there weren't too
MW> many o'scopes around at the time :)  (c) Also if so, what results
MW> did he get?  (d) Finally, what conclusions did he reach about
MW> his results?

JF replies......

    Any responses?
    I've been going through Richard Hull's guide, but have found
no references yet.

     Only one off-list so far. Dr Rzesotarski was kind enough to
tell me what he had gleaned from the CSN which appears at this stage
that Tesla did not (and probably couldn't) make the measurements in
     I think it's probably time to let the cat out of the bag. I have
now scoped a range of coils from 11uH to >300uH with several different
capacitors including an extended foil cap and a single static spark
gap. The scoping was done single-shot. To date and without exception,
all operational (read with gap-in-the-circuit) Q's have measured
around the 10 mark. I was interested in your measurement of 9.8 and I
have yet to hear from anybody who has measured a Q of, say, 12 or
     SO, bearing this in mind, I asked myself, why would a particular
coil's performance improve with a large Lp/small Cp combination? Bear
in mind also that Mark Barton once said on usa-tesla and I will try to
quote as accurately as possible: "don't just try a capacitor that some
program calculated for you. Try different combinations of capacitors
(and by implication Lp) and break rates - there is definitely a PEAK
(my italics) in performance to be found".
     I think the word "peak" is the key here. It implies there is
some intermediate value of Cp/Lp between the obvious extremes that
works best.

     I have two hypotheses :

(1) The Q of the primary (excluding the gap) does indeed increase as
Lp/Cp increases. In fact I measured a Q of 150+ on a Marconi (yeah
yeah I know) Q-meter with the extended foil cap (0.1uF) and the 300uH
coil, and that was hardly with the huge degree of isolation that I
employed on those secondary measurements last year.
    But the point here is - so what? The gap losses are DOMINANT. The
amount of energy lost per 1/2 cycle is independent of L and C.
Additionally, it appears that performance is going to peak at a
certain L/C and then go back down. The other point to note is that Q
embodies another quantity - impedance i.e. Q = Xl/R (=Xc/R), which
leads to the second idea that I think is much more likely....

(2) By varying the L/C ratio the characteristic impedance of the
primary is being altered. In other words, the base of the secondary
coil is "seeing" a different impedance with different values of Lp
and Cp. Why should this matter? The answer is found in 1/4 wave
transformer behaviour (which everyone agrees is a valid model for the
    The next part of this post embodies a large amount of theory that
could fill a chapter if not a book. I will attempt to be brief :

    In a 1/4 wave transformer, the inverse of the feed impedance
is seen at the far end. If you apply a low impedance to one end, you
see a high impedance at the other when looking back in and vice-versa.
Neither end is priveledged. Secondly, if you apply an impedance = to
the Zo of the transformer ( approx. SQRT(L/C) ) to one end, you see
Zo when looking into the far end. As long as the impedance terminating
one end of the secondary matches the impedance seen at that end when
some arbitrary impedance is applied to the other end, an impedance
match is effected, and the VSWR drops to one (energy absorbed by the
load equals energy delivered to the other end).
    The primary spark has a certain resistance (Vp/Ip). The secondary
spark has a certain resistance (ignoring its extra capacitance) of
Vs/Is. Vp/Ip is transformed to some other impedance by the primary
reactances and becomes the feed impedance for the secondary. So the
system is a device for matching a primary spark to a secondary one.
    The secondary spark is not a constant resistance. If it is simply
a corona, it has a high Z (V/I) so the feed impedance of the secondary
is low. An arc to an earthed object on the other hand has a lower V/I
and hence lower resistance, so the ideal secondary feed impedance is
much higher.
     The upshot is that a match is effected only for one set of
secondary spark conditions. I have scoped this using a discharge rod
set at different distances from the secondary terminal and seen it
happening (the differences in VSWR etc.).
     What I am postulating here is that for a given power throughput
(read secondary spark) and a particular secondary coil, there is an
ideal primary configuration that effects a perfect match to give an
arc of a certain length.
     Since impedance is embeded in the Q formula, it is easy to see
how an emphasis might be placed on Q when the real culprit is lurking
     If the above scenario is the real one, it makes the coil an ideal
research tool for an investigation into the charateristics of ionized
gas behaviour since a spark with a certain characteristic is easily
simulated by a resistor that can be used as a line termination.
    Well, that's it for now. I hope this stimulates some debate. I
think it possible that many coils exhibiting the "zener effect" as
Mark calls it when input power is ramped up might overcome this simply
by using a different primary L/C ratio. My final thought is that Lp/Cp
should be proportional to input power. Raising L and reducing C keeps
fr constant (of course as Cp gets smaller, the break rate has to
increase to keep power input up). Try it and see. I certainly will
when I move into an experimental phase later this year. Given Skip's
recent results, I think at least 8 feet from a 230V 10A wall socket
is on.