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Re: Variable Capacitance and Inductance

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Dave,

>At 08:31 PM 5/20/2002 -0500, you wrote: 
>We have discussed on this list individual observations of variations in 
>inductance measurements of coils.  It has brought about sometimes heated 
>debate as to whether this was a malfunction of machinery or a change of 
>atmospheric conditions.  At least one person on this list has proposed to 
>run a test over time to see if there were indeed a variation in inductance 
>in a coil.

Done that ;-)


Humidity (especially rain) seems to affect surrounding structures causing
great losses.  Temperature affects copper losses directly...

>Today I was going through Tesla's writings and came across this article 
>written in 1901.  Apparently Tesla also noticed the variation of capacitance 
>and inductance.  If you have been part of this debate, you will find this 
>article quite interesting.  

Tesla notes that his terminals were very sensitive to surroundings and
elevation.  So are ours ;-))  

In this article, Tesla seems to have resolved his wire length vs resonant
frequency paradox and he has recognized space charge or Medhurst
capacitance as a factor in his experiments.  He is beginning to see the
affects of true transmission line effects as we know them today.  Tesla is
correct in his saying that taking the simple capacitance of a sphere alone
is wrong, as we all know from our early attempts to use such values
ourselves for modern Tesla coils.  The free space capacitance only works in
free space, not on top of a Tesla coil ;-))  Indeed, Tesla was right that a
lot of theory didn't work since capacitance depends so much on surrounding
electrostatic fields rather than the shape and size of the object itself.
It is very significant that he mentions "velocity"!!  He made a giant step
there!!  However, his "law" about elevation was overturned on appeal in the
supreme court of better facts. ;-))  Tesla's stated affects of the moon and
seasons have to be severely questioned as Paul and I's QVAR experiments
show.  Much more mundane factors also have a giant affect which were over
looked in Tesla's experiments.  We also have better equipment nowadays.
Before you trust Tesla's data too much, read the part about the type of
equipment he was using...  Only a genius like him could have gotten that
stuff to work at all!!

At this time, Tesla did not have the tools to calculate the electrostatic
fields and such around his coils.  If he just could have seen the
fields!... it would suddenly have been so clear to him...


Those tools were not available until January 6th, 1999  ;o)))  If he had
those tools, he could have calculated the resonant frequency of his coils
and he could have seen the all important "why" behind it.  However, he had
to guess and he guessed well.  100 years later we can criticize him, but he
really made the best guess he could given the numbers and such he had.
What is important is that Tesla "finally" realized his 'wire length thing'
was "hosed" and he correctly found the basic principles that were truly
controlling his resonant frequencies.  Today we know exactly how elevation
(and every other darn thing!!  (except streamers!!!)) affects the apparent
capacitance of a sphere inside a big secondary coil.  We also know that a
wooden buildings humidity content is a big factor too*...  Computer
programs like E-Tesla, Paul's array of programs, and Rob's new fastTesla
make it all look so easy.  However, fate placed Tesla at ground zero
starting from scratch.  It is interesting to note that our little computer
toys do far more calculations every 1/100 of a second that Tesla did in his
entire life!!!  Even if we have only 1/10,000 his brains, we can all out
calculate him :o)))  God only knows we do too!!  Many of the programs run
for weeks on pretty hefty hardware!!

I wish "we" had listened to what Tesla was saying in this article since a
lot of time was wasted basing calculations on exactly what Tesla is warning
against here.  The Colorado Springs Notes spread like the "shot heard round
the world!"  But the corrections to all the mistakes fell on deaf ears...
Tesla knew well that his calculations were not working in 1999, but he
basically knew why only by 1901.  Unfortunately, we all got the 1999
version...  Richard Hull went a long way in correcting that!!  The
transmission line principles Tesla was talking off were basically forgotten
until the Corums picked it back up in the 1980's.

One thing Tesla could not have predicted is that right up to this day, we
are still correcting the situation he tells of here ;-))  I wonder how
Tesla would rewrite his Colorado Notes today if he knew that is private
experimental notes were going to hit the printing press =:O  There is a
great deal of wonderful first hand accounts in those notes, but conclusions
and analysis would not come "correctly" until years and in some cases a
century later...  The best summary of the results of the CS notes Tesla
makes himself here "it was only subsequently that I actually perceived them"...

BTW - A Tesla coil's measured inductance is different than it's "real
inductance" when operating.  This discrepancy was first noted with E-Tesla
where "I" had to guess at things.  Paul finally figured it out!
Interesting how these events correspond to events exactly 100 years earlier

*Humidity in wooden building affects resonance experiment...


"The modern day experimenter wets the wood around the test coil..."  Note
the many nearby objects that "really" determine terminal capacitance!!


It's the downward red spike...



>New York Sun
>Jan. 30, 1901
>Capacity of Electrical Conductors is Variable
> <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
>Not Constant, and Formulas Will Have to Be Rewritten - Capacity Varies With 
>Abso­lute Height Above Sea Level, Relative Height From Earth and Distance 
>From the Sun.
>Nikola Tesla announced yesterday another new discovery in electricity. This 
>time it is a new law and by reason of it, Mr. Tesla asserts, a large part of 
>technical literature will have to be rewritten. Ever since anything has been 
>known about electricity, scientific men have taken for granted that the 
>capacity of an electrical conductor is constant. When Tesla was 
>experimenting in Colorado he found out that this capacity is not constant - 
>but variable. Then he determined to find out the law governing this 
>phenomenon. He did so, and all this he explained to The Sun yesterday. Here 
>is what he said:
>“Since many years scientific men engaged in the study of physics and 
>electrical research have taken it for granted that certain quantities, 
>entering continuously in their estimates and calculations, are fixed and 
>unalterable. The exact determination of these quantities being of particular 
>importance in electrical vibra­tions, which are engrossing more and more the 
>attention of experimenters all over the world, it seems to be important to 
>acquaint others with some of my observations, which have finally led me to 
>the results now attracting universal attention. These observations, with 
>which I have long been familiar, show that some of the quantities referred 
>to are variable and that, owing to this, a large portion of the technical 
>literature is defective. I shall endeavor to convey the knowledge of the 
>facts I have discovered in plain language, devoid as much as possible of 
>“It is well known that an electric circuit compacts itself like a spring 
>with a weight attached to it. Such a spring vibrates at a definite rate, 
>which is deter­mined by two quantities, the pliability of the spring and the 
>mass of the weight. Similarly an electric circuit vibrates, and its 
>vibration, too, is dependent on two quantities, designated as electrostatic 
>capacity and inductance. The capacity of the electric circuit corresponds to 
>the pliability of the spring and the induct­ance to the mass of the weight.
>“Exactly as mechanics and engineers have taken it for granted that the 
>pliability of the spring remains the same, no matter how it be placed or 
>used, so electricians and physicists have assumed that the electrostatic 
>capacity of a conducting body, say of a metallic sphere, which is frequently 
>used in experiments, remains a fixed and unalterable quantity, and many 
>scientific results of the greatest importance are dependent on this 
>assumption. Now, I have discovered that this capacity is not fixed and 
>unalterable at all. On the contrary, it is susceptible to great changes, so 
>that under certain conditions it may amount to many times its theoretical 
>value, or may eventually be smaller. Inasmuch as every electrical conductor, 
>besides pos­sessing an inductance, has also a certain amount of capacity, 
>owing to the varia­tions of the latter, the inductance, too, is seemingly 
>modified by the same causes that tend to modify the capacity. These facts I 
>discovered some time before I gave a technical description of my system of 
>energy transmission and telegraphy without wires, which, I believe, became 
>first known through my Belgian and British patents.
>“In this system, I then explained, that, in estimating the wave-length of 
>the electrical vibration in the transmitting and receiving circuits, due 
>regard must be had to the velocity with which the vibration is propagated 
>through each of the circuits, this velocity being given by the product of 
>the wave-length and the number of vibrations per second. The rate of 
>vibration being, however, as before stated, dependent on the capacity and 
>inductance in each case, I obtained discordant values. 
>Continuing the investigation of this astonishing phenomenon I observed that 
>the ca­pacity varied with the elevation of the conducting surface above the 
>ground, and I soon ascertained the law of this variation. The capacity 
>increased as the conduct­ing surface was elevated, in open space, from 
>one-half to three-quarters of 1 per cent per foot of elevation. In 
>buildings, however, or near large structures, this increase often amounted 
>to 50 per cent per foot of elevation, and this alone will show to what 
>extent many of the scientific experiments recorded in technical liter­ature 
>are erroneous. In determining the length of the coils or conductors such as 
>I employ in my system of wireless telegraphy, for instance, the rule which I 
>have given is, in view of the above, important to observe.
>“Far more interesting, however, for men of science is the fact I observed 
>later, that the capacity undergoes an annual variation with a maximum in 
>summer, and a minimum in winter. In Colorado, where I continued with 
>improved methods of inves­tigations begun in New York, and where I found the 
>rate of increase slightly great­er, I furthermore observed that there was a 
>diurnal variation with a maximum during the night. Further, I found that 
>sunlight causes a slight increase in capa­city. The moon also produces an 
>effect, but I do not attribute it to its light.
>“The importance of these observations will be better appreciated when it is 
>sta­ted that owing to these changes of a quantity supposed to be constant an 
>electrical circuit does not vibrate at a uniform rate, but its rate is 
>modified in accordance with the modifications of the capacity. Thus a 
>circuit vibrates a little slower at an elevation than when at a lower level. 
>An oscillating system, as used in teleg­raphy without wires, vibrates a 
>little quicker when the ship gets into the harbor than when on open sea. 
>Such a circuit oscillates quicker in the winter than in the summer, though 
>it be at the same temperature, and a trifle quicker at night than in 
>daytime, particularly if the sun is shining.
>“Taking together the results of my investigations I find that this variation 
>of the capacity and consequently of the vibration period is evidently 
>dependent, first on the absolute height above sea level, though in a smaller 
>degree; second, on the relative height of the conducting surface or capacity 
>with respect to the bodies surrounding it; third, on the distance of the 
>earth from the sun, and fourth, on the relative change of the circuit with 
>respect to the sun, caused by the diurnal rotation of the earth. These facts 
>may be of particular interest to meteorologists and astronomers, inasmuch as 
>practical methods of inquiry may result from these ob­servations, which may 
>be useful in their respective fields. It is probable that we shall perfect 
>instruments for indicating the altitude of a place by means of a cir­cuit, 
>properly constructed and arranged, and I have thought of a number of other 
>uses to which this principle may be put.
>“It was in the course of investigations of this kind in Colorado that I 
>first noted certain variations in electrical systems arranged in peculiar 
>ways. These variations I first discovered by calculating over the results I 
>had previously no­ted, and it was only subsequently that I actually 
>perceived them. It will thus be clear that some who have ventured to 
>attribute the phenomena I have observed to or­dinary atmospheric 
>disturbances have made a hasty conclusion.”