|Tesla Coil Builder's Guide to the Colorado
of Nikola Tesla Second Edition
by Richard Hull
This book is available directly from the author.
xxvii, 203 pp., 37 photos, 12 line-art illustrations
The English edition of Nikola Tesla's Colorado Spring Notes: 1899-1900 was published 16 years
ago (Nolit, Belgrade, 439 pp.). Since that time, a number of writers and researchers have made
reference only to certain specific technical points in this remarkable volume, but this Guide is the
first analysis in toto of the detailed experimental work covered. Why is an analysis needed? For
two basic reasons:
As a result of these difficulties, most readers have been frustrated in following the various experimental objectives through the volume and have shied away from the entire reading of it. The Colorado Springs notes have instead typically been scanned for portions that seem to be of special interest. Such an approach excludes the great value to be obtained from the integrated information bearing on the design of very high-voltage, high-power, high-frequency resonators (i.e., Tesla coils and magnifiers).
The Guide is organized in the same fashion as the CSN, by date entry. It is intermixed with Hull's expert advise from his own formidable experience in designing and constructing efficient, high-performance resonators. The practical value of his observations cannot be overstated, and many important guidance criteria are presented for those attempting the construction of any manner of Tesla coils and magnifiers.
Part of the charm of this work appears in the form of summary excerpts of the correspondence between Tesla and George Scherff, his secretary and business manager in New York City. Mr. Scherff came to work for Tesla at the age of 21, in 1895 and their association continued for 35 years. These excerpts provide important information concerning, for example, the equipment and experimental apparatus shipped from New York to Tesla in Colorado Springs as well as presenting a backdrop of the social and competitive milieu. Thus, Hull's volume provides a wealth of information from both an historical and technical perspective.
For those not well acquainted with Tesla coil design and operation, Hull begins by pointing out that, fundamentally, grounding the base end of a vertical coil forces a node at that end, and the coil resonates at its natural 1/4-wavelength frequency. A "good" ground connection is a must. If, however, the coil is ungrounded, and typically placed in an elevated, horizontal position, the coil then self-resonates at its natural 1/2-wavelength frequency with a node forced at the center.
From that introduction we are led through increasingly advanced issues such as "isotropic" capacity—essentially that of an isolated, but connected, conducting body as often employed for a Tesla coil terminal. Hull explains that Tesla's "magnifier" is not the classic Tesla Coil. In an article entitled "Tesla Magnifier Basics" (Electric Spacecraft Journal, Issue 10, pub. 1994), Hull notes that "there appears to be no place in the historical record where the magnifying transmitter was even investigated!" Astonishingly, it has taken between 80 and 90 years to get a "handle" on this three coil system.
The first hint that the three-coil magnifier system required a different form of analysis than simply that of lumped-constant coupled resonant coils came to this writer in September 1975 when Robert Golka, in attempting to mimic Tesla's magnifying transmitter, pointed out that the third or extra coil, need not be coaxially positioned with the secondary driver—it could be positioned at any reasonable distance away from it! This coaxial configuration had heretofore distracted all those examining the amazing photographs of the magnifying transmitter in action Tesla brought back from Colorado Springs at the turn of the century, and turned attention away from the required broad scope of theoretical analysis.
Hull points out that Tesla was also working with the extra coil operating at 1/2 and 3/4 wavelengths but, unfortunately, the results of these tests do not appear in the CSN. These experiments, with the system operating at multiple frequencies, were most propitious as later review has shown. They were performed by Tesla to develop his work on "individualization" techniques leading to his patents on the fundamental AND logic gate (U.S. Patents No. 723,188 and No. 725,605), applied for on his return to New York. The book Nikola Tesla: Guided Weapons & Computer Technology describes Tesla's heretofore unpublished work on this topic.
Multiple-frequency operation also had disturbing, not to mention, threatening, consequences for Tesla working in a wooden structure on the arid plains outside Colorado Springs by causing electric fireballs to frequently break out of the extra-coil resonator (see CSN, Dec. 17-31 entry in association with Photos XI-XII, and Jan. 3 entry in association with Photos XL-XLII). Fire extinguishers are to be seen in some photographs. The mechanism of fireball production was described years later by Tesla to John O'Neill, Tesla's first biographer. The chapter description was omitted from the resulting biography but has been reproduced (Ball Lightning and Tesla's Electric Fireballs). James and Kenneth Corum, Corum Associates, arranged their apparatus according to the chapter description, having two resonators operating together at different frequencies, and fireballs were produced.
For those who have read the CSN, a curious aspect concerning Tesla's section entries on tests to be performed is that the results are not to be found in the daily record. This tends to support a belief held by many that the CSN were not the sole body of records kept by him at the experimental station. The record groups possibly consisted of (a) general correspondence, (b) patent applications, (c) the Colorado Springs notes, (d) notes of experimental results. It is the latter for which scholars have since been searching but without success. Tesla moved all his records to the Wardenclyffe site during its development, and shortly before the tower's destruction in 1917 the plant was ransacked. Records were strewn ankle-deep across the floor. A California inventor who witnessed the destruction and seeing papers blown down the road exclaimed, "History is being carried away by the wind." It is important to realize therefore, that the CSN are not the complete picture of Tesla's efforts at Colorado Springs.
Hull is to be commended on his several references to Tesla working with the magnifying transmitter in a continuous-wave (CW) mode. This aspect of Tesla's work has been essentially overlooked by historians of wireless telegraphy who have assumed, based again upon the photographs of the magnifying transmitter in action Tesla brought back from Colorado Springs, that Tesla's interest was solely in the optimization of damped-wave resonant systems.
Hull concludes by mentioning Tesla's claim of receiving what he believed to be extraterrestrial signals at Colorado Springs experimental station. As expressed by this writer years ago in Nature, it is unfortunate that this claim was not greeted by interested inquiry rather than prompt criticism from the scientific community. The opportunity to obtain details of Tesla's work was thereby lost. It is quite certain that today, if one were to establish an experimental shack on the eastern plains of Colorado Springs and claim to have received extraterrestrial signals, every astronomical group in the world would be knocking at the door.
This book is a veritable storehouse of essential information about the coils and magnifiers designed
and constructed by Tesla at his Colorado Springs experimental station presented in organized
sections with an appendix section of supplemental text and extraordinary photographs. It is a must
for any Tesla coil builder's bookshelf.
While Tesla arrived at Colorado Springs with over 10 years of experience with resonant systems, this doesn't mean that it's necessary to have a similar amount of knowledge for his notes to be understandable. The purpose of this Guide is to cushion the reader a bit and assist in comprehension of what the 19th century Tesla was trying to convey. Tesla was writing to himself and not an audience. Therefore, it is understandable that many of the items he saw as given remained unspoken in his notes, leaving the casual reader mystified as to how certain results and conclusions were obtained.
As has already been stated, I will not teach Tesla coil theory in this present work. I will, however, give a very brief background into why a coil of wire can be made to do the wonderful things that brought Tesla to Colorado Springs in the first place.
Every object, be it a piece of glass, a table, a coil of wire, etc. has a natural resonant frequency. This is determined by the shape and material of the object. Most solid physical objects of any size have resonant frequencies that are low or at least residing within the sonic range, that is to say, 1 to 40,000 oscillations per second. The high frequency oscillations we will be discussing are always electrical in nature and occur within conductors. The nature of a conductor is such that any given length of wire has a naturally occurring electrical resonance. This frequency is given, approximately, by dividing the speed of light by the length of the conductor, and is dependent upon two properties embodied in the conductor, namely inductance and capacitance. Theoretically, when a wire is vibrated electrically at its natural frequency, two voltage peaks and three voltage nodes occur along the length of the conductor as in a sine wave. Likewise, three current peaks and two nodes are also present, but are 90 degrees out of phase with the voltage.
When a wire is wound into a coil its self inductance is altered due to the magnetic fields around the wire interacting with each other. The overall effect of this is to slow down the propagation of electrical energy along the helix of wire, causing the natural resonant frequency of the wire wound into a coil to differ from the theoretical straight-wire example given above. With the property of increased inductance, from winding the long wire on to a short, round form, we gain the advantage of a physically smaller system which can be constructed and worked quite easily.
If we place a quantity of electrical energy into the coil and do it quickly enough, the coil will ring at its natural resonant frequency, much like a bell. Voltage nodes and peaks will appear along the coil. If the coil is floating in free space, it will tend to oscillate at its natural 1/2-wavelength resonant frequency, and each end of the coil will exhibit a voltage peak while a voltage nodal point will exist in the exact center of the coil. If, however, we ground the base of the coil, this is a forced nodal point and the coil will oscillate at its natural 1/4-wave resonant frequency. The results will be enhanced if the energy is pulsed into the coil at its exact resonant frequency. The effect is called resonant rise, and the coil a helical resonator. A standing wave appears on the classic 1/4-wave resonator which has a current peak at its base or ground point and a current node at the top of the coil. Likewise, there exists a voltage nodal point at the ground or base of the coil and a voltage peak at the top. Resonant rise is a function of the current value at the base of the resonator and the "Q" or quality factor of the resonator. This quality factor is determined by the inductance of the coil, its resonant frequency and the AC resistive losses within the coil. This is all that enters into the equation as long as the coil is free and floating in the "perfect vacuum" of interstellar space! To my knowledge, no coil in history has ever completely satisfied the equation for Q!
In the real world, Q is most affected by the coil form that the wire is wound upon, specifically its composition and thickness. There is another "evil" with which Tesla did battle constantly, and never so boldly as when at Colorado Springs, that is known as inter-turn capacitance. In addition to self inductance, a coil of wire also has internal or distributed capacitance created by the proximity of the adjacent turns to one another. Each turn is like a small capacitor plate which interacts capacitively with each turn adjacent to itself. Both the form factor and the internal self capacitance work to reduce the resonator Q. Finally, near effects by things such as the ground, metal objects, etc., all conspire to make the real world Q an almost impossible value to calculate.
Why all the fuss about Q? This is what Tesla terms the "magnification factor" and is directly related to the efficiency of all Tesla coil or similar resonant systems. And, this is what makes a magnifying transmitter into the ultimate Tesla coil.
You will recall I said, "if we place a quantity of energy into the coil . . ." earlier in this section. This was an easy way of avoiding the nasty hands-on stuff and actually linking this cold, dead coil of copper to an active, hot, seething, power source. There are a number of ways to introduce energy into a 1/4-wave helical resonator. Remember, this is usually done very fast and with energy preconditioned to "sympathize" with the resonator. We may couple energy into the resonator inductively. This is what we do in a normal, classic Tesla coil with a primary coil tuned to the 1/4-wave frequency of the resonator which, in this case, is called the secondary. We can also directly couple energy into the resonator by injecting it directly into the base of the coil. This second method is the undisputed, all time, best way to make a Tesla coil perform!!! The first method is of lesser value and involves lost and wasted energy. This is why Tesla abandoned all experiments with regular classic Tesla coils long before he ever dreamed of going to Colorado Springs! There is a third method of coupling energy to our resonator, similar to second, but also involving capacitive coupling. When placed near an operating Tesla system, a "sympathetic" grounded 1/4-wave helical resonator will be energized. Nevertheless, it's the second method of directly coupling the energy into the base of a helical resonator that the magnifier configuration is all about.
People are often heard to exclaim that the magnifier is not a true Tesla coil. Nothing could be farther from the truth. All impact excited helical resonators which experience resonant rise are Tesla coils! Nikola Tesla, at one time or another, probably connected up coils of wire into resonant systems in just about every possible configuration imaginable. It was Tesla who noticed and championed this whole concept. It must be remembered that only one coil is the source of resonant rise in all Tesla resonant systems! All the other circuit components are just so much power pre-conditioning and impedance matching stuff!
I am often pressed to recommend books that will teach a person "all about Tesla coils." I have
only seen one that really impressed me and that was Duane Bylund's, Modern Tesla Coil Theory.
I say this because of the excellent grounding one is given in the theory of resonance and
transmission lines. It is not overly mathematical, but is long in "plain speak" and filled with
examples. I recommend it to anyone as a good introduction to what resonance is all about.
Here, Tesla is calculating his primary inductance using the older "cm" electrostatic units. These are defined in the glossary, but again, 1 cm is equal to 1.1 picofarad of capacitance and 1 nanohenry of inductance. Tesla computes the primary based on a 25 foot radius or 50 foot diameter. He arrives at an inductance value for the single turn primary of 63,900 cm or 63.9 µH. He also gives us a clue to his large working oscillator in the New York laboratory when he computes its inductance and mentions that it is 8 feet in diameter. It is of note to mention that the flat spiral secondary of the New York oscillator was mounted on a vertical frame. The primary was also mounted in the vertical plane on a separate frame that could be moved around the Houston Street lab, allowing primary/secondary coupling to be adjusted at will. I have often thought of placing a larger magnifier coil primary/secondary system in the vertical plane. This would secure two advantages: first, it would save a tremendous amount of floor space, and second, it would not couple much energy into the concrete floor's rebar, as it would be at right angles to the shorted turns created by the floor's reinforcing grid. The fact that he chose a 50 foot diameter for his coil means his lab, still under construction, would just accommodate this size. More about this later on. The building's size may have been dictated by the already existing primary in his New York laboratory! In the correspondence between Tesla and his secretary George Scherff at the New York laboratory, on May 19, the day after Tesla arrives in Colorado Springs, Tesla telegraphs Scherff to inquire as to the length of the two large cables "around the room." Scherff responds in a letter on that date, saying the cables are "162 feet long each." Dividing by (3.1416) we get 51.56 feet in diameter. This was a critical measurement indeed. Tesla asked that these cables be sent to Colorado "express" as he began construction. His pressing need on May 19 was for this measurement in order to calculate the size of the building that he would need to contain his giant oscillator. If things were to proceed rapidly, he would need to give these dimensions to Mr. Dozier, his builder, as soon as possible. On this date, June 7, he is just calculating the primary inductance of these cables when in place. Note that he also calculates for the two cables if used in series as two turns, and arrives at 255.6 µH.
Tesla muses over various receiving schemes and discusses bridge methods for measuring
inductances. In these modern times of digital meters with laser trimmed ranging components, the
bridge box is virtually a thing of the past. It must be remembered that the Wheatstone bridge
gives very accurate readings and was a quintessential part of any good physics lab into the 1970's.
All modern inductance and capacitance meters still rely on the venerable bridge circuit too, but
it is unseen by the end user. . . .
Before building our first magnifier, it was necessary
to master the classic Tesla coil and learn numerous artifices which
Before building our first magnifier, it was necessary to master the classic Tesla coil and learn numerous artifices whichcould later be transferred to the more sophisticated designs. Here, we see the culmination of many Tesla coils built to the classic design, but incorporating many unusual features not often found on such systems. This coil, "Nemesis" is capable of spark outputs of up to 4 times its own 46" secondary coil length! The system uses only .09 ufd capacitance and resonates at 54 kHz. When run to full power potential, it consumes 11-12 kVA of power. The longest spark noted from this system has been 15 feet, point to point.
We see the large classic coil "Nemesis" leaping forward to 12 or 13 feet in a shot from an oblique angle. The power level was only 9 kVA. The sparks constantly impact tool benches and even the system power transformers (lower right)! Notice the large torroid and the rather small 14" x 46" secondary. The secondary is tight wound #18 magnet wire and has an inductance of .11 henry! The primary is 11 turns of 5/8" copper pipe. The coupling is k = .25 which is very tight for a classic Tesla coil system.
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