Ken Herrick’s unique Tesla coil operated on the “tickle principle,” a patented circuit he designed that produces a relatively gradual build-up of secondary voltage to produce awesome sparks a bit more safety.
By Ken Herrick, Contributing Writer
Though sadly no longer operational, Ken Herrick’s novel Tesla Coil (aka “entertainment machine”) produced some satisfying snap-crackle-pops in its day. This photo is the last remaining proof of its existence.
“It all started in 1939. As a kid of 11, many days after school and on weekends I would take the train half way across the Bay Bridge to the “International Exposition” on Treasure Island in the San Francisco bay. Having been a “radio nut” and also pre-pubescent, my goal was not Sally Rand’s innocent teasers on the “Gayway” (not the same meaning as of today). No…it was Tesla Coils! And sermons! It wasn’t the sermons per se since even at that tender age I was skeptical of that kind of thing. But it was sermons as illuminated–no pun–by dramatic emanations from a pair of large Tesla coils operated by the preacher. I realized many years later that the attraction was likely sponsored by the Moody Bible Institute–still in existence, I believe–whose attempt in ‘39 to save souls took them to the Fair. Although I have no memory of their messages I have vivid memory of the dramatic punctuations provided by the pair of Tesla coils–one standing near the preacher and one projecting out from the wall. From that time I’d always wanted to make a coil myself; but I never did anything about it for another half century and more.
For the electrically challenged, a Tesla coil is a transformer, i.e. two coils of wire adjacent to one another between which a magnetic field is created, by a varying electric current introduced into one of them (that one, called the “primary”). The “secondary” responds with a resultant varying current within itself; and an additional crucial factor is present, namely resonance.
Think of a bell, struck by a hammer. The hammer is analogous to the primary’s magnetic field and the bell, imperfectly, to the action of the secondary coil. The bell mechanically-resonates, producing sound. The secondary electrically-resonates, producing, in the case of today’s Tesla coils, a spark. But there is where the analogy breaks down since the bell’s “product”, i.e. the sound, is continuous whereas the Tesla coil’s desired product of today, i.e. the spark, only occurs once per hit of its “hammer”. The Tesla coil today is an entertainment machine; in fact, that’s what I called mine.
The most highly esteemed Nicola Tesla–quite properly so since it is to him that we owe the power coming from our wall sockets and the billions and more induction motors that keep our societies humming, among other things–didn’t want sparks from the numerous giant coils that he had had constructed. What he wanted was broadcast power, to light the world! The coils do emit electromagnetic energy just as do radio stations, and for him sparks would have just been a colossal nuisance. But he was sadly misguided in not appreciating the monumental inefficiency of his expected process, so his wealthy investors ultimately left him. After an otherwise inventive, productive and celebratory career, Tesla–a Serb by birth–died penniless and a bit crazed in New York City.
It will be noticed that all Tesla coils incorporate a smoothly curved metal electrode at the top. The purpose of that element is to inhibit the production of sparks. Tesla wanted that inhibition to be absolute whereas today, Tesla-entertainers want it to be effective only up to a point. That point–and again one could make a pun–is when the voltage built up on the electrode due to the resonant action becomes so great that a spark jumps out into the air. The larger the minimum-radius of curvature existent upon the electrode, the higher the voltage may build up before a spark occurs. If a point per se, i.e. a locality of low radius of curvature, is present there, then the spark will occur sooner and will be weaker.
There are basically two ways to elicit the sparks: “hit it with the hammer” or “tickle it with a feather”. Most coils are hit with the hammer, the hammer being a large stored electric charge repeatedly dumped into the primary coil through an auxiliary spark-gap. But my one successful coil operated on the “tickle it” principle.
The “hammer” requires a very large–and potentially dangerous–voltage to be employed. What I wanted was to use only low voltages, and to do without the annoying but otherwise-necessary auxiliary spark gap. So I utilized, as do other enthusiasts, transistor-operated circuitry. Those methods cause a relatively gradual build-up of secondary voltage, rather as if you could stroke the figurative bell with a feather at its resonant frequency and thus encourage it to emit its sound. There are numerous ways of doing this but, too stubborn, I chose to invent a new way. That way is disclosed in my U.S. patent #6,069,413.
It was only a dozen years or so ago that I really got into this, and I spent way too much time, effort and money pursuing the novel idea. Ultimately I did create a system that worked pretty well, and I was pleased with that. My photo is the only remaining tangible proof, however, except for a couple of left-over secondary coils and the toroidal electrode. Plus piles of documentation, of course, and also the hardware from several so-far-failed attempts at resuscitation. I had “improved” my circuitry over time until the time came when I had improved it once, or more, too often. After that it continually failed and I finally took the entire primary apparatus to an e-waste recycler. [As I mentioned in my prior email, I’d love to hear that snap-crackle-pop again and smell the ozone!]
For those more attuned to electric things, the problem had seemed to lie in how I was driving the power transistors. Since paralleled MOSFET transistors were connected into a daisy-chain encircling the secondary coil, to form the primary circuit, it was necessary to isolate, one from another, the drives to those transistors. I couldn’t get those circuits quite right. And also–and this was a continuing major problem–getting an oscilloscope probe anywhere near the apparatus when it was operating was a real trial due to the strong E-M field surrounding it. Thus I couldn’t effectively trouble-shoot the circuits when they were actually operating. Anyone who’s done trouble-shooting will appreciate that kind of problem.”
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