I have found that a simple series gap can only be used in a non-shunted transformer system under 2kw provided it is air blown and the transformer primary current is limited either inductively or with ballast resistors. Above 2kw a rotary is better. I have further noticed that a rotary- series gap combination is useful at any power level but almost a must over 5kw. The series gaps tend to help the rotary quench better as it come off the contacts or at least it appears so to me. Tesla used this set up in Colorado as well.

I have further noted that a static series gap, properly quenched of course, can deliver far more energy to a coil at critical coupling than a rotary due to less dead time but at high powers only high pessure air blasts and very “hyper” critical gap adjustments make the series static gap work well. As little as 1/64 inch change in gap can really dramatically affect output of the coil. This is an intolerable nuisance to the casual coiler and is usually quickly abandoned for the ease of use of the rotary. I am sure that efforts to develop improved static series gaps at high powers would be a worthwhile endeavor, but it would require repeatable high pressure air volumes and tungsten gaps with massive heat sinks to turn out repeatable, stable results. The static gap has been ignored by the advancing amateur coil builder as he is lured to higher powers and often told religiously that rotary gaps are a must over 1kw plus the rotary appears “high tech” and is often held out as a sign that one has “arrived”.

Rotary gaps give a coil a pure and constant tone like a power saw operating and to many this can be raspy and annoying. The series static gap gives a full throated crashing sound of an electrical arc discharge and can often be louder but due to its variablity doesn't seem so annoying as a rotary tone. Sparks for the two systems can differ too. It has been my experience that the series static gap produces slower, longer lasting, ambling discharges while the non-synchronous rotary based systems tend to give longer, thinner, more frenetic and rapidly dancing discharges. I am sure many may disagree but this is subjective and is not meant to favor one type over the other but to note that each type of gap system has a characteristic “signature” all its own.

I have found, as Tesla did, that using a series gap in series with a rotary greatly improves system performance. I use the rotary only as a break and not a quencher! The series gap is the source of quenching and depletes the energy of the arc, thus the static electrodes of the rotary are set quite close to the flying electrodes. I exhaust the arc with static series gaps and commutate it with the rotary.

I have not mentioned the “quench gap” because all of the amatuer built “quench” gaps that I have seen or read about are not true to the quench gap construction principles, in that they “breathe” or have relatively large plate spacing making them modified series static gaps. Much has been written about this quiet closed system series static gap, but its construction, maintenance and cooling is truly nightmarish. While I have little intimate knowledge of this gap in use, it is said to be superior to the rotary at low to moderate power levels. Most of the literature available on this type of gap is over 70 years old! A very adventurous, well equippped coiler could possibly benefit us all by further investigating this gap system and producing a paper detailing construction using modem materials and methods.

Investigation into higher powers has shown that the toroid enters into the picture as a factor above 4 kw. The proper matching of discharge capacitance to power used and coil size can affect system output. This factor is more important to high power people than the table top coiler. Previous articles in TCBA news have covered this aspect in some detail and it will not be covered here.

In short, there is no best gap. The choice of gap is based on system power, transformer type, budget, ease of maintainance, ease of adjustment and ease of construction. Many high tech materials are available to assist in modern gap construction such as tungsten electrodes, sulfur hexafloride atmospheres, synchronized arc discharge tube systems but the average amateur can't obtain or afford these luxury items. Most of these “techy” items really have little application in the under 10 kilowatt Tesla sytem anyway. So the average Tesla builder should just remember to use the best materials he or she can afford and experiment with one component in the system at a time and bring it to a peak then move on to another. The end result will hopefully be a coil producing a maximum amount of spark for the power placed in it.

Up to this point we have only discussed distribution type transformers but plate transformers are a second type of non-shunted transformer. These are most often encountered at hamfests and almost all are of the open frame or potted type. These can often be purchased very cheaply but rarely appear to have secondary voltages exceeding 6000 volts but may offer high secondary currents near 500ma! Individually these are poor choices for Tesla systems but two or more of these transformers with the primaries hooked in parallel and the secondaries hooked in series will often equal the capabilities of a small pole pig! I have found that an absolute minimum voltage for a Tesla coil circuit is around 5000 volts. Below this the gap adjustment becomes super- critical and quenching it is difficult. Plate transformers should not be overlooked as a good source of high power coil transformers. Just remember to match the secondary currents fairly closely when you hook two or more in series. A 300ma and 400ma secondary pair in series is ok but a 100ma and 500ma match is a bit out of place since 100ma would be the maximum continuous drain that could be available with this combination. A momentary short here could immediately burn out the 100ma transformer.