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Nikola Tesla Spark Gaps

Tesla’s Spark Gaps: A Literature Review

Spark gaps play a central role in many of Nikola Tesla’s devices, most prominently in the circuits powering his famous Tesla Coils. In its simplest form, a spark gap is nothing more than two electrodes with some space in between through which an electric spark passes when the voltage in a circuit reaches a high enough level. However, Tesla took the spark gap, which hte often referred to as a “break”, to a whole new level and devised several ingenious ways to make them more effective. In this post I present a literature review of the various types of spark gaps Tesla employed over the years, how they work, and what the pros and cons of each of them are… in Tesla’s own words!

Yes, you will find a lot of direct quotes from Nikola Tesla in this text. I do this to stay as close as possible to Tesla’s original design specifications, since I have seen a lot of “replications” by experimenters who only follow half of his instructions, and then say the device does not work as advertised. This is what Tesla himself had to say about this:

“Some experimenters who have gone after me have found a difficulty. They said: “No, we cannot produce a constant train of oscillations.” Well, it is not my fault. I never have had the slightest difficulty. I produced constant oscillations and I have described how I produced them. Anyone who has no more than my own skill can do it.” 1

The reason the spark gap deserves such detailed analysis is because it plays a key role in the authentic replication of Tesla’s high frequency experiments, but is one of the hardest parts to procure; it is easy to purchase a high voltage transformer, and it is also easy to buy high voltage capacitors and copper wire, but a properly functioning spark gap built according to Tesla’s design specifications is something that you cannot really buy and will most likely have to construct yourself. Therefore it is crucial to understand the nitty-gritty.

Now that it’s clear why it pays off to study spark gaps, let’s start by looking at the often underestimated subject of how a spark gap actually works, and the main role it plays in many of Tesla’s circuits.

Why use a spark gap?

“The effects which are produced by currents which rise instantly to high values, as in a disruptive discharge, are entirely different from those produced by dynamo currents which rise and fall harmonically.” 2

As you can see from this quote, Tesla referred to the firing of the spark gap as a “disruptive discharge”. So what exactly was “discharging” through his spark gaps?

“The general plan is to charge condensers [capacitors], from a direct or alternate-current source, preferably of high-tension [high voltage], and to discharge them disruptively while observing well-known conditions necessary to maintain the oscillations of the current [resonance].” 2

So Tesla used a high voltage power source, like a generator or a transformer, to charge capacitors. When these capacitors reached a certain voltage, the air (or other discharge medium) between the electrodes of the spark gap gave way, and a disruptive discharge took place. But why would you want to discharge a condenser through a spark gap in the first place? Tesla explains:

With these 44,000 volts I charged my condensers. Then by discharging the condensers, either through a stationary gap or through a gap with a mechanical interrupter, I obtained any frequency I desired, and perfectly undamped waves.” 1

And when Tesla spoke about the problems he had creating a high frequency alternator:

“It appeared desirable to invent a simpler device for the production of electric oscillations. In 1856 Lord Kelvin had exposed the theory of the condenser discharge… I saw the possibilities and undertook the development of induction apparatus on this principle.” 3

This statement is in line with what he mentioned back in his 1891 New York lecture:

“The discharge of a condenser affords us a means of obtaining frequencies far higher than are obtainable mechanically.” 4

So Tesla used condenser discharges as a way to generate the high frequency oscillating currents he used in many of his famous experiments and lectures, as well as undamped or continuous waves, which played an important role in radio transmission. On top of that, these disruptive discharges also enabled extreme power magnification:

“I charged the condenser with 40,000 volts. When it was charged full, I discharged it suddenly, through a short circuit which gave me a very rapid rate of oscillation. Let us suppose that I had stored in the condenser 10 watts. Then, for such a wave there is a flux of energy of (4 x 104)2, and this is multiplied by the frequency of 100,000. You see, it may go into thousands or millions of horsepower.” 1

It is not entirely clear to me where the (4 x 104)2 comes from, but the underlying idea makes sense: you “slowly” fill up a bucket of water [capacitor], and then release all of the water nearly instantaneously [disruptive discharge], creating a massive water pressure [voltage] much greater than the pressure of the water pump [power source].

And according to Tesla, the effects of these discharge currents “are entirely different” than those produced by regular AC. In Tesla’s words:

“The differences of potential at the various points of the circuit, the impedance and other phenomena, dependent upon the rate of change, will bear no similarity in the two cases.” 2

And this brings us to an important and often overlooked point: the rate of change. Many people who try to replicate Tesla’s experiments, including myself at first, believe that the spark gap has to fire at a certain frequency to create the curious effects Tesla describes in his lectures, however…

“When working with currents discharging disruptively, the element chiefly to be considered is not the frequency, as a student might be apt to believe, but the rate of change per unit of time… with low frequency currents it is impossible to obtain such rates of change per unit of time as with high frequencies, hence the effects produced by the latter are much more prominent… Frequency alone in reality does not mean anything, except when an undisturbed harmonic oscillation is considered.” 2

And when a lawyer asked him during an interview in 1916 if the effects “depend upon the suddenness of the discharge”, he replied:

“Yes. It is merely the electrical analogue of a pile driver or a hammer. You accumulate energy through a long distance and then you deliver it with a tremendous suddenness. The distance through which the mass moves is small—the pressure immense.” 1

So maximizing the rate of change of voltage per unit of time, by making your capacitors release their energy as fast as possible, is way more important than making your spark gap fire at the highest frequency possible. In fact…

“It is not, as a rule, of advantage to produce a number of interruptions of the current per second greater than the natural frequency of vibration of the dynamo supply circuit, which is ordinarily small.” 2

So if your high voltage power supply is 50Hz, then a 50Hz firing of your spark gap is fine according to Tesla, as long as the rate of change is high. You can look at the firing of a spark gap as the “ringing” of a bell, which in this case is a conductor. The spark “hits” the bell, which makes it vibrate a certain number of times, until the vibration dies down. So one spark creates several waves. This is how a low spark gap frequency can still generate a high wave frequency in the conductor.

Damped wave
Figure 1. A so-called “damped wave” which occurs when a bell is struck

Since a spark through a gap is essentially an ON/OFF event, it is comparable to a square wave. The rate of change in this case can be thought of as the slew rate and the duration of the pulse as the duty cycle. Looking at it in these terms, you want your slew rate to be as high as possible, by making your duty cycle as short as possible.

While this analogy of a square wave is useful, it is important to keep in mind that a square wave generated by a spark gap differs greatly from a square wave generated with a function generator. While you can easily create a short duty cycle square wave of any frequency using a function generator, this shorter duty cycle does not actually increase the rate of change, it will simply push less energy through the circuit. Making the duty cycle, or discharge time, of a capacitor shorter on the other hand greatly magnifies the rate of change.

disruptive-discharge-vs-function-generator-square-wave
Figure 2. Disruptive discharge vs function generator square wave

For example, say we have a capacitor that is charged to 40.000 Volts. If we discharge this through a spark gap in 10 milliseconds, the slew rate is 4.000 Volts per millisecond. However, if we discharge this same capacitor in 1 millisecond, the slew rate is 40.000 Volts per millisecond. In the latter case the rate of change is ten times greater, while the frequency remains unchanged! And since Tesla mentioned that maximizing the rate of change is one of the main goals of a good spark gap, it seems that one of the main goals of the person designing the spark gap should be to allow as much energy as possible to pass through in the least amount of time possible, and preferably in a consistent way.

It seems Tesla’s main way of achieving this was by “quenching” his sparks…

Quenching

“Quenching” is the art of extinguishing an already established arc. According to Tesla, quenching lets “the fundamental discharges occur in quicker succession.” 5

So proper quenching results in better spark gap performance. The best explanation of quenching I found was by Richard Quick:

“A cold, non-firing, spark gap is “clean”. It contains no plasma, or hot ions. On applying voltage to the gap, a tension is established, and electromagnetic lines of force form… Once the voltage punctures the air (or other dielectric gas) the gap resistance drops. The breakdown ionizes the gas between electrodes, and the arc begins to ablate and ionize the metal electrodes themselves. This mixture of ions forms a highly conductive plasma between the gap electrodes. Without this highly conductive channel through the gap, efficient tank circuit oscillation would be impossible. But the plasma also shorts the gap out. A gap choked with hot ions does not want to open and allow the capacitors to recharge for the next pulse. The gap is gets “dirty” with hot ionized gases, and must be quenched.” 6

In short, the sooner we can extinguish an established arc, the sooner the capacitors can recharge for the next discharge. However, it is easier to start a spark gap than to stop it. Luckily Tesla managed to devise a wide variety of spark gap designs which all dealt with this problem in their own way, let’s have a look!

Static gap

Super simple static spark gap
Figure 3. Super simple static spark gap

As mentioned before, the simplest type of spark gap is two, often movable, electrodes with some air in between through which a spark can pass when the voltage is high enough. These gaps offer no form of active quenching (hence “static”), but because of their simplicity this type of spark gap is the first spark gap I created, and is also the type that many experimenters seem to use.

However, Tesla hardly mentions the use of these simplest of “breaks” and, as we will soon see, resorted mostly to using more complex devices, although he does admit that more complex does not always equal better performance:

“…they [complex spark gaps] may be sometimes a source of trouble, as they produce intermittences and other irregularities in the vibration which it would be very desirable to overcome.” 2

So what do these more advanced spark gaps look like?

Magnetically quenched spark gap

tesla-magnetically-quenched-spark-gap
Figure 4. Magnetically quenched spark gap

The first type we will discuss is the magnetically quenched spark gap, which quenches the spark by the use of a strong magnetic field close to the arc.

“The intense magnetic field… serves to blow out the arc between the knobs as soon as it is formed, and the fundamental discharges occur in quicker succession.” 5

The current in an arc creates a field. The strong magnets, placed at right angles to the arc, pull on the arc and disrupt it, allowing the next charging cycle of the capacitors to begin.

“The rapidity of the interruptions of the current with a magnet depends on the intensity of the magnetic field and on the potential difference at the end of the arc. The interruptions are generally in such quick succession as to produce a musical sound.” 2

So the intensity of the magnetic field, to a large extent, determines the effectiveness of this type of quenching. Therefore Tesla shaped the magnets into points and placed them as close to the arc as possible, as shown in figure 4 above.

When to use it

In which situations should you use magnetic quenching? And does it work equally well for DC and AC sparks? Tesla had several things to say about this:

“The employment of an intense magnetic field is of advantage principally when the induction coil or transformer which charges the condenser is operated by currents of very low frequency.” 5

And…

“The magnet is employed with special advantage in the conversion of direct currents, as it is then very effective. If the primary source is an alternate current generator, it is desirable, as I have stated on another occasion, that the frequency should be low, and that the current forming the arc be large, in order to render the magnet more effective.” 2

So what Tesla makes very clear here is that magnetic quenching works best when you are working with:

  1. DC
  2. Low-frequency AC
  3. Large currents

To increase the amount of current in the arc, which improves magnetic quenching, Tesla recommends connecting the spark gap in a specific way:

Ideal circuit for using magnetically quenched spark gap
Figure 5. Ideal circuit for using magnetically quenched spark gap

“When a magnet is employed to break the arc, it is better to choose the connection indicated diagrammatically in Fig 5, as in this case the currents forming the arc are much more powerful, and the magnetic field exercises a greater influence.” 5

Construction

The pole pieces of the magnet are movable and properly formed so as to protrude between the brass knobs [A B], in order to make the field as intense as possible; but to prevent the discharge from jumping to the magnet the pole pieces are protected by a layer of mica, M M, of sufficient thickness.” 5

It is clear from Tesla’s drawings and description, that the magnets he used were formed into pointy shapes to maximize the magnetic field near the arc and to be able to place the tips of the magnets as close as possible to the arc, with a layer of dielectric material (mica) in between to prevent the spark from flying from the electrodes to the magnets. He even thinned down the ends of the electrodes so the magnets could be placed even closer to the arc!

Magnetically quenched spark gap from 1893 lecture
Figure 6. Magnetically quenched spark gap from 1893 lecture

“The discharge rods d d1, thinned down on the ends in order to allow a closer approach of the magnetic pole pieces” 2

However, in many replications of the magnetically quenched spark gap, round neodymium magnets are used instead of pointy magnets (this), and sometimes there is not even a layer of dielectric in between the arc and the magnets, forcing the experimenter to place the magnets farther away (this). This is not according to Tesla’s specifications, and so it is hard to draw any reliable conclusions from the results of these experiments.

We discussed before that magnetic quenching works well with large currents, but these can also easily damage the electrodes. Tesla has a suggestion:

“When the current through the gap is comparatively large, it is of advantage to slip on the points of the discharge rods pieces of very hard carbon and let the arc play between the carbon pieces. This preserves the rods, and besides has the advantage of keeping the air space hotter, as the heat is not conducted away as quickly through the carbons, and the result is that a smaller E. M. F. in the arc gap is required to maintain a succession of discharges.” 2

So pieces of “very hard carbon” can preserve the electrodes of your spark gap. Do notice however that Tesla does not recommend to use electrodes made entirely out of carbon, like the often used carbon welding rods (like in Karl Palsness’ Hairpin replication here). In figure 4 at the beginning of this post, Tesla actually used brass knobs as electrodes, which can withstand higher temperatures than aluminium, but are about three times more conductive than carbon steel 7.

We learned that magnetically quenched spark gaps work best with large DC currents, however, DC spark gaps don’t always start automatically and need some help, so Tesla came up with a solution:

“When it is desired to start the arc, one of the large rubber handles h h1 is tapped quickly with the hand, whereby the points of the rods are brought in contact but are instantly separated by the springs r r1.” 2

So Tesla could bring the electrodes close together for an instant to start the initial spark, and then a spring would immediately push it back into place. A nice example of such a system can be found in this spark gap.

In an alternative arrangement, Tesla did not place the magnets at right angles, but used them as the electrodes:

“In another arrangement with the magnet I take the discharge between the rounded pole pieces themselves, which in such case are insulated and preferably provided with polished brass caps.” 5

And again in another lecture:

“In another form of discharger, combining the features before mentioned, the discharge was made to pass between two adjustable magnetic pole pieces, the space between them being kept at an elevated temperature.” 2

It is interesting to note here that, like when Tesla mentioned using carbon pieces, he apparently believed it was beneficial to warm up the space between the electrodes, which lowers the breakdown voltage of the gap.

Finally, and this is something not many of us will be able to replicate I’m afraid, Tesla mentioned that he used hydrogen in some of his magnetically quenched spark gaps:

“In this form of break [figure 6], I changed the atmosphere in which the arc was operating. The atmosphere was mostly hydrogen, and with this device I performed my experiments before the Franklin Institute in Philadelphia and the National Electric Light Association in St. Louis.” 1 *

* In the online version of the text an image of an air quenched spark gap is shown near this quote. This is incorrect, since the airflow through the air quenched spark gap would make it impossible to contain the hydrogen. Besides, later Tesla makes reference to the similarity between his design and the Poulsen arc, which also uses hydrogen and is magnetically quenched.

He mentions that this magnetically quenched, hydrogen filled spark gap is the one he used in two of his famous lectures. Tesla continuous in a bitter tone that this design was later attributed to Valdemar Poulsen, whose spark gaps powered radio transmission for years 8.

“This has been used by Poulsen and it is now called the “Poulsen arc” and “Poulsen system.” But, of course, there is no invention in it. I am on record with prior publications, and besides, the hydrogen does not have any other effect except that it lowers the tension under which the device can operate. It has the disadvantage of producing asymmetrical or distorted waves, and the impulses obtained are not best suited for tuning.” 1

So while using hydrogen has advantages, it clearly also has disadvantages. From Tesla’s comment it does not seem like hydrogen is a key component.

Air quenched spark gap

Air quenched spark gap
Figure 7. Air quenched spark gap

Another spark gap Tesla successfully used is the air quenched spark gap, which quenches the arc through a “draught” of warm air which removes hot ions from between the electrodes and physically disrupts the arc, allowing the capacitors to recharge for the next pulse.

I will let Tesla describe the construction and working of his air quenched gap:

Another form of discharger, which may be employed with advantage in some cases, is illustrated in Fig. 3/167.  In this form the discharge rods d d1 pass through perforations in a wooden box B, which is thickly coated with mica on the inside, as indicated by the heavy lines.  The perforations are provided with mica tubes m m1 of some thickness, which are preferably not in contact with the rods d d1. The box has a cover c which is a little larger and descends on the outside of the box. The spark gap is warmed by a small lamp l contained in the box. A plate p above the lamp allows the draught to pass only through the chimney a of the lamp, the air entering through holes o o in or near the bottom of the box and following the path indicated by the arrows. When the discharger is in operation, the door of the box is closed so that the light of the arc is not visible outside. It is desirable to exclude the light as perfectly as possible, as it interferes with some experiments.” 2

So Tesla used a lamp at the bottom of the spark gap to warm the air inside the gap, which is drawn in from the holes that are visible near the bottom of the drawing, but says other methods are acceptable too:

“Instead of using the lamp, it answers the purpose to provide for a draught of warm air in other ways.” 2

Tesla further describes the reason why warming the air in the gap is important, especially in an air quenched spark gap:

“This form of discharger is simple and very effective when properly manipulated. The air being warmed to a certain temperature, has its insulating power impaired; it becomes dielectrically weak, as it were, and the consequence is that the arc can be established at much greater distance. The arc should, of course, be sufficiently insulating to allow the discharge to pass through the gap disruptively. The arc formed under such conditions, when long, may be made extremely sensitive, and the weal draught through the lamp chimney is quite sufficient to produce rapid interruptions. The adjustment is made by regulating the temperature and velocity of the draught.” 2

So the warm air ultimately allows longer arcs to form, and the longer the arc, the easier it is to extinguish it with a draught of warm air. It is important to note that Tesla mentions using warm air several times, and that a “draught” of air should do the trick if the correct conditions are established, so a 12V computer fan blasting away at full speed next to a spark gap is not really following Tesla’s instructions…

To further assist in making the arc “long and unsteady”, Tesla recommended connecting the spark gap in a different manner than the magnetically quenched gap we discussed previously:

Air quenched spark gap circuit
Figure 8. Ideal circuit for air quenched gap

“Instead of the magnet, a draught or blast of air may be employed with some advantage. In this case the arc is preferably established between the knobs A B, in Fig. [8] (the knobs a b being generally joined, or entirely done away with), as in this disposition the arc is long and unsteady, and is easily affected by the draught.” 5

So this setup, which bears some resemblance to the “original” Hairpin circuit, results in smaller currents through the spark gap, and for smaller currents Tesla recommends using aluminium electrodes:

“With small currents through the gap it is best to employ aluminum, but not when the currents are large.” 2

So use the air quenched gap when working with small currents and long, feeble sparks. Also make sure you use aluminium electrodes and warm air. Ideally you should be able to control the temperature and the air velocity inside your air quenched spark gap to find the perfect conditions for your sparks to fly. Tesla did not mention whether these gaps are better used with AC or DC currents.

Series static gap

Series static gap
Figure 9. Series static gap

“This device is now known in the art as the “quenched spark gap.”” 1

In this next type of spark gap, the arc is spread out over multiple conductors, and was called, as you can see from the quote above, a “quenched spark gap” in Tesla’s time. Personally I believe this name is a bit misleading, because in fact this gap is not actively quenched. Richard Quick has a more accurate name for it:

“Gaps of this type are known as “series static gaps”. “Static” in this use refers to the fact that the gap is not actively quenched. The plasma is formed in several locations, and the voltage at each gap is lowered as more electrodes are placed in series. Heat, hot ions, and voltage are distributed.” 6

Distributing the arc over several gaps in series has several benefits according to Tesla:

“In the use of this kind of discharger I have found three principal advantages over the ordinary form. First, the dielectric strength of a given total width of air space is greater when a great many small air gaps are used instead of one, which permits of working with a smaller length of air gap, and that means smaller loss and less deterioration of the metal; secondly by reason of splitting the arc up into smaller arcs, the polished surfaces are made to last much longer; and, thirdly, the apparatus affords some gauge in the experiments. I usually set the pieces by putting between them sheets of uniform thickness at a certain very small distance which is known from the experiments of Sir William Thomson [Lord Kelvin] to require a certain electromotive force to be bridged by the spark.” 5

So the “three principal advantages” of this type of spark gap are:

  1. Less energy lost in the arcs
  2. Electrodes last longer
  3. Offers a way to measure voltage through the gap

Another not mentioned benefit is that smaller arcs are of course a lot less noisy than larger arcs.

It seems like only the first point is of real interest to the experimenter trying to replicate Tesla’s effects, since less energy lost means a bigger rate of change. Having to change electrodes less frequently, measuring voltage, and reducing noise are often not necessarily critical.

Of course Tesla managed to push this spark gap to its limits again, and was able to achieve a remarkable feat:

“With this kind of discharger I have been able to maintain an oscillating motion without any spark being visible with the naked eye between the knobs, and they would not show a very appreciable rise in temperature.” 5

And several years later:

“This device incorporated many spark gaps in series. It had a peculiar feature; namely, through the great number of gaps, I was able, as I have pointed out in my writings, to produce oscillations without even a spark being visible between the knobs.” 1

Tesla also mentioned that it worked best in the “Hairpin” setup indicated in Figure 8:

“I have used it preferably in a disposition similar to that indicated in Fig. [8], when the currents forming the arcs are small.” 5

As to the construction of his series static gap:

It consists of a number of brass pieces c c (Fig. 6), each of which comprises a spherical middle portion m with an extension e below which is merely used to fasten the piece in a lathe when polishing up the discharging surface and a column above, which consists of a knurled flange f surmounted by a threaded stem I carrying a nut n, by means of which a wire is fastened to the column. The flange f conveniently serves for holding the brass piece when fastening the wire, and also for turning it in any position when it becomes necessary to present a fresh discharging surface. Two stout strips of hard rubber R R, with planed grooves g g (Fig. 7) to fit the middle portion of the pieces c c, serve to clamp the latter and hold them firmly in position by means of two bolts C C (of which only one is shown) passing through the ends of the strips.” 5

As you can read in the above passage, Tesla used brass electrodes in this type of gap, and these electrodes were easy to turn in order to twist a fresh piece of brass surface into place, which was one of the main benefits mentioned earlier on.

Richard Quick, who we quoted before in this article, developed a variation of the series static gap which is popular among Tesla Coil builders who are trying to produce large sparks (I will discuss at length in upcoming posts why large sparks are not what Tesla’s main purpose was with his coils). This gap is knows as the RQ gap (link), and is essentially several copper tubes placed inside a cylinder, and a fan blowing some air through it to improve quenching. While this type of gap seems to work well for some experimenters, this design does not really fits Tesla’s idea of having movable and rotatable electrodes.

He did make an observation worth mentioning through:

“The lesson learned was too many gaps in series kills the Q of a spark gap. By adding gaps in parallel, and reducing the number of gaps in series, some Q was regained while power levels increased. This is a valuable hint in spark gap designs.” 6

So there is a limit to the amount of electrodes you can put in series before performance starts to drop significantly.

Rotary spark gap or Mechanical break

Rotary spark gap
Figure 10. Rotary spark gap

Another interesting form of spark gap employed by Tesla was the rotary spark gap, or “mechanical break”. I give the floor to Richard Quick once more:

“The rotary gap is a mechanical spark gap usually consisting of revolving disk with electrodes mounted on the rim. The rotor is spun and the electrodes move in relation to a set of stationary electrodes nearby. As a moving electrode comes near a stationary electrode, the gap fires. As is moves away the arc is stretched and broken. The rotary gap offers the sophisticated coiler the opportunity to control the pulse in the tank circuit. A properly designed rotary gap can control the break rate (bps) and the dwell time.” 6

While Tesla used rotary gaps extensively and even used them in several patents (514,168 & 568,180), he did not seem too enthusiastic about them in 1892:

“I may here mention that I have also used dischargers with single or multiple air gaps, in which the discharge surfaces were rotated with great speed. No particular advantage was, however, gained by this method, except in cases where the currents from the condenser were large and the keeping cool of the surfaces was necessary, and in cases when, the discharge not being oscillating of itself, the arc as soon as established was broken by the air current, thus starting the vibration at intervals in rapid succession. I have also used mechanical interrupters in many ways. To avoid the difficulties with frictional contacts, the preferred plan adopted was to establish the arc and rotate through it at great speed a rim of mica provided with many holes and fastened to a steel plate.” 5

So the benefits were mainly cooling and quenching, which are in fact fair benefits. However, the gaps described in the passage above do not seem to rotate the conductors around, like in a classic rotary gap, but instead the surfaces were rotated or a rim of dielectric material was rotated through the arc.

Tesla also mentioned he had a rotary spark gap in some of his labs during that time:

Rotary spark gap in Tesla's lab
Figure 11. Rotary spark gap in Tesla’s lab

“This is the apparatus I had at 35 South Fifth Avenue and also Houston Street.  It shows the whole arrangement as I had it for the demonstration of effects which I investigated.” 1

However, this too wasn’t exactly his favourite setup:

“That is the way I had it for the production of current effects which were rather of damped character because, at that period, I used circuits of great activity which radiated rapidly… It was a transmission by electromagnetic waves. The solution lay in a different direction. I am showing you this simply as a typical form of apparatus of that period.” 1

It is not 100% clear if he meant the spark gap was the problem, or that the problem lay in the fact that his circuits radiated too much energy as high frequency Hertzian waves. Tesla described another form of rotating gap he used at his Houston Street lab:

Rotary spark gap at Tesla's Houston Street lab
Figure 12. Rotary spark gap at Tesla’s Houston Street lab

“Here I show an apparatus that was installed in the Houston Street laboratory prior to the other break because I wanted to get as high a number of impulses as possible. The drawing dates from the spring of 1896.  It is a break with which I could reach from 15,000 to 18,000 interruptions per second. I used it very much until later I found it was not necessary [to create so many interruptions per second]. That is the innocent device which Marconi thought a great invention.

…It consists of two discs of aluminum, with teeth of aluminum on the side. They were rotated by two motors in opposite directions, and as they rotated they alternately closed and opened the circuit. In some instances I used an uneven number of teeth on one and and even number on the other so that I could produce as many breaks as I desired. I will show you later an apparatus more perfect than this one, and of a different kind, in which I have 24 stationary contacts, and 25 rotating elements that established the contact and broke it, so that by one revolution I obtained 24 times 25, or 600 interruptions [per revolution].” 1

Tesla mention that this rotating gap, which again consists of aluminium electrodes, allowed him to reach a very high firing frequency, up to “18,000 interruptions per second”, but that a high firing frequency turned out to be something that wasn’t necessarily desirable. Nevertheless, it is interesting to hear Tesla mention that the wave frequencies he could obtain with this setup were “from a few thousand up to a million per second.” 1. When asked what the actual frequencies were that he used, he mentioned a spark frequency of “5,000, 6,000—sometimes higher still” 1 and a wave frequency “from 30,000 to 80,000” 1.

Tesla also found out another powerful feature of the rotary spark gap; it allowed him to let the arcs occur at the exact peak of the alternating input, which maximized the amount of voltage through the primary at each spark. This powerful method was patented in 1896.

Patent to break AC at peak of the wave using a rotary spark gap
Figure 13. Patent to break AC at peak of the wave using a rotary spark gap

“This is a form of break which I developed in working with alternators. I recognized that it was of tremendous advantage to break at the peak of the wave. If I used just an ordinary break, it would make and break the current at low as well as high points of the wave. Of this apparatus I had two forms; one in which I drove the break right from the shaft of the dynamo and the other in which I drove it with an isochronous motor. Then, by a movement of these knobs (K K), I would make the adjustments so that the makes would occur exactly at the top of the wave. That is a form of break which is embodied in hundreds of patents and used now extensively.” 1

And later:

“I used this machine [a Westinghouse alternator], as I said, either to produce alternating currents and then interrupt them with a mechanical break at the high peaks of the wave; or, I used alternating currents and interrupted them with an independent rotating break having a great number of teeth.” 1

This last passage hints at two modes of operation: a synchronous mode where the arcs occur only at the peak of the input wave, while in the other mode the arcs are asynchronous and occur whenever the electrodes pass each other. The latter version requires less fine tuning and is therefore easier to construct and control, but will result in more irregular voltage levels per discharge.

One major danger in using a rotary spark gap is that if your tuning is off, or the rotation is disturbed, the electrodes will take too long to pass each other, allowing a voltage to build up in the capacitors that is too high and which can then damage your capacitors and/or your transformer. To prevent this from happening, a simple static safety gap can be put in parallel to the rotary gap. When the voltages reach a point that is dangerously high, the static gap allows a path to safely discharge this pressure, protecting the rest of your circuit.

Liquid dischargers

Tesla also worked on a sub-class of rotary spark gaps which were immersed in a liquid.

“The ideal medium for a discharge gap should only crack, and the ideal electrode should be of some material which cannot be disintegrated… A liquid, especially under great pressure, behaves practically like a solid, while it possesses the property of closing the crack. Hence it was thought that a liquid insulator might be more suitable as a dielectric than air.” 2

So Tesla believed that air was not the ideal medium for discharges, since air does not “crack”, but it actually “breaks” and then creates a conducting path of hot ions, which reduces the performance of the gap. Therefore he started experimenting with different discharge mediums.

Liquid discharger
Figure 14. Liquid discharger

* This image was missing from the online text, but is present in the book

“The discharge gap was filled with a medium which behaved practically like a solid, which possessed the duality of closing instantly upon the occurrence of the break, and which moreover was circulating through the gap at a rapid rate. Very powerful effects were produced by discharges of this kind with liquid interrupters, of which a number of different forms were made. It was found that, as expected, a longer spark for a given length of wire was obtainable in this way than by using air as an interrupting device. Generally the speed, and therefore also the fluid pressure, was limited by reason of the fluid friction, in the form of discharger described, but the practically obtainable speed was more than sufficient to produce a number of breaks suitable for the circuits ordinarily used. In such instances the metal pulley P was provided with a few projections inwardly, and a definite number of breaks was then produced which could be computed from the speed of rotation of the pulley. Experiments were also carried on with liquids of different insulating power with the view of reducing the loss in the arc. When an insulating liquid is moderately warmed, the loss in the arc is diminished.” 2

Tesla observed that his “liquid interrupters” possessed nearly instant quenching capabilities, resulting in very powerful effects and longer sparks. He again mentions the importance of warming the discharge medium to improve performance, in this case by reducing energy losses. Another major benefit of a rotary spark gap, whether discharging through air or a liquid, is that it creates “a definite number of breaks” which can be “computed from the speed of rotation”.

However, despite these benefits, this type of gap was not without downsides:

“A point of some importance was noted in experiments with various discharges of this kind. It was found, for instance, that whereas the conditions maintained in these forms were favorable for the production of a great spark length, the current so obtained was not best suited to the production of light effects. Experience undoubtedly has shown, that for such purposes a harmonic rise and fall of the potential is preferable.” 2

This passage implies that the currents from this spark gap were not harmonic in nature, but quite possibly powerful and extremely abrupt, due to the fast quenching times.

So far Tesla mentioned that he experimented with several liquids, but what liquid did he prefer to use? He mentions this while discussing one of his patents which includes a liquid rotary gap.

CIrcuit Tesla used at the Chicago World Fair
Figure 15. CIrcuit Tesla used at the Chicago World Fair

“This [Figure 15] is the apparatus used in the Chicago Exposition of 1893, at which time I explained for the first time to Professor Helmholtz my plan for transmitting energy… The break was automatically effected by means of a turbine. The oil was circulated by a pump, and the current of oil drove the turbine which effected the make and break. Owing to the fact that the oil used was a very good insulator, rapidly flowing and of great dielectric strength, these make-and-break points were very close together, and the arcs extremely short. The effects were accordingly more intense. Here [T in Fig. 1 of figure 15] is a cooler through which the oil was circulated. The oil was forced through the gaps at great speed, and as it flowed out it was supplied again to the tank and the current driving the turbine…. It had vanes like those of a propeller and constituted a rotary break in the circuit… The rotary gap is shown in detail [Fig. 2 in figure 15]” 1

There we have it: the liquid Tesla used in this type of rotary gap was oil. He also reiterates that arcs created with this type of gap can be made extremely short, resulting in a large rate of change.

Vacuum tubes & gas filled tubes

Figure 16. Vacuum tube spark gap

The final type of spark gap we will discuss is the vacuum tube, in which two electrodes are enclosed in a vacuum. The idea is that a spark in a vacuum is better able to create a single frequency and more undamped waves, where “regular” spark gaps generate fairly broad-band signals and more damped waves, even though Tesla insisted he was able to create undamped waves without a vacuum.

Damped waves in air vs vacuum
Figure 17. Damped waves in air vs vacuum

Tesla did experiment with vacuum tubes, although he initially experienced some difficulties using them.

“The use of the magnet permits, however, of the arc being replaced by a vacuum tube, but I have encountered great difficulties in working with an exhausted tube.” 5

We learned before that warming up the air between the electrodes can lower the breakdown voltage, and a vacuum can have a similar effect.

“The air may be rendered dielectrically weak also by rarefaction. Dischargers of this kind have likewise been used by me in connection with a magnet. A large tube is for this purpose provided with heavy electrodes of carbon or metal, between which the discharge is made to pass, the tube being placed in a powerful magnetic field The exhaustion of the tube is carried to a point at which the discharge breaks through easily, but the pressure should be more than 75 millimetres, at which the ordinary thread discharge occurs.” 2

The lower breakdown voltage due to the vacuum has a similar benefit to the series static gap discussed before: lower voltages create smaller sparks, which in turn make less noise and give the electrodes a longer life span. It is also good to note that Tesla combined his vacuum discharges with a strong magnetic field for better quenching.

However, Tesla believed that a pressurized gas is even more effective than a vacuum.

“Air or other gas under great pressure is of course a much more suitable medium for the discharge gap [relative to a vacuum]. I have carried on long-continued experiments in this direction, unfortunately less practicable on account of the difficulties and expense in getting air under great pressure.” 2

The high-pressure gas was apparently just a lot more difficult to work with. Even though Tesla experimented with vacuum tubes early on -devices which would soon replace nearly all spark gaps- it seems from our investigation that he preferred the rotary spark gap over the vacuum tube in the end.

What happened to spark gaps?

We just discussed an exotic array of spark gaps, all with their own unique characteristics. However, these days it is hard to find spark gaps still in active duty, apart from people building Tesla Coils. Why is that? Here is a brief history of the end of the spark gap era.

“Although the spark gap transmitter was developed and improved to a very large degree, it could never compete with transmitters using valves [vacuum tubes] that were able to create a proper single frequency signal. Spark gap transmitters were inherently wide band and although improvements in the design meant they occupied considerably less spectrum, they could never be as flexible and effective as those using valve based equipment.

As a result of this their use declined in the late 1910s and early 1920s. Although spark transmitters were retained for many years as a last ditch form of emergency communications on distress frequencies, they were ultimately rendered obsolete and their use banned as they caused interference to others.” 9

It is sad to note that in the Spark gap transmitter timeline on the page quoted above, no mention of Tesla is made, even though he invented, perfected, and patented multiple types of ingenious spark gaps and would eventually build the largest spark gap transmitter ever produced on Long Island. The main problem seems to be that Tesla invented and experimented a lot, but didn’t always turn his devices into actual products people could readily buy and use.

So the problem with spark gaps was, as mentioned before, that they generally create a wide range of frequencies as well as damped waves, while radio communication requires a single frequency of undamped or continuous waves of constant amplitude. Again, Tesla said he was able to achieve this with his spark gaps as well:

“I can obtain oscillations of any frequency I desire. I can make them damped or undamped. I can make them of one direction or alternating in direction as I choose.” 1

But he said others had trouble achieving similar results, as we read in a quote I mentioned earlier in this text:

“Some experimenters who have gone after me have found a difficulty. They said: “No, we cannot produce a constant train of oscillations.” Well, it is not my fault. I never have had the slightest difficulty. I produced constant oscillations and I have described how I produced them. Anyone who has no more than my own skill can do it.” 1

In the end, the desired results were easier to achieve using vacuum tubes, and so they replaced spark gaps. These days, vacuum tubes are mostly replaced by solid state devices, like transistors, so sparks have officially left the scene.

Closing thoughts

Wow, this article turned out to be a lot longer than I anticipated! What started off as some research into what spark gap Tesla preferred to use, ended up in a comprehensive literature analysis. So what was Tesla’s preferred spark gap?

Well, as we learned, certain spark gaps work better in certain situations. However, most of the spark gaps discussed here were described in Tesla’s 1892 and 1893 lectures, while Tesla used a liquid rotary gap at the 1893 Chicago World Fair, and has patents containing rotary gaps from 1894 and 1896, so it seems like the rotary gap was the main device Tesla was using in later years.

This makes sense, since the rotary gap, while tricky to construct, allows accurate control over the spark frequency, creates a consistent train of oscillations, has great quenching capabilities, and delivers the highest possible voltage per spark, since it can break the input wave at its peak.

However, if you’re looking at replicating Tesla’s “stout copper bars” circuit, which is what started my journey into spark gap land, it seems more likely that Tesla was using an air quenched spark gap for that particular experiment based on the circuit described in figure 8, so it depends on what your goal is.

Whatever spark gap you decide to use, one thing seems to be a good idea across the board, and that is to warm the medium through which you are discharging. Just don’t perform any transmission experiments using spark gaps, or you risk jamming existing radio transmissions, which could get you (and them) in trouble.

It is worth appreciating the fact that we can now simply purchase a $50 function generator from AliExpress and generate undamped waves of any desired frequency with ease, while this was an incredibly difficult feat to achieve in Tesla’s time when the only method available was a spark gap! I hope reading this information was as useful to you as it was for me putting it together.