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A Brief History of the Tesla Hairpin Circuit / Stout Copper Bars

Nikola Tesla hairpin circuit stout copper bars 1893 lecture

In his autobiography, Nikola Tesla called the Magnifying Transmitter (TMT) his “best invention” 1, but it is also one of his most complex and misunderstood inventions. I decided that the only way to truly understand the TMT, was to retrace all the steps that led up to this invention, and the Hairpin circuit, which is covered in detail in this post, seems to be at the root of it all. This seemingly simple circuit contained more surprises and subtle complexities than I anticipated, so let’s dive in and see what we can learn from the inventor himself!

A journey back in time

Before we discuss how to replicate this unique device, it is important to trace its origin. Tesla first described the Hairpin circuit during his 1891 lecture in New York:

Nikola Tesla hairpin stout copper bars circuit from his 1891 lecture
Figure 1. First mention of Hairpin circuit by Tesla during his 1891 lecture

“In operating devices on the above plan I have observed curious phenomena of impedance which are of interest. For instance if a thick copper bar be bent, as indicated in Fig. [1], and shunted by ordinary incandescent lamps, then, by passing the discharge between the knobs, the lamps may be brought to incandescence although they are short-circuited. When a large induction coil is employed it is easy to obtain nodes on the bar, which are rendered evident by the different degree of brilliancy of the lamps, as shown roughly in Fig. [1]. The nodes are never clearly defined, but they are simply maxima and minima of potentials along the bar. This is probably due to the irregularity of the arc between the knobs.

In general when the above-described plan of conversion from high to low tension is used, the behavior of the disruptive discharge may be closely studied. The nodes may also be investigated by means of an ordinary Cardew voltmeter which should be well insulated. Geissler tubes may also be lighted across the points of the bent bar; in this case, of course, it is better to employ smaller capacities. I have found it practicable to light up in this manner a lamp, and even a Geissler tube, shunted by a short, heavy block of metal, and this result seems at first very curious. In fact, the thicker the copper bar in Fig. [1], the better it is for the success of the experiments, as they appear more striking. When lamps with long slender filaments are used it will be often noted that the filaments are from time to time violently vibrated, the vibration being smallest at the nodal points. This vibration seems to be due to an electrostatic action between the filament and the glass of the bulb.” 2

As you can see from figure 1 and Tesla’s description, this device mainly consists of thick copper bars, hooked up to a high voltage power source, which charges capacitors until they reach a high enough voltage to make them discharge disruptively through a spark gap. Looks like a rather simple circuit, right? Well, there is more to it than meets the eye, and the effects of “nodes on the bar”, hinting at electrical standing waves, and lighting lamps while they’re short circuited, are curious enough results to make this device worth investigating.

Nikola Tesla Chicago World Fair 1893 Hairpin Circuit
Figure 2. A picture taken at the Chicago World Fair of 1893, showing a Hairpin with bulbs connected to it on the right, and “six-pack” Leyden Jar capacitors on the table in the back

The next time Tesla mentioned this circuit was during a lecture two years later in Philadelphia, showing a slightly different setup:

Nikola Tesla hairpin circuit stout copper bars 1893 lecture
Figure 3. Tesla “stout copper bars” circuit from his 1893 lecture

“Referring to Fig. [3]a, B and B1 are very stout copper bars connected at their lower ends to plates C and C1, respectively, of a condenser, the opposite plates of the latter being connected to the terminals of the secondary S of a high-tension transformer, the primary P of which is supplied with alternating currents from an ordinary low-frequency dynamo G or distribution circuit. The condenser discharges through an adjustable gap d d as usual. By establishing a rapid vibration it was found quite easy to perform the following curious experiment. The bars B and B1 were joined at the top by a low-voltage lamp l3 a little lower was placed by means of clamps C C, a 50-volt lamp l2; and still lower another 100-volt lamp l1; and finally, at a certain distance below the latter lamp, an exhausted tube T. By carefully determining the positions of these devices it was found practicable to maintain them all at their proper illuminating power. Yet they were all connected in multiple arc to the two stout copper bars and required widely different pressures. This experiment requires of course some time for adjustment but is quite easily performed.

In Figs. [3]b and [3]c, two other experiments are illustrated which, unlike the previous experiment, do not require very careful adjustments. In Fig. [3]b, two lamps, l1 and l2, the former a 100-volt and the latter a 50-volt are placed in certain positions as indicated, the 100-volt lamp being below the 50-volt lamp. When the arc is playing at d d and the sudden discharges are passed through the bars B B1, the 50-volt lamp will, as a rule, burn brightly, or at least this result is easily secured, while the 100-volt lamp will burn very low or remain quite dark, Fig. [3]b. Now the bars B B1 may be joined at the top by a thick cross bar B2 and it is quite easy to maintain the 100-volt lamp at full candle-power while the 50-volt lamp remains dark, Fig. [3]c. These results, as I have pointed out previously, should not be considered to be due exactly to frequency but rather to the time rate of change which may be great, even with low frequencies. A great many other results of the same kind, equally interesting, especially to those who are only used to manipulate steady currents, may be obtained and they afford precious clues in investigating the nature of electric currents.

In the preceding experiments I have already had occasion to show some light phenomena and it would now be proper to study these in particular; but to make this investigation more complete I think it necessary to make first a few remarks on the subject of electrical resonance which has to be always observed in carrying out these experiments.” 3

You may notice that this time, the capacitors and spark gap have switched place in the circuit compared to the 1891 version. Also, the top shunt bar is now detachable, allowing for more types of experiments to be performed. Tesla also goes into much more detail here explaining his experiments with this device, achieving fascinating results, like lighting several lamps of different voltage ratings at full brightness… while they’re short circuited!

One thing to note is that Nikola Tesla himself never used the term “Hairpin circuit”. In fact, he never really seems to have given this device a name at all, but only refers to it as “stout copper bars”. Wherever the name Hairpin originated, it’s catchy, a lot of people use it already to describe this device, and it is easier to write, which is why I use the name Hairpin in this article.

There is one other time Tesla mentioned the Hairpin circuit, in an 1898 article on the electro-therapeutic benefits of high frequency currents, and it is possibly the most interesting article out of all the ones mentioned here.

“One of the early observed and remarkable features of the high frequency currents, and one which was chiefly of interest to the physician, was their apparent harmlessness which made it possible to pass relatively great amounts of electrical energy through the body of a person without causing pain or serious discomfort… these currents would lend themselves particularly to electro-therapeutic uses.” 4

So the high frequency currents generated from the capacitor discharges were so harmless that they were actually passed through the bodies of real patients, without pain! This explains how experimenters like Karl Palsness, whose Hairpin replication we will discuss at length later on, are able to hold the copper bars while the spark gap is firing, and even light a lamp while its submerged in water and then touching the water, without receiving as much as a shock.

“Now, why is it that in a space in which such violent turmoil is going on living tissue remains uninjured?”, Tesla asks the reader. He continues:

“One might say the currents cannot pass because of the great self-induction offered by the large conducting mass. But this it cannot be, because a mass of metal offers a still higher self-induction and is heated just the same. One might argue the tissues offer too great a resistance. But this again cannot be the reason, for all evidence shows that the tissues conduct well enough, and besides, bodies of approximately the same resistance are raised to a high temperature. One might attribute the apparent harmlessness of the oscillations to the high specific heat of the tissue, but even a rough quantitative estimate from experiments with other bodies shows that this view is untenable. The only plausible explanation I have so far found is that the tissues are condensers. This only can account for the absence of injurious action.” 4

So while Tesla does not seem to have a conclusive explanation for the apparent harmlessness of his high frequency currents, it does help to hear his reasoning and insight on the matter, since I’ve heard many people say in forum posts that “it’s just RF”, suggesting that it is simply a characteristic of radio frequency currents to not injure the human body. However, do not try this at home, because there is also something called Electrosurgery, which is “the application of a high-frequency (radio frequency) alternating polarity, electrical current to biological tissue as a means to cut, coagulate, desiccate, or fulgurate tissue.” Not sure what those last three things are, but they sound awful! Be careful.

Also crucial to note is how Tesla never attributes his peculiar results to an exotic type of energy. There is no mention of scalar waves, longitudinal energy, cold electricity, or any of that stuff people love to throw around. He is simply talking about high voltage, high frequency currents.

So what does his electro-therapy device look like? Tesla actually shows us several different setups in the article, as shown in figure 4 below.

Tesla high frequency oscillators for electro-therapy hairpin circuit
Figure 4. Eight “modes of connection” for Tesla’s electro-therapeutic device

The circuits in figure 4 might be a bit much to take in at one time, so I would like to point your attention now to figure 4.3 specifically, which is in fact a circuit identical to the Hairpin! Let’s see what Tesla had to say about that particular setup..

“One of the prominent characteristics of high frequency or, to be more general, of rapidly varying currents, is that they pass with difficulty through stout conductors of high self-induction. So great is the obstruction which self-induction offers to their passage that it was found practicable, as shown in the early experiments to which reference has been made [The Hairpin from his lectures?], to maintain differences of potential of many thousands of volts between two points — not more than a few inches apart — of a thick copper bar of inappreciable resistance. This observation naturally suggested the disposition illustrated in Fig. [4.3]. The source of high frequency impulses is in this instance a familiar type of transformer which may be supplied from a generator G of ordinary direct or alternating currents. The transformer comprises a primary P, a secondary S, two condensers C C which are joined in series, a loop or coil of very thick wire L and a circuit interrupting device at break b. The currents are derived from the loop L by two contacts c c’, one or both of which are capable of displacement along the wire L. By varying the distance between these contacts, any difference of potential, from a few volts to many thousands, is readily obtained on the terminals or handles T T. This mode of using the currents is entirely safe and particularly convenient, but it requires a very uniform working of the break b employed for charging and discharging the condenser.” 4

So two movable contacts were connected to the conductors, the terminals of which which were then applied to the patient’s skin in an “entirely safe” way! Tesla does mention that the spark gap has to operate very consistently for this to work, or else the waves created on the conductors, and therefore the voltage applied to the patient’s skin, will start to vary. The fact that Tesla required a very uniform working of the spark gap to achieve his results, is what made me research Tesla’s spark gaps in detail, and led me to create a modern version of his air quenched spark gap.

Tesla also mentions another revolutionary effect he could achieve with this device: single wire energy transmission!

“Among the various noteworthy features of these currents there is one which lends itself especially to many valuable uses. It is the facility which they afford for conveying large amounts of electrical energy to a body entirely insulated in space. The practicability of this method of energy transmission, which is already receiving useful applications and promises to become of great importance in the near future, has helped to dispel the old notion assuming the necessity of a return circuit for the conveyance of electrical energy in any considerable amount.” 4

Wow… Tesla already called the need for a return wire an “old notion” in 1898, and still all our electronics require a return wire more than a hundred years later!!

This 1898 article is full of revelations and gives us valuable insight into completely new uses of the Hairpin circuit, but possibly the most interesting part of it is how Tesla shows several different setups of what he seems to view as the same device: some with just one loop of wire, like in figure 4.3, but many come with both a primary and a secondary, like figure 4.2 and 4.4. Yet, the most telling schematic of the bunch is shown in figure 4.5, about which Tesla had the following to say:

“The circuit connections as usually made are illustrated schematically in Fig. [4.5], which, with reference to the diagrams before shown, is self-explanatory. The condensers C C, connected in series, are preferably charged by a step-up transformer… The primary p, through which the high frequency discharges of the condensers are passed, consists of very few turns of cable of as low resistance as possible, and the secondary s, preferably at some distance from the primary to facilitate free oscillation, has one of its ends–that is the one which is nearer to the primary–connected to the ground, while the other end leads to an insulated terminal T, with which the body of the patient is connected. It is of importance in this case to establish synchronism between the oscillations in the primary and secondary circuits p and s respectively.” 4

This sounds an awful lot like a Tesla Coil, complete with a resonantly coupled primary and secondary, where the secondary has one end connected to ground and the other to an insulated terminal. And the most astonishing thing is that Tesla mentions this device in one breath with the Hairpin circuit from figure 4.3! The reason this is of such importance, is because it is like finding a transitional fossil, which describes how the Hairpin eventually evolved into the Tesla Coil.

This might be the reason why some people claim the Hairpin circuit is “just a single turn primary of a Tesla Coil”. Though this might seem like a valid point at first, the Hairpin was clearly used to experiment with electrical standing waves, hence the “nodes on the bar” 2, while standing waves are not required in a Tesla Coil primary, as well as impedance phenomena, which is also not the purpose of a Tesla Coil. So yes, the Hairpin definitely has things in common with a single turn Tesla Coil primary, but they have completely different use cases.

Lecher Lines

When reading forum posts and watching YouTube videos on the Hairpin circuit, there is always someone screaming in ALL CAPS that Tesla is a fraud, because he did not invent this circuit, Ernst Lecher did in 1888 when he invented the Lecher Line. Even though Tesla never actually claims to have invented this circuit, I decided to dig into Lecher’s original paper to see if these assertions had any merit, but when I found out the title was Eine Studie über elektrische Resonanzerscheinungen, I knew I would first have to brush up my German! I even looked into hiring a native speaker to perform the translation, but at 21 pages, that would have cost me a small fortune. Luckily I am Dutch, so German comes fairly naturally to me, and I have several German friends who could help me out with the tricky parts.

It took me a tremendous amount of time, but I finally managed to translate the entire paper (click here for the full translation), which contains a wealth of useful information, and really helps to get a better understanding of the Hairpin circuit. Let’s see how Lecher describes his setup “in its simplest form”:

Orignal Lecher Lines schematic
Figure 5. Lecher Lines schematic taken from the original 1890 article by Ernst Lecher

“A and A’ are square sheet metal plates with 40cm sides; they are connected by means of a 100 cm long wire segment, which is cut in the middle and at F two brass balls of 3 cm in diameter are added (in Fig. 1, only the cross-section of the square plates is drawn). The two brass balls are at a distance of 0,75 cm from each other and are connected using thin wires to the poles of a very strong inductor, whose coil has a length of 35 cm and a diameter of 18 cm; the inductor is fed by four powerful accumulators [batteries], and in some cases by a dynamo. A Foucault mercury interrupter serves as electric break. Across from the plates A and A’ are two plates B and B’ of identical size at a distance of around 4 cm. From these plates B, B’ run two wires against s and s’ and from there parallel until t and t’. The distance between the parallel wires (s to s’) is 10-50 cm; the length st (s’ t’) on the other hand should be at least 400 cm. The diameter of these parallel wires is here and for all experiments in this publication 1 mm. For this first experiment we assume the length [of the wire] to be about 600 cm (drawn too short in the figure ), and the distance of the parallel wires from each other 30 cm. At the end of the parallel wires (t and t’) a cord is connected to each, which extends the length of the wires by about 100 cm and allows for a gentle and comfortable tensioning thereof… Over the wire ends t and t’ I now lay an exhausted glas tube without electrodes g g’, ideally filled with nitrogen and a trace of turpentine vapor; this glass tube starts to light up due to the electrical vibrations in the wires.” 5

It is great that Lecher described his setup in such detail. The “Foucault mercury interrupter” mentioned by Lecher is not to be confused for the spark gap; its function was merely “to rapidly connect and disconnect a direct electric current to create the changing magnetic field needed for induction coils.” 6 In other words, it took the direct current from the batteries and turned it into a pulse current, which then powered the “very strong inductor”.

The metal plates A A’ and B B’ function as the two capacitors we also find in the Hairpin circuit, and because Lecher mentions their specific dimensions and distance from each other, we are even able to calculate their capacitance. Two plates of 40×40 cm, placed at a distance of 4 cm from each other, have a capacitance of 35.4 pF. Since they are placed in series in this circuit, given the short x x’ is present, the total series capacitance in Lecher’s circuit was around 17.7 pF. Lecher also mentions his spark gap was 0,75 cm wide, and since the dielectric strength of air at 25ºC and ordinary atmospheric pressure is 31.300V per cm 7, we can assume that Lecher discharged around 0,75 * 31.300 ≈ 23.500V through his spark gap. Fun facts.

Difference between Lecher Lines and Hairpin circuit

By comparing Lecher’s paper with the writings of Nikola Tesla, we learn that while the circuit diagrams of the Hairpin circuit and the Lecher Lines look similar, and Lecher also lit up lights with it, there are some significant differences. For starters, Lecher did not use “stout copper bars” like Tesla did, but instead used 1 mm wires. Lecher also mentions his wire was more than two times 600 cm, or more than 12 meters, long. Later in the paper he even uses 2 x 20 meter long wires! If we compare this to the Hairpin image in figure 2 at the beginning of this article, we see that Tesla’s bars were a lot shorter than that (looks like the bar is approximately 14 light bulb lengths long, which most definitely is shorter than 12 meter).

Besides, Tesla mentioned that “electromotive forces of many thousand volts are maintained between two points of a conducting bar or loop only a few inches long4, suggesting that his conductors did not need to be that long, possibly because Tesla was able to achieve higher frequencies, and therefore shorter wavelengths, by perfecting the condenser discharge process through the invention of advanced spark gap designs, whereas Lecher used a simple static gap with two brass balls as electrodes.

Finally, the main difference between the Lecher Lines and the Hairpin circuit — and this took me a long time to figure out — is in their purpose: Lecher studied resonance phenomena with his Lecher Lines, while Tesla studied impedance phenomena with his Hairpin circuit. Therefore, Lecher used an electrodeless exhausted tube, which did not have an electrical connection to his conductors, assuring the standing waves were not disturbed, but lit up due to the vibrations in the wires. Tesla, on the other hand, used several incandescent lamps of various voltage ratings, and did connect them electrically to his conductors, since he was showing how high-frequency currents “pass with difficulty through stout conductors”4, causing the current to prefer the resistive path of an incandescent lamp filament over the normally inappreciable resistance of the copper bars.

So did Lecher feel like he invented this circuit? Not really..

“This part of my arrangement is similar to that stated in the beautiful work of Hertz, and was also used in the experiments of Sarasin and De la Rive.”5

Lecher actually credits Hertz, Sarasin and De la Rive for a similar circuit, so the internet crusaders who are fighting to protect Lecher’s primacy and honor at every mention of the Hairpin circuit can hopefully calm down now.

Concluding remarks

In this article we learned that Tesla used his Hairpin circuit to display curious impedance phenomena, single wire energy transmission, apply apparently harmless currents to patients for medical purposes, and, finally, to “create pulsations through metal bars, or pipes, and test for harmonic frequencies and standing waves.”8, as one biographer put it.

We also learned that Tesla does not ascribe these effects to any exotic form of energy. He simply talks about high voltage, high frequency currents. It also became clear that the Hairpin eventually evolved into the Tesla Coil, but that the Hairpin cannot simply be seen as a single turn primary, since the use cases are totally different. The same goes for Lecher Lines, which look similar to the Hairpin in many respects, but differs in several crucial ways, mainly again in its use case.

The information in this article gives us a solid foundation in the journey to understand and eventually replicate the Hairpin circuit, according to Nikola Tesla’s own specifications. There is still one thing we have to cover in more detail before we start the replication and experimentation, one thing which plays a crucial role in understanding why the Hairpin works the way it does, and that is impedance, specifically the so-called “skin effect”. This innocuous sounding term has far reaching consequences for high frequency systems, and also has a surprisingly bumpy past. Click here to read all about impedance and the skin effect.


  1. Tesla, N. (1919). My Inventions: The autobiography of Nikola Tesla. New York, NY: Cosimo Classics
  2. Tesla, N. (1891). Experiments with alternate currents of very high frequency and their application to methods of artificial illumination. Retrieved from
  3. Tesla, N. (1893). On light and other high frequency phenomena. Retrieved from
  4. Tesla, N. (1898). High Frequency Oscillators for Electro-Therapeutic and Other Purposes. Retrieved from
  5. Lecher, E. (1890). Eine Studie über electrische Resonanzerscheinungen. Retrieved from:
  7. De Podesta, M. (2002). Understanding the Properties of Matter. New York, NY: Taylor & Francis.
  8. Seifer, M. (1996). Wizard: The Life and Times of Nikola Tesla: Biography of a Genius. New York, NY: Citadel Press.