Tesla’s Egg of Columbus replica

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There is a legend about Christopher Columbus winning a wager against his critics by making an egg stand on its tip. That story is often used to illustrate the fact that it is often simple to understand the discovery after it already happened. That principle is undoubtedly true in case of famous inventor Nikola Tesla who discovered laws of rotating (rotary) magnetic field. His discoveries lead to the invention of the polyphase induction motor. Although Tesla didn’t discover alternating current, he was the first one to harness it and use it to do useful work efficiently.

Nowadays virtually all electrical motors used in everyday life are based on the principles of rotating magnetic field in one form or another. When Tesla discovered the principles of rotating magnetic field utilisation, he also conceived suitable generators and transformers to provide efficient means of distribution of electrical energy and its transformation. Tesla’s system of distribution of polyphase alternating currents is nowadays universally used for transport of electricity from electrical power generators to end-users.

Tesla originally conceived the idea of the principles of the rotary magnetic field in the February of 1882. During that period young Tesla was working as an electrical engineer in Budapest. In his autobiography Tesla wrote:

One afternoon, which is ever present in my recollection, I was enjoying a walk with my friend in the City Park and reciting poetry. At that age, I knew entire books by heart, word for word. One of these was Goethe’s “Faust.” The sun was just setting and reminded me of the glorious passage, “Sie rückt und weicht, der Tag ist überlebt, Dort eilt sie hin und fordert neues Leben. Oh, da kein Flugel mich vom Boden hebt Ihr nach und immer nach zu streben! Ein schsner Traum indessen sie entweicht, Ach, au des Geistes Flygein wird so leicht Kein korperlicher Flugel sich gesellen!” As I uttered these inspiring words the idea came like a flash of lightening and in an instant the truth was revealed. I drew with a stick on the sand, the diagram shown six years later in my address before the American Institute of Electrical Engineers, and my companion understood them perfectly. The images I saw were wonderfully sharp and clear and had the solidity of metal and stone, so much so that I told him, “See my motor here; watch me reverse it.”

Nikola Tesla

During the following nine years period Tesla worked on perfecting and improving his innovations in the field of alternating current generation, transportation and utilisation. Tesla’s efforts to commercialise his system subsequently lead to a professional confrontation with Thomas A. Edison in so-called “War of Currents”. Tesla’s AC system of electrical energy transmission and utilisation proved to be superior to Edison’s direct current system and he eventually won the “war of currents”. Even in the 21st century virtually all electrical distribution networks in the world use Tesla’s discoveries and innovations.

To better illustrate underlying concepts and ideas behind his inventions Tesla often resorted to showmanship. Very early on, Tesla realised that engineering diagrams and patents were not interesting enough for the majority of people and the same is still true today. During 1890’s and early 1900’s Tesla was renowned for his experiments with electrical currents of high potential and high frequency. He would often let electrical discharges travel over his body, which to most people of his day looked like a miracle.

However, his rotating magnetic field concept was somewhat harder to present because the magnetic field is not visible to a naked eye. Finally, Tesla had an excellent idea of replacing the rotor of the 2-phase induction motor with a copper egg. In that way, the copper egg would rotate in the rotary magnetic field just as any rotor in the induction motor would. As the copper egg gained rotational speed, it was lifted on its main axis as the consequence of gyroscopic effect. Tesla presented his “Tesla’s Egg of Columbus” model in 1893 at the World fair exhibition in Chicago. “Tesla’s Egg of Columbus” fast became popular among the visitors of the World fair exhibition for its ingenuity as well as for its aesthetic appearance. Even today “Tesla’s Egg of Columbus” produces a sense of wonder in the people witnessing the effects of the invisible rotating magnetic field. It also proved to be a great educational tool which, simply conveys basic electro-technical engineering principles to non-engineers.

Conceptually, the “Tesla’s Egg of Columbus” replicas are not difficult to reproduce. However, they usually deviate from the Tesla’s model in a few crucial aspects.

The first difference is that virtually all reproductions use 3-phase AC windings as opposed to Tesla’s original 2-phase system. Although 3-phase reproductions accurately describe Tesla’s general idea of the rotary magnetic field, they also represent historical inaccuracy in the technological aspect. The main reason for using 3-phase windings in the reproductions of “Tesla’s Egg of Columbus” is immediate availability of 3-phase alternating current which is used virtually everywhere in the world and the possibility to use standard star or delta coil configurations.

The second difference is the aesthetical appearance of such modern day reproductions. While it is true that aesthetics of “Tesla’s Egg of Columbus” model is not relevant from the technological point of view, we are of the opinion that the true spirit of Tesla’s ideas has been lost. Everything Tesla ever produced for his public lectures and exhibitions always exhibited a high level of craftsmanship and high aesthetical value. In fact, many Tesla’s professional contemporaries lauded his precision, neatness and even beauty of the models he presented. Virtually all modern “Tesla’s Egg of Columbus” replicas don’t quite have the aesthetic finesse of the Victorian era technological devices.

Currently, there are only a few existing functional models of the “Tesla’s Egg of Columbus” in the world which exhibit technical as well as aesthetical value in the spirit of Tesla’s original model.

One replica is exhibited in the Tesla Museum in Belgrade, Serbia with the size and shape closest to the original which was presented in 1893 at the World fair exhibition in Chicago. Although historically most accurate by its size and shape we find this model simply too modest in its size.

Another model of Tesla’s Egg of Columbus is exhibited in the Technical Museum in Zagreb, Croatia. This model was made as a working prop for the filming of the television series about the life of Nikola Tesla, which was filmed during 1970’s in Yugoslavia. We are much fonder of this version because it is larger than the model exhibited in Tesla Museum in Belgrade. However, this replica is of poor quality and craftsmanship, which is understandable considering the period during which it was made and the meagre budget available at that time.

Two other models of Tesla’s Egg of Columbus are exhibited in Tesla’s Memorial Centre in Smiljan, Croatia and Historical Museum of Croatia in Zagreb, Croatia. Both of those latest model-replicas were designed and made by Zvonimir Rudomino, the owner of RT17 d.o.o. We are proud to say that they are the largest ones in the world both in their size and weight while at the same time exceptional care was given to aesthetics and the conformity to the modern standards of safety.

Our functional replicas of Tesla’s Egg of Columbus

During late 2005 Mr Rudomino was contacted by the people assigned by Croatian government to design, build and equip new Tesla Memorial Centre in Smiljan that was to be opened in the July of 2006 at the date of Nikola Tesla’s 150th birthday. During the preliminary discussions, it was decided that a few working models of Tesla’s inventions should be designed and build. At least four different functional models were to be produced by Mr Rudomino: Tesla’s Egg of Columbus, 2-phase Tesla induction motor, Tesla turbine and Tesla transformer. It was decided that all of the functional models should follow original Tesla’s design as closely as possible, both in technological as well as in the aesthetic details. It was also decided that those functional replicas will have to satisfy all modern safety standards and they would also have to pass safety evaluations by the appropriate governmental institution. By that point, it was evident that the technological challenge would be tremendous given the seemingly conflicting requirements. Until March of 2006, no further talks took place and it was becoming obvious that the whole project would probably be cancelled. However, during early March of 2006 another set of negotiations took place and additional requirements, like automated control of the functional models, were added at the last moment. All of those functional replicas were to be designed from scratch, produced, assembled, tested and approved by authorities in less than four months. Although it was apparent that such task would be an engineering and logistical nightmare, after a brief consideration of all the requirements, we finally agreed and accepted the commissioned job.

During following years several sets of Tesla’s Egg of Columbus were delivered to several customers.

In 2008 we delivered another large Tesla’s Egg of Columbus and large Tesla coil to the Historical Museum of Croatia for their travelling exhibition with the topic of life and work of Nikola Tesla. Due to the lower budget, in this case, we customised the model to reduce overall cost. This model of Tesla’s Egg of Columbus was made of common types of wood, plywood and copper egg was made by spin-forming rather than by machining. Its simple beauty and attention to details gained the attention of exhibition visitors in Madrid, Vienna, Bratislava, Helsinki and other cities.

Several smaller models of Tesla’s Egg of Columbus were delivered to educational institutions in Croatia. These models were customised to be very simple in their appearance and with only the basic finish to minimise the cost.

A few smaller customised models of Tesla’s Egg of Columbus were even delivered to private collectors. In such cases specific design and finish requirements included “rough and worn” finish, very modern looking design of wooden parts and even custom machined copper figurines.

Technological and technical aspects of production

Wooden parts assembly

From the initial discussions and at the later stages it was decided that the functional model of Tesla’s Egg of Columbus had to be the biggest one in the world. It was also agreed that its aesthetical design has to be as close to the spirit of the Victorian era as possible, while at the same time modern materials and technologies had to be used to meet safety standards and retain financial feasibility.

At the earliest stages of the project, we were permitted by the curator of the Technical Museum in Zagreb to carefully examine all available original electrical devices from the Victorian era. We wanted to determine what kind of wood was used during that period, what variety of textures it retained and what were the most commonly used paints and lacquers. After careful considerations, it was decided to avoid modern composite wooden materials because they were not available during the Victorian era. Proper paints and varnishes were also selected in the way that closely matched wooden finishes that were used in the construction of technical devices at the end of 19th century. Another thing to be considered was the durability of the wood from which wooden plate was made because the functional replica is to be used for decades with as less of maintenance as possible. Finally, we settled for the exotic wood iroko because of its hardness (historically, early clocks used cogs made from iroko) and the slight level of greasiness. The slight greasiness of the wood structure should lessen wear of the wooden material if paint and lacquer in the centre of the wood plate eventually wear down after long time use.

At that point, 3D engineering models were made to facilitate discussions with the designers regarding the final appearance of the whole model. It was at that point decided that the base of the entire model should be square shaped to fit existing exhibit stands in the museum. We additionally agreed that the rectangle shape would be more suitable to leave space for patinised brass replicas of the electrical terminals that were at the same time designed and produced for the functional model of the Tesla’s 2-phase induction motor. In that way, the apparent symmetry of the entire operational model was broken and additional electrical terminals provided more “life” to the model. Another addition was large wooden stand-offs that effectively raised stator from the base of the model thus allowing easier manipulation of the stator, and it also provided additional visual dynamics to the whole model.

Once wooden material planks were acquired, they were machined to fit snugly together and were fastened with wooden “cookies” and special glue. In that way, a uniform wooden plate was formed that is very sturdy and it was possible to machine the wooden plate from a single piece of wood. The wooden plate was carved by a precise CNC machining process which allowed high precision, reproducibility and prevented forming of “steps” which would cause the copper egg to jump during its rotation. Although the most demanding parts of the woodwork were CNC machined, the rest of the work was done manually to reduce the total cost of the functional model.

Finally, all of the wooden parts were smoothed with very fine grain sandpaper to remove any remaining irregularities and to provide a high-quality basis for the painting and lacquering. Since it was decided that all of the wood texture should be visible, to keep with the Victorian era designs, the painting was done manually with spirit based paint in the colour tone that was commonly used during the late 19th century. The whole process was finalised by spraying several thin layers of modern lacquer which protects paint and wood from the moisture and the influence of UV light. Thin coats of carefully chosen lacquer also provide a “satin” visual effect which closely matches the look of the woodwork on the devices made at the end of 19th century.

The final assembly of the entire woodwork took place just a few days before the opening of the Tesla Memorial Centre in Smiljan. The design at that time proved to be reliable and the woodwork and its finish don’t show any signs of wear in spite of the unfavourable conditions in the Tesla Memorial Centre in Smiljan (high amount of air moisture, low temperatures during winter periods).

Electrical parts

From the early stages of the project, it was decided that the stator of the Tesla’s Egg of Columbus should be made from modern materials in the form that would ensure low core losses and that would provide a very high level of mechanical stability and sturdiness. Once we settled with final dimensions of the stator, we ordered a custom-made, laser cut, transformer steel, tape core from one of the major producers of distribution transformers. The core itself was already quite firmly formed and tightly wound when we received it but we additionally submerged in the special epoxy resin, vacuumed out all of the air bubbles and baked everything for a few hours. At the end of this stage, core showed excellent electrical and mechanical properties.

The main challenge with this core was its size and weight (> 100 kg) and it had to be manipulated by three people and with the help of a small size hydraulic crane. To prevent any possible damage to the subsequent layers of the lacquered copper wire (“magnet wire”), after the core was initially prepared, a layer of the special glass-fibre insulation tape was placed on the core. Because it was decided to follow Tesla’s original design we had to wound four separate multi-layered sections on the large toroid core and it had to be done manually. It was a tedious job that required three people working simultaneously – two people to manually wind copper wire by manipulating large 20 kg spool and one person to manage small crane. The additional obstacle was the fact that extra insulation had to be placed between layers of wire but in the end, the stator was wound in less than two days.

Once the core was wound, different sections of the windings were interconnected precisely as in the case of the Tesla’s original 2-phase induction motors. The windings were connected to the 3-phase variable transformer (single phase + two phases connected) to slowly raise the applied voltage and test the windings and core gradually. Finally, the full voltage was applied and everything worked as planned. At that time the wooden plate was still in the production stage so an ordinary cardboard plate was improvised and an empty canister of paint was placed in the centre of the stator, slightly above the middle plain of the toroid. The canister immediately started rotating and gaining speed until it was thrown out of the stator with great force.

After the stator was successfully tested, it was time to add one more protective layer of wire for the functional model to be able to pass the safety examination. So another layer of insulation was placed over the entire area of windings. In this case, the transformer insulation textile was chosen because it was to be slightly visible and it had to be a material that was similar to the materials used in the late 19th century. When the layer of insulation textile was wound around windings, it was several times painted over with a shellac solution to give it as much authenticity as possible. The final layer of lacquered copper wire was wound around the whole toroid while the last coat of shellac was still wet. That final winding was at one end connected to protective earthing and its sole purpose is to provide additional security if by some mishap outer most layer of the stator is damaged and if anybody touches it directly by bare hand. After the final winding was added, additional seven coats of shellac were painted over the entire stator to give it more authentic look (lacquered copper wire weren’t widely used during the period when the original Tesla’s Egg of Columbus device was made).

Copper egg

From the very beginning of the project, it was decided that the functional model of Tesla’s Egg of Columbus that was to be exhibited in the Tesla Memorial Centre in Smiljan will have to be the biggest and heaviest one in the world. The main reason for that decision was the fact that functional models would be exhibited in a small house where Tesla grew up and conceived his first innovative ideas. I would usually produce spun copper egg but in this case, I wanted it to be unique so I decided to make it from two halves machined from solid blocks of copper. Each of the two halves would then have a thread that allows them to form a single compact unit.

3D engineering models were designed in just a few days. However, determining the proper method of machining proved to be a daunting task. It soon became evident that 4-axis CNC machine would have to be used for machining (shelling the blocks). The hardest challenge was designing proper holders that would firmly and precisely hold the machined pieces in the correct position. Once the appropriate holders were designed, it took almost 12 hours of slow machining to achieve adequate precision and smoothness of the outer surfaces. The pieces had to be repositioned a few times and precisely aligned and it also contributed to the overall time needed for the entire machining process.

Finally, when both halves of the egg were finished, the copper egg was assembled. The total weight of the egg was ~1366 – 1387 g. When the copper egg was finally tried out in the rotary magnetic field of the finished stator, it was determined that it tended a slight instability regardless of the gyroscopic effect stabilising it when it rotated at high speed (~3000 RPM). I added three additional lead weights and glued them with epoxy resin inside one half of the egg. The total weight of the assembled copper egg with added lead weights was at this point ~1750 g. It was determined that the end of the copper egg on which it stands during its rotation depends primarily on the position of the egg from which rotation started when the stator is turned on (place on the wooden plate and the angle of the egg on all three axes).

Although I considered polishing the copper egg to a high gloss and protecting its surface with a very thin layer of transparent polymer, it was decided to leave surface unprotected to allow a slight natural patinisation to form. In that way additional “feel” of antiqueness and authenticity is achieved, the same way as with the other functional models which had their brass and copper parts deliberately patinised.

Electronics and automation

The final request by the commissioning party was that Tesla’s Egg of Columbus must have interactive qualities. In that sense, we designed a microcontroller centred system which allows visitors to start the rotation of the egg with a press of the button. A user is also allowed to select the rotational direction of the egg. We also added keylock which enables demonstrators to switch the system on and off without any restriction. An additional function of the interactive control system is to act as a time limiter which turns off the rotating magnetic field to allow time for the egg to come to a complete rest. The main reason for such decision was to prevent the copper egg from changing its position in the centre of the wooden plate. Due to the powerful rotating magnetic field produced by the stator, heavy objects like the large copper egg gain relatively high number of rotations per minute (RPM). At high rotational speeds even relatively small imperfections in shape and balance cause slight eccentricity in the orbital movement of the copper egg. Eventually, the eccentric orbital motion of the egg causes it to hit the protective wooden rail mildly. At that point the friction between copper egg and the wooden railing causes it to lose rotational speed and it usually continues its movement around the circumference of the protective wooden rail. Although the described effects were present in the original Tesla’s Egg of Columbus, prevention of even the mild bumping between rotating copper egg and protective railing was necessary to simplify the process of attaining proper safety certificates.


We feel proud of each model and replica that we ever built because each one serves as a reminder of the contribution Nikola Tesla made in the development of the modern civilisation. The fascination with life, work and ideas of Nikola Tesla steadily grows for the past two decades and deservedly so. Nikola Tesla was a sensitive and eccentric man who fathered modern electrical distribution systems and suitable motors. His experiments with electrical distribution systems based on a resonant single-wire and “wireless” systems were forgotten for decades only to be revived in the various forms in the past two decades. Tesla’s bladeless turbines and other turbomachinery are slowly but steadily being developed and used in all branches of industry. Specially modelled micro-Tesla turbines are even designed and introduced in the field of prosthetic medicine. His ideas regarding cheap and non-polluting energy sources inspire many professional and amateur researchers. However, the name of Nikola Tesla is rarely mentioned in textbooks or popular culture.

We choose to commemorate Nikola Tesla’s life, ideas and achievements by producing captivating reminders of the technological wonders that he exhibited to the astonishment of many.

Zvonimir Rudomino
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  1. RT17  February 18, 2019

    Hello Ms Alcorn. Thank you for your interest. We have responded to you by email.

  2. Jane Alcorn  February 13, 2019

    I am interested in obtaining a small working model of Tesla’s Egg of Columbus that can be easily transported for presentations and demonstrations.

    I would appreciate knowing if you can produce a model for that purpose, and what the cost would be.

    I am asking on behalf of the Tesla Science Center at Wardenclyffe, of which I am the president of the Board of Directors.

    Thank you for your assistance.

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