Leo Szilard the Inventor: A Slideshow

by Gene Dannen

A talk delivered at the Leo Szilard Centenary
Eötvös University, Budapest, Hungary
9 February 1998

Copyright © 1998-2018 Gene Dannen. Portions of this talk are based on my article, “The Einstein-Szilard Refrigerators,” in the January 1997 issue of Scientific American, copyright © 1996 Scientific American. All rights reserved.

The text of this talk published in George Marx (Editor), Leo Szilárd Centenary Volume (Budapest:Eötvös Physical Society, 1998) is an edited version containing typographical errors. What appears below is the original text.

Web posted: April 20, 1998
Updated July 26, 2018 to correct the name of Lajos Bihaly

I will talk today about Leo Szilard as an inventor, and especially about his inventions with Albert Einstein.

This is one of Einstein and Szilard’s joint patents, Hungarian patent 102,079. I will leave this slide on the screen for a minute so that you can read it.

Leo Szilard conceived, and filed patent applications on, many of the key inventions of the 20th century. His ideas included the cyclotron, linear accelerator, electron microscope, nuclear chain reaction, and nuclear reactor. Szilard himself did not build all of these devices, or publish these ideas in scientific journals, and so their credit often went to others. As a result, Szilard never received the Nobel Prize, but two of his inventions did. Ernest Lawrence received the 1939 Nobel Prize for the cyclotron. Ernst Ruska received the 1986 Nobel Prize for the electron microscope.

Dennis Gabor, who won the 1971 Nobel Prize for the invention of holography, was one of Szilard’s best friends in Berlin.

“He used to discuss all his inventions with me,” Gabor later recalled. “I was so full of admiration that I felt quite stupid in his presence. Of all the many great men I have met in my life, he was, by far, the most brilliant.”

Gabor also said of Szilard, “Had he pushed through to success all his new inventions, we would now talk of him as the Edison of the twentieth century.”

Why, in fact, were so many of Szilard’s inventions not developed further? It is often said that Szilard was psychologically incapable of pursuing his ideas to completion. I would like to disagree with that answer. It is certainly true that Szilard was willing to drop one idea when a more important idea came along. And he was a person with so many important ideas. But when you are as far ahead of your time as Szilard often was, the obstacles to the acceptance of your ideas can be almost insurmountable. In the case of each of Szilard's inventions, there were specific reasons why they were not developed. Above all other reasons loomed the largest: the lack of a peaceful world in which to pursue them.

It should not be surprising that Szilard’s ideas that found the most commercial interest were also the most mundane. An invention, to be successful, must have obvious, immediate commercial applications. In modern society, these are usually consumer applications.

Refrigerators are one such consumer item. For decades, it was part of the lore of physics that Einstein and Szilard had collaborated on designing household refrigerators without moving parts. But little information about the collaboration was thought to survive. However, as I discovered, the key to the story of the Einstein-Szilard refrigerators was here in Budapest in the memory of Albert Korodi.

This slide shows Albert Korodi in 1994, at the age of 96. He is holding the Tivadar Puskás award of the Scientific Society for Telecommunications, which he received on his 95th birthday. He died a few months after this photograph was taken.

Korodi had a long and eventful life. He won the Hungarian national mathematics competition in 1916, and it was through this competition that he first met Leo Szilard. Korodi and Szilard then became fellow engineering students at the Budapest Technical University. Later he followed Szilard to Berlin, where they lived in the same apartment building and became even closer friends.

Korodi was the primary engineer for the Einstein-Szilard refrigerators, and his knowledge of their development was unique. As a result of information that Korodi provided me, I was able to publish the story of the Einstein-Szilard refrigerators last year in Scientific American magazine.

This slide shows Szilard and Korodi on the roof of the DuPont Plaza Hotel in Washington D.C. in 1963, when Korodi visited the United States.

I will only be able to cover the highlights of Szilard’s inventions in the brief time I have here today. I will focus on refrigerators, but I will also mention some of his other inventions.

This slide shows Albert Korodi in 1934, after the end of the refrigerator work.

For the images I am showing, I am grateful to: Agnes Vadasz of the Hungarian Patent Office, the U.S. Patent and Trademark Office, the Patent Office of the United Kingdom, Björn Lindström of AB Electrolux, Albert and Mihály Korodi, Ernst Ruska, Carol Paulson, and Detlef Lorenz. For help preparing the slides, I am grateful to Angus Crocker.


Szilard filed his first patent in Germany in 1923 on an x-ray sensitive cell. I did not bring a slide of that invention, but it may interest you that he filed the application jointly with fellow Hungarian Imre Patai.

His next inventions were improvements to mercury vapor lamps, which were then at the cutting edge of technology. The Siemens company bought licenses on these inventions, thereby providing Szilard with some much-needed income. This slide shows one of his Hungarian patents on these inventions.

Szilard’s collaboration with Einstein apparently began in the winter of 1925-1926 with a newspaper article. One day Einstein read about the death of an entire family, parents and several children, who had been killed in their beds by the poisonous gases leaking from the pump of their refrigerator.

At the time, such accidents were a growing hazard. Mechanical home refrigerators were starting to replace traditional ice boxes. However, there was no safe refrigerant. The three common refrigerants were methyl chloride, ammonia, and sulfur dioxide. All were toxic, and the amount in a refrigerator could kill its owner.

Einstein was distressed by the tragedy. “There must be a better way,” he said to Szilard.

This was the start of a collaboration that lasted seven years. In all, Einstein and Szilard would file more than 45 patent applications in at least six countries. In the end, none of their refrigerator designs would reach consumers. On the other hand, the income supported Szilard in Germany, and his savings supported his later research in Britain. One of their inventions, the Einstein-Szilard pump, later found a higher use cooling breeder reactors.


At that time, and it is the same today, most refrigerators use mechanical compressor motors to compress a refrigerant gas. “Absorption-type” refrigerators, on the other hand, use heat from a natural gas flame to drive the absorption and release of the coolant from a chemical solution. Szilard and Einstein viewed absorption designs as safest. An award-winning 1922 design by Swedish inventors Platen and Munters, which was marketed by the Electrolux company, had no moving parts whatsoever. Szilard came up with an improvement.

This slide shows that design, as Einstein and Szilard later sold it to Electrolux. A heat source at the lower left drives a combination of gases and liquids through three interconnected circuits. It looks complex because it is. This is a sophisticated application of thermodynamics.

Einstein and Szilard did not stop with one design, however. They produced many ideas, all without moving parts, and in early 1926 Szilard began filing a series of patent applications on their inventions. Szilard quickly negotiated a contract with the Bamag-Meguin company, and by late 1926 development of prototypes began under Szilard’s supervision in the laboratories of Berlin’s Institute of Technology.

Albert Korodi, who had recently received his degree in electrical engineering from the Institute, began working on the refrigerators at this time. Unfortunately, the agreement with Bamag-Meguin lasted less than a year because the company developed financial problems.

Within months, however, Szilard and Einstein reached agreements with two other companies, one German, one Swedish.


In Stockholm, Sweden, the Electrolux company still keeps files on two patents purchased from Szilard and Einstein.

On December 2, 1927, Szilard and Einstein sold a patent application on an absorption refrigerator to Platen-Munters Refrigerating System, a division of Electrolux, for 3,150 Reichsmark, or 750 American dollars. This slide shows that contract. The next slide is a close-up showing the signatures.

Both parties were pleased with the transaction. Electrolux considered the purchase price “very cheap.” For Szilard and Einstein, it was profitable. Adjusted for inflation, the sale price would be roughly $10,000. This slide shows the resulting American patent.


Another, much-different, Einstein-Szilard design produced a partnership with the Citogel company of Hamburg, whose name meant “quick freeze” in Latin.

According to Korodi, the invention was Einstein’s response to the complexity of absorption refrigeration. “Einstein saw how complicated such a system becomes,” Korodi explained, “and proposed a quite simple and cheap system especially suited for small refrigerators.”

This slide shows that design. The device was actually much less complicated that it appears in this diagram. It was extremely simple.

The invention, Korodi recalled, was “a small immersion cooler which could be dipped for instance in a cup of some beverage to be cooled.” Requiring no conventional power source, the clever device operated solely off the pressure of a water tap.

The water tap powered a water-jet pump, producing a vacuum in a chamber from which water and a small amount of methanol were evaporated. The methanol was slowly used up, and the cooler needed to be refilled. Methanol, however, was cheap and readily available.

The cooler worked very well, and in fact a prototype was demonstrated under the Citogel name at the Leipzig Fair in early 1928. Neither Einstein nor Szilard, nor Citogel, however, anticipated a problem with tap-water.

The cooler required reliable tap-water pressure, and the German water system turned out to be highly unreliable. Water pressure at the time varied between buildings, and from floor to floor within buildings. In the end, the variations proved simply too great, and the invention was not marketed.


The most important, and most successful, invention of the collaboration would become known as the Einstein-Szilard electromagnetic pump. This slide shows Einstein and Szilard’s British patent on the pump.

The next slide shows an illustration from their Hungarian patent. It was a fully functional pump without mechanical moving parts of any kind. Instead, a travelling electromagnetic field was used to move a liquid metal. The liquid metal, in turn, could be used as a piston to compress a refrigerant.

You can see the design better in this schematic diagram. At the top is a side view of the pump. At the bottom is the cross section. Alternating current flowing through the coils produced a magnetic field crossing an annular gap running the length of the pump. The liquid metal in the gap was forced at right angles to the field, and was pushed through the pump.

In the fall of 1928, the A.E.G., or German General Electric Company, agreed to develop the pump for refrigeration.

The A.E.G. was a large and powerful company with its own Research Institute, where it established a special department led by two full-time engineers. Albert Korodi was hired to develop electrical aspects of the invention. Another Hungarian engineer friend of Szilard’s, Lazislas Lajos Bihaly, was hired to develop mechanical aspects. Szilard directed the team, with the title of consultant.

This slide shows the floor plan of the third floor of the Research Institute. The director of the Institute was physicist Carl Ramsauer. The Einstein-Szilard refrigerators were developed in the Heat Engineering Laboratory, which I have marked in yellow. The head of the Heat Engineering laboratory was Franz Lauster.

The next slide shows the A.E.G. Research Institute in about 1930. You can see the size of the building from the man walking on the sidewalk at the lower left. The Heat Engineering Laboratory was about here. For these two slides, and information about the Research Institute, I am grateful to Detlef Lorenz.

Korodi, as full-time engineer, and Szilard, as a consultant, received salaries of 500 Reichsmark a month, the equivalent of 120 American dollars. “It was a good salary,” Korodi recalled, at a time when “a car, a Ford, cost 300 dollars.” For Szilard, the A.E.G. contract was even more profitable. Patent royalties, in addition to his consulting fees, eventually brought his income to a comfortable $3,000 a year. This would be roughly $40,000 in today’s dollars.

Now I will show you the actual prototypes. This is an Einstein-Szilard pump, pumping mercury for test purposes. The pump itself is the lower part of the assembly. The glass tubing on the top is simulating the chambers where liquid metal compressed the refrigerant.

Until recently, the only known detail of the Einstein-Szilard pump prototype was its noise. Though expected to be silent, the pump suffered from cavitation — the expansion and collapse of tiny voids or cavities — as the liquid metal was forced through the pump. Dennis Gabor once said that it “howled like a jackal.”

According to Korodi, however, the sound resembled that of rushing water. Furthermore, as described in the A.E.G. final report, the noise was a function of the force and speed of the pump. By a combination of methods — reducing voltage at the start of each stroke, tapering the flux core, and increasing the number of coils of the pump — it proved possible to reduce the noise to acceptable levels even at the lowest design temperature.

This slide shows experimental apparatus for varying the voltage to reduce that noise.

From an engineering viewpoint, the noise problem was mostly cosmetic. The technology was entirely new, and more interesting problems were encountered in working with alkali metals. Because mercury had low electrical conductivity, it was necessary to use a potassium-sodium alloy as the liquid metal. Potassium and sodium were chemically reactive, however, and special equipment was developed to fill the pump without oxidation. Korodi emphasized that there would have been no danger to refrigerator owners. The Einstein-Szilard refrigerator was a sealed system, and the liquid metals were fully contained in welded stainless steel.

This slide shows a nearly-complete refrigerator assembly in the laboratory. The Einstein-Szilard pump is the dark cylinder at the lower left. Above it is the compressor chamber, where the liquid-metal piston compressed the refrigerant. The array of condenser coils operated the same way as those on the back of modern refrigerators. The condenser coils release the refrigerator’s heat to the environment.

The diagrams in this slide show how the pump was mounted in the refrigerator cabinet. The pump is at the lower left. Liquid metal from the pump was pushed upward into the compressor chamber, where it acted as a piston to compress the refrigerant. The rest of the system — you can see the condenser coils — was the same as in standard refrigerators.

In this slide you can see a rear view of a refrigerator cabinet, ready for installation of the Einstein-Szilard apparatus.

Construction of these and other prototypes continued at the A.E.G. Research Institute until August 1932. After all of this work, why was the Einstein-Szilard electromagnetic refrigerator never produced for consumers?

It is true that it had a noise problem, but various methods had succeeded in reducing that noise to acceptable levels. It is true that it was not electrically efficient, but that was not so important in those days. In the U.S.A. at that time, electric companies were promoting the sale of electric refrigerators in order to increase electrical consumption.

It is also true that the world was in economic Depression. The A.E.G., like other companies, was hit hard by the Depression. In 1932, the A.E.G. Research Institute was reduced by half, eliminating all but essential projects. In August, the Heat Engineering Laboratory was closed entirely.

But the main reason lay far beyond Europe, in a refrigerant first demonstrated in 1930 in the U.S.A.. That refrigerant was Freon. Because it was non-toxic, it eliminated the danger from leaks. In just a few years, compressor refrigerators using Freon would became the standard for almost all home kitchens. Only decades later, of course, would it be realized that such chlorofluorocarbons might endanger the ozone layer of the entire planet.

So you see that perhaps Einstein and Szilard had a better idea.


In the short time I have remaining, I will mention some of Szilard’s other patents very briefly.

During his Berlin years, while working on the refrigerators, he also filed for patent on three other major inventions. It is well known that he filed patent applications on the linear accelerator and the cyclotron — two devices that revolutionized 20th century physics.

It is not as well known that Szilard also foresaw, and filed for patent on, the electron microscope. Dennis Gabor later recalled how, in 1928 at the Café Wien in Berlin, Szilard tried to convince him to build such a microscope.

Gabor told Szilard that such a device would be useless. Gabor told Szilard, and I quote, “one cannot put living matter into a vacuum and everything will burn anyway to a cinder under the electron beam.”

Gabor later regretted that he had not taken Szilard’s advice. It was only a few years later that work on developing electron microscopes began at several different laboratories. Szilard filed for patent on the electron microscope on July 4, 1931. This slide shows Szilard’s design from that patent.

In a peaceful world, Szilard might have been able to develop these inventions. But the coming of the Nazi era in Germany forced him to move, for the second time in his life, to a new country.

It was in London, in the fall of 1933, that Szilard conceived his greatest invention. After reading in the newspaper that Ernest Rutherford called the possibility of atomic energy “moonshine,” Szilard conceived the neutron chain reaction. He needed one more piece of information. He needed to know which element would sustain such a reaction.

Szilard knew from experience that a patent was necessary for attracting funding for the necessary experiments. In March 1934, he began filing patent applications.

This slide shows British patent 440,023. This patent, as published, discussed nuclear transmutation only by individual neutrons. Szilard considered the idea of the chain reaction too dangerous for publication. He moved that idea into a separate patent, and assured its secrecy by assigning it to the British Admiralty.

This slide shows that secret patent 630,726, as it was finally published in 1949 after remaining secret for 13 years. In it, Szilard defined the concept of the critical mass, which he called the “critical thickness.” The next slide, from that patent, shows Szilard’s design for a nuclear reactor. The design shows a sphere of a chain-reacting element, and also cooling tubes for the extraction of heat to produce electric power. I ask you to consider how different history might have been if Szilard had published his ideas for German scientists to read.

Of course Szilard was correct that a chain reaction was possible and atomic energy would become a reality. I do not have time to tell how Szilard pushed his idea to its final success in Chicago on December 2, 1942.

In 1955 Szilard and Enrico Fermi, as assignors to the U.S. Atomic Energy Commission, were awarded the joint patent on that first nuclear reactor. When it issued this patent, the U.S. Patent Office compared its significance to the patents by Samuel Morse for the telegraph and Alexander Graham Bell for the telephone.

The next three slides are illustrations from this patent. The first slide shows the uranium-carbon lattice. The second slide shows the distribution of uranium in the pile. And the third slide shows the pile itself with its control rods.

It is important to remember that, when Szilard first proposed the chain reaction, it was regarded as absurd. The doubters included almost every scientist of his time, and most particularly Ernest Rutherford, Niels Bohr and Enrico Fermi. But Szilard pursued the chain reaction with determination and consistent purpose, against all obstacles, to its completion.

I hope that you have enjoyed my slideshow. In conclusion, I would like to ask your help. I have been researching Szilard’s life for many years. My work has been especially difficult because many of his most important documents and letters dated before 1933 are missing. As you probably know, Szilard kept his important papers in suitcases so that he could travel at a moment’s notice. At least one of these suitcases has never been found. Perhaps Szilard left this suitcase in an attic here in Budapest.

Copyright © 1998-2018 Gene Dannen. Portions of this talk are based on my article, “The Einstein-Szilard Refrigerators,” in the January 1997 issue of Scientific American, copyright © 1996 Scientific American. All rights reserved.

Web posted: April 20, 1998 Updated: July 26, 2018
URL: http://www.dannen.com/budatalk.html
Gene Dannen / gene@dannen.com