When the invisible was made visible

10 Feb 2023

Wilhelm C. Röntgen died in Munich 100 years ago, on 10 February 1923. We talk to historian of science Kärin Nickelsen about the discoverer of X-rays.

To mark the 100th anniversary of the death of Wilhelm Conrad Röntgen, Kärin Nickelsen, Professor of History of Science at LMU Munich, tells us what kind of person the discoverer of X-radiation was and how his work shaped science.

When Röntgen took over the Chair of Experimental Physics at LMU in 1900, he was at the high point of his career. Besides a bust of Röntgen, the collage shows part of the certificate of appointment.


Wilhelm Conrad Röntgen researched at LMU for 20 years. How did his time in Munich begin?

When Röntgen took over the Chair of Experimental Physics at LMU in 1900, he was at the high point of his career. Five years earlier, he had discovered X-rays, which in Germany are called “Röntgen rays” in his honor. In 1901, he had received the inaugural Nobel Prize in Physics for this achievement. At LMU, Röntgen sought to get the Chair of Theoretical Physics filled, which had been vacant since the departure of Ludwig Boltzmann in 1894. Eventually, the role was filled by Arnold Sommerfeld, who also deeply influenced physics as a researcher and lecturer.

Did Röntgen continue to study the newly discovered rays at LMU?

Yes, among other interests. Röntgen had always pursued research into very diverse subjects in physics, including thermodynamics, electrodynamics, and solid-state physics. He was particularly interested in the physical properties of crystals, and this continued throughout his time at LMU. He would have been delighted that X-rays would eventually become perhaps the most important tool for explaining crystal structures. The process whereby the double-helix structure of DNA was decoded, for example, was based to a substantial extent on experiments with X-rays.

The discovery of X-radiation was his most important contribution to science. How did he pull off this coup?

According to the standard version of events, it was a coincidence. Like many other researchers, Röntgen was experimenting at this time with so-called cathode rays. This is the name for the rays observed in evacuated glass tubes when you apply a strong voltage between two poles. Electrons fly at high speed from the cathode, the negative pole, to the anode, the positive pole. Upon colliding there, energy is released as radiation. Röntgen apparently noticed that when he switched on the tube, a sheet of paper some distance away that was coated with a fluorescent substance began to glow. And it continued to shine even after he enclosed the tube in cardboard. This could not be explained with the science of the day. Röntgen concluded that radiation of a new kind had been released: “X-rays.”

But we don’t know if it really happened like that, do we?

Precisely. Because Röntgen ordered all his documents and notes to be destroyed after his death, we cannot verify this story.

Röntgen soon realized that the rays not only penetrate cardboard, but tissue as well.

When you lay a hand, for example, between the rays and a photographic plate, you get an image of the bones of the hand, because the rays find it easier to penetrate soft tissue than denser bone matter. And it’s true that Röntgen tested this on his wife – the picture from 1895 is famous today. This ability to see through objects fascinated people. Everything was X-rayed – even feet to see if a shoe fit. X-ray apparatuses became an attraction at fairgrounds. So aside from the great scientific value of the technology, it was also a big hit with the public.

With grave consequences in some cases …

It took a long time before the serious health effects of an overdose of X-rays were discovered. This had fateful consequences for many test subjects and scientists. To take the initial radiographs, body parts were exposed to radiation for up to twenty minutes!

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1:14 | 9 Feb 2023

You alluded to the high scientific value of Röntgen’s discovery. Could you elaborate on that?

Initially, X-rays were just a new kind of radiation beyond the visible spectrum. Several of these had been discovered in the second half of the 19th century. But that they could make the invisible visible without destroying the structure: this is what made these rays special. It works either directly, as in the case of bones, or indirectly via diffraction patterns, as in the case of crystallography. X-rays are used today not only in medicine, but also in materials testing. Moreover, there are special X-ray microscopes and even telescopes that detect X-rays in space. In this way, scientists can obtain insights into astronomical objects such as binary star systems.

Röntgen did not patent his discoveries. And he donated the monetary award for his Nobel Prize to the University of Würzburg. Was he really so modest and uninterested in money?

Based on what we know about Röntgen, he was a quiet and retiring man and found his sudden fame burdensome if anything. But this does not mean that he had an austere lifestyle. When Röntgen won the Nobel Prize, he was already set up for life thanks to his inheritance – his father was a successful cloth merchant. As such, he was not reliant on earning money from patents. This favored the rapid application and spread of the discovery.

Major discoveries were often made by individuals in those days as opposed to teams. It’s said that Röntgen would hunker down in his lab for weeks on end sometimes and even sleep there. Was Röntgen really a lone genius, or did he have the support of others?

The practice of science at the end of the 19th century was quite different from what it is today. Single-authored publications were still very common in physics, as indeed they were in mathematics, chemistry, and biology. This does not mean, however, that the professors worked all alone. They were supported by outstanding craftspeople, for instance, who manufactured research equipment, precision instruments, and labware. Assistants and associate professors were expected to provide help. And their wives took care of the household and sometimes did translations or made corrections.

Röntgen left school without a high-school diploma. Indeed, he’s often cited as an example of why you don’t necessarily have to be a top student to become a successful scientist.

People are fond of rehearsing the story that Röntgen was expelled from school because he wouldn’t betray a classmate who had drawn a caricature of the teacher. However, the story has not been authenticated. It’s quite possible that Röntgen left school to start a career in his father’s company. If so, he never went through with it. Instead, he enrolled at ETH Zurich (then known as the Federal Polytechnic Institute), where he was allowed to study without a high-school diploma. After completing a PhD, he moved to Würzburg as an assistant to August Kundt. His lack of a high-school diploma prevented him from acquiring a habilitation degree there. But when his boss was appointed to a post in Strasbourg, he took Röntgen with him and the university there allowed him to obtain his habilitation without the high-school leaving certificate. After spells in Hohenheim and Gießen, Röntgen was eventually offered a post in Würzburg, where he not only discovered X-rays, but was also elected to the office of rector. He ended his research career in Munich in 1920. Three years later, on 10 February 1923, he died of bowel cancer. If he lived in our time, his cancer might have been discovered at an early stage through the use of X-ray technology.

Kärin Nickelsen is Chair of History of Science at LMU. Her main research interests include the history of experimental life sciences and scientific working processes.

Wilhelm Conrad Röntgen at LMU

Wilhelm Conrad Röntgen was professor at LMU from 1900 to 1920. He was the researcher who was awarded the inaugural Nobel Prize in Physics (in 1901).

When the 16- or 17-year-old Wilhelm Conrad Röntgen was expelled shortly before he was due to take his high-school diploma examinations, nobody could have foreseen his subsequent career as a physicist. The schoolboy refused to say who had drawn the caricature of his teacher on the blackboard. So Röntgen had to take the circuitous route to becoming a renowned professor. After completing a mechanical engineering degree at the Federal Polytechnic Institute in Zurich, for which a high-school diploma was not a prerequisite, he did a postgraduate course in physics. He completed his habilitation degree in Strasbourg, and after various spells at other institutions, he obtained his first professorship in Würzburg in 1888. In 1900, when he was already a celebrated physicist (although not yet a Nobel laurate), he accepted an offer to come to LMU Munich. There, on 10 December 1901, he won the inaugural Nobel Prize in Physics for his discovery of a new kind of radiation, X-rays, which subsequently bore his name in the German-speaking world. He donated the monetary award of 50,000 Swedish kronor to the University of Würzburg to help develop new scientific talent.

© imago images / Photo12

The field of crystal physics had long interested Röntgen, who was born in 1845. He wanted to investigate their structure using rays and light. While at Würzburg, he was already experimenting with cathode rays. And so it was on the evening of Friday, 8 November 1895. Working late in his lab, he was using cathode rays, the exact properties of which were still unknown. By chance, Röntgen discovered that they were capable of passing through materials and thus making an image of their structure. Initially, Röntgen did not want to shout his new discovery from the rooftops, so he chose his wife Bertha as his first test subject. The famous image of her hand with the wedding ring was taken shortly before Christmas, on 22 December 1895. Incidentally, there is a second similar picture of a hand with a ring, which belonged to the anatomist Albert von Koelliker and was taken a few weeks later.

© picture-alliance / akg-images

So-called discharge tubes are important for the experiments. These tubes consist of a cathode and an anode. First, electrons are knocked out of the cathode and are accelerated in the direction of the anode by means of a powerful electric charge, where they excite the atoms to a higher energy level. When these atoms fall back to a lower energy level, they emit the characteristic X-rays. In Munich, Röntgen also experimented with such tubes equipped with platinum cathodes. In the earliest X-ray tubes, the electrodes were fused into a vessel that was only partially evacuated of air. Because the vessel was often shaped like a tube, it acquired the name X-ray tube.


In securing the appointment of Wilhelm Conrad Röntgen, LMU had brought an established physicist to the Chair of Experimental Physics in Munich – incidentally, the only physics chair at the time. The building for the physics institute itself, much of which is still standing today, had opened a short time previously, on 3 November 1894. Housed in the four-story building were workshops, laboratories, offices, an internship room, a large and a small lecture hall, and a room for the university’s physics collection. The institute even had its own plant for generating electricity, along with central heating, electrical lighting, and gas, water, and electrical connections – certainly not the norm in Munich at the end of the 19th century.


Röntgen furnished an office on the 2nd floor, where he had a view of the inner courtyard. He set up a private laboratory next door, as physics professor Joachim Rädler, current occupant of Röntgen’s former office, explains: “Whenever irksome guests came to see him, he quickly retreated to the laboratory next door.” Röntgen inaugurated the golden age of physics in Munich. This would involve not only investigating the structure of matter, of crystals and metals – in other words, the beginnings of solid-state physics – but also incorporating the nature and interactions of the smallest building blocks of matter, the atoms, into physical theories. To this end, Röntgen insisted that the Chair of Theoretical Physics, which had been vacant since the departure of Ludwig Boltzmann, be filled, resulting in the appointment of Arnold Sommerfeld.


The collaboration soon bore its first fruit. A member of Sommerfeld’s research group called Max von Laue, with the help of Röntgen’s doctoral researchers Walter Friedrich and Paul Knipping, demonstrated the interference of X-rays in an experiment with crystals. This proved the wavelike nature of X-rays and therefore their electromagnetic origin. Max von Laue was awarded the 1914 Nobel Prize in Physics for this 1912 discovery. Munich became one of the leading hubs for physics in Europe. Sommerfeld’s lectures drew a lot of students, says Rädler (in the picture above), including many who would go on to become famous physicists, such as Peter Debye, Peter Paul Ewald, Hans Bethe, Werner Heisenberg, and Wolfgang Pauli. The “Munich School” shaped the physics of the first half of the 20th century, and in particular the foundations of quantum physics.


After the death of Wilhelm Conrad Röntgen, LMU honored the physicist by commissioning several busts. Only one of them has survived. This bust was recently installed on the second floor of the physics building, right in front of Röntgen’s former office. It commemorates the world-famous Nobel laureate, who died in Munich 100 years ago, on 10 February 1923.


Faculties: Faculty of Physics

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