HISTORICAL NOTE
Claims of priority – The scientific path to the discovery
of X-rays q
Uwe Busch
Deutsches Röntgen-Museum, Remscheid, Germany
Received 8 November 2022; accepted 10 December 2022
Abstract
Shortly after Röntgen’s publication about a new kind of rays, a dispute about priority claims began. Röntgen was not the
first researcher to produce X-rays nor the first to take X-ray images. An analysis of the history of cathode ray research in
the 19th century reveals ample evidence that researchers before Röntgen had already produced X-rays, albeit without
knowing this. Most of them, for their part, did not claim any priority, some did so rather casually. The GermanHungarian physicist Philipp Lenard, a co-founder of German Physics, considered himself a “true discoverer”. It remains
to be said, however, that he, like many others before him, failed to recognize the character of the new radiation. It was
Wilhelm Conrad Röntgen, with his three scientific publications on X-rays, who laid the foundations for their physical
clarification and paved the way for the success story of their application in a variety of fields that continues to this day.
Keywords: Wilhelm Conrad Röntgen; 100th anniversary of his death; Gas discharge; Cathode rays; Priority claims;
X-rays; Radiating matter; Crookes tube; Hittorf tube; Heinrich Geissler; First X-ray image; Puluj lamp; Unipolar vacuum
tube; Lenard window; Canal rays; Electron
1 Prologue
On February 10, 1923, Wilhelm Conrad Röntgen died of
intestinal cancer in his Munich apartment at the age of
almost 78. In accordance with Röntgen’s testamentary
wishes, his ashes were buried in the Old Cemetery in
Gießen, where his parents had already found their final resting place.
In his last will and testament, he stipulated that a large
part of the letters addressed to him and unpublished scientific records were to be destroyed. Prior to this, in the summer of 1922, he and his housekeeper Kätchen Fuchs had
begun burning the mail addressed to him in his country
house in Weilheim, some of which had been stored for years.
He had kept letters that were important to him in a special
cabinet in his Munich apartment and always kept them
q
locked and even sealed when he was away. That is why
he insisted on helping to burn the letters in the stove in
the living room. Later, he asked his friends to burn the letters
they had received from him as well. Not all of them complied with this request.
This undertaking was probably not planned in advance,
but rather due to his emotional state. The war had been lost
for Germany and in 1919, Röntgen’s wife had died after a
long and serious illness. On April 1, 1920, Röntgen was
retired from his position as professor at the University of
Munich. Due to the dangerous post-war political turmoil surrounding the founding of the Bavarian Council Republic in
1919, Röntgen spent much of his time at his country home in
Weilheim, Bavaria. Large parts of his considerable assets
were unfortunately lost due to inflation in the post-war years.
As a retired civil servant, however, he received regular
On the 100th anniversary of the death of Wilhelm Conrad Röntgen (1845–1923).
Z Med Phys xxx (2023) xxx–xxx
https://doi.org/10.1016/j.zemedi.2022.12.002
www.elsevier.com/locate/zemedi
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
pension payments and therefore did not have to suffer any
hardship. Nevertheless, all this may have influenced his
decision. Another reason could be that Röntgen certainly
also wanted to avoid his documents falling into the hands
of colleagues and envious people who could use them
against him. For early on, there were rumours that Röntgen
was not the first discoverer of the rays at the Physics Institute
of the University of Würzburg, but that his institute servant
Marstaller was the chance discoverer.
This is the subject of a discussion that continues to this
day on social media about the priority claims of the “true
discoverers” of X-rays.
2 The route to the discovery- Research on
cathode rays in the 19th century
The history and the claims about the discovery of X-rays
are a history of research on gas discharges in the 19th century. Gas discharge tubes were a popular attraction in vaudeville shows and fairgrounds in the last decades of the 19th
century because of their mysterious luminous phenomena.
For the pleasure of viewing the luminous phenomena, special gas discharge tubes were built that contained various fluorescent crystals arranged in flower bouquets.
In the second half of the nineteenth century, gas discharge
became increasingly a subject of science. A new field of
research emerged, which was steadily expanded by numerous innovative technical developments and skills. These
technical innovations included, from the 1850s onwards,
powerful high-voltage supplies, such as spark inductors,
and especially the further development of the art of glass
blowing. Here, the glassmakers from Thuringia in particular
led the way. The town of Gehlberg became a center for the
production of thermometers and technical glass articles for
measuring instruments as late as the 19th century. The Gundelach company was also the first to build special X-ray
tubes after the discovery became known.
The development of modern mercury air-pumps made it
possible to create the high gas dilution ratio in the tubes
required for the experiments and the scientific analysis of
the glow phenomena. However, low-pressure gas discharges
are based on quite complex physical processes that cannot be
easily explained. Therefore, gas discharges remained a rather
exotic topic until the last decades of the 19th century. Nevertheless, before Röntgen started working on cathode rays,
many scientists had already done research in that field. They
all aimed to fathom the physical nature of the observable
phenomena of luminosity. Until Röntgen’s research in
1895 and two years later, however, the nature of cathode
rays remained unclear. This motivated Röntgen to conduct
his own research.
In the following, some of the important researchers and
their contributions to the study of cathode rays in the context
of possible priority claims for the discovery of X-rays are
presented [1].
2.1 Michael Faraday (1791–1867), Royal Institution
London: First scientific research on glow discharges
(1831–1838)
Michael Faraday (Fig. 1) was an English scientist, inventor, assistant to Humphry Davy (1778–1829) and member of
the Royal Society. He is considered one of the most important experimental physicists of the 19th century. In addition
to his discoveries about electrolysis, electromagnetic induction, diamagnetism, and the invention of the terms ‘anode’
and ‘cathode’, he investigated electric current in evacuated
glass tubes during the years 1831–1838. He had discovered
that the sparks observed under atmospheric pressure change
Figure 1. Michael Faraday by Thomas Phillips oil on canvas,
1841–1842 35 3/4 in. x 28 in. (908 mm x 711 mm) Purchased,
1868 NPG 269. (Michael Faraday – Wikipedia Wikimedia
Commons).
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
to a glow when the tube is conventionally evacuated to a
pressure of about 10 mm mercury down to 0.01 mm mercury
(13 mbar–0.013 mbar). At high DC voltages of 10,000 to
15,000 volts, he noticed glow discharges starting at the cathode and ending at the anode for various gases such as oxygen, hydrogen, and gas mixtures such as air (Fig. 2). When
he lowered the gas pressure, typically to about 0.05 mm mercury, he noticed a dark spot between the negative glow and
positive column, also known as the “Faraday dark space”
[2].
2.2 Francis Hauksbee (1660–1713), Royal Society
London: Electrostatic Repulsion (1709)
Despite his intensive research into gas discharges, Faraday, however, never mentioned anything about mysterious
rays, that would indicate the existence of a radiation penetrating matter. Evidence for the existence of a penetrating
type of radiation was mentioned by Francis Hausksbee in
1706. A British scientist, Fellow, and curator of experiments
at the Royal Society in London, Francis Hauksbee was best
known for his work on electricity and electrostatic repulsion.
Using a modified electrifying machine, he succeeded in
sucking out the air in a spherical container using mercury
and electrostatically charging the sphere by rubbing it with
3
his hand (Fig. 3). This produced a rather bright light. This
phenomenon is similar to St. Elmo’s fire and later became
the basis for the development of glow lamps and mercury
vapor lamps. In his treatise “Physico-Mechanical Experiments” (London 1709), he described a special phenomenon
he recognized in an Experiment concerning the ’Produclion
of Light in an exhausted Glass. “... My Hand was no sooner
applied to that part of the Globe which was lined with the
Sealing-Wax, but I saw the shape and figure of all the parts
of my Hand (which touch’d the convex Surface of the Glass)
distinctly and Perfectly upon the concave Superficies of the
Wax within.” [3]
2.3 Julius Plücker (1801–1868) and Heinrich Geißler
(1814–1879), University of Bonn – Discovery of elementspecific line spectra (1865)
The beginning of a systematic exploration of lowpressure gas discharges is closely linked to the activities of
the German mathematician und physicist Julius Plücker
(Fig. 4) and the glassblower Heinrich Geißler at the Institute
of Physics, University of Bonn. Plücker was fascinated by
Faraday’s research, with whom he also kept up a lively
correspondence, and he and his doctoral student Johann
Wilhelm Hittorf experimented with gas discharge tubes.
Figure 2. Schematic diagram and photograph of a direct current glow discharge showing the following structures: The cathode glow, i.e.,
the first violet glowing region immediately adjacent to the cathode; the cathode dark space (Hittorfscher Dunkelraum), i.e., the darker band
between the two brighter regions adjacent to the cathode, where the electrons already have energy above the maximum of the excitation
energy and can no longer excite the gas as efficiently; the negative glow, i.e., the second larger purplish glowing region next to the cathode,
whose brightness gradually decreases toward the anode; the Faraday dark space, i.e., the region without glow near the center of the tube; the
positive column, i.e., the pink glowing region filling the right side of the tube and representing a plasma with hot electrons. This region is
characterized by low electric field strength and vanishing space charge and ensures current transfer between cathode and anode. In the
photo, striation, i.e. alternating bright and dark bands, can be seen in the positive column, caused by an instability in the plasma; the anode
glow, i.e. the bright layer on the far right in front of the anode (Gas discharge - Category:Gas discharge tube - Wikimedia Commons).
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
Figure 3. Plate VII ’A continuation of the Experiments on
Attrition of Glass’ and ’Some further Experiments relating to
Electricity of Glass’ from Physico-Mechanical Experiments
various subjects, presented to the Royal Society on October
1709 (Francis Hauksbee – Wikipedia Wikimedia Commons
the
the
on
26,
First alone and later with Hittorf, he first investigated the
magnetic deflection of cathode rays. Together they made
many further important discoveries. Both showed that any
gas could be made to discharge when a certain voltage
was applied, each with an element-specific line spectrum.
In 1862, Plücker pointed out that the same element may
exhibit different spectra at different temperatures. The success of the experiments carried out was due in no small part
to the ingenious glassblower Heinrich Geißler (Fig. 5). In
1865, Plücker and Hittorf described their research results
in a publication of the Royal Society in London. [4] The
results were groundbreaking for future research on atoms
and the development of spectral analysis.
Figure 4. Julius Plücker (1801–1868). Lithographie von Rudolf
Hoffmann, 1856. (Julius Plücker – Wikipedia Wikimedia
Commons).
2.4 Johann Wilhelm Hittorf (1824–1914), University of
Münster: Discovery of cathode rays (1869)
For further investigations on discharge phenomena, Wilhelm Hittorf (Fig. 6) improved his apparatus. At the World
Exhibition in Paris 1867, he acquired the spark inductor
invented by the German-French mechanic and instrument
maker Heinrich Daniel Rühmkorff (1803–1877). With this
device, later referred to as the Rühmkorff inductor, Hittorf
was able to generate a pulsating voltage of about 100,000
volts from a DC voltage of 15 volts. Hittorf also set about
improving the vacuum and developed a special discharge
tube called the Hittorf tube (Fig. 7). Shortly after Plücker’s
death in 1869, Hittorf discovered and described the main
properties of the rays originating from the cathode and
extending to the anode. Hittorf found that cathode rays propagate in a straight line, produce fluorescence when they
strike the glass wall, and that a metal foil in the path of
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
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Figure 5. Heinrich Geissler (1814–1879). (Heinrich Geißler
(Glasbläser) – Wikipedia Wikimedia Commons).
the rays casts shadows. Taking into account Plücker’s results
on magnetic deflection, he explained the motion of the cathode rays from the cathode as a simple negative current. First
in 1869 [5] and then until 1884, Hittorf published four
groundbreaking papers on the study of the passage of electricity through gases. In this way, he succeeded in exploring
the main properties of cathode rays, which were so named
by the German physicist Eugen Goldstein in 1876. Hittorf’s
pioneering research was an important step on the way to the
discovery of X-rays [6].
Figure 6. Johann Wilhelm Hittorf. Taken from the Festschrift on
the occasion of Johann Wilhelm Hittorf’s 80th birthday. Leipzig:
Barth 1904. Adolf Heydweiller, Münster (Johann Wilhelm Hittorf –
Wikipedia Wikimedia Commons).
2.5 William Crookes (1832–1919), Royal Society
London: Linear beams and Radiant Matter (1879)
of tube emit blue light and the photographic plates show
’foggy’ shadows [...]” [7].
During his investigations on the conduction of electricity
in low-pressure gases, the British chemist, physicist and
parapsychologist William Crookes discovered that the negative electrode (cathode) emits rays as the pressure decreases.
In his research with heavily evacuated glass tubes, Crookes
was able to detect the cathode rays as a shadow-casting cross
by using a special design of the anode (made of aluminium)
(Fig. 8). When cathode rays strike glass or other materials, a
fluorescent glow is produced. He pioneered the construction
and use of vacuum tubes to study the physics of plasma in
1879. Crookes is believed to have observed radiating atoms
or matter in a special state. He called this radiant type of
matter a fourth state of matter. During his experiments he
also noticed veils on photographic plates. “[...] the walls
2.6 Eugen Goldstein (1850–1930), Urania Berlin: Canal
rays (1886)
Building on the work of Plücker and Hittorf, Eugen Goldstein from Berlin conducted extensive pioneering gas discharge experiments, for which he had almost 2,000 tubes
manufactured by 1885, some of which he paid for out of
his own pocket. In 1876, he found that rays were emitted
perpendicular to the cathode surface. Goldstein first called
them “cathode rays” and understood them as a fundamental
phenomenon of electricity, but interpreted them – in the tradition of Hermann Helmholtz – as ether phenomena. With
William Crookes he had a scientific dispute about the corpuscular nature of cathode rays.
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
Figure 8. Crookes shadow cross tube (Crookes tube two views File:Crookes tube two views.jpg - Wikimedia Commons).
other physicists in their search for atomic beams and mass
spectroscopy.
Figure 7. Gas discharge tube of the Hittorf type. The first
experiments with X-rays took place with tubes that were initially
still cylindrical. The end of the glass bulb acted as an anti-cathode,
but the radiation was still quite diffuse. (Archiv Deutsches
Röntgen-Museum).
In 1886 Goldstein discovered the anode rays, which are
produced by positively charged ions after impact ionization
in the cathode ray tubes (Fig. 9). These ions are accelerated
towards the cathode, pass – if the cathode is provided with
holes – through these holes (“channels”) due to their inertia
and can be detected behind the cathode by their luminous
phenomena. Goldstein therefore called them “canal rays”
(German “Kanalstrahlen”). Since the canal rays went in
the opposite direction as the “cathode rays”, Goldstein speculated that these rays consisted of positively charged particles. About 15 years later, the physicist Wilhelm Wien
proved that these rays were positive ions from the gas.
The nine-page 1886 Academy paper went unnoticed at the
time, especially since the channel ray did not particularly
stand out among the variety of gas discharge phenomena
[8]. This changed with the discovery of the X-rays, when
the channel rays also gained importance for a short time.
However, Goldstein’s work had opened new avenues for
2.7 Johann Puluj (1845–1918), University of Prague:
Puluj Lamp (1881)
Johann Puluj was an Austrian-Hungarian physicist and
electrical engineer of Ukrainian nationality. In 1876, he
received his doctorate from the University of Strasbourg
under Röntgen’s mentor August Kundt. Puluj’s scientific
interests included the study of cathode radiation. He investigated and described their interaction with magnetic fields
and showed similarities to electric currents in solids. He published his results in four articles between 1880 and 1882.
Puluj, however, rejected the Crookes hypothesis of a fourth
state of matter to explain the cathode rays. He described the
idea as “transcendental” with an inclination towards spiritualism. Referring to the work of Hittorf, he proposed a more
materialistic and mechanical explanation. According to his
hypothesis, physical particles should be mechanically
released from the electrodes. They are negatively charged
and move in a straight line at high speed in the tube to the
positive electrode. Together with the residual molecules of
the gas, they mediate the electrical conduction of electricity
between the electrodes in the discharge tube.
In the course of his research, he developed luminescent
lamps, which were later called Puluj lamps (Fig. 10). Puluj
was the first to incorporate an additional electrode, the
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Figure 10. Puluj lamp. Courtesy of Museum Experimentalphysik
Innsbruck.
scientists knew each other, Röntgen, like Lenard, never later
cited Puluj’s research.
2.8 Arthur W Goodspeed (1860–1943), University of
Pennsylvania: Unrecognised first radiograph (1890)
Figure 9. Canal rays: Anode-ray tube showing the rays passing
through the perforated cathode and causing the pink glow above it
(Anode ray - Wikipedia Wikimedia Commons).
so-called anticathode, into the tube in addition to the cathode
and anode. He published the results of his research on
“Radiant Electromatter” between 1880–1882 in the Vienna
Reports and in 1883 in a monograph [9]. The phenomena
he described were probably indeed X-rays, and his lamp
was in principle a prototype of an X-ray tube. Puluj, however, resumed his research only after the publication of
Röntgen’s first treatise on the new rays in January 1896.
He published his first paper on X-rays already on February
13, 1896 [10]. Puluj was one of the first physicists to recognise the potential of X-rays for medical diagnostics and produced numerous X-ray images.
With his research, Puluj certainly laid an important foundation for Röntgen’s discovery of X-rays. Therefore, some
journalists called him the discoverer of X-rays [11,12]. Puluj
himself acknowledged Röntgen’s priority. Although both
On the evening of February 22, 1890, American physicist
Arthur Goodspeed and photographer William N. Jennings
(1860–1945) were experimenting with high-voltage discharges in the physics laboratory at the University of Pennsylvania. As early as the fall of 1889, they had begun
comparing various forms of artificial electrical discharges
generated by various devices with Jennings’ earlier photographs of natural lightning. That evening, they photographed lightning and tuft discharges generated by spark
generators. They also placed coins and weights on unexposed photographic plates. After they finished their experiments, there were still some unexposed photographic
plates in corresponding photographic cassettes on their
experimental table. Goodspeed took some of Crookes discharge tubes from his collection, hooked them up, and
showed Jennings the fascinating glow in the tubes. Enthusiastic and engrossed in conversation about cathode radiation,
they forgot about the unexposed photo cassettes lying next to
the tubes.
A few days later, Jennings reported that while developing
all the photographic plates, even those that were not directly
exposed, he noticed something strange. Circular structures
overlaid with spark marks appeared on one plate. The structures were completely different from those on the directly
exposed photographic plates. Neither the photographer nor
the physicist could explain this strange effect. Was it a manufacturing defect? Goodspeed put the negative aside along
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
with other strange plates. As time passed, no one gave it a
second thought. Six years later, Arthur Goodspeed also
learned of the discovery of new, penetrating rays by the German physicist Roentgen in Würzburg. Goodspeed remembered the plate put aside and studied it again. The logical
explanation could only be that he and Jennings had unknowingly taken the first X-ray picture of coins on February 22,
1890 (Fig. 11).
Goodspeed then repeated his experiments under the same
conditions, with the same apparatus and tubes, and he
obtained comparable results. At the American Philosophical
Society meeting on February 21, 1896, Goodspeed gave a
remarkable experimental lecture on X-rays and told the story
of this first “radiation accident” in history, almost exactly six
years ago. At the end of his lecture, Goodspeed said: “Now,
gentlemen, we wish it clearly understood that we claim no
credit whatever for what seems to have been a most interesting accident, yet the evidence seems quite convincing that
the first Röntgen shadow picture was really produced almost
exactly six years ago to night, in the physical lecture room of
the University of Pennsylvania” [13]. In a letter to the
German-American medical physicist and Röntgen biographer Otto Glasser (1895–1964) dated February 15, 1929,
Goodspeed regretted not having investigated the findings:
“[...] The accidental roentgen effect which W. N. Jennings
and I produced in 1890 was real and authentic. Because of
our laxity in not following the matter up we do not claim
any credit whatsoever [...]” [14].
2.9 Nikola Tesla (1856–1943), New York:
Phosphorescent lamps and an unidentified radiograph
(1894)
Tesla on the internet, who repeatedly post the claim that
Tesla is the true discoverer of X-rays. As early as 1891,
Tesla experimented with high-frequency gas discharges.
He then developed a unipolar gas discharge tube and in January 1894 he undertook experiments on photography with
phosphorescent lamps. On a photograph of the American
writer, humorist, entrepreneur, publisher, and lecturer Marc
Twain, an adjusting screw inside his camera lens is said to
have been visible on the picture [15]. Whether Tesla did further early research on this phenomenon cannot be proven, as
unfortunately much of his work was lost when his laboratory
in New York burned down on March 13, 1895. Tesla, however, did not begin to work intensively on X-rays until after
Röntgen’s announcement (Fig. 12). He published numerous
papers on X-rays in the course of 1896, including their biological effects. The New York Herald reported on February
23, 1896, that Tesla had also succeeded in taking the first Xray image of the human brain. From today’s perspective,
however, this was a false report. Tesla took early radiographs and sent a collection to Röntgen, who thanked Tesla
and congratulated him on the excellent images.
2.10 Philipp Eduard Anton Lenard (1862–1947),
University of Bonn: Passing of cathode rays through thin
foils (1894)
I am the mother of X-rays. Just as the mid-wife is not responsible for the mechanism of birth, so was Roentgen not responsible
for the discovery of X-rays since all the groundwork had been
prepared by me. Without me, the name of Roentgen would be
unknown today. [16]
There are a large number of followers of the SerbianAmerican inventor, physicist and electrical engineer Nikola
Figure 11. X-ray image of two American coins. (Archiv Deutsches
Röntgen-Museum).
Figure 12. Telegram from Tesla to Röntgen dated March 14, 1896
(Archive Deutsches Röntgen-Museum).
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
After his habilitation on water electricity with Heinrich
Hertz (1857–1894) in Bonn, Philipp Lenard worked on other
electrical phenomena, such as thunderstorm electricity. His
special interest, however, was in the study of the passage
of cathode rays through thin layers of material. As early as
1892, Hertz had discovered that a thin metal sheet transmitted cathode rays. Hertz used quite thin, very soft and porous
gold, silver and aluminium bookbinding sheets, but showed
that the cathode rays passed not only through the holes but
also through the material itself, i.e., the metal of the sheets.
This prompted Lenard to insert a thin, vacuum-tight aluminum window in the wall of a tube through which the cathode rays could pass from the vacuum of the tube into the air.
In addition to the different penetrating powers, Lenard was
later able to demonstrate the photographic effect and the
magnetic deflection of cathode rays. In 1894, he published
his results in the Annalen der Physik [17].
His research and the use of the “Lenard window” provided the first opportunity to study cathode rays outside
the discharge tube (Fig. 13). Lenard’s results contributed
to further research into the physical nature of cathode rays.
Using his research results, the British physicist Joseph John
Thomson (1856–1940) later in 1897 succeeded in proving
that cathode rays are made of electrons. This embittered
Lenard greatly. But he was even more embittered by the discovery of X-rays in November 1895, since Röntgen had
received the essential information for the cathode ray experiments as well as a discharge tube directly from Lenard.
After Röntgen became famous for the discovery of X-rays,
Lenard accused him of having robbed him of the discovery
[18]. The dispute was further fuelled by the award of the first
Nobel Prize to Wilhelm C. Röntgen, especially since the
Nobel Committee had initially thought of awarding Lenard
and Röntgen the first Nobel Prize in 1901. Lenard later
received the Nobel Prize in Physics in 1905 “for his work
on cathode rays”. The conflict, which had been simmering
for decades, flared up again during the National Socialist
era in Germany in the late 1930s. Because of his early membership in the Nazi Party (NSDAP), Lenard, with the help of
the National Socialist press and some party members, succeeded in stirring up considerable controversy over Röntgen’s position in science. During this period, Lenard, not
Röntgen, was credited with the discovery of X-rays by the
Third Reich. Lenard also emphasized this in the preface to
his four-volume work on German physics published in
1936. Instead of “X-rays” he used the term “highfrequency rays” [19].
2.11 Wilhelm Hallwachs (1859–1922), Technical
University Dresden: The missed chance (1895)
Another excellent experimental physicist also narrowly
missed the discovery of X-rays. Wilhelm Hallwachs
9
Figure 13. Lenard discharge tube (Archive Deutsches RöntgenMuseum).
(Fig. 14), who had studied under August Kundt in Strasbourg and Friedrich Kohlrausch (1840–1910) in Würzburg,
was, like Röntgen, a master of experimental physics who
designed and built many devices himself. In 1893, he
became a full professor of electrical engineering at the Technical University of Dresden. There he succeeded August
Toepler (1836–1912) as full professor of physics in 1900.
His greatest scientific achievement was the fundamental
study of the external photoelectric effect discovered by
Heinrich Hertz in 1887, i.e., the escape of electrons from surfaces when irradiated with suitable light. For this reason, the
photoelectric effect was also called the “Hallwachs effect”
for a short time. In any case, the effect opened the door to
quantum mechanics [20]. Albert Einstein (1879–1955) later
realized that this photoelectric effect can only be explained if
one assumes that the light hits the metal in individual portions – as “light particles”, or “photons”. The energy of each
photon is thereby determined by its wavelength.
In the estate of senior engineer Horst Beger, former chief
designer at the Koch & Sterzel X-ray company in Dresden,
the son-in-law of Wilhelm Hallwachs reports on his
father-in-law’s early experiments with cathode rays and his
regret about the course of fate. “My father-in-law Hallwachs
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U. Busch / Z Med Phys xxx (2023) xxx–xxx
Figure 14. Wilhelm Hallwachs (1859–1922) (Wikimedia commons. Wilhelm Hallwachs | Niels Bohr Library & Archives (aip.
org)).
had worked in the same field (X-rays) and found satisfaction
in experimentation, but he had not been able to achieve success. At that time, Gundelach [21] (Fig. 15) had built five
tubes and delivered them to five universities; the matter
was on the trail in the small circle of physicists. The tube
for Hallwachs arrived defective - until it was returned
repaired, by which time Röntgen had published. My
father-in-law once told me, rather sadly, that immortality
had thus passed him by” [22].
2.12 Wilhelm Conrad Röntgen (1845–1923): University
of Würzburg: Scientific research on X-rays (1895)
Probably in the spring of 1894, Röntgen became aware of
Lenard’s work on cathode rays. Röntgen later never divulged
what measurements he intended to make. In an interview
with the American reporter H. J. Dam from McClure’s
magazine, he said: “I have been for a long time interested
in the problem of the cathode rays from a vacuum tube as
studied by Hertz and Lenard. I had followed theirs and other
researches with great interest, and determined, as soon as I
Figure 15. Advertisement for X-ray tubes by the Gundelach
company from May 1896 (Courtesy of Glasmuseum Gehlberg).
had the time, to make some researches of my own. This time
I found at the close of last October (1895). I had been at
work for some days when I discovered something new“ [23].
On Friday, November 8, 1895 late in the afternoon Röntgen set up some experiments on cathode rays in his laboratory at the University of Würzburg (Fig. 16). He used a
simple ionising Crookes type or Hittorf tube, an induction
coil, and a modern vacuum pump. “A discharge from a large
induction coil is passed through a Hittorf’s vacuum tube, or
through a well-exhausted Crookes’ or Lenard’s tube. The
tube is surrounded by a fairly close-fitting shield of black
paper; it is then possible to see, in a completely darkened
room, that paper covered on one side with barium platinocyanide lights up with brilliant fluorescence when brought
into the neighborhood of the tube. Whether the painted side
or the other be turned towards the tube. The fluorescence is
Please cite this article as: U. Busch, Claims of priority – The scientific path to the discovery of X-rays, Z Med Phys, https://doi.org/10.1016/j.zemedi.2022.12.002
U. Busch / Z Med Phys xxx (2023) xxx–xxx
11
which caused a deflection of the cathode rays in the
Crookes tube. In 1897, he repeated the experiment with
different anode materials and different gases, but the
deflection remained the same. To determine the massto-charge-ratio of these particles, he equated the magnetic deflection with the electric one. After the Irish
physicist Johnstone Stoney (1826–1911) had already
proposed in 1878 the existence of an electric charge carrier of uniformly large charge associated with atoms and
coined as “electron”, Thomson succeeded in providing
the first experimental proof of the electron on April
30; 1897. “As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as
if they were negatively electrified, and are acted on by a
magnetic force in just the way in which this force would
act on a negatively electrified body moving along the
path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried
by particles of matter” [25]. He found that each of the
particles had the same small mass, about 1/2000 of the
mass of a hydrogen atom. In this way he had discovered
the electron as a negatively charged elementary particle.
It should not remain unmentioned here that in the late
1920s J. J. Thomson’s son George Paget Thomson
Figure 16. Röntgen’s glass plate for investigating permeability
with increasing layer thickness (Archiv Deutsches RöntgenMuseum).
still visible at two meters distance. lt is easy to show that the
origin of the fluorescence lies within the vacuum tube” [24].
3 Epilogue
3.1 Joseph J. Thomson (1856–1940), Cavendish
Laboratory Cambridge University: 1/2000 of the
hydrogen atom (1897)
The clarification of the physical nature of cathode radiation finally took place at the Cavendish Laboratory in Cambridge, UK. Sir Joseph John Thomson (Fig. 17), British
physicist, president of the Royal Society, teacher of Ernest
Rutherford and winner of the Nobel Prize in physics in the
year 1906, succeeded in deciphering the nature of the cathode rays by conducting different experiments. Thomson was
the first to recognize that all the cathode rays have the same
origin and that cathode rays were charged particles or, as he
called them, “corpuscles”. To prove this, he used magnets,
Figure 17. Joseph J Thomson (1856–1940) steel engraving from
1896. Taken from The Electrician, 1896 - The Electrician, 1896,
p.120 ISBN 0-87942-238-6, (File:JJ Thomson.jpg - Wikimedia
Commons) Wikimedia Commons.
Please cite this article as: U. Busch, Claims of priority – The scientific path to the discovery of X-rays, Z Med Phys, https://doi.org/10.1016/j.zemedi.2022.12.002
12
U. Busch / Z Med Phys xxx (2023) xxx–xxx
(1892–1975) demonstrated that an electron can be diffracted
like a wave. For the discovery of the wave-like properties of
the electron, made at the University of Aberdeen, George P
Thomson received the Nobel Prize in Physics in 1937,
together with the American physicist Clinton Davisson
(1881–1958), who had made the same discovery independently in 1927 at the Bell Telephone Laboratories.
4 Conclusion
Despite numerous observations on the existence of a radiation penetrating matter in the 19th century, no questions or
not the right questions were asked from the scientists to
explain the observed phenomena. The phenomena remained
unrecognised and uninterpreted until Wilhelm Conrad Röntgen tried to really question the observations.
It is true that Röntgen was not the first to generate and
observe X-rays but it was him who got to the bottom of
the observations in his experiments and sought and found
a scientific explanation for the new phenomena of invisible
rays. Röntgen was the first to clearly identify the new phenomenon and to describe its properties precisely in the
course of his meticulous research. He was thus also the first
of the scientists to deliberately generate X-rays and make
radiographs in order to make the invisible visible (Fig. 18).
Röntgen was therefore awarded the first Nobel Prize in
Physics “in recognition of the extraordinary services he
has rendered by the discovery of the remarkable rays subsequently named after him”. Interestingly, among the 12 candidates nominated for the first Nobel Prize in Physics, the
committee of the Royal Swedish Academy of Sciences had
initially given preference to two proposals. Wilhelm Conrad
Röntgen had the most advocates of all on his side. Philipp
Lenard was nominated only by Silvanus P. Thompson
(1851–1916), the London authority on cathode rays and
X-rays. Nevertheless, the five-member committee, chaired
by the Swedish astronomer and physicist Klas Bernhard
Hasselberg (1848–1922), proposed that the prize be awarded
equally to Röntgen and Lenard. In justifying its decision, the
Nobel Committee speculated whether Lenard might not have
discovered X-rays before Röntgen. In the committee’s view,
however, this was not evident in Lenard’s work. This discussion contained the explosive material that was long debated.
In the end, at its plenary session on November 12, 1901, the
entire Academy in Stockholm finally overrode the recommendation of the prize committee and decided to award
Röntgen an undivided prize, emphasizing the achievements
of Lenard and other scientists mentioned in this article in
the study of cathode rays and stressing that “Röntgen’s work
on cathode rays led him, however, to the discovery of a new
and different kind of rays” [26]. In his presentation speech,
Swedish historian and president of the Royal Swedish Academy of Sciences, Clas Theodor Odhner (1836–1904),
emphasised in particular the great significance of Röntgen’s
discovery for medical practice.
The important question of the physical nature of X-rays,
which Röntgen was unable to answer, later similarly gave
rise to two bearings in the discussion on clarifying the nature
of light: those who saw in X-rays as having a wave character
like Charles Glover Barkla (1877–1944) and those who
advocated a corpuscular theory like William Henry Bragg
(1862–1942).
Declaration of Competing Interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
Figure 18. Wilhelm Conrad Röntgen in his physics institute in
Würzburg portraits by Nicola Perscheid (1864–1930) for the
making of the Röntgen Monument by Reinhold Felderhoff (1865–
1919), which was erected on the Potsdamer Bridge in Berlin in
1898. (Archiv Deutsches Röntgen-Museum).
I would like to thank Prof. Dr. Jürgen Reichenbach,
University Hospital Jena for his numerous valuable suggestions and proposals as well as Dr. Arpan K. Banerjee,
Retired consultant radiologist, Chairman International
Society for the History of Radiology (ISHRAD), Birmingham, UK, for reviewing the manuscript.
Please cite this article as: U. Busch, Claims of priority – The scientific path to the discovery of X-rays, Z Med Phys, https://doi.org/10.1016/j.zemedi.2022.12.002
U. Busch / Z Med Phys xxx (2023) xxx–xxx
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Please cite this article as: U. Busch, Claims of priority – The scientific path to the discovery of X-rays, Z Med Phys, https://doi.org/10.1016/j.zemedi.2022.12.002