Early history of x rays

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Early History of X Rays
by ALEXI ASSMUS

The discovery of X rays
in 1895 was the
beginning of a
revolutionary change
in our understanding
of the physical world.

IN THE WINTER of the year of his ﬁftieth birthday, and the year

following his appointment to the leadership of the University

of Würzburg, Rector Wilhelm Conrad Roentgen noticed a barium

platinocyanide screen ﬂuorescing in his laboratory as he

generated cathode rays in a Crookes tube some distance away.

Leaving aside for a time his duties to the university and to his

students, Rector Roentgen spent the next six weeks in his labora-

tory, working alone, and sharing nothing with his colleagues.

10 SUMMER 1995 zycnzj.com/http://www.zycnzj.com/


Wilhelm Conrad Roentgen (1845–1923).
(Courtesy of AIP Emilio Segré Visual
Archives)

Three days before Christmas he that jolted the fin-
brought his wife into his laborato- de-siècle disci-
ry, and they emerged with a photo- pline out of its
graph of the bones in her hand and of mood of finality,
the ring on her finger. The Würzburg of closing down
Physico-Medical Society was the first the books with
to hear of the new rays that could ever more precise
penetrate the body and photograph measurements, of
its bones. Roentgen delivered the losing itself in de-
news on the 28th of December 1895. bates over statistical
Emil Warburg relayed it to the Berlin mechanics, or of try-
Physical Society on the 4th of Janu- ing to ground all
ary. The next day the Wiener Press physical phenomena in
carried the news, and the day fol- mathematically precise
lowing word of Roentgen’s discovery fluctuations of the ether.
began to spread by telegraph around All three discoveries, X rays,
the world. uranium rays, and the elec-
On the 13th of January, Roentgen tron, followed from one of the
presented himself to the Kaiser and major experimental traditions in the
was awarded the Prussian Order of second half of the nineteenth
the Crown, Second Class. And on the century, the study of the discharge
16th of January the The New-York of electricity in gases. All three
Times announced the discovery as contributed to a profound transfor-
a new form of photography, which mation of physics. In the 20th cen-
revealed hidden solids, penetrated tury, the discipline has been ground-
wood, paper, and flesh, and exposed ed in the study of elementary Forms of tube used by Roentgen
the bones of the human frame. “Men particles. in 1895–1896 for the production
of science in this city are awaiting As with the invention of in- of X rays.
with the utmost impatience the candescent light
arrival of English technical journals bulbs, the study
which will give them the full par- of electrical dis-
ticulars of Professor Roentgen’s discharge through
covery of a method of photographing gases was made
opaque bodies,” The New-York possible by the
Times began, and it concluded by pre- development of
dicting the “transformation of mod- improved vacu-
ern surgery by enabling the surgeon um technology
to detect the presence of foreign in the 1850s. Ear-
bodies.” (Jan. 16, 1896, p. 9) ly on, English
The public was enthralled by this scientists were
new form of photography and curi- investigating the
German Museum, Munich

ous to know the nature of the new patterns of light
rays. Physicians put it to immediate and dark that ap-
use. Physicists sat up and took no- peared in sealed
tice. The discovery of X rays was the lead-glass tubes.
first in a series of three discoveries The patterns in

zycnzj.com/http://www.zycnzj.com/ BEAM LINE 11


Roentgen’s apparatus for studying the
ionization of air by X rays, 1906.

transparent to ultra-violet light.
When Heinrich Hertz found that he
could pass the rays through metal
foil, a fellow German scientist, Philip

German Museum, Munich
Lenard, began to study them more
carefully. Lenard designed a tube
with a thin aluminum window
through which the rays could
emerge, and he measured how far
they could travel and still induce
these partially evacuated tubes were fluorescence. Defined in this way,
stimulated by a voltage drop between the range of the cathode rays was six
a cathode and an anode: typically to eight centimeters. Lenard’s ex-
there was a dark space, called periments inspired Roentgen to won-
Crookes’ dark space; then a glow, der if the rays in an attenuated form
called negative light; then another really traveled farther, and he
dark space, this one called Faraday’s; planned experiments to see if a
and a final glow of positive light. If sensitive electroscope would mea-
the air in the tube was exhausted un- sure a discharge at four times the
til the first dark space expanded to distance Lenard had identified.
fill the entire tube and all glows dis- This line of work was outside
appeared, then the rays emitted from Roentgen’s usual research pursuits,
the cathode could be investigated. which had by this time gained him
The rays cast shadows, and were great stature in German science. Son
deflected by magnetic fields, but of a cloth manufacturer and mer-
appeared to be immune to the ef- chant from the Rhine province,
fects of static electric forces. Roentgen was not a particularly
As was to be characteristic of the diligent student in his youth. He
new ray physics to come—the phys- eventually made his way to the
ics of cathode rays, X rays, alpha rays, Polytechnic in Zurich, where he
beta rays, gamma rays, and N rays— obtained a diploma in mechanical
the nature of the cathode rays was in engineering in 1868 and a doctor-
dispute, the British favoring a stream ate one year later. In Zurich he
of particles, those on the Continent became an assistant to August Kundt
preferring to think of them as some and moved along with him to the
sort of disturbance of the ether. (The University of Würzburg, and then on
British position, and the research pro- to the Physical Institute at Stras-
gram developed by J.J. Thomson at bourg. His first move on his own was
the Cavendish Laboratory to study to the chair of physics at Giessen
ionization in gases, would result in in Hesse in 1879, from which he
the discovery of the electron. But our received many offers to go elsewhere.
story does not take us that way). The path upward in the German
A strong reason for believing that university system was to follow calls
the cathode rays were particles was to universities of higher and higher
Sir Joseph John Thomson, 1856–1940. the observation that they would stature, and finally to obtain an
(Courtesy of the AIP Niels Bohr Library) not pass through matter that was institute of one’s own. Roentgen



refused the calls until the Universi- centimeters that Lenard had found
ty of Würzburg offered him the to be the maximum distance for
Directorship of their Physical Insti- which cathode rays maintain their
tute. In 1894 he was elected Rector power to induce fluorescence. Roent-
at Würzburg. In his inaugural ad- gen recognized the effect as wor-
dress, given the year before his dis- thy of his undivided attention and
covery of X rays, Roentgen stated devoted the next six weeks to its
that the “university is a nursery of uninterrupted study.
scientific research and mental edu- Historians have speculated about
cation” and cautioned that “pride in why Roentgen was the first to rec- Phillip Lenard, 1862–1947. (Courtesy of
one’s profession is demanded, but not ognize the significance of this effect. Ullstein Bilderdienst and the AIP Niels
professional conceit, snobbery, or The equipment, a cathode ray tube Bohr Library)
academic arrogance, all of which and a fluorescing screen, had been in
grow from false egoism.”* use for decades. In 1894 J.J. Thomson
Roentgen’s pride could rest in the had seen fluorescence in German-
over forty papers he had published glass tubing several feet from the
from Strasbourg, Giessen, and discharge tube. Others had noted
Würzburg. These early interests fogged photographic plates. But
ranged widely—crystals, pyroelec- before Lenard’s work, the object of
trical and piezoelectrical phenomena, study was always the effects inside
and the effects of pressure on liquids the tube itself, and stray ultra-ultra-
and solids—but did not yet include violet light could be used to explain
electrical discharges in gases. He had the fogging of photographic plates.
taken his turn at measuring the Lenard’s great interest was in prov-
Demonstration by Crookes that cathode
specific heat ratios of gases using a ing, in contradiction to the British, rays travel in straight lines: a) cathode;
sensitive thermometer of his own the ethereal nature of cathode rays, b) aluminum cross and anode; d) dark
making. He was an exact experi- and he was the first to study the shadow; c) fluorescent image.
menter who often made his own
apparatus—a skill learned during his
training as an engineer in Zurich—
and he was able to measure ex-
tremely small effects, surpassing
even Faraday’s measurement of the
rotation of polarized light in gases.
Roentgen turned to a new interest
in October of 1895: the study of cath-
ode rays. In the course of repeating
the experiments of Hertz and Lenard,
he happened to notice a glowing flu-
orescent screen set off quite some
distance from the Crookes’ tube he
was operating. The screen sat much
farther away than the six to eight
*Quoted in “Wilhelm Conrad Roentgen,”
Dictionary of Scientific Biography
(New York: Scribner’s, 1975), p. 531.



effects of the rays in air or in a sec- shadowy pictures they produce:
ond glass tube into which he directed bones in a hand, a wire wrapped
them. around a bobbin, weights in a box,
Roentgen, a meticulous and ob- a compass card and needle hidden
servant experimenter, made the away in a metal case, the inhomo-
obvious tests on the new X rays: geneity of a metal. The ability of the
Were they propagated in straight new rays to produce photographs
lines? Were they refracted? Were they gave them great popular appeal and
reflected? Were they distinct from brought Roentgen fame. Many arti-
cathode rays? What were they? Like cles appeared in photography jour-
Heinrich Rudolf Hertz, 1857–1894. the cathode rays, they moved in nals, and The New-York Times in-
(Courtesy of Deutsches Museum and straight lines. Roentgen was unable dexed the new discovery under
AIP Emilio Segrè Visual Archives) to refract them with water and car- photography. Since the rays exposed
bon bisulphide in mica prisms. Nor photographic plate, the public as-
could he concentrate the rays with sumed they were some form of light.
ebonite or glass lenses. With ebonite The physicist Roentgen concurred.
and aluminum prisms he noted the Accepting Lenard’s claim that cath-
possibility of refracted rays on a pho- ode rays were vibrations of the ether,
tographic plate but could not observe Roentgen compared the new rays to
this effect on a fluorescent screen. them and forwarded the opinion that
Testing further, he found that X rays the two were ethereal, although dif-
O, Röntgen, then the news is true, could pass freely through thick lay- ferent from visible, infra-red and
And not a trick of idle rumour, ers of finely powdered rock salt, ultra-violet light in that they did not
That bids us each beware of you, electrolytic salt powder, and zinc reflect or refract. He suggested that
And of your grim and graveyard humour. dust, unlike visible light which, cathode rays and X rays were longi-
because of refraction and reflection, tudinal vibrations of the ether rather
We do not want, like Dr. Swift,
To take our flesh off and to pose in is hardly passed at all. He concluded than transverse ones.
Our bones, or show each little rift that X rays were not susceptible to Now that their existence was
And joint for you to poke your nose in. regular refraction or reflection. established, it was easy enough to
Roentgen found that the X rays experiment with the new X rays.
We only crave to contemplate originate from the bright fluores- Roentgen himself published only
Each other’s usual full-dress photo;
cence on the tube where the cathode three papers on the subject, but oth-
Your worse than “altogether” state
Of portraiture we bar in toto! rays strike the glass and spread out. ers jumped quickly into the field.
The point of origin of the X rays And not just physicists. Thomas
The fondest swain would scarcely prize moves as the cathode rays are moved Edison used modified incandescent
A picture of his lady’s framework; by a magnetic field, but the X rays light bulbs to produce the new rays.
To gaze on this with yearning eyes themselves are insensitive to the He boasted to reporters that any-
Would probably be voted tame work! magnet. Roentgen concluded that one could make photographs of
No, keep them for your epitaph,
they are distinct from cathode rays, skeleton hands; that was mere child’s
these tombstone-souvenirs unpleasant; since Lenard’s work had shown that play. Within a month of Roentgen’s
Or go away and photograph cathode rays passing through the announcement doctors were using
Mahatmas, spooks, and Mrs. B-s-nt! tube maintained their direction the X rays to locate bullets in human
but were susceptible to magnetic flesh and photograph broken bones.
—Punch, January 25, 1896 deflection. Dr. Henry W. Cattell, Demonstrator
Roentgen justified calling the new of Morbid Anatomy at the Univer-
phenomena rays because of the sity of Pennsylvania, confirmed their



Henri Poincaré, 1854–1912. (Courtesy of
AIP Emilio Segrè Visual Archives)

importance for the diagnosis of
kidney stones and cirrhotic livers and
commented that “The surgical
imagination can pleasurably lose it-
self in devising endless applications
of this wonderful process.” (The
New-York Times, Feb. 15, 1896, p. 9).
In the ﬁrst six months after their dis-
covery Viennese mummies were un-
dressed, doctors claimed to have pho-
tographed their own brains, and the transverse ethereal vibrations: light,
human heart was uncovered. By 1897 uranium rays, X rays. Uranium rays
the rays’ dangerous side began to were given off by certain minerals,
be reported: examples included loss and they needed no apparatus to pro-
of hair and skin burns of varying duce them, but they shared certain
severity. properties with X rays. They exposed
Electricians and physicists specu- photographic plates and they caused
lated on the nature of these X rays. gases to conduct electricity.
Albert Michelson thought they British physicists weighed in on
might be vortices in the ether. the side that X rays were impulses in
Thomas Edison and Oliver Lodge the ether rather than continuous
suggested acoustical or gravitation- waves. Lucasian Professor of Math-
al waves. But the rays ability to pho- ematics at Cambridge, Sir George
tograph was decisive, and serious Gabriel Stokes, and his colleague and
thinkers settled on three possibili- director of the Cavendish Laborato-
ties, all of them of electromagnetic ry, J.J. Thomson, committed them-
origin: the waves were very high fre- selves to the impulse hypothesis in
quency light; they were longitudinal 1896. It was consistent with their
waves (Roentgen’s initial suggestion); conception of cathode rays as parti-
or they were transverse, discontin- cles (Thomson was to announce the
uous impulses of the ether. discovery of the corpuscle or electron
Quite early on the hypothesis that one year later.) The abrupt stop of a
they were longitudinal waves was charged particle would result, after a
discarded, despite the support of tiny delay, in the propagation out-
Henri Poincaré and Lord Kelvin. The ward of an electromagnetic pulse.
crux of the question was whether the With Thomson’s exact measurement
waves were polarizable. If so they of the charge-to-mass ratio and H.A.
could not be longitudinal waves. Lorentz’ successful theory of the
Although the early experiments on electron, which explained many
polarization were negative or unclear, intriguing phenomena, Continen-
with the discovery of another ray, tal physicists began to accept, to
Henri Bequerel’s uranium rays for Lenard’s dismay, cathode rays as
which he claimed to have found po- material particles and X rays as
First X ray made in public. Hand of the famed
larization, those on the Continent impulses in the ether. anatomist, Albert von Kölliker, made during
set up a convincing typology. It went Soon new results began to Roentgen's initial lecture before the Würzburg
from lower to higher frequency come in. Two Dutch physicists, Physical Medical Society on January 23, 1896.



Arnold Johannes Wilhelm Sommerfeld,
1868–1951. (Courtesy of the AIP Niels
Bohr Library)

director of the Cavendish Laboratory,
had separated α rays, stoppable by
metal foil or paper sheets, from the
more penetrating β rays. In 1900,
Rutherford had identified the βs as
high-speed electrons: deflected in a
magnetic ﬁeld they showed the cor-
rect charge-to-mass ratio. A third
component of the uranium rays,
undeviable and highly penetrating,
was discovered by Paul Villard at the
Hermann Haga and Cornelius Werd, Ecole Normale Superieur in Paris.
announced that X rays could be dif- Rutherford named these γ rays. In
WE WANT TO KNOW fracted, and a Privatdozent at Göt- her 1903 thesis Marie Curie made
tingen named Arnold Sommerfeld these comparisons: γ rays to X rays;
carried out a mathematical analy- β rays to cathode rays; and α rays to
If the Roentgen rays, that are way ahead,
Will show us in simple note, sis of diffraction to show that their canal rays. (Canal rays were streams
How, when we ask our best girl to wed, results could be explained in terms of positively charged molecules.)
That lump will look in our throat. of aperiodic impulses. In 1904, A few years later another story
Charles Glover Barkla, a student of came out. The British scientist
If the cathode rays, that we hear all about, both Stokes and Thomson at Cam- William Henry Bragg announced in
When the burglar threatens to shoot, bridge, showed that X rays were 1907 that X rays and γ rays were not
Will they show us the picture without any doubt,
plane polarizable while experiment- in fact ether waves, but rather par-
Of the heart that we feel in our boot.
ing with secondary and tertiary ticles, a neutral pair at that: electron
If the new x-rays, that the papers do laud, X rays. (These were produced by plus positively charged particle.
When the ghosts do walk at night, directing X rays against solids.) Bragg’s serious research began at a
Will show ’neath our hat to the world abroad As X rays began to show, more and late age, 41, after twenty pleasant
How our hair stands on end in our fright. more, the properties of light, urani- years at the University of Adelaide,
um rays provided new mysteries. Australia, where he played golf and
If the wonderful, new, electric rays,
Will do all the people have said, They themselves were composed of hobnobbed with government of-
And show us quite plainly, before many days, three sorts of distinct rays: α, β, and ficials. He announced his new in-
Those wheels that we have in our head. γ rays. What were these? Suddenly tellectual work in a Presidential
physics, which had seemed to some Address to the Australian Associa-
If the Roentgen, cathode, electric, x-light, to be coming to a conclusion, was tion for the Advancement of Science
Invisible! Think of that! during which he made a critical
faced with unexplainable, qualita-
Can ever be turned on the Congressman bright
And show him just where he is at. tive discoveries. They were not “in review of Rutherford’s work, ques-
the sixth place of the decimals,” as tioning the law of exponential
Oh, if these rays should strike you and me, Michelson had predicted. At the decrease for the absorption of α rays.
Going through us without any pain, international congress on physics, For two and a half years he published
Oh, what a fright they would give us to see staged in Paris in 1900 by the French a paper every few months, work that
The mess which our stomachs contain! Physical Society, fully nine percent led him to make the radical state-
of the papers delivered were on the ment that X rays were particles. His
—Homer C. Bennett,
American X-ray Journal, 1897 new ray physics. idea was based on two facts: (i) X rays
In 1899 Ernest Rutherford, another excite fewer gas molecules in their
student of Thomson’s and the man path than would be expected from
who would become his successor as a wave-like disturbance, and (ii) the



velocity of the electrons excited by
X rays is greater than could be giv-
en to them by a wave. By this time
Bragg and his physicist son were back
in England, and their theory caused
great controversy even in the country
where particles were in favor and
where exotic modeling of physical
phenomena was well tolerated. Their
most vociferous opponent was wondered whether such a discipline Roentgen picture of a newborn rabbit made
Charles Barkla, who argued that the distinct from chemistry existed!) by J. N. Eder and E. Valenta of Vienna, 1896.
ionization of matter was a secondary When in 1899 Roentgen was of- (Burndy Library, Dibner Institute, Cambridge,
effect not needing to be directly fered a position at Munich and Massachusetts.)
attributable to the wave-like nature the chance to build up physics there,
of X rays. We will return later to the he accepted. Five years later, in
problem of the concentration of X- negotiations with the minister of
ray energy, unexplainable in terms education over another possible
of waves, as it bears on Louis de move, this time to the Reichsanstalt,
Broglie’s insight into the wave nature Roentgen received, in return for a
of matter. pledge to stay in Munich, a second
institute, for theoretical physics, to
complement his existing institute
X RAYS AS A PROBE OF THE Radiographs of tropical ﬁsh made
for experimental physics. When Emil by J.N. Eder and E. Valenta of Vienna,
STRUCTURE OF MATTER
Cohn and Emil Weichert succes- Jaunary 1896 and presented to
Before we turn to our ﬁnal act in sively declined the offer of a position, Roentgen. (Burndy Library, Dibner
the almost thirty year drama to unit was given to Privatdozent Som- Institute, Cambridge, Massachusetts.)
derstand the nature of X rays, let us merfeld, who joined Roentgen in
turn aside to follow another direc- Munich and shared his desire to build
tion that the work on X rays took, up physics there to the quality of the
a shift from the investigation of the institutes in Göttingen, Berlin, and
nature of X rays to their use in prob- Leipzig. In the work on quantum
ing the structure of crystals and of theory of the next two decades,
atoms. That story will take us back Munich would join Copenhagen and
to Roentgen and the center for phys- Göttingen as the main centers on the
ics he built up at Munich. While Continent.
at Würzberg, Roentgen had been Sommerfeld was initially unen-
agitating for an extra position in thusiastic about assistant Max von
physics. He wanted a position for Laue’s idea that regularly spaced
theoretical physics, a newly emerg- atoms in a crystal might act as a dif-
ing specialty of German origin that fraction grating for X rays, the fine
followed by several decades the crys- distances between the atoms serving,
tallization of physics itself in the as no hand- or machine-ruled grating
mid-nineteenth century. (In 1871 could, to diffract ultra-high frequen-
James Clerk Maxwell hesitated in cies. If, of course, that is what one
giving his support to the creation of thought X rays were! Sommerfeld,
a Physical Society in London. He pushing the impulse hypothesis, was



White radiation Laue diffraction pattern from the protein
trimethylene dehydrogenase (an enzyme that catalyzes the
conversion of trimethylamine to dimethylamine and
formaldehyde) recorded on SSRL beam 10–2 with a 5 msec
X-ray exposure. The photograph was taken by Scott Matthews
and Scott White of Washington University, St. Louis, and Mike
Soltis, Henry Bellamy, and Paul Phizackerly of SSRL/SLAC.

engaging in dis- Later others would suggest that the
cussions with crystal itself imposed structure on
Johannes Stark the incoming radiation. Laue pub-
over the quan- lished a rather long article on his
tum nature of X theory of diffraction in the Enzyk-
rays. Stark was lopadie der Mathematische Wis-
one of the few senschaften, and much later (1941)
physicists who he went on to publish a 350-page
in 1911 took se- review of the subject, Roentgen-
riously Einstein’s strahlen-Interferenzen, in which he
suggestion that light comes in quan- included the effects of electron
ta of energy. Applying the notion to interference.
X rays, Stark was able to assign them Perhaps as was ﬁtting for an early
a frequency and to explain the high proponent of relativity and a defender
velocity of electrons that had been of Einstein throughout the Nazi pe-
excited by X rays, one of the phe- riod, Laue made little of quantum
nomena that so exercised Bragg and theory and remained skeptical of the
Barkla. Copenhagen interpretation through-
Laue persisted in asking that the out his life. He became director of
experimentalists try out X rays on the Kaiser Wilhelm Institute in the
crystals. A student of Max Planck’s years before World War II, resigning
(in fact, his favorite), Laue had his position in 1943, at which time
worked on a theory of the interfer- the Institute was directed towards
ence of light in plane parallel plates. the building of an atomic bomb un-
By 1912 his specialty had become the der the leadership of Werner Heisen-
theory of relativity, but he was not berg. After the war Laue worked to
averse to following Sommerfeld in rebuild German science. In the fall
working on a theory of diffraction. of 1946 he helped create the German
Laue’s guess was that it would be Physical Society in the British Zone,
only the secondary X rays, not the and worked to revive the ﬁrst of the
chaotic Bremsstrahlung identified national bureaus of standards, the
with the initial deceleration of Physikalische-Technische-Reich-
electrons, that would interfere con- sanstalt. Towards the end of his life
structively in the crystal. In April he assumed the directorship of the
1912 Walther Friedrich and Paul now one of several Kaiser Wilhelm
Knipping shone secondary X rays on Institutes, this one devoted to elec-
copper sulfate and zinc sulfate sur- trochemistry in Berlin-Dahlem. Laue
faces and found that dark spots in died in an auto accident at the age of
successive circles appeared on pho- eighty-one.
tographic plates placed behind them. Laue was representative of the
At this time both the nature of X rays German talent for institution build-
and the structure of crystals was a ing in the support of science and the
puzzle. Laue’s analysis of the situa- German fascination for fundamen-
tion was to identify five distinct tal principles and theories. Those
wavelengths of incoming X rays who would apply Laue’s idea and
between 1.27 and 4.83 × 10 −9 cm. build on Friedrich and Knipping’s



Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William Lawrence
Bragg, 1890–1971. (Courtesy of the AIP Niels Bohr Library)

experimental demonstration were using a photographic plate or an
the British, specifically the Braggs ionization chamber (depending on
and Henry Moseley. In view of the the strength of the incoming rays) as
German results the Braggs had come a detector—the Braggs proceeded
to believe that X rays were of an elec- with the ﬁrst measurements in X-ray
tromagnetic nature, but they insist- spectroscopy. By 1913, just a year af-
ed that the rays must have some sort ter they had pioneered the method,
of dual existence as they were able crystal analysis with X rays had
to concentrate their energy. But the become a standard technique. The
continuing puzzle as to their nature results not only gave insight into the
did not stop the Braggs from recog- structure of crystals but also into the
nizing the practicability and impor- nature of the anti-cathode that
tance of a new field of study, X-ray produced the rays.
crystallography. The first person to notice that X
The new field was pioneered by rays can be characteristic of the sub-
the Braggs. They were inspired by the stance that emits them was Charles
Cambridge theorists who argued that Barkla, the opponent of the Braggs in
a diffraction grating imposes a struc- the matter of X rays as neutral par-
ture on an inhomogeneous pulse of ticles and a professor at the Univer-
white light, picking out, as if in a sity of Edinburgh who spent over
Fourier transform, the wavelengths forty years examining the properties
into which the beam can be decom- of secondary X rays. Between 1906
posed. William Henry Bragg and his and 1908 he had noticed that ele-
son, William Lawrence Bragg, argued ments emit secondary X rays with
by analogy that the crystal, by dint a penetrating power in aluminum
of the distance between planes of that is distinct for each element. To
atoms, imposes a similar structure distinguish between the hardness
on an inhomogeneous pulse of X of the characteristic rays, he intro-
rays. When the X rays are reflected duced the terminology K and L rays.
off two successive planes of atoms in It was for this discovery that he was
the crystal, they interfere construc- awarded the Nobel Prize in 1917.
tively if the difference in the distance (His subsequent work earned Barkla
traveled is equal to an integral num- the reputation as something of a sci-
ber of wavelengths. Thus the famous entific crank.) What the Braggs no-
Bragg condition ticed (see figure on next page) was
that a pattern of multiple peaks with
n λ = 2d sin θ, varying intensities was produced no
matter what the crystal (shifted only
where d is the distance between by the varying distances between
planes and θ is the angle of reﬂection. planes of atoms) as long as the ele-
Using an X-ray tube and a colli- ment of the anti-cathode remained
mating slit to produce the incom- the same. In other words, the pattern
ing rays; using various minerals, was analogous to spectral lines emit-
quartz, rock salt, iron, pyrite, ted by gases in the optical frequen-
zincblende, and calcite, as three- cies. The person to explore this anal-
dimensional diffraction gratings; and ogy to its fullest was Henry Moseley,



B1
II B2
B
Ionization

C1 C A C2 A2
A1

5 10 15 20 25
I B2 Degrees
C2 A2
C3? B3

0 10 20 30
Degrees
a young researcher working in gave on Friday on X rays. It was
One of the earliest examples of X-ray Rutherford’s Manchester laborato- rather anxious work, as Bragg, the
spectroscopy. The Braggs made a ry during the time when Niels Bohr chief authority on the subject (Pro-
seredipitous discovery: while studying fessor of Leeds) was present, and
was visiting regularly.
the scattering of X rays off of crystals as I had to be cautious. However it
Moseley’s two grandfathers had proved quite successful and I
they noticed that a distinctive pattern of
peaks appeared for each of the different
been fellows of the Royal Society, and managed to completely disguise
anti-cathodes being used to produce his father had founded a school of my nervousness. I was talking
the rays. What had initially started out as zoology at Oxford. Mosely himself chiefly about the new German
a study of the structure of crystals led to was perhaps the only important experiments of passing rays
an investigation of the atomic structure atomic physicist to be educated at through crystals. The men who did
of the anti-cathode elements. [Bragg Oxford. In the fall of 1910 he came the work entirely failed to under-
and Bragg, PRS, 88A, 413 (1913).] stand what it meant, and gave an
to work as a demonstrator under explanation which was obviously
Rutherford, his salary being paid by wrong. After much hard work Dar-
a Manchester industrialist. He was win and I found the real meaning
assigned a research problem to which of the experiments.*
everyone knew the answer: how
many β particles are emitted in the For a time the Braggs, Moseley,
radioactive disintegration of radium and Darwin continued on the same
B (Pb214) to radium C (Bi214). On find- track, even though Rutherford pre-
ing the answer everyone expected, sented difficulties which were final-
one, he proved his competency as an ly overcome by Moseley’s persistent
experimentalist. However, his next enthusiasm and by Bragg’s offer to
experiments would not be so cut and Moseley of a visit to Leeds to teach
dried, nor would they receive the him the techniques of X-ray spec-
ready approval of Rutherford. Like troscopy. Some of the questions they
the Braggs, and quite independent- pursued were the old ones about the
ly of them, Moseley was stimulat- nature of X rays. How to reconcile
ed by the photographs of Friedrich the corpuscular nature of the rays
and Knipping, and felt that Laue had with their ability to interfere? Bragg
misinterpreted them as evidence of had compared this conundrum in the
five homogeneous X rays. He teamed electromagnetic theory of X rays to
up with Charles G. Darwin, grand- the physical impossibility of a spread-
son of the famous evolutionist, and ing circle of water waves, caused by
turned to, as he said, the “real mean- the fall of a rock, to excite another
ing” of the German experiments. rock to jump the same distance the
The Laue dots connoted the struc- wave-producing rock had fallen.
ture of the crystal, not the structure The new questions concerned the
of the incoming rays. When pre- elements. In July of 1913 Bohr paid a
senting his results to a Friday phys- visit to Manchester and discussed
ics colloquium which father Bragg atomic structure with Moseley,
attended, Moseley discovered the Darwin, and George Hevesey. The
similarity in their understanding of discussion revolved around the sim-
H. G. J. Moseley in Balliol-Trinity
the phenomena, and afterwards he ilarity, and possible differences,
Laboratory, Oxford, circa 1910.
(Courtesy of University of Oxford, wrote to his mother:
Museum of the History of Science and I have been lazy for a couple of *Nov. 4, 1912. Quoted in J.L. Heilbron,
the AIP Niels Bohr Library) days recouping after the lecture I H.G.J. Moseley, p. 194.



Charles G. Darwin, L. M. Thomas, and
Gregory Breit. (Courtesy of the
Goudsmit Collection)

between the atomic weight of an
element (A) and its nuclear charge
(Z). Geiger’s and Marsden’s scatter-
ing experiments and Rutherford’s
theory had proposed that the newly
discovered nucleus held a charge half
that of the atomic weight. A Dutch
lawyer and would-be interpreter of
Mendeleev’s table, Van de Broek, had
suggested that the nuclear charge of
an element set its place in the table.
Now the frequency of characteris- 3
tic K rays gave another quantity with νK = ν (Z − 1)2 .
α 4 0
which to mark the elements. What
Moseley interpreted his formula
would the X-ray spectroscope have
as a vindication of Bohr’s theory,
to say about those places in the table
which at the time was being pub-
where the atomic weights did not fol-
lished in three lengthy and famous
low in increasing order the serial
papers “On the Constitution of
numbers: between nickel and cobalt;
Atoms.” Moseley argued, not quite
between argon and potassium; and
convincingly, that his results could
between iodine and tellurium?
be used to support the quantization
Would the hardness of the K rays or-
of an electron’s angular momentum.
der the elements by atomic weight
Frederick A. Lindeman, a fellow Eng-
or by nuclear charge?
lishman working with Walther
Moseley used an ingenious de-
Nernst on the Continent but with
vice of G. W. C. Kaye’s to exam-
his eye on the same chair of phys-
ine the K rays from copper, nickel,
ics as Moseley, Clifton’s chair at Ox-
cobalt, iron, manganese, chromi-
ford, criticized both Bohr and Mose-
um, and titanium. By putting the
ley. He was working out of an already X-ACTLY SO!
different elements which served as
successful tradition which applied
anticathodes on a magnetized truck
the condition of quantized frequen- The Roentgen Rays, the Roentgen Rays,
and rail inside the evacuated cham-
cies to the motion of atoms to pre- What is this craze?
ber, Moseley was able to change The town’s ablaze
dict specific heats in a solid and to
anti-cathodes with an external With the new phase
the motions of molecules in a gas
magnet without disrupting the in- Of X-ray’s ways.
to predict the patterns of rotational
tegrity of the chamber. After
and vibrational infrared spectra. (A I’m full of daze,
switching from detecting the K rays
quantum theory of molecular spec- Shock and amaze;
by ionization to detecting them by
tra preceded one of atomic spectra!) For nowadays
photography, his work went quick-
More successful than Moseley’s I hear they’ll gaze
ly, and in several weeks he showed Thro’ cloak and gown—and even stays,
argument in favor of Bohr’s atomic
that the ranking of elements by K These naughty, naughty Roentgen Rays.
theory was his help to the chemists
rays followed their ranking by nu-
in sorting out the confusions of the
clear charge, Z. The relation was —Wilhelma, Electrical Review,
rare earths. In November of 1913,
simple as well. The darker of the April 17, 1896
Moseley moved to Oxford where
two primary K lines, K α , fit the
he worked with equipment at the
form



Arthur Holly Compton, 1892–1962.
(Courtesy of the AIP Niels Bohr Library)

brought to a halt frequency of X rays emitted. By 1916,
by the end of the the Duane-Hunt law was the best
World War I. The way to determine h, Planck’s con-
Braggs’ work stant, although neither Millikan nor
continued after Duane then subscribed to the view
the war. The el- that energy came in discrete quan-
der Bragg revivi- tized units.
fied the Royal Arthur Holly Compton also
Institution, initially interpreted his results on
where Sir Hum- X-ray scattering from electrons as a
phry Davy and cut-off relation that was governed in
Michael Faraday this case by Planck’s constant rather
had made their than as proof of the quantum nature
chemical and of radiation. Compton, who was later
electrical dis- to run the Manhattan Project’s Met-
coveries, by es- allurgical Laboratory at Chicago
tablishing a re- during World War II, received his
search school for Ph.D. from Princeton just before the
the analysis of First World War for work on X-ray
organic crystals. diffraction and scattering. After
Electrical Laboratory but with no This work would become central to several years spent at Westinghouse
salary. Moseley began a thorough in- the developing ﬁeld of molecular bi- Manufacturing Company working
vestigation of Mendeleev’s table us- ology. During the war the elder Bragg on ﬂuorescent lamps, he spent a year
ing X rays, and moving from calcium worked for the Navy board to eval- at Cambridge’s Cavendish Laborato-
to zinc and then to the rare earths, uate inventions and to promote re- ry where he developed a friendship
lanthanum to erbium. George Ur- search with military applications. with J.J. Thomson and carried out an
bain, a Professor of Chemistry in Like many British and U.S. scientists investigation into the orderly change
Paris at one of the Grands Ecoles had he eventually found himself work- of X-ray frequency with scattering
been engaged for years in fraction- ing on problems of submarine de- angle as the X rays scattered from
ating the elemental rare earths in tection. Moseley, with his Eton pa- electrons. As a new professor at
competition with Carl Auer von triotism, practically forced himself Washington University in St. Louis,
Welsback, who performed his frac- upon the Royal Engineers along with Compton published a mass of data
tionations in an Austrian castle. his friend Henry Tizard. Tizard sur- on the relation between X-ray fre-
Urbain recognized the power of vived the Great War and subse- quency and scattering angle taken
Moseley’s technique and paid him quently led the minds of British sci- with a Bragg crystal spectrometer. In
a visit with precious samples of the entists into World War II. Moseley, 1922, a year after he had taken the
last four rare earths, thulium, ytter- however, died at Gallipoli in the bat- measurements, and along with Peter
bium, lutecium, and celtium. He was tle of Sari Bari. Debye in Germany who had seen his
astounded at how quickly Mose- As Europe engaged itself in the results in the Bulletin of the Na-
ley’s X-ray spectrometer could de- Great War, interesting work on X tional Research Council, Compton
termine that celtium was not his rays began to come out of the Unit- accepted Einstein’s light quantum
sought after new element, but was ed States. Following Robert Mil- and by extension the X-ray quantum.
only a combination of lutecium and likan’s work on the photoelectric The explanation of the Compton-
ytterbium! effect, William Duane at Harvard effect then became a simple scatter-
The Braggs’ work on crystals and gave an exact law that related the en- ing of two elastic particles.
that of Moseley’s on elements was ergy of cathode ray electrons to the



Niels Bohr, 1885–1962. (Courtesy of the
AIP Niels Bohr Library

Few physicists had taken Einstein
seriously when he predicted the
light quantum in 1905. Bohr had
pooh-poohed the idea. But by 1921
evidence was mounting and so was
Einstein’s fame. The de Broglie
brothers, Maurice and Louis, were
two others who had learned from the
studies of X rays of the dual nature
of radiation, and Louis was inspired
to suggest that matter too might have vinced most physicists that they
this dual nature. Maurice de Broglie’s were waves. The Braggs, not quite
interest in the quantum had been giving up, insisted that they had the
sparked by his secretaryship of the properties of both waves and parti-
first Solvay Conference called in cles. By 1922 the startling explana-
1911 by chemist Walther Nernst to tion by Compton of his scattering
introduce the quantum concept to experiments—X-ray energy was
physical scientists, and he decided to concentrated into particle points—
investigate the energies of electrons helped convince the science com-
excited by K and L frequency X rays. munity to take Einstein’s notion of
He found the old problem over which light quanta seriously. And finally
Bragg and Barkla had argued: X rays the work on X rays by the de Broglies,
can concentrate their entire energy and the younger brother’s desire to
and pass it on to electrons. And like put on an equal footing light and
Bragg he concluded that X rays act matter, gave Louis de Broglie the
both as waves and as particles. His courage to suggest that even the good
younger brother, Louis, in a spirit old electron (the cathode ray parti-
of unification, longed to treat light cles!) partook of wave qualities.
and matter as equals. Both could be
understood as particles following
ADDENDUM
waves, he proposed. A “mobile” of
light or X rays or of matter followed The early history of X rays follows
along behind an “onde ﬁctive.” another path that I have not covered
So the discussion of X rays had here. As the physicists wondered
come around full circle. They were about the nature of X rays and used
discovered in Roentgen’s laborato- them to probe the structure of crys-
ry as this newcomer to cathode rays tals and atoms, medical doctors used
was trying to puzzle out his coun- them to probe the human body and
tryman Lenard’s challenge to the to diagnose and treat disease. Roent-
British. Lenard believed cathode rays gen by presenting an X-ray photo-
to be ethereal. The British thought graph of his wife’s hand to the
them particles. Soon X rays became Würzburg Physical and Medical
the new mystery. Were they elec- Society in January of 1896 began the
tromagnetic waves or were they practice of radiology. A month later
neutral pairs of particles? By 1913 a German doctor used an X ray to
the interference of X rays had con- diagnose sarcoma of the tibia in the



Louis de Broglie, 1892–1987.
(Courtesy of the AIP Niels Bohr Library)

right leg of a young boy. The military Two of the three discoveries that
first used X rays in Naples in May of helped shake physics out of its fin-
1896 to locate bullets in the forearms de-siècle malaise, X rays and
of two soldiers who had been wound- radioactivity, would be taken up,
ed in Italy’s Ethiopian campaign. almost immediately, by doctors in
Radiology would be advanced their medical practice. And as
by the strong tradition of medical physicists began to require substan-
research in France. Antoine Béclère tial funds to continue their quest to
set up the first X-ray machine in discover the smallest structures of
which a patient was strapped and matter, the link between physics and
moved around for complete X rays of medicine would be pushed. Ernest
the chest. For those taking pictures Lawrence regularly raised money for
he introduced safety equipment, lead his laboratory’s cyclotrons by vir-
aprons and lead rubber gloves. He tually promising cures for cancer: “It
pioneered the first use of radiography is almost unthinkable that the man-
of the stomach in 1906. The patient ifold new radiations and radioactive
had first a meal of bismuth. Through substances [produced by his cy-
the work of Béclère and others the clotrons] should not greatly extend
practice of medical diagnosis changed the successful range of application
significantly. Soon to follow was the of radiation therapy.”*
use of X rays to treat cancer. The rays
An informal moment at an informal *Quoted in J.L. Heilbron and Robert
of the chemists and physicists
conference called by Paul Ewald Seidel, Lawrence and his Laboratory,
in 1925. From left: Paul P. Ewald, seemed to inspire doctors: α, β, and γ (Berkeley, University of California Press,
Charles G. Darwin, H. Ott, rays were also beamed at cancerous 1989) p. 215.
William L. Bragg, and R. W. James. tumors.
(The Isidor Fankuchen Collection)

FOR FURTHER READING

John Heilbron, H.G.J. Moseley:
The Life and Letters of an Eng-
lish Physicist (Berkeley, U.C.
Press, 1974).
Bruce Wheaton, The Tiger and
the Shark: Empirical Roots of
Wave-Particle Dualism (Cam-
bridge, Cambridge University
Press, 1983)
One can also find good bibli-
ographies in the numerous articles
in the Dictionary of Scientific
Biography under the individual
scientists discussed above.


Early history of x rays

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