(1852 - 1908)
French physicist who discovered radioactivity through his investigations
of uranium and other substances. In 1903 he shared the Nobel Prize for
Physics with Pierre and Marie Curie.
He was a member of a scientific family extending through several generations,
the most notable being his grandfather Antoine-Cesar Becquerel (1788-1878),
his father, Alexandre-Edmond Becquerel (1820-91), and his son Jean Becquerel
Education and training.
After his early schooling at the Lycee Louis-le-Grand, Henri received
his formal scientific education at the Ecole Polytechnique (1872-74)
and engineering training at the Ecole des Ponts et Chaussees (Bridges
and Highways School; 1874-77). In addition to his teaching and research
posts, Becquerel was for many years an engineer in the Department of
Bridges and Highways, being appointed chief engineer in 1894. His first
academic situation was in 1876 as assistant teacher at the Ecole Polytechnique,
where in 1895 he succeeded to the chair of physics. Concurrently, he
was assistant naturalist to his father at the museum, where he also
assumed the physics professorship upon his father's death.
Electricity, magnetism, optical phenomena, and energy were major areas
of physical investigation during the 19th century. For several years
the young man's research was concerned with the rotation of plane-polarized
light by magnetic fields, a subject opened by Michael Faraday and to
which Henri's father had also contributed. Henri then concerned himself
with infrared radiation, examining, among other things, the spectra
of different phosphorescent crystals under infrared stimulation. Of
particular significance, he extended the work of his father by studying
the relation between absorption of light and emission of phosphorescence
in some uranium compounds.
By 1896 Henri was an accomplished and respected physicist--a member
of the Academie des Sciences since 1889--but more important than his
research thus far were his expertise with phosphorescent materials,
his familiarity with uranium compounds, and his general skill in laboratory
techniques, including photography. Together, these were to place the
discovery of radioactivity within his reach.
Systematic study of radiation.
At the end of 1895, Wilhelm Rontgen discovered X rays. Becquerel learned
that the X rays issued from the area of a glass vacuum tube made fluorescent
when struck by a beam of cathode rays. He undertook to investigate whether
there was some fundamental connection between this invisible radiation
and visible light such that all luminescent materials, however stimulated,
would also yield X rays. To test this hypothesis, he placed phosphorescent
crystals upon a photographic plate that had been wrapped in opaque paper
so that only a penetrating radiation could reach the emulsion. He exposed
his experimental arrangement to sunlight for several hours, thereby
exciting the crystals in the customary manner. Upon development, the
photographic plate revealed silhouettes of the mineral samples, and,
in subsequent experiments, the image of a coin or metal cutout interposed
between the crystal and paper wrapping. Becquerel reported this discovery
to the Academie des Sciences at its session on February 24, 1896, noting
that certain salts of uranium were particularly active.
He thus confirmed his view that something very similar to X rays was
emitted by this luminescent substance at the same time it threw off
visible radiation. But the following week Becquerel learned that his
uranium salts continued to eject penetrating radiation even when they
were not made to phosphoresce by the ultraviolet in sunlight. To account
for this novelty he postulated a long-lived form of invisible phosphorescence;
when he shortly traced the activity to uranium metal, he interpreted
it as a unique case of metallic phosphorescence.
During 1896 Becquerel published seven papers on radioactivity, as Marie
Curie later named the phenomenon; in 1897, only two papers; and in 1898,
none. This was an index of both his and the scientific world's interest
in the subject, for the period saw studies of numerous radiations (e.g.,
cathode rays, X rays, Becquerel rays, "discharge rays," canal
rays, radio waves, the visible spectrum, rays from glowworms, fireflies,
and other luminescent materials), and Becquerel rays seemed not especially
significant. The far more popular X rays could take sharper shadow photographs
and faster. It required the extension in 1898 of radioactivity to another
known element, thorium (by Gerhard Carl Schmidt and independently by
Marie Curie), and the discovery of new radioactive materials, polonium
and radium (by Pierre and Marie Curie and their colleague, Gustave Bemont),
to awaken the world and Becquerel to the significance of his discovery.
Returning to the field he had created, Becquerel made three more important
contributions. One was to measure, in 1899 and 1900, the deflection
of beta particles, which are a constituent of the radiation in both
electric and magnetic fields. From the charge to mass value thus obtained,
he showed that the beta particle was the same as Joseph John Thomson's
recently identified electron. Another discovery was the circumstance
that the allegedly active substance in uranium, uranium X, lost its
radiating ability in time, while the uranium, though inactive when freshly
prepared, eventually regained its lost radioactivity. When Ernest Rutherford
and Frederick Soddy found similar decay and regeneration in thorium
X and thorium, they were led to the transformation theory of radioactivity,
which explained the phenomenon as a subatomic chemical change in which
one element spontaneously transmutes into another. Becquerel's last
major achievement concerned the physiological effect of the radiation.
Others may have noticed this before him, but his report in 1901 of the
burn caused when he carried an active sample of the Curies' radium in
his vest pocket inspired investigation by physicians, leading ultimately
to medical use.
For his discovery of radioactivity, Becquerel shared the 1903 Nobel
Prize for Physics with the Curies; he was also honoured with other medals
and memberships in foreign societies. His own Academy of Sciences elected
him its president and one of its permanent secretaries.
There is no full-length biography of Becquerel in English. The article
on him by Alfred Romer in the Dictionary of Scientific Biography, vol.
1 (1970), pp. 558-561, provides further details.