French physicist

Antoine-Henri Becquerel's landmark research on x rays and his discovery of radiation laid the foundation for many scientific advances of the early twentieth century. X rays were discovered in 1895 by the German physicist Wilhelm Conrad Röntgen, and in one of the most serendipitous events in science history, Becquerel discovered that the uranium he was studying gave off radiation similar to x rays. Becquerel's student, Marie Curie, later named this phenomenon radioactivity. His later research on radioactive materials found that at least some of the radiation produced by unstable materials consisted of electrons. For these discoveries Becquerel shared the 1903 Nobel Prize in physics with Marie and Pierre Curie. Becquerel's other notable research included the effects of magnetism on light and the properties of luminescence.

Becquerel was born in Paris on December 15, 1852. His grandfather, Antoine-César Becquerel, had fought at the Battle of Waterloo in 1815 and later earned a considerable reputation as a physicist. He made important contributions to the study of electrochemistry, meteorology, and agriculture. Antoine-Henri's father, Alexandre-Edmond Becquerel, was also scientist, and his research included studies on photography, heat, the conductivity of hot gases, and luminescence.

During his years at the Ecole des Ponts et Chaussées, Becquerel became particularly interested in English physicist Michael Faraday's research on the effects of magnetism on light. Faraday had discovered in 1845 that a plane-polarized beam of light (one that contains light waves that vibrate to a specific pattern) experiences a rotation of planes when it passes through a magnetic field; this phenomenon was called the Faraday effect. Becquerel developed a formula to explain the relationship between this rotation and the refraction the beam of light undergoes when it passes through a substance. He published this result in his first scientific paper in 1875, though he later discovered that his initial results were incorrect in some respects.

Although the Faraday effect had been observed in solids and liquids, Becquerel attempted to replicate the Faraday effect in gases. He found that gases (except for oxygen) also have the ability to rotate a beam of polarized light. Becquerel remained interested in problems of magneto-optics for years, and he returned to the field with renewed enthusiasm in 1897 after Dutch physicist Pieter Zeeman's discovery of the Zeeman effect—whereby spectral lines exposed to strong magnetic fields split—provided new impetus for research.

In 1874 Becquerel married Lucie-Zoé-Marie Jamin, daughter of J.-C. Jamin, a professor of physics at the University of Paris. She died four years later in March 1878, shortly after the birth of their only child, Jean. Jean later became a physicist himself, inheriting the chair of physics held by his father, grandfather, and great-grandfather before him. Two months prior to Lucie's death, Becquerel's grandfather died. At that point, his son and grandson each moved up one step, Alexandre-Edmond to professor of physics at the Musée d'Histoire Naturelle, and Antoine-Henri to his assistant. From that point on, Becquerel's professional life was associated with the Musée, the Polytechnique, and the Ponts et Chaussées.

In the period between receiving his engineering degree and discovering radioactivity, Becquerel pursued a variety of research interests. In following up his work on Faraday's magneto-optics, for example, he became interested in the effect of the earth's magnetic field on the atmosphere. His research determined how the earth's magnetic field affected carbon disulfide. He proposed to the International Congress on Electric Units that his results be used as the standard of electrical current strength. Becquerel also studied the magnetic properties of a number of materials and published detailed information on nickel, cobalt, and ozone in 1879.

In the early 1880s Becquerel began research on a topic his father had been working on for many years—luminescence, or the emission of light from unheated substances. In particular he made a detailed study of the spectra produced by luminescent materials and examined the way in which light is absorbed by various crystals. Becquerel was especially interested in the effect that polarization had on luminescence. For this work Becquerel was awarded his doctoral degree by the University of Paris in 1888, and he was once again seen as an active researcher after years of increasing administrative responsibility.

When his father died in 1891 Becquerel was appointed to succeed him as professor of physics at the museum and at the conservatory. The same year he was asked to replace the ailing Alfred Potier at the Ecole Polytechnique. Finally, in 1894, he was appointed chief engineer at the Ecole des Ponts et Chaussées. Becquerel married his second wife, Louise-Désirée Lorieux, the daughter of a mine inspector, in 1890; the couple had no children.

The period of quiescence in Becquerel's research career ended in 1895 with the announcement of Röntgen's discovery of x rays. The aspect of the discovery that caught Becquerel's attention was that x rays appeared to be associated with a luminescent spot on the side of the cathode-ray tube used in Röntgen's experiment. Given his own background and interest in luminescence, Becquerel wondered whether the production of x rays might always be associated with luminescence.

To test this hypothesis Becquerel wrapped photographic plates in thick layers of black paper and placed a known luminescent material, potassium uranyl sulfate, on top of them. When this assemblage was then placed in sunlight, Becquerel found that the photographic plates were exposed. He concluded that sunlight had caused the uranium salt to luminesce, thereby giving off x rays. The x rays then penetrated the black paper and exposed the photographic plate. He announced these results at a meeting of the Academy of Sciences on February 24, 1896.

Through an unusual set of circumstances the following week, Becquerel discovered radioactivity. As usual, he began work on February 26 by wrapping his photographic plates in black paper and taping a piece of potassium uranyl sulfate to the packet. However, because it wasn't sunny enough to conduct his experiment, Becquerel set his materials aside in a dark drawer. He repeated the procedure the next day as well, and again a lack of sunshine prompted him to store his materials in the same drawer. On March 1 Becquerel decided to develop the photographic plates he had prepared and set aside. It isn't clear why he did this—for, according to his hypothesis, little or no exposure would be expected. Lack of sunlight had meant that no luminescence could have occurred; hence, no x rays could have been emitted.

Surprisingly, Becquerel found that the plates had been exposed as completely as if they had been set in the sun. Some form of radiation—but clearly not x rays—had been emitted from the uranium salt and exposed the plates. A day later, according to Oliver Lodge in the Journal of the Chemical Society, Becquerel reported his findings to the academy, pointing out: "It thus appears that the phenomenon cannot be attributed to luminous radiation emitted by reason of phosphorescence, since, at the end of one-hundredth of a second, phosphorescence becomes so feeble as to become imperceptible."

With the discovery of this new radiation Becquerel's research gained a new focus. His advances prompted his graduate student, Marie Curie, to undertake an intensive study of radiation for her own doctoral thesis. Curie later suggested the name radioactivity for Becquerel's discovery, a phenomenon that had until that time been referred to as Becquerel's rays.

Becquerel's own research continued to produce useful results. In May 1896, for example, he found uranium metal to be many times more radioactive than the compounds of uranium he had been using, and he began to use it as a source of radioactivity. In 1900 he also found that at least part of the radiation emitted by uranium consists of electrons, particles that were discovered only three years earlier by Joseph John Thomson. For his part in the discovery of radioactivity Becquerel shared the 1903 Nobel Prize in physics with Curie and her husband Pierre.

Honors continued to come to Becquerel in the last decade of his life. On December 31, 1906, he was elected vice president of the French Academy of Sciences, and two years later he became president of the organization. On June 19, 1908, he was elected one of the two permanent secretaries of the academy, a post he held for less than two months before his death on August 25, 1908, at Le Croisic, in Brittany. Among his other honors and awards were the Rumford Medal of the Royal Society in 1900, the Helmholtz Medal of the Royal Academy of Sciences of Berlin in 1901, and the Barnard Medal of the U.S. National Academy of Sciences in 1905.

See also Geochemistry