Julius Elster and Hans Friedrich Geitel pioneered research in ion conduction in gases, atmospheric electricity, photoelectric effects, and radioactivity.
The late 1890’s and early 1900’s were a fertile period for discoveries in radioactivity, which is the emission of energetic particles and radiant energy by certain atomic nuclei. These early discoveries were the building blocks for nuclear physics and the nuclear age, which began in the mid-1940’s.
Julius Elster (left) and Hans Friedrich Geitel.
Radioactivity was first observed in 1896 by the French physicist Antoine-Henri Becquerel. He found that uranium ore emitted radiation strong enough to blacken covered photographic plates and to discharge a charged electroscope, a device for detecting the presence of electricity and whether it is positive or negative by means of electric attraction and repulsion. In 1898, Pierre and Marie Curie, French physicists and chemists, announced the discovery of two new radiation-emitting elements: polonium and radium.
In 1903, Ernest Rutherford, an English physicist born in New Zealand, made the discovery that radiation from such elements is composed of three different kinds of energetic rays: alpha rays
Natural radioactivity is the property—possessed by roughly fifty elements, including radium, thorium, and uranium—of spontaneously emitting alpha or beta rays
The existence of radioactivity cannot be discerned without the aid of instruments. The most useful procedures for the detection and measurement of alpha and beta particles and gamma-ray photons are based on the fact that gases become electrical conductors as the result of exposure to radiation from radioactive substances. Because there is a strong electrical field in its immediate vicinity, a rapidly moving charged particle ejects orbital electrons from the atoms or molecules of a gas through which it passes, thus converting them into positive ions. The expelled electrons usually remain free for some time, although a few may attach themselves to other atoms or molecules to form negative ions. The passage of a charged particle through a gas results in the formation of a number of ion pairs and free electrons.
The study of radioactivity and the successful use of radiation as a research tool or for other purposes depends on its quantitative detection and measurement. The quantities most often needed are the numbers of particles (electrons, photons, beta) arriving at a detector per unit time and their energies. When a charged particle passes through matter, it causes excitation and ionization of the molecules of the material. This ionization is the basis of nearly all instruments used for the detection of such particles and the measurement of their energies.
German physicists Julius Elster and Hans Friedrich Geitel discovered a method of counting alpha particles from the visible scintillations the particles produced on a zinc sulfide screen. Radiation of alpha particles can produce luminescence in zinc sulfide. This luminescence is not uniform; rather, it consists of a large number of individual flashes, which can be seen under a magnifying glass. Each alpha particle produces one scintillation, so the number of alpha particles that fall on a detecting screen per unit time is given directly by the number of scintillations counted per unit time. The screen is dusted with small crystals of zinc sulfide containing a very small amount of copper impurity. The counting is done with the use of a microscope with a magnification of about thirty. This method works well for counting alpha particles in the presence of other radiations because the zinc sulfide screen is comparatively insensitive to beta and gamma rays.
During this period of major discovery, Elster and Geitel were actively conducting research on radioactivity in rocks, springs, and air. Elster was on the faculty of the Gymnasium, Wolfenbüttel, Germany, from 1881 to 1919. Geitel was a teacher at Strosse Schule in Wolfenbüttel. Their joint research concerned ion conduction in gases, atmospheric electricity, photoelectric effects, and radioactivity. Elster, in addition to his work with Geitel, built the first photoelectric cell, the first photometer, and the Tesla transformer. Also, he was the first to determine the electrical charge on falling raindrops in 1899. Elster demonstrated that lead is not radioactive of itself, and he also discovered the presence of radioactive substances in the atmosphere that easily break down into unstable elements, which are responsible for atmospheric conductivity. Geitel built the first cathode tube and discovered selective photoelectric effect free energy left in an atom after the transformation of radioactive elements. He built a photometer, invented the photocell with Elster, formulated a law of radioactive fallout, and originated the concept of atomic energy.
One of Elster and Geitel’s first collaborations took place in 1880, when they carried out a systematic study of the electrification of hot bodies. From this early work and their perfection of instruments for detection and measurement, Elster and Geitel moved to a determination of other sources of radiation. A basic fact is that the earth is heated from within by the energy released when uranium, thorium, and other radioactive elements naturally undergo nuclear disintegration. As this disintegration takes place, radioactive elements
Elster and Geitel discovered in 1901 that it is possible to produce excited radioactivity from the atmosphere, without further agency, simply by exposing a highly charged wire to a negative potential in the atmosphere for many hours. They found that the radioactivity may dissolve with exposure to acids and that the wire would be left unchanged. This discovery, according to Elster and Geitel, had important bearing on the theory of atmospheric electricity.
In 1903, Elster and Geitel discovered a property of alpha rays that proved of great importance in radioactive measurement. If a screen coated with small crystals of phosphorescent zinc sulfide is exposed to alpha rays, a brilliant luminosity is observed. Further, the study of penetrating radiation had its origin in their observations that there was a definite transport of charge to the insulated system even after all possible precautions had been taken to reduce electrical leakage over the insulators. The order of magnitude of this residual ionization corresponded to the production of about twenty pairs of ions per cubic centimeter.
Elster and Geitel’s discoveries—although less heralded than those of some of their contemporaries— constituted major contributions to nuclear physics and particularly to the understanding and detection of the omnipresence of radioactive elements in the environment.
Elster and Geitel’s discoveries led to the realization that not all nuclei are stable. Radioactive nuclei disintegrate spontaneously, releasing energy in the process. Understanding why this takes place was one of the great advances in physics during the first half of the twentieth century. The field of geology was particularly affected. In natural radioactivity, the unstable nucleus emits several types of high-energy particles and also releases energy in the form of electromagnetic waves similar to light energy. This process results in radioactive decay,
One of the more important early discoveries stemming from Elster and Geitel’s work was made by Bertram Borden Boltwood,
Only a few of the natural radioactive isotopes are of geologic importance. Some are useful in determining the ages of objects; others are sources of radioactive heating of the earth. Those that have proved most useful are carbon 14, potassium 87, uranium 235, uranium 238, and thorium 232. The requirements for use in dating are a reasonable rate of decay (half-life), retention of daughter isotopes, and the existence of common minerals containing the parent element.
The ability to detect radiation also made possible the location of uranium ores. The early work by Elster, Geitel, Becquerel, the Curies, and Rutherford resulted in the development of the most widely used radiation detection instrument, the Geiger-Müller counter (also known as the Geiger counter).
The early work of Elster and Geitel led to the discovery in the second decade of the twentieth century that the atomic nucleus is a source of large quantities of energy. Humans have learned to make nuclear energy available in many different ways: for medical therapy, for power in industry, for energy to propel submarines and ships, for research in biological sciences, and for weapons with great destructive power.
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Badash, Lawrence. “Becquerel’s Blunder.” Social Research 72 (Spring, 2005). This article, part of an issue devoted to “fruitful errors,” describes Becquerel’s discovery of radioactivity. -
Curie, Marie. Radioactive Substances. 1904. Reprint. Mineola, N.Y.: Dover, 2002. A translation from the French of Curie’s classic thesis presented to the Faculty of Science in Paris. Very informative on the early days in the study of detection and discovery of radioactive bodies by one of the researchers of the time. Accessible to advanced high school students with a background in science. -
Mann, Wilfred B., R. L. Ayres, and S. B. Garfinkel. Radioactivity and Its Measurement. 2d ed. Elmsford, N.Y.: Pergamon Press, 1980. Chronicles the discoveries concerning radioactivity. An ideal source for advanced high school and lower-level college students as well as interested lay readers with some background in physics. Presents mathematical computations in an easy-to-follow format. Includes illustrations and reference list. -
Rayner-Canham, Marelene F., and Geoffrey W. Rayner-Canham. A Devotion to Their Science: Pioneer Women of Radioactivity. Philadelphia: Chemical Heritage Foundation, 2005. A collection of biographical essays on twenty-three women involved in atomic science research in the early part of the twentieth century, including Marie Curie as well as many lesser-known scientists whose stories are rarely told. -
Romer, Alfred. The Discovery of Radioactivity and Transmutation. Vol. 2 in Classics of Science. New York: Dover, 1964. Collection of essays and original articles in various areas of radioactivity taken from scientific writings by Becquerel, Rutherford, Pierre and Marie Curie, and others. Includes illustrations, photographs, and footnotes. -
Rutherford, Ernest, James Chadwick, and C. D. Ellis. Radiations from Radioactive Substances. Reprint. London: Cambridge University Press, 1951. Scholarly text suited for college students and historians of science. Includes illustrations and references. -
Taton, René. Science in the Twentieth Century. New York: Basic Books, 1966. A very good reference for both lower- and upper-level college students. Features sections devoted to mathematics, physical science, earth science, the universe, biology, and medicine. Copiously illustrated. Includes references in each section.
Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity
First Practical Photoelectric Cell Is Developed
Boltwood Uses Radioactivity to Determine Ages of Rocks
Geiger and Rutherford Develop a Radiation Counter
Gamow Explains Radioactive Alpha Decay with Quantum Tunneling
First Artificial Radioactive Element Is Developed