Elster, Johann Philipp Ludwig Julius

(b. Bad Blankenburg, Germany, 24 December 1854; d. Bad Harzburg, Germany, 6 April 1920)

experimental physics.

Elster’s scientific work can be discussed only with that of Hans Geitel; they jointly carried out and published almost all of their investigations from 1884 to 1920. They were teachers of mathematics and physics at the Herzoglich Gymnasium in Wolfenbüttel, near Brunswick.

Elster and Geitel studied together from 1875 to 1877 in Heidelberg, and until 1878 in Berlin. Elster then returned to Heidelberg, to study with Georg Quincke, under whom he received the doctorate in 1879 for his dissertation “Die in freien Wasserstrahlen auftretenden elektromotorischen Kräfte.” After taking the examination to become a teacher he went in 1881 to Wolfenbüttel, where Geitel had been teaching since 1880. In 1884 they began collaborating on scientific works, which eventually totaled almost 150. They were especially concerned with the following problems, which were then new; electrical phenomena in the atmosphere, the photoelectric effect and thermal electron emission, photocells and their use in photometry, various aspects of radioactivity, and the development of apparatus and methods for the measurement of electrical phenomena in gases. The vast scope of Elster and Geitel’s pioneering work in all these areas can be determined from the contemporary literature and is emphasized in textbooks of both the nineteenth and the twentieth centuries. Many results of their investigations are now part of the accepted foundations of the areas covered. Today, in the age of Geiger and Müller counters, of cloud and bubble chambers, and of electronics, their methods of measurement are no longer employed; but until 1920 they were crucial to research in the respective areas.

Elster and Geitel’s first joint work was concerned with the electrification of flames (1884). This was followed by the first investigations of electrical processes in thunderclouds,¹ the development of electricity in rain, and the “dispersion of electricity” in the atmosphere and its dependence on the electric field of the earth and the measurement of that field. The ionization of gas was not yet known; it arose from J. J. Thomson’s discovery of the conductivity of air induced by roentgen rays (1896). With the theory of gas ions, a rational treatment of the phenomena of atmospheric electricity was finally possible; a comprehensive presentation of the results obtained up to 1901 is in a report by Geitel² (the investigations continued until 1905). Measurements of atmospheric electricity were made in the Austrian Alps in 1891–1893; on Mt. Brocken, Mt. Säntis, and Mt. Gornergrat in 1900; during the total solar eclipses in Algeria in 1900 and on Majorca in 1905; and on Spitsbergen and Capri in 1902.

A series of twenty investigations on the photoelectric effect began in 1889 with the discovery that negatively charged magnesium filaments, freshly ground with emery, are discharged not only by ultraviolet light but even by “dispersed evening daylight.”³ The investigations of the sensitivity of the photocathodes to visible light led to the actinoelectric series—rubidium, potassium, sodium, magnesium, thallium, zinc—and to the discovery (1910) of the sensitivity of the hydrogenized potassium cathode, which was found to extend into the infrared range.⁴ By use of a photocathode of a fluid, absolutely smooth potassium-sodium alloy, the dependence of the photoelectric current on the polarization of the light was discovered,⁵ as was the existence of a “normal” and a “selective” photoelectric effect, which later became of decisive importance in the electron theory of metals. The Elster-Geitel photocell was for decades the photometric instrument of physics and astronomy.

In 1887 Elster and Geitel discovered the “electrification of gases by means of incandescent bodies,”⁶ a finding that later was very significant in thermionics. Their finding of the emission of negative electricity from incandescent filaments was decisive in the proof that Thomson’s “corpuscles” (electrons) are a constituents of all matter.

Soon after the discovery (1896) of radioactivity Elster and Geitel began to study Becquerel rays,⁷ in order to determine the origin of the energy of these rays. Crookes had proposed the hypothesis that the air molecules with the greatest velocity stimulated the rays; energy was therefore extracted from the surrounding air. Elster and Geitel placed uranium in a glass vessel that was then evacuated: even at the highest vacuum the radiation remained constant. They also placed uranium and a photographic plate in a container: the blackening of the plate was independent of the pressure. Therefore the radiation could not be stimulated by the air.

Mme. Curie suggested another hypothesis: the radioactive emission was a fluorescence of the uranium, which was excited by a very penetrating radiation that fills all of space. She therefore named the new phenomenon la radioactivité, i.e., “activated by radiation.” Elster and Geitel showed, however, that the intensity of the uranium radiation above the earth is the same as it is in a mine 852 meters below the surface. They also investigated whether uranium emitted stronger Becquerel radiation when under the influence of cathode rays. For this purpose they developed a new Lenard cathode-ray tube, which let pass into the atmosphere an intense electron beam with a cross section of several square centimeters. (They closed off the discharge tube with a copper net covered with a very thin aluminum foil; the cathode rays escaped through the net’s interstices.) The result was negative. They also demonstrated that Becquerel radiation is independent of the temperature of the uranium and of the compound in which it occurs. They concluded from these and other experiments that the radioactive emission is not the consequence of an external influence, but can only be a spontaneous release of energy by the atom. They inferred “that the atom of a radioactive element behaves like an unstable compound that becomes stable upon the release of energy. To be sure, this conception would require the acceptance of a gradual transformation of an active substance into an inactive one and also, logically, of the alteration of its elementary properties.”⁸ With this statement radioactivity was defined for the first time as a natural, spontaneous transformation of an element attendant upon the release of energy.

A magnetic deflection of the Becquerel rays was sought, but it could not be demonstrated unequivocally; nevertheless, the question raised here for the first time was answered definitively by Giessel (1899) and Mme. Curie (1900). Elster and Geitel did not succeed because in their positive tests beta rays were measured, while in their negative tests gamma rays were measured—this distinction, however, was not yet known. Elster and Geitel had already announced in 1898 that they had obtained new, highly radiant substances from the chemical treatment of Joachimstal pitchblende. Polonium, just discovered by the Curies, was immediately prepared by their methods. A short while later Mme. Curie and G. Bémont made known their discovery of radium. At the same time Elster and Geitel communicated their finding that lead extracted from pitchblende is highly radioactive; the radioactivity of ordinary lead and the amount of radium D, E, and F that it contains were repeatedly investigated in later studies. In 1899 the ionization of air by Becquerel rays was examined for the first time: the radiation produced equal numbers of positive and negative ions in the air mass between the electrodes, not at the electrodes themselves, and thus resembled the effect of roentgen rays, which J. J. Thomson and Ernest Rutherford had demonstrated for the first time in 1896. The influence of the Becquerel rays on spark and brush discharge in the air under various pressures was also described.

In 1899 Elster and Geitel had observed and demonstrated that uranium potassium sulfate glows with constant intensity, completely independently of all external influences. They then investigated the visible fluorescence that Becquerel rays excite in many crystals (“radioactive luminous paint”). After the three types of radioactive Becquerel rays (alpha, beta, gamma) were discovered, such experiments could be carried out separately with each of these types. In this process they discovered⁹ (at exactly the same time as Crookes) the scintillation of zinc sulfide by alpha rays: the appearance of a flash of light as each alpha particle enters the crystal. The scintillation method was important in radioactivity research until 1920.

With the experience they had gained in radioactivity investigations, Elster and Geitel set themselves the question¹⁰ of whether the ionization of the atmosphere results from radioactive material within it. Geitel had shown that the ion content of a quantity of air hermetically sealed off from the outside becomes constant after some time; since both positive and negative ions disappear from the air, for example, through recombination to neutral molecules, an ionizing source must be present. Hence a wire one meter long was suspended in the air at a potential of — 2,000 volts against earth; after several hours it was radioactive. Under definite, accurately determined experimental and measurement conditions, its activity was found to be proportional to the concentration of the radium emanation (radon) of the free atmosphere. (This is known as the Elster-Geitel activation number.)¹¹ This simple method provided information on the distribution of the emanation in the atmosphere over land and water, its dependence upon the height, upon meteorological data, and upon the earth’s local electric field and its high concentration in narrow valleys and caves. Next came extensive measurements of the radioactivity of rocks, lakes, and spring waters and spring sediments, especially at health spas. ¹² In 1913 Ernest Rutherford wrote: “The pioneers in this important field of investigation were Elster and Geitel and no researcher has contributed more to our knowledge of the radioactivity of the earth and the atmosphere than they have.”¹³

Elster and Geitel, as inseparable in their life as in their work, were called “the Castor and Pollux of physics.” They set up their physics laboratory in their residence, which also contained an astronomical telescope, terraria with tropical animals, and all kinds of natural history collections. They took vacation trips together to investigate the electricity in mountain and sea air and to measure the radioactivity of rocks, springs, and spas. When Geitel received a call to the University of Breslau in 1899, it was obvious that he would go there only with Elster. Elster was also considered for a professorship at Breslau, but they both finally rejected the offers: they feared that in Breslau they would not have the independence and quiet for their research that they had in Wolfenbüttel.

Skilled in designing and making equipment, they constructed all of their apparatus but refused to take out patents on their inventions (for example, the electrometer and the photocell). In 1889 they obtained a great deal of financial support from the Elizabeth Thompson Science Fund, of Boston, Massachusetts.

Both were good teachers, beloved by their students; Geitel especially understood how to train students to think independently. Elster’s absent-mindedness was almost legendary: When told that he had placed a stamp of too high a value on a letter, he crossed it out and stuck one of the correct value next to it. And the door of his apartment had a large opening and a small one cut out at the bottom because he had a large dog and a small dog.

The general respect in which Elster and Geitel were held is shown by the large (719 pages) Festschrift that was dedicated to them in 1914–1915, a gift from older and younger physicists on their sixtieth birthdays. Among the authors who contributed original works were Max Born, Laue, Lenard, Gustav Mie, Planck, R. W. Pohl, Regener, and Sommerfeld.

See also Geitel article in Vol. 5.

NOTES

1. “Observations on the Electrical Processes in Thunder-Clouds,” in Philosophical Magazine, 20 (1885).

2. H. Geitel, Anwendung der Lehre von den Gasionen auf die Erscheinungen der atmosphärischen Eletrizitat (Brunswick, 1901), 27 pp.

3. “Entladung negativ elektrisierter Körper durch Sonnen-und Tageslicht,” in Annalen der Physik, 38, (1889), 497.

4. “Über gefarbte Hydride der Alkalimetalle und ihre photoelektrische Empfindlichkeit,” in Physikalische Zeitschrift, 11 (1910), 257; “Über den lichempfindlichen Effekt im Infrarot und einige Anwendungen hochempfindlicher Kaliumzellen,” in Physikalische Zeitschrift, 12 (1911), 758.

5. “Abhängigkeit der Intensität des photoelektrischen Stromes von der Lage der Polarisationsebene des erregenden Lichtes zu der Oberfläche der Kathode,” in Sitzungsberichte der Berliner Akademie der Wissenschaften (1894); Annalen der Physik, 55 (1895), 684, and 61 (1897), 445; Physikalische Zeitschrift, 10 (1909), 457.

6. “Elektrisierung der Gase durch glühende Körper,” in Annalen der Physik, 31 (1887), 109, and 37 (1889), 315; Sitzungsberichte der Akademie der Wissenschaften in Wien, 97 (1888), Ila, 1175.

7. “Versuche an Becquerel-Strahlen,” in Annalen der Physik, 66 (1898), 735, and 69 (1889), 83.

8.Jahresberichte des Vereins für Naturwissenschaft zu Braunschweig, 10/12 (1902), 39; Annalen der Physik, 69 (1899), 83.

9. “Über die durch radioaktive Emanation erregte szintillierende Phosphoreszenz der Sidotblende,” in Physikalische Zeitschrift, 4 (1903), 439.

10. “Analogie im elektrischen Verhalten der natürlichen Luft und der durch Becquerel-Strahlen leitend gemachten,” ibid. 2 (1901), 590; “Radioaktivität der im Erdboden enthaltenen Luft,” ibid., 3 (1902), 574.

11. “Radioaktive Emanation in der atmosphärischen Luft,” in Sitzungsberichte der Bayerischen Akademie der Wissenschaften, 33 (1903), 301–323; and in Physikalische Zeitschrift, 4 (1903), 522.

12. “Sédiments radioactives des sources thermales,” in Archives des sciences physiques et naturelles, 4th ser., 19 (1905), 5.

13.Handbuch der Radiologie, II (Leipzig, 1913), 563.

BIBLIOGRAPHY

In addition to the works cited in the text and the notes see the following.

I. Original Works. Among the writings published by Elster and Geitel are Ziele und Methoden der luftelektrischen Untersuchungen (Wolfenbüttel, 1891); and Ergebnisse neuer Arbeiten über atmosphärische Elektrizität (Wolfenbüttel, 1897).

II. Secondary Literature. Elster’s work is discussed in Handbuch der Radiologie, Vol. I (Leipzig, 1920), Vol. II (1913); and R. Pohl and P. Pringsheim, Die lichtelektrischen Erscheinungen (Brunswick, 1914). An obituary is E. Wiechert, in Nachrichten der Akademie der Wissenschaften zu Göttingen (1921), 53–60.

Walther Gerlach