Jan Lacki, Uni Genève
The year 2012 marks the hundredth anniversary of the discovery of cosmic rays by the Austrian Victor Franz Hess. It gives the physics community the opportunity to look back at a century of scientific investigations in a field that offered physics some of its most exciting discoveries, making early research in particle physics possible and offering today a way to extend it beyond energies achievable in our accelerators.
As it happened often in the history of discoveries, that of cosmic rays came as a surprise, uncovering a new realm of physical phenomena way beyond what was initially imagined. It was nonetheless the outcome of a sustained effort following a clear rationale. It originated in the field of atmospheric electricity pioneered mainly by Austrian and German investigators at the turn of the 19th century. It was known since at least the observations of Charles Augustin Coulomb (1736-1806) that charged electroscopes loose spontaneously their charge (1785) but the phenomenon went under close scrutiny only a century later. At Vienna University, Franz Exner (1849-1926) established from the middle of the eighties on a successful tradition of research in Luftelektrizität while in Germany the fundamental achievements and insights of the remarkable tandem formed by the Gymnasium teachers Julius Elster (1854-1920) and Hans Geitel (1855-1923) inspired the work of many local and foreign researchers 1.
At the beginning of the 20th century, the community of investigators in atmospheric electricity included scholars from most (Western) Europe countries and even beyond (Canada). One of the most noteworthy was Albert Gockel (1860-1927) from Freiburg (CH) University 2. The SPS-Communications featured recently a paper on the initial years of the Freiburg Institute of Physics and in particular the opposition of scientific styles between the Institute long-term director Joseph de Kowalski and his assistant and then colleague Gockel 3. Here, I want to take a closer look at Gockel’s life-long interest in atmospheric electricity phenomena, and in particular at his substantial contribution to the discovery of cosmic radiation.
In order to understand Gockel’s achievements one has first to recall what were at the time the research trends and the main issues in the field of atmospheric electricity. After Elster and Geitel concluded that the spontaneous discharge of electrometers was due to the presence of ions in the atmosphere (1900) came the question of the their origin. The recent discovery of radioactivity (1896) and of its ionizing properties on gases led Elster and Geitel to investigate its presence in the air (1901-1902): the radioactive gaseous emanations corresponding to decay products of active minerals they detected made them conclude that it was indeed the primary cause of the conductibility of air. It was the time when one investigated the natural radioactivity of soils, rocks and air: its effects were examined underground, at ground level and in mountain heights, in land and in seas. While one learned more and more about the radioactive substances and their decay products, hypotheses on the location of the sources responsible for atmospheric ionization were getting more precise. When it turned out that the radioactive decay products of the emanations (Radium A, Thorium A and Actinium A) in the atmosphere could hardly account alone for its total ionization, the direct outgoing radiation from the active substances in the Earth crust came to be considered as the next candidate. Contemporary experiments conducted by Canadian teams on ionization of air in sealed vessels showed on the other hand that, in spite of shielding, the enclosed air was still ionized by a very penetrating radiation different from the radiation originating from the vessels walls or nearby artificial or natural formations (1903). For some time it was commonly accepted that the ionizing radiation from substances in the Earth’s crust could explain it all. At the very end of the first decade it was realized however that this radiation, given its decreasing strength with distance, could not account for the ionization of the atmosphere at least in its higher layers. After a series of investigations in situations where the effects of the direct radiation from the ground could be ruled out, it became more and more evident that a new source of radiation of non-ground origin was involved. Most remarkably, instead of decreasing rapidly with altitude, the penetrating radiation was found, after an initial albeit slower than expected fall, to rise again. In 1912, Victor Hess brought finally indisputable evidence that the radiation had to come from outside the atmosphere, hence was genuinely of extraterrestrial origin. His series of celebrated balloon observations in 1911-1912 which enabled him to reach this conclusion were preceded by extensive attempts by some other investigators who, to various degrees, reached similar conclusions. Among the latter Albert Gockel deserves particular attention: with a series of balloon flights almost two years before Hess’, he preceded the Austrian on many key observations. However, due to reasons that will be discussed shortly, it was not given to Gockel to bring forth evidence as strong as Hess’. Lack of material means and support, including from his home institution, ruled him out of the game just when the investigations reached their climax and that first priority claims were issued. Let us have a closer look at Gockel’s research and his early conclusions about a penetrating radiation of possibly non-terrestrial origin.
Atmospheric conductivity and more generally atmospheric electric phenomena were Gockel’s life-long interest. He devoted to them most of his time from his initial research years till the end of his life with the exception of some circumstantial work he did to obtain his academic degrees or fulfill his institutional duties. Right after his Ph. D. in Heidelberg (1885) Gockel went to teach at the Ladenburg Gymnasium where he initiated a systematic study of atmospheric phenomena with special emphasis on thunderstorms 4. In 1896 he was hired as assistant in the newly created Physics Institute of the Freiburg (CH) University. Serving under the directorship of Joseph de Kowalski, the applied research oriented almighty head of the Institute, Gockel had certainly less time for his interests because of the work for his habilitation 5. However, according to his list of publications 6, he managed somehow to still devote time to his favorite investigations following closely the latest developments in atmospheric electricity and related phenomena. Gockel took early an active part in the most advanced research over the ionization of air and natural radioactivity. It was not easy given the adverse institutional conditions that he had to confront because of his non-compliance with the strong local preference for applied research dictated by Kowalski. In spite of his apparently successful career (he became extraordinary professor in 1903 and ordinary in 1910) Gockel worked practically without help and with a severely limited research budget 7. In spite of this, he managed nonetheless to produce original contributions extending and sometimes going beyond those of the best scholars in the field. Indeed, in the years when cosmic rays were finally discovered, Gockel was one of the most accomplished and among the top Luftelektrizität researchers and his contributions, often published in the Physikalische Zeitschrift which was then hosting the best publications in the field, were widely known and systematically quoted.
Restricted financial means could seriously hinder one’s research as, besides the expenses occasioned by field research at various geographical locations, the techniques and instruments used were sometimes of considerable sophistication. Today one remembers mostly the ionization cavities sitting on top of electrometers, instrumental in the study of the ionizing radiation, but Gockel used, as many others, a broader range of techniques and devices, many resulting from the pioneering attempts of Elster and Geitel 8 and perfected by their followers. To study the distribution of radioactive substances, Gockel used the capture of the positive active ions over extended negatively charged suspended wires: the activity of the deposit was estimated on the basis of the rate of discharge of special electrometers. To investigate the ion density of air, Gockel measured the intensity of the ion current between the plates of condensers: to secure a regular intake of ions, the latter were aspired into the condenser by engine or hand driven air pumps. His investigations of the intensity of the ionizing radiation were finally based on the rate of ion production in ionization cavities containing sealed air: here the issue was to secure as small electrical capacity as possible to make minute charge variations due to ionization as detectable as possible. All of these techniques required often electric batteries as sources of electric potential and, at the final stage of the experimental setups, very sensitive electrometers: Gockel followed the trend from the first electrometers of Elster and Geitel to the much improved devices designed on purpose by Theodor Wulf (1868-1946) for the study of the penetrating radiation (1909). Given the experimental setups involved, one can then better appreciate the challenge that air-borne balloon experimentation represented then: one had to solve problems of restricted space, varying pressure and of large temperature swings.
The dedication of Gockel to his research made him use any opportunity to extend and improve his observations. Gockel’s experiments were geographically broadly distributed, not only over a substantial part of Europe, but they covered also Turkey and some countries of North Africa 9. He did his investigations underground in caves and tunnels (Simplon), in the mountains and glaciers of the Alps (Briener Rothorn, Zermatt 10, Jungfraujoch), in land and water, in lakes (Bodensee) and seas (Mediterranean), etc. But his most important results were obtained studying the ionizing radiation at high altitudes using balloon flights, among the very first to be done with this purpose 11. Gockel’s first flight took place in December 11, 1909, in a time when the hypothesis of ionizing radiation coming from Earth was largely undisputed. The material conditions of the this flight are perfectly illustrative of Gockel’s limited research means: he benefited from the generosity of the East section of the Swiss Aeroclub that funded the flight of its balloon, the Gotthard, on the occasion of the International Balloon Week. Besides the pilot, another scholar, Alfred de Quervain, then research associate at the ETH and associate director of the Schweizerische Meteorologische Centralanstalt, took part in the flight. De Quervain, himself a balloon pilot, was familiar with the flight activities of the Aeroclub and, as we shall see, proved instrumental in making Gockel’s further flights possible 12.
Filled with coal-gas, the Gotthard reached a final altitude of 4500 meters before landing after more than four hours of flight. Gockel did measures of ion density but what caused real surprise were his results on the variation of the ionizing radiation with altitude obtained using the standard setup made of Wulf’s electrometer coupled to a ionization chamber. Gockel observed initially a decrease in accordance with the hypothesis of radiation originating from ground, but its rate with altitude was much smaller than expected; at higher altitudes Gockel even recorded back an increase. The main conclusion of the account Gockel published some months later in the Physikalische Zeitschrift 13, was accordingly that:
Das Resultat der Messungen ist demnach, dass in der freien Atmosphäre zwar eine Verminderung der durchdringenden Strahlung eintritt, aber lange nicht in dem Masse, wie man es erwarten könnte, wenn die Strahlung in der Hauptsache vom Boden ausgeht 14.
This was a startling result to be matched only by the observations that Theodor Wulf obtained on the top of the Eiffel tower during next Easter 15. It is fair to say that all the ensuing measures and in particular those of Hess were prompted by the need to confirm or disprove Gockel’s and Wulf’s observations. One could indeed have some doubts about the reliability of Gockel’s observations as he himself did not hesitate to spell-out in his report. Because of the circumstances of the flight, Gockel could not rule out systematic errors spoiling his results. In fact, two potential sources of error could account for a slower than expected rate of decrease and (perhaps) the even more surprising increase. During the initial climbing phase, the Gotthard went through a layer of clouds that could have caused a deposit of active substances captured by an initial charge that the balloon could have picked up when at ground. Next, the electrical isolation of the electroscope could have been faulty because of the atmospheric conditions met during the flight. Gockel’s setup did include a second monitoring device whose isolation could be checked thus giving information about the integrity of his main measuring device. However, as he reported in his paper, the landing took place in dense fog: the balloon landed in a snow covered forest and the passengers went through considerable hardship to unload the scientific devices and bring them to a safe place. When Gockel could at least check the isolation, he found that the monitoring device had been damaged during transportation making a check meaningless. In his paper, Gockel discussed carefully both sources of error and gave rather convincing reasons why his results should be trusted the same in spite of the adverse circumstances. Still he himself did not consider them as sufficiently warranted and felt they needed confirmation. Interestingly enough, in the eighties, a team of Freiburg physicists measured the ionization radiation in a situation comparable to Gockel’s using modern techniques. Thanks to their results it could be shown that Gockel’s results were consistent with the modern ones provided one corrected for the effect of short range radiation due to the radioactive decay products of Radon present in the air 16 that Gockel did not take away. This is in particular true of the values which corresponded to an increase. He himself preferred not to grant too much significance to the latter because the balloon was then in a phase of rash motion making the measures difficult.
Gockel did two more flights, on October 15th, 1910, and April 2nd, 1911. Again, he was relying on ad hoc arrangements and good will. The October flight was again courtesy of the Zürich Aeroclub section, while the April one was a regular showcase flight of the Aeroclub with five other passenger on board 17. I refer the reader to Gockel’s account for details: suffice it to say here that the results he obtained made Gockel confident about what he observed during the first flight. He wrote 18:
Ich habe schon früher eine Mitteilung veröffentlicht, wonach es sich bei einer Ballonfahrt herausstellte, dass die Gammastrahlung in der Atmosphäre nur in geringem Masse mit der Höhe abnahm. Die damaligen Versuche waren, weil am Schluss die Isolation des Apparates infolge eines Unfalles nicht geprüft werden konnte, nicht ganz einwandfrei. Ich habe mittlerweise Gelegenheit gehabt, die Versuche bei zwei weiteren Fahrten zu wiederholen und bin, wie ich hier gleich bemerken will, zu denselben Resultaten wie früher gekommen 19.
The same year he wrote in a more assertive way in a contribution to the Bulletin de la Société Fribourgeoise des Sciences Naturelles 20:
In einem Gefäss dicht eingeschlossene Luft zeigt […] ein gewisses Leitungsvermögen. Dasselbe wird zum grossen Teil durch eine radioaktive Strahlung verursacht, die von aussen kommt. [...] Die verbreitetste Annahme ist die, dass diese Strahlung ausgeht von den radioaktiven Produkten, welche sich im Erdboden befinden. […] Um zu erforschen, wie sich die Intensität dieser Strahlung mit der Höhe ändert, hat der Vortragende bei 2 Ballonfahrten die Intensität dieser Strahlung gemessen. Das Resultat war, dass selbst in Höhen von 4500 m die Abnahme der Strahlung nur sehr gering ist. […] Es muss diese Strahlung daher zum Teil entweder aus der Atmosphäre oder von einem Gestirn ausserhalb der Erde kommen 21.
A hasty reading of the last sentence could make one reach the conclusion that Gockel preceded Hess in his discovery of the extraterrestrial radiation. I shall come back to this later. For now, let us discuss the circumstances of the second flight.
Gockel’s papers and correspondence conserved in the archive of Freiburg University contain some letters from de Quervain that reveal important facts that went until now unnoticed by Gockel’s biographers and commentators of his scientific activity. It turns out that the second flight was initially planned to be a hydrogen filled balloon one and could have reached more than 7000m. Indeed, in a letter from Friday, October 7, 1910, de Quervain informs Gockel that a scientific flight up to 7000m will be readily available and that Gockel is offered to use it to his scientific purposes. Reconstructing the context from the somewhat lacunary information provided by de Quervain’s letter, it seems that de Quervain and the director Maurer of the Meteorologische Centralanstalt were of great help in the organization of the Gordon-Bennet balloon competition which took place in Zürich in the preceding year 1909 22. The organizing Swiss Aeroclub offered, to thank them, a free flight they could use for their scientific purposes and scheduled to take place sometime in Fall. The whole affair took an exciting turn when it appeared that the Aeroclub balloon Gotthard could, for this occasion, be filled with costly Hydrogen (moreso than conventional but heavier coal-gas) because the latter could be decanted from a balloon of the "Luftschiffgesellschaft Luzern" at a reduced price, a filling staying hence in the financial framework of the initial offer of the Aeroclub. In a second letter from Sunday, October 9, de Quervain acknowledges Gockel’s readiness to take part in the flight and gives him some details about its planning. In particular, he confirms that the flight will reach in the best case a bit above 7000m altitude, going up in a way adapted to the comfort of the crew and the requirements of the measures, that he and Gockel will be the sole passengers and that oxygen will be brought with. De Quervain indicates also that the flight will happen after forthcoming Wednesday. However, just the next day, in a third and final letter from Monday, October 10, de Quervain informs Gockel that the flight is cancelled because the "Generaldirektion in Paris" has withdrawn its agreement to the Hydrogen filling of the Gotthard. Manifesting his frustration, sorry de Quervain proposes Gockel a substitution conventional flight with coal-gas filled balloon to take place in the next days. This is then the second flight from October 1910 that Gockel eventually did and reported about.
Documents are missing that could disclose with certainty the actors and reasons for this last minute cancellation, to start with the identity of the "Generaldirektion". There are no conserved records in the archives of the Fédération Aeronautique Internationale which was federating the national aeroclubs 23 nor in those of the Swiss Aeroclub or its local branches. However, there are some serious clues: there was indeed at the time in Luzern a recently created successful enterprise of public flights run by the French industrialist Jean Kapférer and the French Compagnie générale transaérienne with which Kapférer was associated. It is most probably the "Luftschiffgesellschaft Luzern" mentioned by de Quervain. The Compagnie was operating a dirigible named "Ville de Lucerne" during summer which was to return to south of France to be kept safely in winter. During inaugural summer 1910, the dirigible was transporting up to 48 passengers a day, and effected around 60 flights 24. Winter and a seasonal stop of its operations approaching (scheduled on October 9), was this the source of Hydrogen that de Quervain and Maurer were counting on? There are at present no alternative hypotheses available. Following this thread, a change of plans occurred which made the Paris based general direction of the Compagnie générale transaérienne change its mind. One does not have further clues.
One can as well only speculate about what would have happened had Gockel climbed up to 7000m as originally planned. If all had gone well (altitude reached and instruments operational in spite of intense cold) it is likely that Gockel would have observed a strong increase of ionizing radiation, much stronger than in his first flight. What would have been then the impact of this on Gockel’s views on the nature of the radiation? Would that have made him precede Hess’ strong conclusions that came almost two years after? We shall never know. But we can be sure about one thing: as the cancelled flight episode illustrates, Gockel’s scientific agenda was heavily dependent on the courtesy of associations and individuals with no specific scientific interest. Consequently, he could not move on with his research as efficiently as Hess who could rely on firm support from the Austrian scientific authorities. Hubert Schneuwly, the author of a short scientific biography of Gockel hints at the lack of research means as an explanation why Gockel lost the discovery race against Hess, to the effect that his contributions went more and more overlooked in the consecutive years. There are other factors to be considered, but there is no doubt that Gockel suffered from the unfavorable professional context which he could compensate for only with ad hoc initiatives he had no full control on, running the risk of disappointments and slow-downs.
In the last years of the second decade and especially after World War I ended and scientific activity resumed with many new and better equipped competitors around (most notably Hess and the German Kolhörster), Gockel's until then most prominent place in the community started to weaken. This was unfortunate since this was also the time of the first priority conflicts. Worse, Gockel died in 1927 which definitely prevented him to claim his share 25. Today, with hindsight and knowledge of the context, we can better assess his achievements and their impact. How is one then to evaluate Gockel’s role in the discovery of cosmic radiation? It is clear that he deserves to be listed among the main discoverers of a source of ionizing radiation that one cannot ascribe to the Earth's crust. From this point of view, his contribution is unjustly overlooked, even by historians who try today to achieve a historically more faithful account of cosmic rays discovery drawing attention to some of its unsung heroes. This in no way diminishes Hess’ merits: in a series of methodically planned and carefully executed experiments, Hess achieved a level of experimental sophistication and accuracy that none of his predecessors reached before him, included Gockel. Hess also stated very clearly and provided decisive evidence that the source of the unknown radiation had to be extraterrestrial and beyond Earth atmosphere. As expressed in some of his later publications, Gockel felt until the end of his life uneasy with the idea of an extraterrestrial source of ionizing radiation and favored rather the hypothesis of a yet unidentified but still intra-atmospheric or solar radiation. Would have the results of his eventually aborted 7000m flight had made him change his later views? No one will ever tell.
Be it as it may, and beyond the cosmic ray discovery affair, Gockel’s scientific activity makes him certainly one of the most original Swiss physicists in the first decades of 20th century: it belongs to us to remember and promote his fundamental contributions.
1 For an overview of the history of the discovery of cosmic rays by his main protagonist see V. F. Hess, The Electrical Conductivity of the Atmosphere and Its Causes, New York, 1928; see also William F. G. Swann, History of cosmic rays, Am. J. Phys., vol. 29 (1961), 811-819; B. Rossi, Cosmic Rays, New York, 1964; Y. Sekido and H. Elliot (eds.), Early History of Cosmic Ray Studies, Reidel, 1984; Q. Xu and L. M. Brown, The early History of Cosmic Ray Research, Am. J. Phys., vol. 55 (1987), 23-33 and L. M. Brown and L. Hoddeson (eds.), The Birth of Particle Physics, Cambridge Univ. Press, 1986. For a recent and detailed account see P. Carlson and A. De Angelis, Nationalism and internationalism in science: The case of the discovery of cosmic rays, Eur. Phys. J. H, vol. 35 (2010), 309-329. Retracing in particular the early history before Hess’ discovery, this article gives due credit to some of usually overlooked scholars who made Hess’ discovery possible, with particular emphasis on the work of the Italian Domenico Pacini (to the point of overshadowing a bit other important precursors such as Gockel). I refer the reader to this article for many details only alluded to in the present work.
2 For a brief scientific biography of Gockel see H. Schneuwly, Albert Gockel et la découverte de rayons cosmiques in "Défis et Dialogue / Herausforderung und Besinnung", Vol. 13, Editions Universitaires Fribourg, 1991; a further list of publications on Gockel and his work can be found here. This useful web site includes links to pdf files of several Gockel’s key articles, reports and books.
3 See R. Catinaud, Which physics for a new institute? Albert Gockel, Joseph Kowalski and the early years of the Fribourg Institute of Physics, SPS-Communications no. 36 (2012), 24-27.
4 Gockel’s early interests led him to publish in 1895 a book entitled Das Gewitter (Commissions-Verlag und Druck von J. P. Bachem, available in pdf format) which brought him at the time some fair recognition.
5 Gockel’s habilitations work considered the relation between polarization and conductibility in salts, see Schneuwly’s biography of Gockel, loc. cit.
6 See the list at the end of Gockel’s obituary by A. Reichensperger, in Bulletin der Naturforschenden Gesellschaft Freiburg/Schweiz, vol. 28 (1927), 227.
7 See Schneuwly, also Catinaud, loc. cit.
8 For some detailed descriptions, see Gockel’s survey of techniques and results, Die Luftelektrizität – Methoden und Resultate der neueren Forschung, Verlag von S. Hirzel, 1908, available as pdf file.
9 See for instance his extensive report Luftelektrische Untersuchungen, Kommissionsverlag der Universitätsbuchhandlung, Freiburg (Schweiz) (1902), to be found here.
10 In Zermatt he collaborated with Wulf.
11 For a penetrating analysis of the role of balloon flights in the discovery of cosmic rays, see Ch. A. Ziegler, Technology and the Process of Scientific Discovery: The Case of Cosmic Rays, Technology and Culture, vol 30, (1989), 939-963.
12 Interestingly enough, de Quervain, better known today for his achievements in geophysics and meteorology, was instrumental in the establishment of the Jungfraujoch scientific station which, as is well known, served also as a cosmic ray laboratory.
13 Luftelektrische Beobachtungen bei einer Ballonfahrt, Phys. Zeit. Vol. 11 (1910), 280-282.
14 Bold mine: The result of the measures is thus that in free atmosphere there occurs indeed a decrease of the penetrating radiation, but by far not of the importance one should expect if the radiation comes mainly from the Earth.
15 Wulf’s Eiffel tower measures albeit at only circa 300m were considered rightly important because of their superior reliability given the stability of the location and altitude of the measuring setup (contrary to balloon observations).
16 See Hansruedi Völkle, Albert Gockel und die kosmische Strahlung, Bull. Soc. Frib. Sc. Nat., vol. 97/98 (2008-09), 105-114.
17 Rememberig the delicate experimental setup Gockel used, this tells us much about the less than perfect conditions of his measurements.
18 Messungen der durchdringendern Strahlung bei Ballonfahrten, Phys. Zeit., vol. 12 (1911), 595-597.
19 Bold mine: I have had since then the opportunity to repeat my investigations during two other flights and reached, as I shall explain here, the same results as before.
20 Luftelektrische Messungen bei einer Ballonfahrt, Bull. Soc. Frib. Sc. Nat., vol. 19 (1911), 20-28.
21 Bold mine: This radiation has then to come, in part, or from the atmosphere, or from a body outside the Earth.
22 The Gordon-Bennet yearly balloon competition consisted in flying with a balloon as far as possible. In 1908, starting from Berlin, a Swiss crew won the competition: in accordance with the rules, the country of the winning crew had to organize the next event and so Zürich was chosen for 1909. Many sources report that manifestation raised considerable popular interest and so contributed to the growth of the Swiss Aeroclub.
23 It was funded in 1905 with its direction in Paris; it was the organizer of the Gordon-Bennet competition (its name has since changed to The International Air Sports Federation based in Lausanne).
24 See Airship and Balloon News, August 27, 1910, also http://flightglobal.com/pdfarchive/view/1910/1910%20-%200699.html. One can find also some information on http://www.kamov.net/airline/aero-association-lucerne-next-oldest-airline-after-delag/ and http://www.cabanus.e-monsite.com/pages/les-dirigeables/histoire-des-dirigeables.html for some postcard images (content as of August 12, 2012).
25 See Carlson and de Angelis, loc.cit.
[Released: September 2012]