Cathode Rays
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Cathode Rays
Theodore Arabatzis
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The detection of cathode rays was a by-product of the investigation of the discharge
of electricity through rarefied gases. The latter phenomenon had been studied since
the early eighteenth century. By the middle of the nineteenth century it was known
that the passage of electricity through a partly evacuated tube produced a glow in
the gas, whose color depended on its chemical composition and its pressure. Below
a certain pressure the glow assumed a stratified pattern of bright and dark bands.
During the second half of the nineteenth century the discharge of electricity
through gases became a topic of intense exploratory experimentation, primarily
in Germany [21]. In 1855 the German instrument maker Heinrich Geißler (1815–
1879) manufactured improved vacuum tubes, which made possible the isolation
and investigation of cathode rays [23]. In 1857 Geissler’s tubes were employed by
Julius Plücker (1801–1868) to study the influence of a magnet on the electrical discharge. He observed various complex and striking phenomena associated with the
discharge. Among those phenomena were a “light which appears about the negative
electrode” and a fluorescence in the glass of the tube ([9], pp. 122, 130).
The understanding of those phenomena was advanced by Plücker’s student and
collaborator, Johann Wilhelm Hittorf (1824–1914), who observed that “if any object is interposed in the space filled with glow-light [emanating from the negative
electrode], it throws a sharp shadow on the fluorescent side” ([5], p. 117). This effect
implied that the “rays” emanating from the cathode followed a straight path. Furthermore, Hittorf showed that those rays could be deflected by the action of a magnet.
In 1876 they were dubbed cathode rays (Kathodenstrahlen) by Eugen Goldstein
(1850–1930) [2, 24]. Thus, by the late 1870s cathode rays had been identified and
some of their main observable properties had been established.
The nature of cathode rays remained a controversial subject for some years to
come. There were two opposing views concerning their constitution. The first view
was maintained by British and French scientists, who identified cathode rays with
streams of charged particles. A well-known advocate of that view was the British experimentalist William Crookes (1832–1919). Crookes studied electrical discharges
through highly rarefied gases: “[T]he exhaustion carried out [is so high] that the
dark space around the negative pole . . . entirely fills the tube.” ([1], p. 6) Under
those conditions the behavior of cathode rays could be studied in isolation, without
interference from other discharge phenomena. Thus, Crookes determined, in a particularly clear manner, several properties of cathode rays: their “power of exciting
phosphorescence” (p. 7), their propagation in straight lines (p. 12), their power to
cast shadows (p. 15), their capacity to “exert strong mechanical action where they
strike” (p. 17) and to “produce heat when their motion is arrested” (p. 24), and
their deflection by a magnet (p. 20). He put forward the hypothesis that cathode
rays were charged molecules, “molecular bullets”, which he justified on the basis
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Cathode Rays
of their magnetic deflection and their capacity to perform mechanical work. Furthermore, from the direction of their magnetic deflection he inferred that they were
negatively charged. Several years later, in 1895, Jean B. Perrin (1870–1942) would
arrive at the same conclusion by means of a different experiment [8].
Another eminent scientist who defended the particulate interpretation of cathode
rays was Arthur Schuster (1851–1934). In 1884 he suggested that they were negatively charged atoms [10]. In 1890 he calculated the upper and lower bounds of their
charge to mass ratio (e/m), based on measurements of their magnetic deflection and
an estimate of their velocity. The lower limit was close to the charge to mass ratio
of electrolytic ions. The upper limit was three orders of magnitude higher ([11],
pp. 546–547).
The second view concerning the nature of cathode rays was advocated by some
German physicists, who identified them with processes in the ether. Their main
argument was that cathode rays have some of the properties of light-waves. For
instance, they both travel in straight lines and produce fluorescence. The ethereal
interpretation of cathode rays received additional support in 1883, when Heinrich
Hertz (1857–1894) failed to deflect them by an electric field [3,22]. In the following
years, new experimental facts were discovered which seemed to undermine further
the interpretation of cathode rays as charged particles. In 1892 Hertz showed that
they could penetrate thin sheets of metal (e.g., gold, silver, aluminum) [4]. In 1893
his student, Philipp Lenard (1862–1947), built upon Hertz’s work to investigate the
behavior of cathode rays outside the vacuum tube. He devised a tube with a thin
metallic “window” facing the cathode. The cathode rays passed through that window
and, thus, Lenard could measure their mean free path outside the tube. As it turned
out, it was much longer than that of atoms and molecules. Furthermore, he showed
that their absorption depended only on the density of the absorbing substance [7].
Thus, different experimental results supported different accounts of the nature of
cathode rays. Furthermore, the evidential import of some of those results was ambiguous. On the one hand, the magnetic deflection of cathode rays, which indicated
that they were charged particles, was compatible with an ethereal interpretation of
their nature. It was conceivable that the magnetic field altered the state of the ether
so as to produce a deflection of the rays ([17], p. 285). On the other hand, the capacity of cathode rays to pass through thin metallic sheets, which suggested that they
were waves in the ether, could be accommodated by the hypothesis that cathode rays
were charged particles. In 1893 J. J. Thomson (1856–1940) argued that the capacity
in question was only apparent: what really happened, according to Thomson, was
that the material bombarded by cathode rays turned into a source of cathode rays
itself.
The cathode ray controversy was resolved by Thomson in 1897. He had studied
electrical discharges in gases since 1883 and the discovery of X-rays by Wilhelm
Conrad Röntgen (1845–1923) rekindled his interest in cathode rays. In a lecture
to the Royal Institution on 30 April 1897, Thomson argued that cathode rays were
composed of minute, sub-atomic particles that he named “corpuscles”. Their small
size followed, according to Thomson, from Lenard’s results concerning their mean
free path outside the cathode ray tube. A further indication of their small size was
Cathode Rays
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provided by Thomson measurements of their mass to charge ratio, which turned out
to be very small in comparison to the corresponding ratio of hydrogen ions [12].
A few months later, in October 1897, Thomson presented his case for the particulate interpretation of cathode rays in more detail [13]. He reported a novel result
favoring that interpretation: the deflection of cathode rays by an electric field. Furthermore, he reported a series of measurements of the mass to charge ratio (m/e)
of cathode ray particles, whose purpose was to enable him to figure out their identity. He obtained those measurements by means of two different approaches. The
first one was based on measurements of the charge carried by cathode rays, the heat
produced by their impact on a target, and the effect of a magnetic field on their trajectory. A combination of those data led to an estimate of m/e. The guiding idea
behind the second approach was to place cathode rays under the influence of an
electric and a magnetic field and to adjust the intensity of the latter “so that the electrostatic deflexion [sic] was the same as the magnetic” ([13], p. 309). It was then
possible to calculate m/e on the basis of directly measurable parameters. Thomson
obtained the following value: m/e = H 2 l/F Θ, where H and F were, respectively,
the intensities of the magnetic and the electric fields, l the length of the region under the influence of the field, and Θ the angle of electric (or magnetic) deflection.
Both methods indicated that the value of m/e was three orders of magnitude smaller
than “the smallest value of this quantity previously known, and which is the value
for the hydrogen ion in electrolysis” ( [13], p. 310). Furthermore, the value of m/e
was independent of the material of the cathode and the chemical composition of the
gas within the cathode ray tube. This independence suggested to Thomson that the
“corpuscles” were universal constituents of all material substances.
In the early months of 1897 analogous results of the charge to mass ratio of
cathode rays were reported by Emil Wiechert (1861–1928) and Walter Kaufmann
(1871–1947). Those physicists, however, drew different conclusions from their experiments. Wiechert identified the constituents of cathode rays with disembodied
charges [14, 15]; and Kaufmann suggested that the unexpectedly large ratio of e/m
refuted the particulate interpretation of cathode rays [6]. According to our knowledge today, the cathode rays are nothing but swiftly moving electrons.
Primary Literature
1. W. Crookes, On Radiant Matter, Nature 20, 419–423, 436–440 (1879).
2. E. Goldstein, Vorläufige Mittheilungen über elektrische Entladungen in verdünnten Gasen,
Königliche Preussische Akademie der Wissenschaften zu Berlin. Monatsberichte, 279–295
(1876).
3. H. Hertz, Versuche über die Glimmentladung, Annalen der Physik 19, 782–816 (1883); Engl.
transl. Experiments on the Cathode Discharge, in H. Hertz, Miscellaneous Papers (London,
1896), pp. 224–254.
4. H. Hertz, Ueber den Durchgang der Kathodenstrahlen durch dünne Metallschichten, Annalen der Physik und Chemie 45, 28–32 (1892); Engl. transl. On the Passage of Cathode
Rays Through Thin Metallic Layers, in H. Hertz, Miscellaneous Papers (London, 1896),
pp. 328–331.
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5. W. Hittorf, Ueber die Elektricitätsleitung der Gase, Annalen der Physik 136, 1–31, 197–234
(1869); Engl. transl. On the Conduction of Electricity in Gases, in Physical Memoirs, Selected
and Translated from Foreign Sources Under the Direction of the Physical Society of London,
vol. 1 (London, 1891), pp. 111–166.
6. W. Kaufmann, Die magnetische Ablenkbarkeit der Kathodenstrahlen und ihre Abhängigkeit
vom Entladungspotential, Annalen der Physik und Chemie 61, 544–552 (1897).
7. P. Lenard, Ueber Kathodenstrahlen in Gasen von atmosphärischem Druck und im äussersten
Vacuum, Annalen der Physik und Chemie 51, 225–267 (1894).
8. J. Perrin, Nouvelles propriétés des rayons cathodiques, Comptes Rendus 121, 1130 (1895);
Engl. transl. New Experiments on the Kathode Rays, Nature 53, 298–299 (1896).
9. J. Plücker, Ueber die Einwirkung des Magneten auf die elektrischen Entladungen in verdünnten
Gasen, Annalen der Physik 103, 88–106 (1858); Engl. transl. On the Action of the Magnet upon
the Electrical Discharge in Rarefied Gases, Philosophical Magazine (4th ser.) 16, 119–132
(1858).
10. A. Schuster, The Bakerian Lecture: Experiments on the Discharge of Electricity through Gases.
Sketch of a Theory, Proceedings of the Royal Society of London 37, 317–339 (1884).
11. A. Schuster, The Bakerian Lecture: The Discharge of Electricity through Gases. (Preliminary
Communication), Proceedings of the Royal Society of London 47, 526–561 (1889–1890).
12. J. J. Thomson, Cathode Rays, Proceedings of the Royal Institution 15, 419–432 (1897).
13. J. J. Thomson, Cathode Rays, Philosophical Magazine (5th ser.) 44, 293–316 (1897).
14. E. Wiechert, Ueber das Wesen der Elektrizität, Schriften der Physikalisch-Ökonomischen
Gesellschaft zu Königsberg 38, 3–12 (1897).
15. E. Wiechert, Experimentelles über die Kathodenstrahlen, Schriften der PhysikalischÖkonomischen Gesellschaft zu Königsberg 38, 12–16 (1897).
Secondary Literature
16. P. F. Dahl, Flash of the Cathode Rays: A History of J J Thomson’s Electron (Bristol: Institute
of Physics Publishing, 1997).
17. O. Darrigol, Electrodynamics from Ampère to Einstein (Oxford: Oxford University Press,
2000).
18. E. A. Davis and I. J. Falconer, J. J. Thomson and the Discovery of the Electron (London: Taylor
& Francis, 1997).
19. I. Falconer, Corpuscles, Electrons and Cathode Rays: J. J. Thomson and the ‘Discovery of the
Electron’, British Journal for the History of Science 20, 241–276 (1987).
20. S. M. Feffer, Arthur Schuster, J.J. Thomson, and the discovery of the electron, Historical Studies in the Physical Sciences 20, 33–61 (1989).
21. E. Hiebert, Electric Discharge in Rarefied Gases: The Dominion of Experiment: Faraday,
Plücker, Hittorf, in A. J. Kox and D. M. Siegel (eds.), No Truth Except in the Details. Essays
in Honor of Martin Klein (Dordrecht: Kluwer, 1995), pp. 95–134.
22. G. Hon, H. Hertz, ‘The electrostatic and electromagnetic properties of the cathode rays are
either nil or very feeble.’ A case-study of an experimental error, Studies in History and Philosophy of Science 18, 367–382 (1987).
23. F. Müller, Gasentladungsforschung im 19. Jahrhundert (Berlin: GNT, 2004).
24. M. Hedenus, Der Komet in der Entladungsröhre. Eugen Goldstein, Wilhelm Foerster und die
Elektrizität im Weltraum (Stuttgart: GNT 2007).