Crookes also found that the cathode rays all have identical properties, independent of the various materials involved. Furthermore, another way to test whether an effect consists of particles would be to determine whether it can push things, whether it imparts momentum. Accordingly, Crookes devised a small metal wheel with paddles, on glass rails, such that the paddles would be hit by the cathode rays. He found that when the rays hit the paddles, the wheel spun and moved, showing that the rays impart momentum. Crookes also showed that the rays could be deflected by a strong horseshoe magnet, and that two parallel streams of rays repel each other like electrified bodies (like the pith balls in Coulomb's experiment).
In 1879, Crookes concluded, on experimental grounds, that cathode rays were “a fourth state of matter” (not solid, liquid, or gas), a kind of “Radiant Matter.” He argued: “The molecules shot from the negative pole may be likened to a discharge of iron bullets,” bullets extremely minute and moving at extraordinary speed and encountering resistance from gases. He described the rays as “the little indivisible particles which with good warrant are supposed to constitute the physical basis of the universe.”12
In 1884, Arthur Schuster argued that cathode rays consist of negatively charged particles produced when molecules near a negative cathode were broken up.13 He inferred that scarce but significant experiments seemed to suggest that the particles carried a constant charge. By experimenting with magnetic deflections, by 1890, Schuster estimated the upper and lower bounds of e/m, the charge to mass ratio.14 He quoted a statement by Hermann von Helmholtz, which said “we cannot avoid concluding” that electricity behaves as consisting of “atoms of electricity.” But Schuster did not think that atoms were divisible or had removable pieces; that might sound too much like the old and supposedly crazy alchemy. Schuster later recalled that “The separate existence of a detached atom of electricity never occurred to me as possible, and if it had, and I had openly expressed such heterodox opinions, I should hardly have been considered a serious physicist, for the limits of allowable heterodoxy in science are soon reached.”15
Since the 1870s, George Johnstone Stoney had argued that there are material units of electricity, positive and negative, permanently attached to atoms. He imagined that they rotated around atoms, and in 1891 he called these tiny units “electrons.”16
Still, another way to argue that cathode rays were particles smaller than atoms would be to prove that they can penetrate materials that are opaque to atoms. This was demonstrated in Bonn in 1892 by Hertz and his student Philipp Lenard.17 But they argued that since cathode rays penetrated matter, they were some sort of waves, like sound passing through a wall, not particles. Lenard showed that as the rays passed through a metal foil, they deviated, fanning out. (This result could instead be interpreted like bullets ricocheting through a fence.) Lenard also manipulated rays outside of glass tubes, showing that they were not an effect of rarefied gas, but were self-standing phenomena.18 Lenard also studied the rate at which the rays diminish in brightness as they travel distances in gas. He found that regardless of the gas in which they moved, the rays exhibited the same magnetic deflection.19 Thomson interpreted Lenard's findings as suggesting that corpuscles were smaller than atoms because they crossed between many molecules without hitting them.
In 1895, Jean Perrin showed experimentally that the negative charge invariably accompanies the cathode rays, a result that, he said, supports the claim that the rays consist of particles, not waves.20 Thomson acknowledged that these results influenced his work. Furthermore, in April of 1896, Gustav Jaumann published experimental results showing the electrostatic deviation of cathode rays.21 So contrary to widespread claims, Thomson was not really the first to do that.22
Yet none of these scientists identified all the properties of electrons (or “ions” or “corpuscles”); they made mistakes in some of their characterizations and conjectures. Then again, so did J. J. Thomson. For example, he claimed that the corpuscles were the only constituents of atoms—he was wrong.
Thus, prior to 1897, various physicists had demonstrated that cathode rays seemed to consist of negatively charged particles smaller than atoms because: the rays traveled in straight paths, cast shadows, penetrated thin foils, imparted momentum, and could be deflected magnetically. Some of those physicists also estimated the charge to mass ratio of such particles, and they conjectured that these were components of atoms.
What's left of the claim that J. J. Thomson discovered the electron? One might argue that Thomson was the first to measure the charge to mass ratio. That argument was propagated by several influential physicists including Oliver Lodge, Norman Campbell, and Robert A. Millikan. But they were mistaken.23 By 1890, Schuster had published upper and lower estimates. In 1896 and in March of 1897, prior to Thomson's work, Pieter Zeeman measured and published the charge to mass ratio of negatively charged particles and concluded that they were about a thousand times smaller than charged atoms.24 Zeeman described his findings as “direct experimental evidence” of the existence of the charged material particles.25 And, in January 1897, also prior to Thomson's work, Emil Wiechert too showed that cathode rays consist of charged particles, “electric atoms,” that are smaller than ordinary molecules. He also calculated upper and lower bounds of e/m. Wiechert declared: “Here we are not dealing with atoms as we know them in chemistry, because the mass of the moving particles turns out to be 2000 to 4000 times smaller than the mass of a hydrogen atom, the lightest of the known chemical atoms.”26
One might imagine that maybe Thomson's measurements of e/m were just the most accurate at the time. But in 1897, Walter Kaufmann obtained values that were considerably closer to the value that we now recognize.27 Moreover, Thomson's values did not clearly seem to converge toward a constant.28 His largest reported value was about five times greater than his smallest. And the results from his electric-magnetic deflection method compared to his magnetic method differed by about 20 percent. This discrepancy was so large that one of Thomson's biographers commented that it would have been unacceptable even in a grocery transaction.29
What about the Nobel Prize? After all, Thomson was awarded the prize in 1906, so it might seem that physicists agreed that his contribution was outstanding. However, his prize was not awarded for having discovered any subatomic particle. It was awarded for his work on “the conductivity of gases”—cathode rays. Moreover, Lenard was awarded the Nobel Prize in 1905, for his work on cathode rays. (And, Lenard later claimed that his early experiments proved the existence of electrons.)30
Regardless, another way in which a few physicists and historians have defended Thomson's reputed discovery is by claiming that even if Thomson's work of 1897 did not prove the existence of the electron, his later work did. In 1899, Thomson showed two more ways of measuring the charge to mass ratio, roughly matching the earlier results.31 Moreover, he managed to measure the charge alone of the corpuscles, which in turn led to a calculation of their mass.32 Those were immensely valuable contributions. Still, as noted by historian Theodore Arabatzis, those are just two properties of electrons, among many others, so why should the identification of those properties in particular give someone the title “discoverer of the electron”? (Moreover, the values of mass and charge were estimated earlier by Joseph Larmor and H. A. Lorentz.33) Physicists who already recognized the existence of electrons were impressed by the accuracy of Thomson's new measurements, but they did not construe them as a qualitative discovery. For example, Arthur Schuster recalled about Thomson's important lecture of 1899: “It at once carried conviction, and though to those who had followed the gradual development of the subject, it only rendered more certain what previous experiments had already plainly indicated, the scientific world seemed suddenly to awake to the fact that their fundamental conceptions had been revolutionised.”34 In 1900, Pierre and Marie Curie did not characterize Thomson as having discovered the electron; instead they esteemed him as having “completed” the ballistic theory of William Crookes.35
One more way to defend Thomson's alleged role is to argue that his work, at least, was the key contribution that finally convinced most scientists to accept electrons. This approach implies that the credit for being the discoverer of something consists not merely in being the first to make the claim, but in being the one to actually succeed in persuading the majority of the community of peers.
For example, Charles Coulomb was not the first to propose “Coulomb's law,” as others such as Joseph Priestley proposed it earlier. Likewise, Charles Darwin was not the first to propose that species evolve by natural selection. In a book of 1831, on the breeding of trees to build boats, Patrick Matthew argued that the superabundance of offspring against pressing competition and circumstances, makes species change.36 His conjectures seem to have been ignored. But after Darwin's success, Matthew claimed credit, and he described himself as the “Discoverer of the Principle of Natural Selection.” Yet he did not get the credit. In Darwin's opinion, speaking generally, “all the credit” for discoveries goes to whomever succeeds in convincing readers.37
Yet there were several physicists who helped to gradually convince the community, or ascertain phenomena, regarding electrons. Some showed that cathode rays were rectilinear; others showed that the rays transfer mechanical momentum, like bullets; still others showed that the rays consist of particles smaller than atoms; and so forth. The problem with claiming that J. J. Thomson convinced everyone is that, again, this claim does not hold up historically. Plenty of physicists remained unconvinced. In 1897, for example, George FitzGerald considered Thomson's claim that the corpuscles were the fundamental constituents of all matter, and noted the dramatic implication that physicists were now on “the track of the alchemists,” moving toward “a possible method of transmutation of matter.”38 FitzGerald also considered other plausible hypotheses, including that negative corpuscles were just charges devoid of matter. Similarly, John Zeleny, a young physicist who worked in a laboratory in Berlin at the time, reported that “nobody in Berlin” believed the announcement that the electron was a corpuscle.39 Max Planck too, in 1900, still disdained material electrons as a dubious hypothesis. Only by the 1910s, after the works of many experimenters, did nearly all physicists accept the notion that matter is made of particles such as atoms and electrons. As late as 1914, one of Thomson's own students, Owen Richardson, who intensively investigated electrons, commented that a series of discoveries over “the last fifteen years” had finally established their existence.40 Ernest Rutherford had a similar view.
Just as some chemists doubted the existence of Marie Curie's radium before she finally isolated it, physicists could doubt electrons until one was isolated. Thus in 1899, for example, the chemist Henry Armstrong criticized Thomson's work as not showing that corpuscles were separable from atoms; and further, even the physicists who then did appreciate Thomson's work did not construe it as the discovery of a new particle.41 The isolation of a single electron was accomplished finally by Robert A. Millikan in 1911. Incidentally, Millikan did not even believe that Thomson alone discovered the electron.42
Looking back at the “discovery of the electron,” Theodore Arabatzis proposed to construe that expression properly as describing a complex process that led to the consolidation of physicists' belief in electrons. In that sense, he concluded that J. J. Thomson did not discover the electron, he just contributed to the process of its acceptance.
J. J. Thomson was not the first to believe that cathode rays consist of particles, and he was not the first to experimentally substantiate that conjecture. This was clear to physicists and engineers in the early 1900s. In 1906, Edmund Fournier D'Albe published a historical presentation of the electron theory in which he argued: “It has not been heralded by a flourish of trumpets, nor has it been received with violent opposition from the older schools. No one man can claim the authorship of it. The electron dropped, so to speak, into the supersaturated solution of electrical facts and speculations.”43 Yet decades later, it became commonplace in textbooks of physics and chemistry to claim simply that “in 1897 J. J. Thomson discovered the electron.” What was it about that claim that made it so appealing, so common, and easy to echo? I really don't know, but one might hazard to guess a few reasons. Maybe Thomson's image suited textbooks: a proper Englishman with eyeglasses and trimmed mustache, hunched over his experimental apparatus, a flattened flop of black hair over his prominent forehead. Perhaps this image of a serious-looking, beady-eyed gentleman who paid attention to the smallest details might insinuate to young students that they too, in their own disciplined way, could someday discover important things? And the name—J. J.—does its phonetic repetition help people to remember it? Marginal human tidbits to gradually shape students into physicists? Maybe Thomson seemed appropriate for being regarded as a discoverer because he was acknowledged to be a gentleman. (By contrast, William Crookes seemed more eccentric: he claimed to have witnessed spirits. He claimed to hear rapping sounds, see objects moving by themselves, even see persons levitate while standing or float up on chairs, sometimes in full daylight. He claimed that invisible beings communicated by writing, even with a floating pencil. During séances in which Crookes held onto the hands and feet of a medium, he claimed to have seen small, luminous hands floating in the air, some that even touched him and pulled at his coat. In darkness dimly lit, he claimed to see, speak with, and repeatedly touch, a beautiful apparition called “Katie King.” She wore white robes, and Crookes touched her hands, neck, put his ear to her chest to hear her heartbeat and lungs, and followed her into a cabinet.44)
I think that a stronger reason why the Thomson claim became common is that it gives a simple and definite story of origins: the electron is such an important object in physics textbooks, such a seemingly solid, active, and forceful thing, that it begs for a single discoverer plus a plain date when it was found. By giving a seemingly straightforward provenance to the invisible electrons, teachers can avoid the tortuous intricacies of how such entities were actually identified, how physicists gradually and contentiously became convinced of their existence.
Myths disguise our ignorance. The simple discovery story of electrons conveniently hides the complications of physics and history, to enable students to promptly imagine these invisible objects, to just use them in explanations and calculations. If we were to try, instead, to tell students the actual process by which from the 1870s until 1913 physicists struggled to identify these components of matter and electricity, it would take a fair bit of work. So instead, textbooks portray a simplistic vignette. But as historians have delved into the matter, they have increasingly rejected the attribution of the electron's discovery to any one individual.
Still, some writers have concocted new and subtle ways in which it might still be fair to say that Thomson did discover the electron, at least in a weak sense of the term “discover.” For example, the philosopher Peter Achinstein has argued that by elucidating the notion of discovery, on the basis of three main criteria, we might well say that Thomson discovered the electron, because: Thomson discovered something that really exists, he knew that he had done so, and he had some priority in that realization. Yet Achinstein admitted that the question remains complex, and opinions differ.45 Looking at this issue, Bruce Hunt, a historian of physics and electricity (and my colleague at the University of Texas), commented that even if we do apply Achinstein's own criteria, we would hardly conclude that Thomson discovered the electron. Instead, it would then seem that Zeeman, or perhaps Lenard or Wiechert did so. 46
As for me, I see no reason at all to regard J. J. Thomson as the discoverer of the electron. And I think that, pedagogically, there are better stories we can tell about how physicists came to know that cathode rays consist of subatomic matter that is negatively charged. In particular, students will more readily understand how physicists came to think of cathode rays as negatively charged material particles thanks to the results on the experiments in which the rays cast shadows and impart momentum (Crookes).
And, they came to think that such matter was negatively charged in view of magnetic deflection (Crookes, Schuster, Perrin). And, they came to appreciate the subatomic size of electrons thanks to the experiments of various physicists (including Crookes, Lenard, Zeeman, Wiechert, Thomson, and Kaufmann). And later they isolated such entities (Millikan).
You might still want to know how the legend of J. J. Thomson started. Thomson had some part in the matter, for in old age he described his own achievements in ways that did not fairly acknowledge the contributions of others. But also, historians of physics, such as Isobel Falconer and E. A. Davis, have pointed out that apparently it was Thomson's former students who began to promulgate the narrow, simplistic discovery tale.47 Graeme Gooday has pointed out that one of Thomson's colleagues wrote an influential article, in a manual of electrical engineering published in 1931, in which he claimed that J. J. Thomson, as a “disinterested seeker after the truth,” discovered the electron in 1897 and that from that one key discovery there followed many practical devices.48 But even this latter claim was false. Technologies (such as television) that are often attributed to the discovery of the electron actually originated from learning how to control cathode rays, rather than knowing what cathode rays are.
I think that the electron discovery story also developed to satisfy an explanatory craving. As readers and students of history, we want to know who discovered what and when. We know that for ages people did not know the invisible nature of electricity, and we expect that with technological improvements someone eventually managed to see what had been hidden for so long: the invisibly small. A similar case was discussed years ago by Thomas Kuhn. He rightly analyzed historical problems of pinpointing the discovery of oxygen, in the 1770s, and he commented: “Though undoubtedly correct, the sentence, ‘Oxygen was discovered,’ misleads by suggesting that discovering something is a single simple act assimilable to our usual (and also questionable) concept of seeing. That is why we so readily assume that discovering, like seeing or touching, should be unequivocally attributable to an individual and to a moment in time.”49
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