DID THOMSON DISCOVER THE ELECTRON?
Having proposed an account of discovery and disposed of some others, we are now in a position to take up this question. To begin with, I think my account helps us to see why we refrain from attributing this discovery to some of the other physicists mentioned.
For example, claims made about the electron by Crookes, Larmor, and Lorentz, even if many were correct, were primarily theory-driven, not experimentally determined. This is not to say that Thomson had no theoretical beliefs about electrons. Falconer and Feffer[247] claim that he probably believed that they are not discrete particles with empty spaces between them, but certain configurations in an all-pervading ether. But that is not enough to put him in the same category as some of the more theoretically driven physicists. The question is whether Thomson was the first to know that electrons exist from observations of them or their direct effects.Let me divide this question into three parts. First, in 1897 did Thomson know that electrons exist? Second, if he did, did he know this from observations of electrons or of their direct effects? Third, was he the first to know this from such observations? If the answer to all three questions is “yes,” then Thomson retains the honor usually accorded to him.
In 1897 did Thomson know that electrons exist? Well, what did he claim to know in 1897? Here is a well-known passage from his October 1897 paper:
As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.[248]
Thomson continues: “The question next arises, What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision.
To throw some light on this point, I have made a series of measurements of the ratio of the mass of these particles to the charge carried by it” (384).Thomson then proceeds to describe in some detail two independent experimental methods he employed to determine the mass-to-charge ratio. At the end of this description he concludes: “From these determinations we see that the value of m/e is independent of the nature of the gas, and that its value 10-7 is very small compared with the value 10-4, which is the smallest value of this quantity previously known, and which is the value for the hydrogen ion in electrolysis.”
He continues:
Thus, for the carriers of electricity in the cathode rays m/e is very small compared with its value in electrolysis. The smallness of m/e may be due to the smallness of m or the largeness of e, or to a combination of these two. That the carriers of the charges in the cathode rays are small compared with ordinary molecules is shown, I think, by Lenard's results as to the rate at which the brightness of the phosphorescence produced by these rays diminishes with the length of the path travelled by the ray. (392)
After a little more discussion of Lenard’s experimental results, Thomson concludes: “The carriers, then, must be small compared with ordinary molecules.”
In sum, in 1897 Thomson claimed to know these facts:
1. That cathode rays contain charged particles. (This he claimed to know from his experiments showing both the magnetic and the electrostatic deflection of the rays.)
2. That the ratio of mass to charge of the particles is approximately 10, which is much smaller than that for a hydrogen atom. (The 10-7 ratio he claimed to know from experiments of two different types involving magnetic and electrical deflection.)
3. That the particles are much smaller than ordinary molecules. (This he claimed to know from his own experiments yielding a mass-to- charge ratio smaller than that for the hydrogen atom, together with Lenard's experiments on the distance cathode rays travel outside the tube, which is much greater than that for hydrogen ions.)
Did he know these facts? He certainly believed them to be true: he says so explicitly.
Are they true? True enough, if we don't worry about how much to pack into the notion of a particle. (Clearly Thomson had some false beliefs about his particles, in particular that they lacked wave properties.) Was he justified in his beliefs? His experimental reasons for claims 1 and 2 are quite strong, that for the smallness of the particles is perhaps slightly less so (but I think better than Heilbron alleges in his article on Thomson in the Dictionary of Scientific Biography, 367). A reasonable case, I think, can be made that Thomson knew the facts in question in 1897.To be sure, there are other claims Thomson made in 1897 concerning which one might not, or could not, attribute knowledge to him. Perhaps one of the former sort is the claim that the charged particles are constituents of all atoms. Indeed, Thomson's explicit argument here appears a bit more tentative and less conclusive than those for the three other claims. It is simply an explanatory one to the effect that if atoms are composed of the particles whose existence he has already inferred, then this would enable him to explain how they are projected from the cathode, how they could give a value for m/e that is independent of the nature of the gas, and how their mean free path would depend solely on the density of the medium through which they pass. In general, explanatory reasoning does not by itself establish the claims inferred with sufficient force to yield knowledge. And finally, there are the claims that the particles are the only constituents of atoms and are arranged in accordance with a model of floating magnets suggested by Mayer. Both claims, being false, are not claims that Thomson or anyone else could know to be true.
Like knowing that there is a person in the ditch, however, not every belief about that person needs to be true or known to be true. If in 1897 Thomson knew that cathode rays contain charged particles, whose ratio of mass to charge is 10-7 and that are much smaller than ordinary molecules, then I think it is reasonable to say that in 1897 he knew that electrons exist at least in the weaker of the two senses discussed earlier.
He knew of the existence of things that happen to be electrons.Electrons are the charged particles in question. Knowing these particular facts about them entails knowing that they exist. Whether he knew that electrons exist in the stronger sense is a question I will postpone for a moment.
The second of my three questions is whether Thomson knew what he did from observations of electrons or their direct effects. I suggest the answer is clearly “yes.” Those were electrons in his cathode tubes, and they did produce fluorescent effects and others that he observed in his experiments. Despite various theoretical assumptions, his conclusions about electrons are primarily experiment driven.
The final of the three questions concerns priority. Was Thomson the first to be in the appropriate epistemic state? Was he the first to know that electrons exist in the weaker sense of this expression? Was he the first to know of the existence of things that happen to be electrons? He was clearly not the first to know of the existence of cathode rays which happen to be, or to be composed of, electrons. But that is not the issue here. Was he the first to know, by experimental means, of the existence of the things that happen to be the constituents of cathode rays, that is, electrons? That would be a more important question, albeit a question of discovery in the weaker sense. How do you demonstrate the existence of the constituents of cathode rays? Not simply by showing that cathode rays exist. Thomson demonstrated their existence by showing that charged particles exist comprising the rays, and he did so by means of experiments involving the direct effects of those charged particles. Was he the first to do so?
The answer I would offer is a less than decisive “maybe.” Other physicists, including Schuster, Perrin, Wiechert, and Lenard, had conducted experiments on cathode rays which yielded results that gave support to the claim that the constituents of cathode rays are charged particles.
Moreover, these experiments involved observing the electron’s direct effects. It might be argued that although these other physicists provided such experimental support, that support was not strong enough to produce knowledge. One might claim that Thomson’s refinements of Perrin’s experiment, and more important, his achievement of producing electrostatic deflection of the rays, and his determination of m/e, showed conclusively, in a way not shown before, that cathode rays contain charged particles. (This is what Thomson himself claims in his October 1897 paper.) If this is right, then one can say that, in the weaker sense of discovery, Thomson discovered the electron. Although others before him had produced experimental evidence of its existence, he was the first to produce evidence sufficient for knowledge.This, however, is a controversial priority claim. It was vehemently denied by Lenard, who claimed that his own experiments prior to Thomson’s conclusively proved the existence of electrons.[249] It was also denied, albeit less vehemently, by Zeeman, who claimed that he determined the ratio of mass to charge before Thomson.[250] Finally, Emil Wiechert makes claims about the constituents of cathode rays that are fairly similar to Thomson's in a paper published in January 1897, before Thomson's papers of April and October of that year.[251] In this paper Wiechert explicitly asserts that cathode rays contain charged particles that are much smaller than ordinary molecules, and from experiments involving magnetic deflection of cathode rays he determines upper and lower bounds for the mass-to-charge ratio of the particles. Unlike Thomson, however, Wiechert did not produce electrostatic deflection, he did not obtain two independent means for arriving at his determination of mass to charge, and he did not produce precise values. The issue, as I have defined it, is simply this: even though others had provided some experimental evidence for the existence of charged particles as the constituents of cathode rays, were Thomson's experiments the first to conclusively demonstrate this? Were they the first on the basis of which knowledge of their existence could be correctly claimed? If so, he discovered the electron. If not, he didn't.
One might make another claim. Relativizing discovery to the individual, one might say that Thomson first discovered the electron (for himself) in 1897, whereas others had done so a bit earlier. One might then say that Thomson was among first to discover the electron (for himself). Perhaps this is what Abraham Pais has in mind when, as reported in the New York Times (April 29, 1997), he claims that Thomson was a, not the, discoverer of the electron. The others Pais mentions are Wiechert and Kaufmann.