STRONG DISCOVERY

What about the stronger sense of discovery, the sense in which if I dis­cover gold, then I know it is gold? To those seeking to deny the title “discoverer of the electron” to Thomson, one can concede that he did not know that the constituents of cathode rays have all the properties that electrons do.

There is a further problem here with the question of whether Thomson knew that the constituents of cathode rays are electrons. Putting the ques­tion that way presupposes some established concept of electron. And the question appears to be whether what Thomson discovered (in the weaker sense) fits that concept and whether Thomson knew this. By analogy, to ask whether I discovered gold (in the stronger sense) is to presuppose that these objects satisfy some established concept of gold and whether I know that they do. Which concept of electron is meant in the question about Thomson? In 1897 there was no established concept. Stoney, who introduced the term “electron,” used it to refer to an elementary elec­tric charge, but Thomson was not talking about this. Nor was his claim that the constituents of cathode rays are Lorentz's electrons, which in 1895 Lorentz claimed were ions of electrolysis.

So the question “did Thomson know that the constituents of cathode rays are electrons?” is, I think, ambiguous and misleading. Instead, I sug­gest, it is better simply to ask what facts, if any, about the constituents of cathode rays Thomson knew, when he knew them, and when others knew them.

Briefly, let's take four central claims that Thomson made about cathode ray constituents in his October 1897 paper: first, they are charged par­ticles; second, their ratio of mass to charge is approximately 10^-7; third, they are much smaller than ordinary atoms and molecules; and fourth, they are constituents of atoms. Earlier I said that it is reasonable to sup­pose that Thomson knew the first three of these facts in 1897, but not the fourth. He came to know them during that year as a result of his exper­iments with cathode rays. I also said that one might claim that Thomson was the first to demonstrate conclusively that the constituents of cathode rays are charged particles, though this is controversial. At least he was among the first to do so.

With regard to the second claim—that the ratio of mass to charge of these particles is approximately 10^-7—Wiechert had arrived at upper and lower bounds before Thomson. In defense of Thomson, one might say that his determinations were more precise and were based on two inde­pendent experimental methods.

With respect to the third claim—that the cathode particles are much smaller than atoms and molecules—perhaps Lenard is correct in claiming knowledge of this prior to Thomson. Indeed, Thomson made important use of Lenard's absorption results in his own arguments that cathode par­ticles are smaller than atoms. And if Wiechert's arguments are sufficiently strong, he too has some claim to knowledge before Thomson.

Finally, the fourth claim—that cathode particles are constituents of atoms—is, it is probably fair to say, one that Thomson did not know the truth of in 1897, although he gave explanatory arguments in its favor.