TWO PROBLEMS WITH IDENTIFYING J. J. THOMSON AS THE DISCOVERER

Heroes are falling in this age of revisionist history. Thomas Jefferson, according to one recent authority, was a fanatic who defended the excesses of the French Revolution. Einstein was not the saintly physicist we were led to believe but was mean as hell to his first wife.

I would like my heroes to retain their heroic status. However, my aim in this chapter is not to defend Thomson's reputation but to raise the more general question of what constitutes a discovery. My strategy will be this. First, I want to discuss why anyone would even begin to doubt that Thom­son discovered the electron. Second, I want to suggest a general view about discovery. Third, I will contrast this with several opposing positions, some of which allow Thomson to retain his status, and others of which entail that Thomson did not discover the electron; I find all of these opposing views wanting. So who, if anyone, discovered the electron? In the final part of this chapter I will say how the view I develop applies to Thomson and also ask why we should care about who discovered the electron, or anything else.

Let me begin, then, with two problems with identifying Thomson as the discoverer of the electron. The first is that before Thomson's experiments in 1897 several other physicists reached conclusions from experiments with cathode rays that were quite similar to his. One was William Crookes. In 1879, in a lecture before the British Association at Sheffield, Crookes advanced the theory that cathode rays do not consist of atoms, “but that they consist of something much smaller than the atom— fragments of matter, ultra-atomic corpuscles, minute things, very much smaller, very much lighter than atoms—things which appear to be the foundation stones of which atoms are composed.”^[235]

So eighteen years before Thomson's experiments, Crookes proposed two revolutionary ideas essential to Thomson's work in 1897: that cath­ode rays consist of corpuscles smaller than atoms, and that atoms are composed of such corpuscles.

Another physicist with earlier views about the electron was Arthur Schuster. In 1884, following his own cathode ray experiments, Schuster claimed that cathode rays are particulate in nature and that the particles all carry the same quantity of electricity.^[236] He also performed experiments on the magnetic deflection of the rays, which by 1890 allowed him to compute upper and lower bounds for the ratio of charge to mass of the particles com­prising the rays. Unlike Thomson (and Crookes in 1879), however, Schuster claimed that the particles were negatively charged gas molecules.

Philipp Lenard is still another physicist with a considerable claim to be the discoverer of the electron. In 1892 he constructed a cathode tube with a special window capable of directing cathode rays outside the tube. He showed that the cathode rays could penetrate thin layers of metal and travel about half a centimeter outside the tube before the phosphorescence produced is reduced to half its original value. The cathode rays, therefore, could not be charged molecules or atoms, since the metal foils used were much too thick to allow molecules or atoms to pass through.

Other physicists as well, such as Hertz, Perrin, and Wiechert, made important contributions to the discovery. Why elevate Thomson and say that he discovered the electron? Why not say that the discovery was an effort on the part of many?

The second question is: Even assuming that Thomson discovered something, was it really the electron? How could it be, since Thomson got so many things wrong about the electron? The most obvious is that he believed that electrons are particles or corpuscles (as he called them), and not waves. In a marvelous twist of history, Thomson's son, G. P. Thomson, received the Nobel Prize for experiments in the 1920s demonstrating the wave nature of electrons. Another mistaken belief was that electrons are the only constituents of atoms. Still others were that the charge carried by electrons is not the smallest charge carried by charged particles, and that the mass of the electron, classically viewed, is entirely electromagnetic, a view Thomson came to hold later. Why not deny that Thomson discov­ered anything at all, since nothing exists that satisfies his electron theory? To deal with these issues something quite general needs to be said about discovery.