Two Kinds of Electron?

In the 1930s physicists were confronted by two conflicting values for the charge of the electron as measured with great precision by two different methods. Could it be that there are two kinds of (negative) electrons, differing in charge by a fraction of a percent? The experiments were reconciled when a small systematic error in one of the experiments was identified and taken into account using elementary methods. This is physics at its best. Let’s go back to 1933, when I was beginning graduate work in physics at the University of Chicago. The neutron and the positron had been discovered in 1932. The new quantum mechanics was only eight years old, the fission of uranium had not yet been discovered, and nuclear physics was just ready to take off. Only five of the major independent fundamental constants had been directly measured to within about ± 0.1%. They included the speed of light c, the Faraday F, the charge of the electron e, the ratio of the electron’s charge to mass e/m, and the ratio of Planck’s constant to the charge of the electron h/e. Avogadro’s number NA was notoriously difficult to measure. In 1909, Perrin1 published a summary of 14 different ways of determining it. Rayleigh estimated a value from a study of the blue sky. Perrin himself used a static measurement of the variation with height of the distribution of small particles suspended in a liquid, and he later obtained a value for NA by applying Einstein’s theoretical study of the Brownian motion of measurable particles in a liquid. However, a very useful equation involving NA follows directly from the definitions of the quantities involved. In the electrolysis of a monovalent substance, the Faraday F is the total charge necessary to deposit NA ions, each with charge e. Therefore