In a 1747 letter, Benjamin Franklin wrote
… it is demonstrated and discovered, both here and in Europe, that the Electrical Fire is a real Element, or species of Matter, not created by the Friction, but collected only.
So far we have, successfully, followed Franklin’s lead and modeled this “Electrical Fire” as a conserved quantity that can flow from place to place. But what exactly comprises this electrical fluid? In fact, even after Maxwell’s death, physicists still had not answered the question of whether Franklin was correct and there was one electrical fluid with different properties, or whether du Fay was correct and there were two different kinds of fluid that cancel one another out when they occur in equal amounts. The purpose of this chapter is tell the story of how this completely changed over the course of the thirty years from 1879 to 1909.
We begin this chapter with a novel experiment conducted by a young American graduate student, Edwin H. Hall, whose 1879 results showed a clear asymmetry in the flow of positive and negative charge. Most of his results suggested that negative charge flows through conductors, but some metals showed the opposite.
Meanwhile a number of scientists, especially the British chemist Sir William Crooks, began working with partially evacuated glass tubes containing electrodes. These would glow when connected to a high voltage circuit, and they also observed an asymmetry between the positive and negative terminals, as rays of light appeared to extend from only the negative terminal. These cathode rays could be deflected by a nearby magnet.
These experiments let to the hypothesis that there was some sort of negatively charged particle, called an electron. This led some physicists to start thinking of the electrical fluid as some sort of collection of negatively charged particles. This led Joseph John Thomson, Oliver Heaviside, and Hendrik A. Lorentz to finally consider what the electromagnetic force would be on a moving charged particle.
In 1897 J.J. Thomson successfully measured the radius of curvature of cathode rays when placed in a known magnetic field. At this time, the electron went from being just an idea, to being a particle with a measured charge to mass ratio.
This led to a flurry of different experiments, each with the goal of independently measuring the electron charge. By 1909 these varied techniques would agree, within only a few percent, on the value of charge of an electron. Physicists now knew the values of the most fundamental constants of nature to within a few percent.
So, who was right? Franklin or du Fay? Is there one fluid or two?
Protons are 1800 times more massive than Electrons, yet carry the same magnitude of charge, so du Fay clearly guessed correctly. Electrons comprise his resinous fluid, and it exactly cancels out the heavy vitreous fluid of protons.
On the other hand, perhaps Franklin Franklin’s was right. Positive charge does not flow with respect to the matter, rather it is the matter. Negative charge is the excess of electrons, and positive charge is their absence. We do not talk about electrons and protons in semiconductors, but rather about electrons and holes. Under this picture, a century and a half after Franklin’s arbitrary choice of positive charge, physicists found that he chose backwards.
 The Collected papers of Benjamin Franklin (New Haven: Yale University Press, 1961), 142. See also, J.L. Heilbron, Electricity in the 17th and 18th centuries: a study of Early Modern Physics, (Berkley: University of California Press, 1979), 330.