Democritus, a Greek philosopher of the 5th century B.C., was the first to propose that all things are made of indivisible particles called atoms. His hypothesis was only half right. The things we call atoms today are in fact made up of three different kinds of particles: protons, neutrons, and electrons. Electrons are much smaller than the other two particles. Under the influence of the electronic force, electrons orbit the nucleus of the atom, which contains protons and neutrons.

Protons and electrons both carry electric charge, which causes them to be attracted to one another. In most atoms, there are as many electrons as there are protons, and the opposite charges of these two kinds of particle balance out. However, it is possible to break electrons free from their orbits about the nucleus, causing an imbalance in charge. The movement of free electrons is the source of everything that we associate with electricity, a phenomenon whose power we have learned to harness over the past few hundred years to revolutionary effect.