The first particle accelerators to be built were linear accelerators, or linacs. In a linac, particles are introduced at one end of the machine, accelerated by a changing electrical field, and then caused to collide with a target at the opposite end of the machine. The first machine of this type was invented by John Cockcroft and E. T. S. Walton at the Cavendish Laboratory in the late 1920s. Cockcroft and Walton developed a method for producing very high voltages in an evacuated glass tube. Protons--hydrogen atoms stripped of their electrons--were introduced at one end of the tube. They were accelerated to very high velocities by the large potential difference within the tube. As the protons traveled from one end of the tube to the opposite end, they reached energies of several hundred thousand electron-volts. An electron-volt is the unit used to measure the energy of...