The term accelerators most commonly refers to particle accelerators, devices for increasing the velocity of subatomic particles such as protons, electrons, and positrons. Particle accelerators were originally invented for the purpose of studying the basic structure of matter, although they later found a number of practical applications. Particle accelerators can be subdivided into two large sub-groups: linear and circular accelerators. Machines of the first type accelerate particles as they travel in a short line, sometimes over very great distances. Circular accelerators move particles along a circular or spiral path in machines that vary in size from less than a few feet to many miles in diameter.

The simplest particle accelerator was invented by Alabama-born physicist Robert Jemison Van de Graaff (1901-1967) in about 1929. The machine that now bears his name illustrates the fundamental principles on which all particle accelerators are based.

In the Van de Graaff accelerator, a...