The largest accelerators are used to study particles smaller than atoms; smaller accelerators are used to study atomic nuclei and make radioactive materials. There are two kinds of particle accelerators: linear accelerators and circular accelerators. Linear accelerators move the particles in a line, while circular accelerators move the particles in a circle. One example of a particle accelerator is the Large Hadron Collider at CERN. It is very big, stretching through two countries. Another example is the Tevatron in the United States. The cathode ray tube, used in 20th century televisions and computer monitors is also a simple accelerator for electrons.