The best known and one of the most successful devices for acceleration of ions to millions of electron volts is the cyclotron, which was invented by E. O. Lawrence in 1929. The first working model produced 80-keV protons in 1930. A cyclotron, as well as a linac, uses multiple acceleration by a radio frequency electrical field. However, the ions in a cyclotron are constrained by a magnetic field to move in a spiral path. The ions are injected at the center of the magnet between two semicircular electrodes called "Dees". As the particle spirals outward it gets accelerated each time it crosses the gap between the Dees. The time it takes a particle to complete an orbit is constant, since the distance it travels increases at the same rate as its velocity, allowing it to stay in phase with the RF. As relativistic energies are approached, this condition breaks down, limiting cyclotrons in energy. However, cyclotrons are still in use all over the world for nuclear science studies, radioisotope production, and medical therapy.