subatomic particles in a constant magnetic field .
It consists of two hollow semicircular electrodes , called dees, in a large evacuated cylindrical box. An alternating {{link=electric field">electric field between the dees continuously accelerates the particles from one dee to the other, while the magnetic field guides them in a circular path. As the speed of the particles increases, so does the radius of their path, and the particles spiral outward. In this manner, a cyclotron can accelerate protons to energies of up to 25 million electron volts.