A cyclotron is a type of particle accelerator, first developed in the 1930s, consisting of two D-shaped cavities in a constant magnetic field. There is a gap between the cavities, so they form a circle with a missing stripe in the middle. A radioactive source placed in the center of the cyclotron—in the gap between the cavities—provides particles that will be accelerated. When a charged particle is emitted by the radioactive source, it is accelerated by a voltage placed across the gap. The magnetic field bends the particle's path so it travels in a circle. When the particle circles back to the gap, it is traveling in the opposite direction. At that point, the voltage across the gap is reversed so the particle is accelerated further. The voltage is alternated so that the particle is accelerated each time it crosses the gap. As the particle speeds up, its path is bent less by the magnetic field and it travels in an increasingly larger circle. Eventually, it spirals out of the cyclotron moving at a high speed. The largest cyclotron currently in use is TRIUMF at the University of British Columbia in Vancouver, Canada.