A cyclotron has two large D-shaped dipole magnets designed to produce a semicircular region of uniform magnetic field, pointing uniformly downward. The two D's were placed back-to-back with their straight sides parallel but slightly separated. An oscillating voltage was applied to produce an electric field across this gap. Particles injected into the magnetic field region of a D trace out a semicircular path until they reach the gap. The electric field in the gap then accelerates the particles as they pass across it. The particles now have higher energy so they follow a semicircular path in the next D with larger radius and so reach the gap again. The electric field frequency must be just right so that the direction of the field has reversed by their time of arrival at the gap. The field in the gap accelerates them and they enter the first D again. Thus the particles gain energy as they spiral around. The trick is that as they speed up, they trace a larger arc and so they always take the same time to reach the gap. This way a constant frequency electric field oscillation continues to always accelerate them across the gap. The limitation on the energy that can be reached in such a device depends on the size of the magnets that form the D's and the strength of their magnetic fields.
The cyclotron was conceived by Ernest O. Lawrence in 1929 as a way of obtaining high energy particles without having to use an extremely long linear accelerator. It was developed by Lawrence and his colleagues and students at the University of California in the 1930s. Once the synchrotron principle was developed, it was found to be a much cheaper way to achieve high energy particles than the cyclotron and so the original cyclotron method is no longer used.