Common to all accelerators is the use of electric fields for the acceleration of charged particles; however, the manner in which the fields are applied varies widely. The most straightforward type of accelerator results from the application of a potential difference between two terminals. To obtain more than about 200 kV of accelerating voltage, it is necessary to use one or more stages of voltage-doubling circuits. The first such device was built by J. D. Cockcroft and E. T. S. Walton in 1932 and was used for the first transmutation experiments with artificially accelerated particles (protons). Cockcroft-Walton accelerators are still widely used today, sometimes as injectors to much larger accelerators.