Early patent for an accelerator that made use of the charge-changing, or tandem, principle to double the energy of a beam of particles was awarded to Willard H. Bennet of Ohio State University in 1937. In this copy of Bennet's original patent drawing a beam of negative ions (dashes) is accelerated to a terminal (160), where electrons are removed from the negative ions by collisions with a thin coil (167). The resulting positive ions are accelerated by the second gap and strike the target (151) with an energy corresponding to acceleration through an accelerator with twice the voltage, Bennet's proposal was apparently forgotten, and it was left to Luis W. Alvarez of the University of California Berkeley to demonstrate the feasibility of the charge-changing idea in 1951.