Two model constant‐frequency cyclotrons based on the principle of L. H. Thomas, as extended by David L. Judd, are described. Both accelerated electrons to speeds of half that of light in magnetic fields of three‐fold azimuthal periodicity. Three 60°‐wide wedge‐shaped electrodes, driven 120° out of phase, provided an energy gain per revolution of 3eV0, whereV0is the peak electrode‐to‐ground voltage. Electrons were accelerated to 75 kv withV0=23 v, implying a minimum of one thousand revolutions in the cyclotron. The beam reached full energy without axial loss and it was demonstrated that essentially all of the circulating current will emerge from this type of accelerator without the use of additional deflecting systems. The success of this development program has shown the feasibility of a high‐current, high‐energy cyclotron based on the Thomas principle.