As part of the program to develop high current accelerators several model Thomas cyclotrons employing three‐phase rf were built. The three phase signal was produced by a phase generator which was excited by a crystal oscillator. Each phase was amplified and applied to the corresponding dee, which was kept in tune by means of servomechanisms. Balanced dee voltages require a symmetrical resonator with equal inter‐dee capacitances; stability of the servomechanism requires that the effective inter‐dee capacitances be zero. This last condition was fulfilled by means of neutralizing transmission lines connected from dee stem to dee stem. Thomas cyclotrons appear to be capable of producing megawatts of beam power with an rf efficiency in excess of 70&percent;.