A six-sector spiral ridge FFAG accelerator was constructed and successfully operated to accelerate electrons from 35 to 180 kev kinetic energy. Acceleration was by betatron action, supplemented by r-f acceleration when desired. The design was based on magnetostatic and orbit computations performed with the Illiac digital computer. Subsequent performance was found to be in good accord with these computations. Tuning coils permitted variation of the basic parameters about the design values suggested by the computations. The theoretical basis of the computational work, the constructional features of the accelerator, and the magnetostatic measurements are described. Tests with sn operating model are reported, comprising a resonance survey, injection studies, perturbation studies, and the use of r-f acceleration. The frequencies of radial and axial betatron oscillation at the nominal operating point were nu /sub x/ = 1.40 and nu /sub y/ = 1.12, respectively. The resonance survey indicated this operating point to be centrally located within a region of relatively large intensity which was bounded by the resonances nu /sub y/ = 1.0, nu /sub x/ = 1.5, and (less markedly) 2 nu /sub y/ -- nu /sub x/ = 1. Injection from a deflectorstructure with a thin septum permitted efficient injection tomore » be achieved either by concomitant rapid acceleration of the injected electrons or, alternatively, by use of a timedependent radial electric field applied as a perturbation. Experiments with a protracted injection pulse permitted the observation of phenomena attributable to space-charge effects. A suitable frequency-modulation schedule permitted successful acceleration of a substantial fraction of stacked electrons through the transition energy. A modulator, with negative-feedback stabilization, which permits protracted injection is described. The principles of Parzen's theory of perturbations, which was found to account satisfactorily for the results of the perturbation experiments, are discussed. (auth)« less