A series generator in the operating range below maximum voltage possesses both d.c. negative resistance, (-V/I), and incremental negative resistance, (-dV/dI). It is shown that the incremental negative resistance is equal to Kn-R, where n is the speed, K the volts per r.p.m. per unit of field current, and R is the armature circuit resistance. Since the series generator has the incremental resistance property, it will support oscillations if the tuned circuit has the proper constants. It is shown that a generator load consisting of a separately excited motor can be represented by a condenser of sufficient size (in the order of farads), that oscillations at mechanically feasible frequencies of 0.2 to 1.5 cycles per second can be obtained. It is shown analytically that the magnitude of the capacitance varies directly with the moment of inertia of the motor armature and inversely with the square of the induction factor. It is shown experimentally that the frequency of oscillation of the circuit depends upon the motor induction factor and moment of inertia, and slightly upon the circuit resistance. The amplitude of oscillation is controlled by the circuit resistance.