It is not the purpose of this paper to discuss pro and con the advisability of building generators that develop " a sine wave under all conditions of load,") as sometimes stipulated in specifications. Suffice it to say that designers and users of alternaing-current generators have been content up to the present time with simply a rough approximation to a sine wave. The purpose of this paper may be said to be three-fold; first, to show some potential waves that have more or less close relation

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... the evolution of a-c. generators; second, to show how load and other conditions affect the no-load or open-circuit wave; third, to exhibit waves of several generators that have supplied commercial systems, large and small, for many years. The exhibit, for the most part, pertains to generators that have open slots in the armature and form-wound coils. In connection with definite pole generators of high periodicity and high voltage, and at the same time of small capacity, or more exactly, small capacity per pole, it is of great importance that poles shall be so shaped that there shall be a sinusoidal distribution of flux in the air gap. The three waves, Curves 1, 2 and 3, showing the potential as affected by shape of pole, were taken many years ago during a study of the problem. The machine was a 60-cycle, 150-kw., 6600-volt, three-phasc, belt-driven generator which happened to be available for the purpose. The only changes made were on the pole faces. It is probable that a better wave could have been obtained by an additional trial. Curves 4 and 5 show what was accomplished on another alternator by a slight change in the curvature of the pole face, 749