is somewhat less than one-eighth of a period later when both coils are active. The field thus is not only rotating, but also to a slight extent pulsating, and these pulsations give rise to parasitic currents which contribute nothing to the driving force and only waste power. By certain methods of grouping the wires it is possible to reduce these pulsations to a tolerable amount, but a better way still is to build the motor for three-phase current. The combined action of three currents mutually one-third of a period apart in point of time results in the production of a sensibly constant flux revolving at constant speed. A mechanical analogy of the kind of irregularity to be expected in the cases of two-phase and three-phase motors is furnished by deep well pumps. With such pumps it is important that the rate of flow of water in the delivery pipe shall be as uniform as possible, because with a great length of delivery pipe the column of water, alternately accelerated and retarded if the flow is not uniform, throws considerable stresses on the machinery. A pump with one cylinder only is, therefore, even if a large air-vessel is used as equaliser, not so satisfactory as a pump with two cylinders and cranks set at 90 degrees. More
