POLYPHASE CURRENTS. 25

secondary coil will be proportional to the numerical sum of thevariations in the strength of the exciting current per unit of time;whence it follows that for a given variation any prolongation ofthe primary current will result in a proportional loss. In orderto obtain rapid variations in the strength of the current, essentialto efficient induction, a great number of undulations are employ-ed ; from this practice various disadvantages result. These are:Increased cost and diminished efficiency of the generator; morewaste of energy in heating the cores, and also diminished outputof the transformer, since the core is not properly utilized, thereversals being too rapid. The inductive effect is also very smallin certain phases, as will be apparent from a graphic representa-tion, and there may be periods of inaction, if there are intervalsbetween the succeeding current impulses or waves. In producinga shifting of the poles in a transformer, and thereby inducingcurrents, the induction is of the ideal character, being alwaysmaintained at its maximum action. It is also reasonable to as-sume that by a shifting of the poles less energy will be wastedthan by reversals.