This book represents the experience of seven years' teaching of alternating currents, and almost every chapter has been sub jected repeatedly to the test of class-room use. The authors have endeavored to include in the text only those things which contribute to the fundamental understanding of the subject and those things which are of importance in the engineering practice of to-day. It may be taken for granted that the authors are deeply in debted to Mr. C. P. Steinmetz, Whose papers are unique in their close touch with engineering realities. W. S. F.