Since the appearance of the original edition in 1892, one com plete revision of this book has been made and corrections introduced from time to time. The rapid progress of electrical engineering has now brought about changes so radical that another thorough revision is necessary. The arrangement and object of the book remain the same, but much matter, more or less obsolete, has been eliminated. For example, constant-current motors are no longer used and various types of constant-current machines have lost some of their prominence compared with many modern examples of electrical apparatus; nevertheless a large number of, these machines are still in use, so that considerable matter relating to them is retained in the present edition and may have renewed importance due to the introduction of ?aming arcs. To a large extent direct connection has taken the place of belting, and the maximum as well as average size of electrical machines has greatly increased. These changes are only partly true of motors, the use of belting and motors of moderate size being still common practice. For that reason it is now necessary to consider the two classes separately in some respects. A large amount of new and amended material has been introduced, such as the management of alternating-current generators and motors both single and polyphase, also that of railway motors. In the preparation of this new edition a great deal of the work has. Been done by Morton Arendt, e.e., of the Electrical Engineering Department of Columbia University, who has rendered much valuable assistance in the obtaining and arrangement of new ma terial, illustrations and data, also in the labor of proof-reading.