The main object of the authors of this work has been to bridge the gap existing between the elementary text-books on magnetism and simple current electricity, Ohm's Law, etc., and so many of the more advanced books which deal with the design point of view. Not only the authors, but also many of their friends, have often found difficulty in recommending to students a book which would, without containing many details of a nature more or less superfluous to the purpose, present in a concise form to second-year students at Universities and Technical Schools the theoretical principles underlying Direct Current Electrical Engineering.<br><br>No attempt has been made to avoid the use of the Calculus where it simplifies proofs, since it is an essential to all students of engineering; but, in this book, only a very elementary knowledge of that subject is demanded.<br><br>With a view to simplification, only typical examples and cases are considered, as, for instance, in the chapter on instruments, since, if these are thoroughly mastered, other types of instruments and machines will be readily understood.<br><br>Thanks are due to Messrs. Metropolitan Vickers Electrical Company and the Reason Manufacturing Company for valuable help with drawings and information.<br><br>In spite of careful revision, errors easily creep into a work of this nature and, in such a case, the authors will be grateful to have any of these pointed out to them or to receive suggestions as to a second edition, if that should be required.