PREFACE THE Electrical Engineer is now confronted with a variety of new problems for whose solution he must look to the full development of the science of Electro-Dynamics. This science in the near future will assume the same relation to the electric motor that the science of Thermo-Dynamics already bears to the steam engine. And since no branch of dynamics has a better claim to be called an exact science than electro-dynamics, it will be able to offer not only convincing but final solutions of these problems. To apply the principles of electro-dynamics to the Direct-Current Motor is the aim of this book. Writing for electrical engineers particularly, I take for granted a certain acquaintance with the use and design of motors, but as the book is intended to be of service to engineers generally, unexplained technicalities have been avoided as far as possible. There are now so many excellent text-books on electricity and magnetism, that I offer no apology for omitting discussion of elementary principles here. I have not considered it necessary to deal with the subject of self-induction, except in connection with the question of sparking. The advanced student will perceive the analogy between the law of acceleration given in Chapter VII. and that for the rise of current in an inductive circuit, and may be tempted to pursue the subject for himself. The numerical accuracy attempted has been limited to that attainable with an ordinary ten-inch slide rule, on which all the examples have been worked out. Importance is attached to the graphic method of solution, and the diagrams are intended to serve as exercises for the student, who should work out similar problems with different data by the same methods. I have to thank many friends for assistance, particularly Mr. H. S. Hering, for allowing me to use the results of his tests on electric cars; Mr. L. H. Parker, for providing me with particulars of the construction and performance of the electric locomotives on the Baltimore and Ohio Railroad; Mr. H. P. Curtiss, for placing at my disposal the outcome of his experiments on the Buffalo and Niagara Falls Electric Railway; and the Railway Department of the General Electric Company, for furnishing me with valuable information and data. MCGILL UNIVERSITY, MONTREAL : C. A. CARUS-WILSON. THE DIRECT-CURRENT MOTOR CHAPTER I THE INDUCTION FACTOR WHEN a straight conductor carrying a current is placed in a uniform magnetic field in a plane at right angles to the magnetic lines, it experiences a force urging it to move in that plane at right angles to its own length. If H is the intensity of the field in lines per square centimetre, the length of the conductor in centimetres, and i the current in the conductor measured in amperes, the force on the conductor is given by The sign of ƒ can be changed by reversing the sign of either H or i; hence the direction of the force can be reversed by altering the sign of the field or of the current. If both are altered at the same time, the direction of the force is unchanged. B |