PHYSICS TOWER, SMITH, TURTON, AND COPE [Illustration: (_See p.441_) THREE-COLOR PRINTING _Y._ Yellow impression; negative made through a blue-violet filter. _R._ Crimson impression; negative made through a green filter. _RY._ Crimson on yellow. _B._ Blue impression; negative made through a red filter. _YRB._ Yellow, crimson, and blue combined; the final product. (Courtesy of Phototype Engraving Co., Philadelphia.)] PHYSICS BY WILLIS E. TOWER, M. SCI. (Univ. of Illinois) HEAD OF THE DEPARTMENT OF PHYSICS, ENGLEWOOD HIGH SCHOOL, CHICAGO CHARLES H. SMITH, M. E. (Cornell) HEAD OF THE DEPARTMENT OF PHYSICS AND ASSISTANT PRINCIPAL, HYDE PARK SCHOOL, CHICAGO CHARLES M. TURTON, A. M. (Syracuse) HEAD OF THE DEPARTMENT OF PHYSICS, BOWEN HIGH SCHOOL, CHICAGO IN COLLABORATION WITH THOMAS D. COPE, Ph.D. (Pennsylvania) ASSISTANT PROFESSOR OF PHYSICS, UNIVERSITY OF PENNSYLVANIA BASED UPON PRINCIPLES OF PHYSICS BY TOWER, SMITH and TURTON WITH 7 PLATES AND 448 OTHER ILLUSTRATIONS PHILADELPHIA P. BLAKISTON'S SON & CO. 1012 WALNUT STREET COPYRIGHT, 1920, BY P. BLAKISTON'S SON & CO. PREFACE In the preparation of this text, the _pupil_, his experience, needs, and interests have been constantly kept in mind. The order of topics, illustrations, and problems have been selected with the purpose of leading the _pupil_ into a clear understanding of the physical phenomena continually taking place about him. The recommendations and conclusions reached by the "New Movement in the Teaching of Physics" have been incorporated into the book as a whole. These conclusions indicate that the most efficient teaching in physics involves a departure from the quantitative, mathematical methods of presentation that were in general use a dozen or more years ago, toward a method better adapted to the capabilities, interests, and requirements of the young people in our physics classes. The older methods are effective with a portion of the student body which has the greater mathematical ability and training, but they discourage a large majority of the pupils who are not gifted or prepared for severe mathematical analysis. For this reason, many of the more difficult mathematical demonstrations often given in physics texts are omitted. Most of the problems involve only the units employed in practical every-day measurements. The portions of Mechanics that are ordinarily so difficult for the average pupil are not taken up until he has covered considerable ground with which he is more or less familiar and not until he has become somewhat accustomed to the methods of study and the technical terms of the subject.