THE TALKING MOVIE, OR VITAPHONE This amazingly perfect device combines the standard motion-picture projector and the phonograph. The projector and the phonograph turntable are both driven by a single motor provided with control equipment to insure a speed constant to .1 per cent, a maximum variation of only 1 part in 1000. On the turntable is a record made to synchronize exactly with the picture film. The vibrations of the phonograph needle move a tiny coil in a magnetic field, thereby generating an electric current having the frequencies of music or speech. This current is amplified and then operates loud speakers. The records (which are electrically produced) and the films are so marked that they may easily be started in exact synchronism. All parts of this apparatus are made with the most extreme accuracy UNIV. OF BY ROBERT ANDREWS MILLIKAN, PH.D., Sc.D. DIRECTOR OF THE NORMAN BRIDGE LABORATORY OF AND HENRY GORDON GALE, PH.D. PROFESSOR OF PHYSICS IN THE UNIVERSITY OF CHICAGO BEING A REVISION OF THE AUTHORS' "PRACTICAL PHYSICS" WILLARD R. PYLE, B.S. HEAD OF THE DEPARTMENT OF PHYSICS, MORRIS HIGH SCHOOL PREFACE Never in the history of the world has any subject grown so rapidly, or changed so radically, as has physics during the past thirty years. A field which used to be thought dry is now attracting universal interest. The authors have perhaps been unusually favorably situated, both for appreciating the changes and for estimating their bearing upon the elementary presentation of the subject. In the preparation of this book they have endeavored to eliminate everything that is outgrown, without sacrificing any of the invaluable and otherwise unattainable results of twenty years of experience, not merely by themselves but by the tens of thousands of physics teachers who have assisted them in their efforts to improve the teaching of physics. Their chief aim from the beginning has been "to present elementary physics in such a way as to stimulate the pupil to do some thinking on his own account about the hows and the whys of the physical world in which he lives." Hence as to subject matter they have included in this book only such subjects as touch closely the everyday life of the average pupil. In a word, they have endeavored to make it represent the everyday physics which the average person needs to help him to adjust himself to his surroundings, and to interpret his own experiences correctly. As to method they have endeavored to present a systematic, carefully chosen, and well-tested course as distinct from an encyclopedic array of heterogeneous facts. This course, in its shortest form, is carried by the larger print. It represents the indispensable backbone of such a presentation as would generally be given in schools which have the least time to spend on physics. It is not too difficult to be 675180 |