Physics

Front matter

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Preface

Within the last twenty years the methods of teaching physics have been revolutionized. The reaction against the loose and desultory methods previously in vogue was started by the emphasis given to laboratory work in the new books which then appeared. The movement gained impetus from the influence brought to bear on 'the schools by the Harvard Entrance Requirements and the Report of the Committee of Ten. This pressure on the schools resulted in a demand for closer observation by the students, involving careful measurements.

As far as science is concerned, the most important result of this introduction of laboratory work into the schools has been the development in the public mind of a widespread recognition of the fundamental principle that knowledge is real and living to the individual, only when it is founded on personally observed facts and personal experience.

Nevertheless, during the past few years it has become clear that the present methods of applying this principle to instruction in physics have failed to arouse in the students that real enthusiasm for the pursuit of the subject, which is indispensable for the mastery of its principles. This has been found true in spite of the fact that, in their own way, boys and girls have by nature and disposition the keenest interest in physical phenomena.

In recognition of this fact, many attempts have been made to develop a better system of instruction. Such a system ought to give the mental training that has been so much emphasized; but it ought also to inspire in the boys and girls a living enthusiasm for the subject, and to develop in them the scientific habit of mind, the ability to utilize their knowledge, and a just appreciation of the significance of natural phenomena. If physics can be so taught as to develop in the student these elements of power, so vital to his future career, there can be no doubt that in due time its educative value will be properly appreciated and its popularity restored.

iii

IV PHYSICS

This book is the contribution of two of the fraternity of teachers toward the attainment of this end. The method of instruction herein set forth has been developed after long experience and much experimenting with high school and college classes. The greater part of the book has been in manuscript for more than two years, and has been used in connection with class teaching. The results obtained have been so encouraging that the book is submitted to other teachers, in the hope that it may be of service to them.

In a recent work, President G. Stanley Hall comments at length on the decline of interest in physics in the high schools, and consequently also in the colleges, and he' suggests several remedies. 1 He attributes this failure of physics to "the violence done to the nature and needs of the youthful soul by the present methods and matters/ ' He points out that this violence consists in: 1. Neglect of the hero-ology of the science, of historical and biographical references, so that the learner is not made to "feel vividly a sense of growth." 2. "The rage to apply mathematics to the boy's brain processes," instead of appealing to his interest in concrete things. 3. The failure to realize that "very much thoroughness and perfection violates the laws of youthful nature and of growth." The young student "wants only answers that are vague, brief, but above all suggestive." 4. Neglect of the practical side of the subject, which is the side that appeals most strongly to the youth. "He is chiefly interested in the 'go' of things."

The methods of instruction which have proved helpful to us, and which are embodied in this book, are in harmony with many of President Hall's suggestions. We have endeavored to strengthen the presentation of the subject, and aid the teacher in three ways: / I. By arousing interest. II. By developing the scientific habit of i thought. III. By presenting some of the principles from the historical standpoint. Some of the ideas that have guided us in this endeavor are the following:

I. Interest. Interest is rarely stimulated in youth by elegant

1 Adolescence, by G. Stanley Hall, Vol. II, pp. 154 seq., New York, Appleton, 1905.

PREFACE V

and abstract mathematical treatment; nor is it often aroused by rigorous logical demonstrations. It is aroused by beginning with some concrete thing that goes — something which is already familiar. Interest may be sustained by basing the discussion on these familiar and concrete things. Nothing helps more than to have the student feel that you are discussing with him something concerning which he already knows a little, and of w T hich he has long been desirous of knowing more. The authors believe that the adoption of an informal style and the use of arguments that are physical, rather than mathematical, will also be helpful ; for they have been mindful of the success of the great teachers, Faraday and Tyndall, in imparting scientific ideas to untrained minds in this way. Mathematics is an excellent servant but a very bad master; so equations are used only where they are clearly a help to the student, and the development of each is carefully presented with the aid of physical, rather than mathematical concepts.

The aim has been to show the student that knowledge of physics enables him to answer many of the questions over which he has puzzled long in vain. He is approached with the attitude: What do the forces of Nature do for us, and how do they do it? His self-activity is stimulated by this questioning attitude of text-book and teacher, and he is urged to investigate independently at home. His interest is not killed at the start by attempting to cram him with definitions of things to which he has no corresponding concept, such as indestructibility, impenetrability, and the like. Nor is he deceived by attempted definitions of undefiriable concepts, such as mass, force, time, and space. On the contrary, the attempt is made to implant the concept and create the demand for its name, or definition, which is withheld Until the need for it is apparent.

II. The Scientific Method. Although interest may be obtained through the technical applications of physics, the teaching must not consist in descriptions of these only, any more than of descriptions of laboratory apparatus only. The attainment of scientific principles is always the purpose or end of the argument; not inventions, nor yet laboratory experiments. Science must

VI PHYSICS

be shown to consist in that body of organized knowledge which makes invention possible. Beginning arguments with inventions, or general observations of phenomena, may not be the logical order, but it is more nearly the order in which Nature herself teaches, and the result of the argument does not lose in definiteness, clearness, or accuracy, provided the laboratory is continually held up as the final court of appeal where all doubtful questions are settled.

Each chapter in this book is a continuous argument toward some principle or principles, and the entire book is an argument toward the conclusions stated in the last chapter. This treatment is intended to develop and foster the habit of scientific thinking. The attempt is made (1), to interest the student in observing carefully and accurately first the familiar things about him, and then the things in the laboratory; (2), to interest him in detecting analogies and similarities among the things observed; (3), to train him in keeping his mind free from bias and in drawing conclusions tentatively; (4), to make him see the value of verifying the conclusions and accepting the result, whether it confirms or denies. his inferences. The arguments in the various parts of the book are not all alike; there are many forms in which the scientific method may be used.

We have tried deliberately to give the student the impression that science leads to no absolute results — that, at best, it is merely a question of close approximation; of doing the best we can, and accepting the result tentatively, until we can do better. This attitude places the teacher also in the position of a learner and prohibits him from making use of didactic or dogmatic statements; for these are the bane of science as well as of other things. Science instruction that does not develop mental integrity, freedom of the personal judgment, and tolerance, fails in a very vital spot.

III. History. References are given to books in which the biographies of the great men of science may be read, and the student is urged to read them and report. The arguments used by some of the great thinkers have been briefly sketched, and the methods devised by them for reaching conclusions have been given. The attempt has been made to present them as they live

PREFACE VU

in the ideas which they have handed down to us; to picture their mental processes and attitude, and to show how one thing leads to another as the subject develops in the discoverer's mind.

We wish to call the attention of our colleagues to several practical points. In the first place, although each chapter is a continuous argument, the paragraphs are headed in black type, so that the important steps are well marked; and a summary and set of questions are added at the end of each chapter, to assist the student in fixing the subject-matter in mind. The teacher will, we think, find these latter very helpful to his pupils in both advance and review work.

In the second place, the continuity of the treatment is not interrupted by the insertion of descriptions of laboratory and lecture experiments in fine type. Judged from our own experience, such experiments, thus inserted, confuse rather than assist the student. It goes without saying, that we expect both laboratory f and lecture experiments to be given in connection with this book; x J but every laboratory experiment made by the student, and' every experimental demonstration by the teacher should have a definite ) relation in time, place, and subject matter to the general argument \ as presented in the text. An experiment is simply an incumbrance and a source of distraction to the student unless its rela- ( tion to the general scheme of the lessons in the classroom is per- J fectly obvious. A detailed description of a lecture experiment which he has not seen is of relatively small value to the student, and ordinarily there is no interest or profit to him in obtruding on his attention the distracting details of setting up and manipulating the apparatus. If such description of an experiment occurs in the text book, while the teacher chooses to make it with some other style of apparatus, different in its details, his con- / fusion is all the worse, for his attention is distracted from the( principle to be illustrated, and lost in the details of the apparatus.

On the other hand, when the student is to make an experiment himself in the laboratory, he must be given many details in order that he may manipulate, observe, and record successfully and without boss of time. It is the province of the laboratory manual to give these details, for they can not be included in a text

VU1 PHYSICS

book without encumbering it to the exclusion of important theoretical matter, and destroying its unity. We have therefore preferred to leave the choice of illustrative experiments largely in the hands of the teacher, who may thus select them according to his individuality, his equipment, and the circumstances and limitations of his class and community.

/ We have based the argument wherever possible on the pupils' 1 experience, expecting this to be supplemented by the teacher with lecture demonstrations and laboratory experiments, chosen in accordance with the conditions which he has to meet and with his own taste and judgment. But when a particular kind of experimental evidence is necessary to the argument, it has been used, without manipulatory details and in uniform type with the other subject matter.

In the third place, many of the old and familiar landmarks of the elementary physics text do not appear in these pages. Among these may be mentioned the division of levers into classes; the wedge; the classification of equilibrium as stable, unstable, and neutral; specific gravity as distinguished from density; the electrophorous and the electrostatic machine; the concave and convex mirrors; multiple reflection; and the formulas concerned with the radii of curvature of lenses. These have been omitted because they seem of less interest and importance than the following new subjects which we have been able to introduce in the space thus saved: The use of graphical methods and of vectors; the discussion of efficiencies of engines, both prac' tical and theoretical; the relations among electrostatic charge, current, and magnetic field; the meaning of harmony; the nature of spectra; the reasons for the electromagnetic theory of light; and the electron theory of matter. We also believe that the presentation of the subjects of rotary motion and of optical instruments will be found much simpler and more satisfactory than those usually given.

The problems are also an innovation. They include no cases of forces a, b, and c, meeting at a point q, etc., but are, as far as possible, real, concrete cases, such as occur in actual practice, and which every boy or girl ought to know how to meet.

PREFACE IX

( They also contain many of the subjects usually placed in the text I and there explained; for example, the pulleys, distillation, and the iWheatstone bridge. We hope that this form of problem will ^interest the student, as most of them are problems in whose solution he can see some use.

Other devices for catching and holding the interest are the questions and the suggestions to students at the end of each chapter. We hope that these latter will be stimulating to the students and serve as hints which will lead them to suggest for themselves other home experiments. Are not such experiments, clumsy though they be, yet made with a genuine interest in finding out something — in getting the answer from Nature herself — far more useful than many that are made in some laboratories?

The illustrations are also a novelty. Great pains have been

"taken to have every picture a photograph of a real thing, for a

photograph is always more interesting than a woodcut. It is

believed that these will add much to the interest of the work.

We have been favored with the original photographs for many of these illustrations, by the firms and individuals mentioned on page x, whom we wish to thank for their courtesy.

We also desire to express our thanks to Professor R. D. Salisbury of "the University of Chicago, Editor-in-Chief of the Lake Science Series for many valuable suggestions, and to Messrs. A. A. Knowlton of the Armour Institute of Technology, J. H. Kimmons of the Austin, Chicago High School, and C. Kirkpatrick of the High School, Seattle, Washington, for aid in the reading of the proof.

Many of the line diagrams are new and have been designed and executed with much thought and care, so as to present the essential ideas without complication by unnecessary details.

That great difficulties are involved in the working out of a method of instruction differing in principle from that in general use must be apparent to every one. We know better than any one else can that we have not produced a perfect book. This might be approximated by the concerted action of all teachers of physics. We therefore hope that members of the teaching fraternity will regard the result of our work as a first approximation, and will

X PHYSICS

join with us in making a united effort to lift our subject up to its proper place, and to inspire our young friends with an adequate appreciation of its interest, its majesty, and its grandeur. To this end we appeal to our colleagues to give us the benefit of their experience by sending us suggestions and criticisms, which will be gratefully received and carefully considered.

Charles Rtborg Mann, George Ransom Twiss.

ACKNOWLEDGMENT OF ILLUSTRATIONS

* Plate I. The Lake Shore and Michigan Southern Railway. Large

copies of this picture in color may be obtained for 50 cents, by

applying to Mr. A. J. Smith, General Passenger Agent, Cleveland,

Ohio.

Fig. 11. The Electric Vehicle Co., Hartford, Conn. Figs. 16, 17, 18. The Eastman Kodak Co., Rochester, N. Y. Fig. 19. Pawling, Harnischfeger & Co., Milwaukee, Wis. Plate II, and Figs. 70, 156, 157, 158. The Niles-Bement-Pond Co.,

New York.

Fig. 31. The Manitou and Pike's Peak Railroad Co,, Manitou, Col. Fig. 51. Crowe Bros., House Movers, Chicago, 111. Plates III, IV, VI. The Allis-Chalmers Co., Milwaukee, Wis. Fig. 61. The Bausch and Lomb Optical Co., Rochester, N. Y. Figs. 65, 72. The Ingersoll-Sargeant Drill Co., New York. Fig. 74. The Chicago Bridge and Iron Works Co., Chicago, 111. Fig. 77. The Century Co., New York.

Figs. 91, 92. The Whitlock Coil Pipe Co., Hartford, Conn. Fig. 100. The Otto Gas Engine Co., Philadelphia, Pa. Fig. 101. Mr. Alfred Stieglitz, New York.

148, 151. The Westinghouse Electric Co., Pittsburg, Pa. " The Electric Spark in Nature," page 205. Mr. M. I' Anson,

Newark, N. J.

Fig. 162. The Electric Controller and Supply Co., Cleveland, Ohio. Figs. 168, 169, 170. The Electric Storage Battery Co., Philadelphia,

Pa.

Plate VIII. The University of Chicago, Chicago, 111. Figs. 236, 237. Wm. Scheidel & Co., Chicago, IU.

Table Of Contents

PAGE

Introduction 11-14

CHAPTER I Motion, Velocity, Acceleration

Motion of a train — How velocity is measured — Units — Graphical representation of velocity — Analytical representation of velocity— ^Slope — Changing velocity — Acceleration — Graphical and analytical representation of acceleration — Measurement of acceleration — Summary — Questions — Problems — Suggestions to students 15-32

CHAPTER II Mass and Energy

Production of acceleration —Acceleration and force — Different bodies having the same acceleration — Mass — Masses compared by forces — Relation of force, mass, and acceleration — Unit mass — Weight — Galileo'? experiment — Weight and mass — Density — Work, force and distance — Unit work — Energy, how measured — Efficiency — Kinetic and potential energy — Newton's laws of motion — Power — Engineering units — Summary — Questions — Problems — Suggestions to students 33-56

CHAPTER III Composition and Resolution op Motion

Up-grade — Composition of motions — Vectors — Motions at right angles — Motions not at right angles — Vector solutions — Analytical solution — Traveling crane — Resolution of motions — Force vectors — Balanced forces — Mechanical advantage — Summary — Questions — Suggestions to students 57-72

6 CONTENTS

Chapter IV Page

How rotation is produced —Moment of force — The lever — Work done by the lever — The lever principle — Parallel forces — Weight and center of mass — Equilibrium — Stability, how measured — Determination of the center of mass — Mechanical advantage of a composite machine — The law of machines — The screw — The equal arm balance — Review — Summary — Questions — Problems — Suggestions to students 73-96

CHAPTER V Rotation

Flywheels — Angular measurement and units — Correspondence with linear measurements and units — Moment of inertia and mass — Determination of moment of inertia — Conditions for circular motion, centripetal force — Bursting wheels — Distribution of mass — Moment of mass — Railroad curves — Spinning tops — Summary — Questions — Problems — Suggestions to students 97-111

Chapter VI

Pumps — Air has weight — Torricelli's experiment — Pascal's experiment — Mercurial barometer — Characteristics of fluids — Pascal's principle — Hydraulic machines — Free level surface of a liquid — Gases — Air pump — Guericke — Density of air — Theory of pumps — Archimedes' principle — Flotation — Buoyancy — Determination of density — Boyle and his law — Summary — Questions — Problems — Suggestions to students 112-136

CHAPTER VII Heat

Heat and work — Thermometers — Temperature scale — Gases — Change of volume at constant pressure — Change of pressure at constant volume — Air thermometer — Absolute temperature — Expansion of solids and liquids — Heat quantity — Gram calorie — Specific heat — Steam — Evaporation — Pressure and temperature of saturated vapor — Boiling point — Superheated vapor — Critical temperature — Humidity — Dew — Latent heat — Water and climate — Summary — Questions — Problems — Suggestions to students 137-157

CONTENTS 7

Chapter VIII Page

Conduction and convection — Applications — Radiation —

Diffusion — Evaporation — Gaseous pressure — Effect of

heating — Kinetic hypothesis — Radiation — The ether —

Prevost's theory of exchanges — Absorption — Absorbing

power of water vapor — Radiation and absorption — Heat

and light — Summary— Questions — Problems — Suggestions

to students 158-170

CHAPTER IX Heat and Work

Mechanical equivalent of heat — Gas is heated when compressed — Gas cools when it expands and does work — Liquid air — Cooling by evaporation — Manufactured ice — The steam engine — Work done by the steam — The pressure-volume graph — Back pressure — Lower pressure at exhaust — Condensers — Higher boiler pressure — Heat energy consumed — Efficiency and temperature — Comparison of efficiencies — The triple expansion engine — The gas engine — The steam turbine — Summary — Questions — Problems — Suggestions to students 171-189

CHAPTER X Electricity

Transmission of power — Generators — Early knowledge of electricity — Gilbert — Electrification — Conductors and insulators — Repulsion — Discharge — Electroscope — Both . bodies equally charged — Two kinds of charge — Polariza- . tion — Charging by influence — Charge on the outside of a conductor — Coulomb's law — Leyden jar — Condensers — Operation of a condenser — Discharge of condenser is oscillatory — Lightning — Summary — Questions — Problems —

tions to students 190-210

CHAPTER XI Magnetism

Lodestone and compass — Magnetic curves — Magnetic field — Like poles — Unlike poles — Permeability — Magnetic circuit — Earth's magnetism — Unit pole — Law of magnetic force — Chief characteristics of magnets — Electric currents — Voltaic cell — Electromagnetism — Magnetic field of the current — Electromagnets — Telegraph — Relay — Grounded

Contents

PAGE

wires — Electric bell — Galvanometers — A suggestive experiment — Motors — Motor parts — From toy to practical machine— Ampere's theory of magnetism — Magnetic field of moving charges — Energy of a magnetic system — Summary — Questions — Problems — Suggestions to students ... 21 1-243

CHAPTER XII Induced Currents

Sources of current — Current and magnetic field — Faraday's discovery — Current induced by a moving magnet — Number of lines of force changed — Currents induced by currents — Iron core — Laws of induced currents — The dynamo principle — The dynamo — Magnetos — Alternating current dynamos — The induction coil — The transformer — Alternating current motors — The telephone — Summary — Questions — Problems — Suggestions to students 244-264

CHAPTER XIII The Electric Current at Work

Pressure and current in the arc lamp — Current strength — Resistance — Laws of resistance — Ohm's law — Ammeters and voltmeters — Electric power — Watt meters — Arc light plant — Incandescent lamps, parallel distribution — Incandescent light plant — Heating effects of the current — Joule's law — Heat loss in transmission — Three wire system — Alternating current transmission — Divided circuits — Shunts — Arc lamp regulation — Lifting magnets — Voltaic cells — Energy of the cell — Polarization of cells — The ion hypothesis — Commercial cells — Electrolysis — Faraday's laws — Electroplating — Storage batteries — Retrospect — Summary — Questions — Problems — Suggestions to students . . . 265-299

CHAPTER XIV Wave Motion

Water waves — Origin of waves — Characteristics of waves — What waves tell us — Wave motion — Wave length — Period — Phase — Velocity of propagation — Waves of simple shape — Complex waves — Waves of different shapes — Stationary waves — Summary — Questions — Problems — Suggestions to students 300-316

CONTENTS 9

Chapter XV Page

Uniform circular motion — Displacement and force — The sine curve — Period, mass, and force constant — Pendulum — Uses of the pendulum — The Foucault pendulum — Summary — Questions — Problems — Suggestions to students 317-327

CHAPTER XVI Sound

Sources of sound — Sound a wave motion — Sound waves longitudinal — Velocity of sound — Resonance — Noise — The piano — Pitch — Musical intervals — Laws of strings — Vibrating rods — Tuning forks — Organ pipes— -Ah columns as resonators — Intensity — Summary — Questions — Problems — Suggestions to students 328-341

CHAPTER XVII The Musical Scale

Development of the musical scale — The related triads — The vibration numbers — The major scale — The complete scale — The tempered scale — Standard pitch — Forced vibrations — The ear — Beats — Discord due to beats — Summary—Questions — Problems — Suggestions to students . . . 342-356

CHAPTER XVIII Harmony and Discord

Wave shape and tone quality — The vibrating flame — Musical tones complex — How musical tones are possible — Fundamental and overtones — Overtones of strings — Resonators — How the ear perceives a complex tone — Related tones — Chimes — Summary — Questions — Problems — Suggestions to students 357-368

CHAPTER XIX Light

What does light do for us — Direction — Image by a pin hole — Image by a lens — The eye — How light is changed in direction — Refraction — Index — How the lens forms the image — Reflection — Diffuse reflection — Summary — Questions — Problems — Suggestions to students 369-383

10 CONTENTS

Chapter XX Page

Principal focus — Image of a point source — Construction of the image — Lens angle — Size and distance of the image — Virtual image — How the eye is focused — Spectacles — The simple microscope — The camera — Stops — Spherical aberration — The telescope — The concave lens — The opera glass — The compound microscope — Magnification — Resolution — Summary — Questions — Problems — Suggestions to students ' 384-402

CHAPTER XXI Color

Newton's experiment — Interference fringes — Wave length and color — Interference in white light — Dispersion — The spectrum — Bright-line spectra — Measurement of dispersion — Achromatic lens — Spectrum analysis — How the eye perceives color — Mixing colors — Colors of ordinary objects — Paints and dyes— Mixing pigments — Summary — Questions — Problems — Suggestions to students 403-422

CHAPTER XXII Velocity of Light

What we can learn from the velocity of light — Galileo's method — Fizeau's method — The velocity determined — Velocity of electric waves — Wireless telegraphy — Light and electricity operate through the same medium — The complete spectrum — Summary — Questions — Problems — Suggestions to students 423-434

Chapter XXIII

Light waves start at an electrically charged particle — Other such particles — Cathode rays — Action of magnet — determination of — — Comparison with the ion of electrolysis — The electron — Radioactivity — The X-rays — The nature of white light — Conclusion 435-446

INTKODUCTION

It has been said that man made his start on the long road toward enlightenment when he learned how to make a fire. For many centuries, our ancestors groped at a snail's pace along this road where we of the twentieth century are advancing by leaps and bounds.

By slow and painful steps, prehistoric man learned to use fire in order to keep himself warm, to cook his food, to get metals out of their ores, and to forge them into rude tools and weapons of defense. By means of signal fires on the hilltops, he sent his first wireless messages across the valleys. The magnetic force of the lodestone and the electric attraction of amber were known to the ancients, and the fact that steam pressure can be made to produce motion was known in the early centuries of our era. Why was it that so many centuries elapsed before man learned to subdue these forces of nature and make them do his will? Now we have the steam engine, the electric dynamo and motor, the power printing press, the power loom, the telephone, the wireless telegraph. By means of these and countless other inventions, one man can do the work of hundreds, the continents are linked together, darkness is turned into light, time and space are vanquished.

We can best realize how important are these inventions when we try to think how we should get on without them. And yet this great development of miracle working machinery has come within the space of three centuries, and the greater part of it within seventyfive years!

The stories of how these marvelous inventions came to be, of the struggles of the men who brought them into being, and of the pail tient researches and brilliant discoveries of the men of science who established the foundation principles upon which all these inventions rest are among the most important and most interesting chapters of history.