LIBRARY OK THK UNIVERSITY OF CALIFORNIA. < ; i KT c ) K Ct). - Accession w .847u>/<5_ Class EXPERIMENTAL PHYSICS BY WILLIAM ABBOTT STONE, A.B. INSTRUCTOR IN PHYSICS AT THE PHILLIPS EXETER ACADEMY BOSTON, U.S.A., AND LONDON GINN & COMPANY, PUBLISHERS CTJe &tj)enaettm 1899 COPYRIGHT, 1897, BY WILLIAM ABBOTT STONE ALL RIGHTS RESERVED PREFACE. THIS book is the result of an experience of nearly ten years in teaching Experimental Physics to classes consist- ing of students who were preparing for college and of students who were not preparing for college. Most of the experiments are quantitative, some are qualitative. Qualitative experiments serve to stimulate the interest of the student, and to prepare his mind for a better understanding of quantitative experiments. A beginner in Physics should know something about that which he is expected to measure before he attempts to measure it. This knowledge is readily acquired from qualitative experiments. To show the aim of the work, I have put at the begin- ning of each experiment a concise statement, not of the* result, but of the object of the experiment ; and at the end of each experiment, questions for the purpose of help- ing the student unfold the result of the experiment from his record. The general results of the experiments are enforced by numerous examples, many of which have been drawn from Harvard Examination Papers. The experi- ments are often stepping-stones, each to the next. The book contains only two or three experiments which require students to work in groups ; for my experience IV PREFACE. has shown that students get the greatest benefit from a laboratory course in Physics by working each for himself. My purpose has been not only to teach the student something about Physics, but also, as Physics yields itself readily to this purpose, to teach him the importance of distinguishing between facts and inferences from these facts, to lead him to weigh facts carefully, and then to use his judgment impartially in drawing inferences. The teacher cannot use too much care in impressing upon the mind of the student the limitations which he must con- stantly put upon his statements and the danger of mak- ing generalizations from imperfect data. The student should not think that he is discovering laws of nature for himself. I am under obligations to numerous authors and espe- cially to my former teacher, Dr. E. H. Hall. Among many friends to whom my thanks are due are Professor G. A. Wentworth for his interest, encouragement, and sugges- tions, Professor J. A. Tufts for valuable assistance in reading both the manuscript and the proof sheets and for the great pains he has taken in looking after the English of the book, and Mr. Frank Rollins for reading the proof sheets and for valuable suggestions about the experiments. I shall be grateful for any corrections or suggestions. EXETER, N. H., February, 1897. W> A< S ' THE LABORATORY AND THE APPARATUS. THE laboratory should be a well-lighted room, provided with a sink, tables, and gas or gasoline, if experiments in heat are to 'be attempted. The room should also contain cases or cupboards, or a large closet, in which to store the apparatus when not in use. Some of the more common tools ought always to be at hand, such as a screw-driver, a hammer, a saw, a vise, a gimlet, a soldering-iron. An intelligent carpenter can make many of the pieces of apparatus. In Boston, the L. E. Knott Apparatus Company, 16 Ashburton Place, are prepared to furnish the apparatus called for in this book. Some of the apparatus can doubtless be obtained from Walmsley, Fuller & Co., 134-136 .Wabash Avenue, Chicago ; Eimer & Amend, 205 Third Avenue, New York; Queen & Co., 1010 Chestnut Street, Philadelphia. A sufficient supply of apparatus to provide for a division of twelve students working at a time will cost about $400. This estimate is based upon the fact that certain pieces of apparatus, such as balances and air-puinps, can be used in common by two or more students. Some teachers may find useful the following list, which shows the experiments in this book that are similar to the exercises in the revised Harvard list or that are equivalent to them : HARVARD EQUIVALENT HARVARD EQUIVALENT KXERCISES. EXPERIMENTS. EXERCISES. EXPERIMENTS. 1. 1, 2, 3. 9. 108. 2. 7. 13. 109. 3. 13. 14. 110. 4. 9. 15. 110. 5- 6. 17. 88, 89. 6. 10. 18. 92. 7. 11, 12. 19. 93. VI THE LABORATORY AND THE APPARATUS. HABVABD EQUIVALENT HARVARD EQUIVALENT EXERCISES. EXPERIMENTS. EXERCISES. EXPERIMENTS. 21. 95. 43. 49. 22. 96, 98. 44. 51. 23. 97. 45. 56. 24. 100. 46. 52. 25. 102. 47. 72. 26. 59, 60. 48. 78. 28. 61. 49. 77. 29. 62. 50. 129. 30. 63, 64, 65. 51. 139, 140, 141. 31. 66, 67, 68. 52. 142. 32. 25. 53. 138. 33. 28. 54. 143. 35. 106. 55. 144, 145. 36. 112. 56. 152, 153. 37. 113. 57. 154. 39. 36, 37, 38. 58. 147, 148, 149, 150. 40. 40. 59. 151. 41. 41. , The experiments in the foregoing list are drawn largely from the Har- vard Pamphlet, but in several cases with modifications. In general, apparatus has been recommended like that devised by Dr. E. H. Hall. For the discussion of the experiments valuable suggestions have been derived from Hall and Bergen's Text-Biok of Physics, Hall's Lessons in Physics, and Worthington's Physical Laboratory Practice. EXPERIMENTAL PHYSICS. CHAPTER I. MENSURATION, HYDROSTATICS, AND PNEU- MATICS. 1. Purpose. The purpose of this course in physics is to lead the student to observe carefully, experiment intelligently, record accurately, judge impartially, and infer justly. 2. Directions for Note - Taking-. In a note -book, which must be his constant companion in the laboratory, the student should keep a record of the experiments which he performs. This book must contain the original records of the work done. No records should be made on scraps of paper, to be copied later into the note-book. A blank book about 8f inches long by 7 inches wide, and containing about 250 pages of good unruled paper, suit- able for pen or pencil, is recommended. The binding should be strong, with leather corners for the covers and a leather back. The first leaf should be left blank for the name and title, and the remaining pages numbered like the pages of a printed book, the even numbers on the left- hand page, the odd on the right. _84732 2 EXPERIMENTAL PHYSICS. The records of the measurements and of the obser- vations should be put on the left-hand pages, while on the right-hand pages must be put computations and inferences. The notes, which may be written either with a black lead-pencil or in ink, should not be altered after they are made. An obvious error may be corrected by writing between the lines, but the original record should not be obscured in any way other than by drawing a line through the erroneous statement. At the top of the left- hand page must be placed the number and the object of the experiment with the date, and also the names of the pieces of apparatus used. The notes must not be crowded. It is a good plan to make illustrative diagrams and sketches. Not more than one experiment should be recorded on a page. As long as the student does not depart from the general rules laid down, he is at liberty to follow any system of note-taking that may seem best to him. In developing his method of keeping notes, it is well for him to ask himself frequently, "Will my notes tell another person just what I have done ? " Let the student make his notes concise, yet so clear that another in reading the record cannot fail to understand it. 3. Directions for Performing- Experiments. Before beginning work, the student should read with care the directions which accompany the experiment. In these directions attention will be called to the precautions which should be taken in the proper performance of the experiment. A precaution once noted will be rarely men- tioned again, but should be taken whenever applicable. MENSURATION. All measurements and other necessary data must be recorded. On the following pages, questions in connec- tion with the experiments will be frequently asked. The questions must not be answered by a simple " yes " or " no," but by a declarative sentence. When the question is in parentheses, however, the student should not record the answer in his note-book, but should be able, when called upon, to give it orally. 4. Mensuration. With a meter stick, having inches on one side, measure the length, breadth, and thickness, as accurately as you can, of one of the table-tops in the laboratory. Get the dimensions in feet and fractions of a foot, also in meters and fractions of a meter. Record the measurements in your note-book, on page 2. After con- sulting your record, answer the following questions : How many inches are there in each of the three dimensions ? How many centimeters ? How many milli- meters ? In your records of numerical data and results, use decimal fractions only. Experiment 1. To find the volume of a solid of regular Apparatus. A meter stick ; a rectangular block of wood. Directions. (a) Measure and record the length, breadth, and thickness of the block in inches and fractions of an inch, taking four measurements of the length, one along each of the four edges running in the direction of the length, four of the breadth, and four of the thickness. Find the average length, breadth, and thickness. EXPERIMENTAL PHYSICS. In making the measurements, place the meter stick on its narrow side (Fig. 1) to make the ends of the gradu- m \ I ! I I i i 1 2 3 4 5 C, 1 1 1 1 1 1 II 1 1 1 1 1 1 I 1 1 ' 1 ' I ! jj 7 8 19 60 1C l 1 1 1 l 1 1 1 18 ; FIG. l. ations come close to the block ; do not use the divisions at the ends of the stick, as the ends may be worn. Find the product of the numbers that express the average length, breadth, and thickness. This product will be the number of cubic inches in the block. (b) Using the same care as in (a), find the dimensions in centimeters and fractions of a centimeter. From the average length, breadth, and thickness, find the number of cubic centimeters in the block. NOTE. The block will be needed for the next two experiments. Experiment 2. To find the weight of a wooden block by means of a spring balance. Apparatus. The block of Exp. 1; a spring balance of 8-ounce capacity (Fig. 2) ; thread. Directions. Taking care that the balance frame touches nothing, hang it by its ring from a hook or other suit- able support. Place your head in such a position that the line of vision passes by the end of the pointer and is per- pendicular to the face of the balance. Observe whether the pointer is opposite the line marked (zero). If the MENSURATION. pointer is not opposite the zero line, note how much above or below it is. By means of a piece of fine thread hang the block on the hook of the balance, and observe the new position of the pointer. In computing the result, make a cor- rection for the "zero error" of the balance, that is, the error arising if the pointer, when no weight is hung on the hook, is not exactly in front of the zero line. Strive to read carefully to the tenths of the smallest divisions. Does the weight of the thread make any difference in the indications of the balance ? Making use of the result obtained in Exp. 1 (a), what do you find to be the weight in ounces of one cubic inch of the block ? FIG. 2. Experiment 3. To find the weight of a wooden block by means of a platform balance. Apparatus. The block of Exp. 1; a platform balance; metric weights. Directions. Put the rider on the zero notch of the balance scale. Wipe the pans dry and clean. Set the pans swinging, and add bits of paper till they swing evenly. On the left-hand pan lay the block ; on the right-hand pan put weights (Fig. 3). Use the rider in making the final adjustments. Trust the indications of a swinging balance only. (Why ? ) Find how many grams and fractions of a gram the block weighs. 6 EXPERIMENTAL PHYSICS. Making use of the results obtained in Exp. 1 (5), what do you find to be the weight in grams of one cubic centimeter of the block? Experiment 4. To find the volume of a solid of irregu- lar shape. Apparatus. A 100 CC graduate (a cylindrical glass vessel marked off into cubic centimeters) ; a piece of lead. Directions. Fill the graduate about half full of water and note the exact level of the water. Into the water put the piece of lead (Fig. 4). Be sure that it is entirely beneath the surface. (Why ?) If air-bubbles cling to the lead, remove by shak- ing, but take care not to spill any water. Note the level at which the water now stands. As the surface of the water is highest at its edge where it .meets the graduate, get the level by sighting along a horizontal line that just grazes the lowest part of the surface. How many cubic centimeters of water are displaced ? Is the volume of water displaced the same as the volume of the lead ? How many cubic centimeters does the lead contain ? 5. Quantity ; Unit ; Numerical Value. In the ex- periments already performed we have made measurements of length, volume, and weight, and in our subsequent work, we shall often make measurements of other magni- tudes, such as temperature, friction, and electrical resistance. Measured magnitudes are called quantities. Every quantity is expressed by a phrase consisting of two parts : one of these is the name of a certain known quantity FIG. 4. DENSITY. which is taken as a standard of reference, and which is of the same kind as the quantity to be expressed ; and the other is a number which shows how many times the standard is to be taken in order to make up the required quantity. The standard quantity is called the unit, and the number the numerical value of the quantity. There are as many units as there are different kinds of quantities to be measured. In this book the cubic inch is taken as the unit of volume and the ounce as the unit of weight for the English System, except for the measurement of large quantities, when the cubic foot and the pound are used ; for the Metric System the cubic centimeter is taken as the unit of volume and the gram as the unit of weight. The distance from one end of the Capitol at Washing- ton to the other is just 751 feet. The phrase " 751 feet " tells the number of units of a particular kind contained in the distance mentioned. In this case the foot is the unit, while 751 is the numerical value of the quantity. QUESTIONS. What is the unit and what is the numerical value in each of the following expressions of quantity : 72 feet ? 10 meters ? 202 cubic centimeters ? 8 grains ? 7 ounces ? 100 cubic feet ? 10 inches ? DENSITY. 6. Density. The final result of Exp. 2 gave the weight in ounces of one cubic inch of the block, while that of Exp. 3 gave the weight in grams of one cubic centimeter of the block. Definition. By the density of a substance is meant the weight of one unit of volume of the substance. 8 EXPERIMENTAL PHYSICS. NOTE. Later in the course we shall limit the meaning of the word weight in this definition to that expressed by the word mass, and we shall show that a platform balance and not a spring balance should be used in our method of getting the density in the English System. The final result of Exp. 2, then, gave the density of the block in the English System ; the final result of Exp. 3, the density in the Metric System. The student should consult Exps. 1, 2, and 3, and on the right-hand page of his note-book, the one opposite the last left-hand page that has been written on, give a brief account of the method of finding the density of a rect- angular block of wood, which shall apply to either the English System or the Metric. When the Metric. System was planned, it was decided to take the density of water as unity, that is, to take the weight of a cubic centimeter of water as the unit of weight, and to call it the gram, hence A cubic centimeter of water weighs a gram. NOTE. Strictly speaking, however, in order that a cubic centimeter of water may weigh a gram, the water must be pure and it must have a certain temperature a few degrees above freezing ; but for our purposes common water at any ordinary temperature will give results sufficiently accurate. 7. Precautions to be taken in Measurements and Computations. With a meter stick, such as you have used, suppose a rectangular block to have been measured in centimeters with the following results : MEASUREMENTS MEASUREMENTS MEASUREMENTS OF LENGTH. OF BREADTH.