Modern Physics 
 Laboratory Manual 
 
 Projects and Problems 
 
 for 
 
 Secondary Physics 
 
 Prepared by the Physics’ Teachers' Association 
 of Minneapolis 
 
 
 A. VV. HURD 
 
 North High 
 
 J. H. SANTEE 
 
 North High 
 
 J. V. S. FISHER 
 South High 
 
 J. R. TOYVNE 
 
 East High 
 
 EARL SWEET 
 Central High 
 
 
 H. J. ROHDE 
 
 Central \ High P 
 
INDEX 
 
 1. English and Metric Units of Measurement. 
 
 2. Diagonal Scale. 
 
 3. Construction of Vernier Caliper. 
 
 4. Application of Vernier Caliper. 
 
 5. Micrometer Caliper. 
 
 6. Density of a Regular Solid. 
 
 7. Principle of Moments. 
 
 8. Center of Gravity. 
 
 9. Liclined Plane. 
 
 10. Block and Tackle. 
 
 11. Parallelogram of Forces. 
 
 12. Government Barometer. 
 
 1 3. Boyle’s Law. 
 
 14. Water and Gas Pressures. Open Manometer. 
 
 15. Archimedes’ Principle. 
 
 16. Specific Gravity of a Solid Heavier Than 
 
 Water. 
 
 7. Specific Gravity of a Solid Lighter Than 
 Water. 
 
 Specific Gravity of a Liquid by Loss of Weight 
 Method. 
 
 Specific Gravity of a Liquid by Hare's Method. 
 l Hooke’s Law', 
 breaking Strength of Wires, 
 jesting Fixed Points of a Thermometer. 
 
 Viect of Pressure on Boiling Point, 
 efficient of Linear Expansion. 
 
 Lata of Heat Exchange. 
 
 26. Spe\ ific Heat of a Metal. 
 
 27. Dew) Point and Relative Humidity. 
 
 28. Heat\ of Fusion. 
 
 29. Heatj of Vaporization. 
 
 30. Imaglcs in a Plane Mirror. 
 
 Copyri 
 
 ■ight\ 92 
 
 921 by Henry J. Rohde, Earl Sweet, J. 
 Mail Address: A. W. Hurd, 
 
 31. Images in a Concave Mirror. 
 
 32. Index of Refraction. 
 
 33. Focal Length of a Convex Lens. 
 
 34. Images Formed by a Convex Lens. 
 
 35. Astronomical Telescope. 
 
 36. Erecting Telescope. 
 
 37. Vibration Rate by Siren Disc Method. 
 
 38. Vibration Rate by Resonator Method. 
 
 39. Velocity of Sound Resonator Method. 
 
 40. Law of Lengths of Vibrating Strings. 
 
 41. Law of Tension of Vibrating Strings. 
 
 42. Speed of Sound in a Metal. 
 
 43. Magnetic Fields. 
 
 44. Theory of Magnetism. 
 
 45. Distribution of Magnetism. 
 
 46. Electroscope. 
 
 47. Relation of Static to Current Electricity. 
 
 48. Magnetic Effects of an Electric Current. 
 
 49. Electric Bell Circuits. 
 
 50. Cells in Series and Parallel. 
 
 51. Relation of P. D. and R. 
 
 52. Law of Induced Currents. 
 
 53. Testing of Fuses. 
 
 54. Resistance of Lamps in Series. 
 
 55. Resistance of Lamps in Parallel. 
 
 56. Wheatstone Bridge. 
 
 . 57. Electrical Energy in a Circuit. 
 
 58. Cost Per Hour for Flatiron, etc. 
 
 59. Efficiency of an Electric Stove, etc. 
 
 60. Efficiency of a Gas Burner. 
 
 61. Telephone. 
 
 62. Study of Motor and Dynamo. 
 
 63. Efficiency of Electric Motor. 
 
 64. Transformer. 
 
 R. Towne, J. H. Santee, J. V. S. Fisher, A. W. Hurd. 
 Norih High School, Minneapolis. 
 
PROBLEM No. 1 
 
 To Measure Carefully in English and Metric Units the Length and Breadth of a Sheet of 
 
 Paper and Compare These Units. 
 
 Write the words, “top,” “bottom,” “right,” 
 and “left,” on the corresponding edges of the 
 paper. Measure these edges, first in English 
 units and then in metric units. Avoid using 
 the end of the ruler and be careful to place it 
 parallel with the edge of the paper. 
 
 Express the English units in terms of 
 inches, giving approximately hundredths of 
 an inch. Express the metric units in terms 
 of centimeters, estimating to tenths of milli- 
 meters (hundredths of centimeters). 
 
 Record all fractions in decimals. Find the 
 number of centimeters in an inch. Compare 
 the “computed” value with the “accepted” 
 value, as given in your text-book. Carry the 
 division to the third decimal place. 
 
 From the average results obtained for the 
 width and length, compute the area of the 
 sheet of paper in square inches and also in 
 square centimeters. Find the number of 
 square centimeters in a square inch and com- 
 pare with the accepted equivalent. 
 
 Results : 
 
 1. Width in inches. 
 
 (Top).. . (Bottom).... Average 
 
 2. Width in cm. 
 
 (Top) .... (Bottom) .... Average 
 
 3. From these averages determine the num- 
 ber of cm. in an inch 
 
 4. Length in inches. 
 
 (Right) (Left) .... Average. . . . 
 
 5. Length in cm. 
 
 (Right) (Left).... Average.... 
 
 6. From these averages, determine the 
 
 number of centimeters in an inch 
 
 7. Average number of centimeters in an 
 
 inch (computed) 
 
 8. Number of centimeters in an inch (ac- 
 cepted) 
 
 9. Difference 
 
 10. Per cent of error 
 
 Diff. 
 
 accepted=Per cent of error. 
 
 o 
 
 v/U 
 
 
 100 
 
11. Area in square cm 
 
 12. Area in square inches 
 
 13. Number of square cm. in a square inch 
 
 (computed) 
 
 I 
 
 14. Number of square cm. in a square inch 
 
 (accepted) 
 
 15. Difference 
 
 16. Per cent of error 
 
 a. Why is it not desirable to begin measur- 
 
 ing from the end of the rule? 
 
 b. Give the dimension in inches of the 
 
 famous French 75 millimeter gun. 
 
 c. 39.37 inches equals 100 cm. 
 
 .3937 inches equals 1 cm. 
 
 1 inch 
 
 .3937 
 
 d. What does the answer to (c) express? 
 
PROBLEM No. 2 
 
 To Measure the Distance Between Points to Hundredths of a Centimeter, by Means of a 
 
 Diagonal Scale. 
 
 1. What pail of an inch would this be? 
 
 Draw a diagonal scale in your note-book 
 as follows: Construct eleven parallel lines, 
 four centimeters long and two millimeters 
 apart. Cross these lines by perpendiculars, 
 one centimeter apart. Divide those portions 
 of lines one and eleven, falling between the 
 last two perpendiculars on the right, into ten 
 equal parts each and call the left side zero. 
 
 Now join with a straight line, the point 
 marked no-tenths on the horizontal line num- 
 bered one with the point marked one-tenth 
 on the horizontal line numbered eleven. Join 
 the remaining tenths by lines parallel to the 
 one just drawn. Notice that the lines are 
 one-tenth of a centimeter farther to the 
 right on line eleven than on line one. Each 
 line passes over ten equal spaces in moving 
 one-tenth to the right. 
 
 ' 2. Figure out what part of a centimeter it 
 moves to the right in crossing one space. 
 
 Ask your instructor to locate the points to 
 be measured. 
 
 Mark them with letters as A. B. 
 
 Adjust a pair of dividers to the exact dis- 
 tance without touching the points to the pa- 
 per. Place the dividers on the top horizon- 
 tal line with the left leg of the divider on 
 such a centimeter division that the right leg 
 will fall in the last centimeter division con- 
 taining the diagonal scale. From this, read 
 the v ’’stance in centimeters and tenths. Then 
 move 'oth legs of the dividers down the scale 
 until ti e right leg exactly strikes an inter- 
 section. Now get the reading to hundredths 
 of a centimeter. 
 
 3. Could you read the third decimal by es- 
 timating a part of one space? If so, how? 
 
 4. Would this estimate increase the accu- 
 racy of your measurement? Why? 
 
 5. How may you be sure that the dividers 
 have not moved while you were taking the 
 reading? 
 
 Average several readings for the final val- 
 ue and tabulate all of them. 
 
 6. Why do we measure things? Would it 
 not be better to estimate such things as mon- 
 ey. gas, electricity and land? 
 
 7. How does an estimate differ from a 
 guess ? 
 
 r 
 
. 
 
 
 
 
 . 
 
 ' 
 
 _ 
 
 •it! ;»i< 
 
 " 
 
 •• ■ 1 
 
PROBLEM No. 3 
 
 To Make a Vernier Caliper. 
 
 Along the middle of a card draw a straight 
 line, AB. Beginning at a point about 2 cm. 
 from the left end, lay off along AB a scale of 
 centimeters. To do this, stand the centi- 
 meter rule on edge, so that one of the centi- 
 meter marks of the scale is at the starting 
 point. Mark a fine dot with a sharp pencil 
 opposite each succeeding centimeter mark 
 for about 12 centimeters. 
 
 Be sure that each dot is accurately in line 
 with the mark on this scale and through each 
 one erect perpendiculars to AB. 
 
 Number them 0. 1, 2, 3, etc., at their top. 
 
 On the other side of the line AB lay off in 
 the same way a scale of 10 divisions, each di- 
 vision being 9 mm. long. This scale must 
 start at the same point (0) as the first scale. 
 
 The line marked 10 on this new scale will 
 then be found to coincide with the 9 line of 
 the centimeter scale. This new scale is call- 
 ed the vernier. 
 
 About 5 mm. to the left of the zero divi- 
 sion draw a line, CD, perpendicular to AB on 
 the side having the vernier scale. With a 
 scissors cut along this line and also along AB 
 to the right. Cut the edges smooth. Now 
 you have a model vernier caliper by which 
 the diameter of a round object may be meas- 
 ured. 
 
 (1) When the jaws are closed, what is the 
 distance between the 1 mark of the vernier 
 and the 1 mark of the scale? 
 
 (2) How will you know when the jaws are 
 0.1 cm. apart? 0.1 mm.? 
 
 % 
 

 
 
 
 '••V . i <:n-/F ■ <j ' r.T 
 
 
 
 ' 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
PROBLEM No. 4 
 
 To Measure the Dimensions of a Metal Cylinder With a Vernier Caliper and Determine 
 the Value of Pi From the Measurements Obtained. 
 
 By the use of a vernier caliper, measure- 
 ments may be made accurately to tenths of 
 a millimeter. 
 
 1. On the caliper, notice that 10 equal divi- 
 sions on the movable scale, which is called 
 the vernier, cover 9 millimeter divisions on 
 the fixed scale. 
 
 2. By what part of a scale unit (mm.) does 
 each division of the vernier differ from that 
 of the fixed scale? 
 
 The first line on the vernier is called the 
 zero line, or reading line. Place line No. 1 
 (second line) of the vernier exactly opposite 
 line No. 1 (second line) of the scale. 
 
 3. By what part of a millimeter are the 
 jaws separated? 
 
 4. Explain how you determine this. 
 
 Place the lines No. 2 of the scale together. 
 
 5. Now, what is the distance between the 
 jays? Observe this distance when other 
 corresponding lines are opposite. 
 
 To see if you understand how to read the 
 scale on this caliper, set it at the following 
 values and if in doubt, take them to the in- 
 structor for verification: 1.6 mm., .4 mm., 
 
 16.7 mm. 
 
 Cut off a narrow strip of note-book paper 
 and wran it around the cylinder until it over- 
 laps. Then, thrust a sharp needle through 
 the overlapping portion. Measure the dis- 
 tance between these points as accurately as 
 possible. Repeat for three readings. Next, 
 measure the diameter of this cylinder in 
 three different places. 
 
 6. Why do you take each measurement 
 three times? 
 
 Using the average values, compute the 
 value of Pi and compare it with the accented 
 value (3.1416). Determine the per cent of 
 ' error. Put this data in a convenient TABU- 
 LAR FORM. Be sure to record the number 
 of your evlinder. 
 
 7. Read the vernier scale on the barometer 
 as accurately as you can and record this val- 
 ue in your report. 
 

PROBLEM No. 5 
 
 To Measure the Diameter of the SAMP] Cylinder With a Micrometer Caliper. 
 
 Caution: Never force the screw tight as 
 it will injure the instrument. Use ratchet 
 where provided and where not, use extreme 
 care. 
 
 The micrometer is a precision measuring 
 instrument, which is used to obtain more ac- 
 curate results than can be obtained with the 
 vernier. On an inner sleeve, there is a longi- 
 tudinal scale of millimeter divisions, while 
 on the outer revolving sleeve, there is a cir- 
 cular scale divided into 50 equal parts. In 
 this instrument, the distance between the 
 threads of the screw (called the pitch) is 
 such that it is necessary to turn the revolv- 
 ing sleeve through two complete revolutions 
 to separate the jaws a distance of one milli- 
 meter. 
 
 1. What distance will the jaws be opened 
 when the outside sleeve is turned through 
 one of its divisions ? Turn it through one of 
 these divisions and holding the caliper to a 
 strong light, notice the distance the jaws 
 have opened. 
 
 2. What distance is covered by one com- 
 plete turn of the sleeve ? 
 
 Set the micrometer at the following values 
 and if in doubt, have the instructor verify 
 each : 2.20 mm., 2.02 mm., 0.22 mm., 5.36 mm. 
 
 Before making a measurement with the 
 caliper, it is always necessary to take what 
 is known as the “zero reading;” that is, to 
 determine whether or not the zero line on 
 the ciicular scale is opposite the zero line on 
 the longitudinal scale. If they are not oppo- 
 site. then make a record of the reading, 
 which must be added to, or subtracted from, 
 each reading made with this particular cali- 
 per. It is also important that the same pres- 
 sure be exerted at the jaws for each meas- 
 urement, otherwise the results will be erron- 
 eous. To get this even pressure, a ratchet 
 head is sometimes attached to the instru- 
 ment. Where this device is lacking, hold 
 the head of the screw as loosely as possible 
 when setting the caliper, thus allowing the 
 fingers to slip as soon as contact is made and 
 so avoiding undue pressure. 
 
Measure the thickness of several small ob- 
 jects, such as a sheet of note book paper, the 
 diameter of your hair, etc. Notice if these 
 objects are of uniform dimensions. Record 
 your results. 
 
 Also measure the diameter of the cylinder 
 used in Problem 3 and compare this value 
 with the result obtained when you used the 
 vernier. 
 
 3 What degree of accuracy can be ex- 
 pected of this caliper? 
 
 4. What degree can be estimated? 
 
 5. What is meant by the pitch of a screw ? 
 
 6. What is the pitch numerically equal to 
 in this micrometer? 
 
PROBLEM No. 6 
 
 To Find the Density of a Regular Solid, Whose Volume Can Be Found by Direct Measui 
 
 ment. 
 
 (mass or 
 
 The density of a body is its (weight per 
 unit volume — in the metric system usually 
 expressed in grams per cubic centimeter. 
 
 (mass or 
 
 1. Given the (weight and volume, how is 
 the density computed? 
 
 In this experiment, do as careful work as 
 you can and see how accurate a result you 
 can get. Be sure you have a regular solid 
 whose measurements may be obtained eas- 
 ily. Measure the dimensions with a 30 cm. 
 rule (use a pair of outside calipers with the 
 30 cm. rule, if you have them), a vernier or 
 micrometer caliper, getting readings in cen- 
 timeters to at least two decimal places. Take 
 three readings of each dimension, being care- 
 ful to take no two in the same place. Com- 
 pute the volume from the average readings, 
 indicating clearly the method of computa- 
 tion. 
 
 To find the weight, use a beam balance and 
 set of weights. Take a complete set of 
 weights and try to keep it intact. (Do not 
 let others borrow single weights from you 
 and do not borrow them from others. A set 
 of weights with missing weights is worthless 
 in itself for weighing purposes.) In using 
 the balance, see that the pointer is at the 
 middle of the scale with no weights on either 
 pan. Each balance has an adjustable nut to 
 use in securing proper equilibrium in the be- 
 ginning. Tf this is not sufficient, use paper. 
 
 After you have secured the desired equilibri- 
 um. place the object you wish to weigh on 
 the left hand scale pan and the largest 
 weight less than the weight of the body that 
 you find will most nearly balance it on the 
 ri Hit hand scale pan. Add weights, always 
 using the largest possible ones, working 
 down step by step from the larger to smaller, 
 finally making use of the sliding weight, if 
 your balance is provided with one, until the 
 pointer again rests at the middle of the arc. 
 
 The sum of the weights plus the reading in- 
 dicated by the sliding bob will be the ap- 
 proximate weight of the object. Follow 
 these directions unless otherwise directed. 
 
Compute the density as indicated in your j 
 answer to (1). 
 
 Make a neat complete tabulation of results 
 which will show clearly the general method 
 of procedure. It should contain each meas- 
 urement of dimensions to hundredths of cen- 
 timeters, averages, volume (showing method 
 of computation), weight, density (showing 
 method of computation), and the accepted 
 value of the density of the substance used as 
 recorded in a table of densities. 
 
 2. Mention at least two reasons why you 
 would not vouch for the accuracy of your 
 final result. 
 
 3. How does the weight of the substance 
 you used compare with that of water? 
 
 4. Give reasons for your answer to (3). 
 
 5. If the body you have used in this ex- 
 periment is non-porous and insoluble in water 
 and you have a cylindrical glass vessel gradu- 
 ated in cubic centimeters, suggest a method 
 for finding the valume of the body, other 
 than by measurement of its dimensions. 
 
PROBLEM No. 7 
 
 To Test the Principle of Moments and to Work Out the Law of the Lever. 
 
 A moment of a force is defined as the prod- 
 uct of the force and the perpendicular dis- 
 tance from the fulcrum to the line in which 
 the force acts. 
 
 1. Slip the meter stick through the clamp 
 and fix the knife edge exactly at the 50 cm. 
 mark. If the meter stick will not balance 
 horizontally on the knife edge, place a clip 
 or wire at such a position that the meter 
 stick will balance and keep the clip there 
 throughout the experiment. 
 
 Call the fulcrum F. 
 
 2. Suspend by a loop of thread a 100 gram 
 weight on the right side of the bar at a place 
 where it will exactly balance 200 g. placed 
 at a definite place on the left. 
 
 Make a straight line diagram and record 
 values in figures on the sketch. 
 
 4 o o k /cL = /o c x sd 
 
 Find the moment of each force and, using 
 the equation below the diagraam, test its 
 truth. 
 
 3. Repeat the test, placing the weights at 
 other positions and record the same way. 
 
 4. Suspend a 50 g. weight and 100 g. 
 weight at different points on the right side 
 of the fulcrum and balance with the 200 
 gram weight on the left. 
 
 Record on a sketch similar to the first one. 
 
 5. Repeat the last trial, using the 100 
 gram and 200 gram weigRts on the right 
 side, balancing the 500 gram weight on the 
 left. 
 
 Make a diagram for each trial. 
 
 6. Repeat, using an unknown weight such 
 as a lead cylinder on one side; a known 
 weight on the other. Form an equation, let- 
 ting x equal the unknown weight, and solve 
 for. 
 
 Verify by weighing on a beam balance. 
 
Questions. 
 
 A. Which method of weighing do you 
 consider more accurate and why? 
 
 B. A boy weighing 95 lbs. is 4 ft. from 
 the fulcrum on a see-saw. Two boys are 
 balancing him on the other side of F, one 
 weighing 60 lbs. 2 ft. from F, the other 
 weighing 70 lbs. How far from F is the last 
 boy? 
 
PROBLEM No. 8 
 
 To Prove That a Body Acts as if Its Weight Is Collected at Its Center of Gravity. 
 
 Find the line of the center of gravity of a | 
 tapering rod about one meter long by bal- 
 ancing it on a sharp edge support. Mark the 
 position after seeing that the fulcrum edge 
 and the edge of the rod are perpendicular to 
 each other. Hang a 200 g. weight (W) by 
 means of a light thread 10 cm. from the 
 tapering end of the rod and balance again. 
 
 Mark the position of the fulcrum. Measure 
 and record the distances from the fulcrum 
 to the weight (W) and from the fulcrum to 
 the center of gravity. Call these distances 
 d x and d 2 respectively. 
 
 It is evident that the moment of the 
 weight (W), which is equal to W times d, 
 must be balanced by the moment of a weight 
 on the other side of the fulcrum. Let us as- 
 sume that such a weight (X) is at the cen- 
 ter of gravity. Then the moment of such a 
 weight is equal to X times d 2 . From the 
 principle of moments of a lever W times d, 
 equals X times d 2 . Solving for X, a weight 
 at the center of gravity is calculated. Re- 
 peat with the weight (W) at 20 and 30 cm. 
 from the end. Average the computed weight 
 at the center of gravity. 
 
 Weigh the rod and compare its weight 
 with the computed weight at the center of 
 gravity. Tabulate results showing weight 
 W, d 1( d 2 , computed weight at center of 
 gravity X, average and weight of rod. 
 
 1. What does this experiment prove con- 
 cerning where the weight of a lever is con- 
 centrated V 
 
 2. Which trial should give the best result? 
 
 Why? 
 
 3. A boy weighing 120 lbs. uses a see-saw 
 12 ft. long, which weighs 140 lbs. and which 
 has its center of gravity in the middle. If 
 the boy sits 1 ft. from the end, where is the 
 fulcrum from this end, in order to produce 
 balance ? 
 

 
 
 
 
 
 
 
 
 
 * 
 
 
 
 . 
 
 ' 
 
 ■ 
 
 
 
 
 
PROBLEM No. 9 
 
 To Compute and to Compare the Efficiency of an Inclined Plane at Different Angles. 
 
 The efficiency of a machine is the ratio of 
 the output to the input. 
 
 Set up a board to be used as an inclined 
 plane at about 15 degrees. Measure off some 
 convenient length, 50 or 100 cm., along the 
 lower edge of the plane from the base. 
 
 Measure height to the bottom of board at the 
 length taken. Adjust the plane until the 
 height to the marked off length will make a 
 15 degree angle. Weight a car; add a known 
 weight for load in the car; place this car on 
 the inclined plane and attach- weights by 
 means of a cord passing over a pulley. In 
 this way determine the effort required to 
 pull the car up slowly and uniformly, apply- 
 ing the force parallel to the face of the plane. 
 
 Repeat with different angles, say 20° and 
 30°. 
 
 Compute the input and the output for the 
 different grades. The work put in, the in- 
 put, is equal to the product of the effort and 
 the measured length. The work accom- 
 plished, the output, is equal to the product of 
 the total load and the measured height. 
 
 Compute the efficiency for each angle. Tabu- 
 late the results showing weight of car, total 
 load, effort, length, height, input, output, ef- 
 ficiency. 
 
 1. What is gained by using an inclined 
 plane ? 
 
 2. The efficiency in pulling a 100 ton train 
 up an incline, the height of which is 2 feet 
 for each length of 100 feet, is 90 %. How 
 much must the engine pull? With no fric- 
 tion, what would have been the pull? 
 

 - 
 
 •li. - o' '•» 
 
 •• ; • 
 
 . 
 
 • ; 
 
 r> 
 
 : .3 
 
 
PROBLEM No. 10 
 
 To Find the Efficiency of a Block and Tackle. 
 
 Arrange any combination of pulleys you 
 may choose. Attach a heavy weight to the 
 movable block. Put suitable weights on the 
 ends of the free rope till they just pull the 
 heavy weight up slowly. Make three trials. 
 
 Count the number of ropes that support 
 the weight. This number is called the Me- 
 chanical Advantage. 
 
 1. If the small weights move through a 
 distance of 60 centimeters, how far does the 
 heavy weight move? 
 
 It is obvious that the small weights do 
 work on the large weight in making it move 
 slowly upward. To find what is called the 
 input, or work expended, when the force or 
 effort works through a certain distance, ob- 
 tain the product of the force and distance. 
 
 The output, or work accomplished, is found 
 similarly by obtaining the product of the 
 weight and the distance it moves in the same 
 operation. If there are six strands of rope, 
 the force will move through a distance six 
 times as great as the weight. 
 
 Make a simple diagram, indicating The 
 weight lifted, the force acting and actual 
 arrangement of cord used. 
 
 2. How much work will be accomplished 
 (Output) under the conditions stated in 1 ? 
 
 3. Under the same conditions, how much 
 work will be expended (Input) ? 
 
 4. Efficiency being the ratio of the work 
 accomplished or output, to the work expend- 
 ed, or input, compute the efficiency of the 
 pulleys. 
 
 Were the efficiency of the pulley 100%, 
 the two quantities formed in (2) and (3) 
 would be equal. 
 
 5. Why is not efficiency 100%? 
 
 6. If the efficiency of the pulleys were 
 100%, what would be the ratio of the weight 
 to the force with six strands? 
 
 7. Supposing the set you have to be strong 
 enough and the efficiency to be 80%, how 
 much weight could you lift with a force of 
 250 lbs.? 
 
 8. Is the work obtained from a set of pul- 
 leys greater or less than the work put in? 
 
 9. What is gained by using a set of pul- 
 leys? 
 
• -J 
 
 1 ; i i 
 
 ■ 
 
 ■ • • 
 
 . 
 
PROBLEM No. 11 
 
 To Find the Resultant of two Non-parallel Forces and to Compare With Their Equilibrant. 
 
 Method A: Arrange the apparatus ac- 
 cording to the diagram* 
 
 Place the balances down on the table, ad- 
 justing the system so that each balance reg- 
 isters more than half but less than full scale. 
 
 If the balances do not register zero with 
 no tension, an allowance must be made for 
 the amount of error. This is called the zero 
 error. 
 
 Place a sheet of paper from notebook un- 
 der the junction of the cords, and transfer 
 the angles to the paper. This should be done 
 without disturbing the set up. One way is 
 by marking two dashes under each cord, af- 
 terward drawing lines through each pair. 
 
 Read each balance carefully and record the 
 value on the line running to that balance. 
 The paper may then be removed and the ten- 
 sion on balances released by unfastening any 
 one of them. 
 
 Finish the experiment as follows using rul- 
 er and compass and a sharp pointed pencil. 
 
 Mark the junction of the three straight 
 lines O. Mark the other ends A, B, C. Choos- 
 ing any two of the three, say OB and OC, 
 construct a parallelogram to scale as follows, 
 letting Yu" equal 1 ounce: — 
 
 Lay off on the lines OB and OC as many 
 units as the reading of the spring balances 
 indicate. With each of these points as a 
 center and the opposite side as a radius, de- 
 scribe arcs cutting each other. Mark the 
 intersection F. Draw OF and measure it 
 carefully to scale of the drawing. 
 
1. Is OF equal to OA? 
 
 2. Is OF opposite OA? 
 
 3. What is the name of OF relative to the 
 forces OB and OC? 
 
 4. What is the name of OA relative to the 
 forces OB and OC? 
 
 5. State in a single sentence two things 
 which you have shown to be true in the ex- 
 periment regarding relation of resultant and 
 equilibrant. 
 
PROBLEM No. 12 
 
 To Explain the Principle of the Barometer and Read a U. S. Government Barometer, Hav- 
 ing a Vernier Scale. 
 
 I. Describe Torricelli’s experiment and an- 
 swer the following questions: — 
 
 1. How long a tube is used in performing 
 the experiment? Why? 
 
 2. Why was mercury used instead of 
 water ? 
 
 3. What kept the mercury supported in 
 the tube? 
 
 4. How is a barometer like a Torricellian 
 tube? 
 
 5. What does an area of “low pressure” on 
 a weather map indicate? 
 
 6. When the usual statement, e.g. “the 
 pressure today is 74.25 cm.,” is made, what is 
 the interpretation so that it really means 
 pressure? (Cm. are not units of pressure, 
 are they ?) 
 
 7. What does the word “pressure” mean 
 in Physics? 
 
 8. Calculate the downward “force” of the 
 atmosphere on a table 3 meters long and 2 
 meters wide, when the barometer reads 
 74.39 cm. 
 
 II. To read the barometer. 
 
 1. The cup at the bottom must first be ad- 
 justed so that the little ivory pointer which 
 marks the zero of the linear scale, just 
 touches the surface of the mercury in the 
 cup. (The cup is adjusted by means of a 
 set screw at the bottom. Be sure you see 
 the pointer before adjusting the set screw.) 
 
 2. After the cup is adjusted correctly, no- 
 tice the movable scale toward the top, which 
 is adjustable by means of another set screw 
 at the side. This movable scale should be 
 adjusted so that its lower surface is just I 
 tangent to the convex surface of the mer- 
 cury in the tube. 
 
 3. The reading can now be obtained in cm. 
 and tenths of cm. by noting the reading on 
 the fixed scale opposite the bottom of the 
 movable scale. The purpose of the movable 
 scale is to get a reading accurate to hun- 
 dredths of a cm. 
 
4. To do this, note which line on the mov- 
 able scale most nearly coincides with a line 
 on the fixed scale. The number of this line 
 (on the movable scale) gives the number of 
 hundredths of cm. For example, suppose 
 that the reading opposite the bottom of the 
 movable scale is more than 74.2 cm. and yet 
 not 74.3, i.e., between 74.2 cm. and 74.3 cm. 
 Now on looking to see what line of the mov- 
 able scale most nearly coincides with a line 
 on the fixed scale, suppose it to be the sev- 
 enth. The number of hundredths indicated 
 is, therefore, seven. The correct reading is 
 74.27 cm. 
 
 The reading in inches to hundredths is ob- 
 tained similarly. 
 
 This form of scale is called a vernier scale. 
 It is so made that ten divisions on the mov- 
 able scale is equal to nine divisions on the 
 fixed scale. By carefully studying the scale, 
 the truth of its principle described above will 
 be understood. 
 
 III. Read the barometer as suggested in 
 both cm. and inches and draw neatly and 
 accurately a figure showing all of the mov- 
 able scale and enough of the fixed scale to 
 show plainly one reading, either cm. or 
 inches. This drawing will show whether 
 or not you know how to read the scale. 
 (Try it roughly on scratch paper first, 
 so as not to spoil your page. It will be 
 easier to make it many times the actual 
 size.) 
 
 IV. Read the barometer every day for three 
 days and neatly tabulate readings. 
 
 V. Why does a barometer read differently 
 on different days? 
 
 2. Why is the cup at the bottom adjust- 
 able? 
 
 3. What is the effect of altitude on a ba- 
 rometer reading? Why? 
 
PROBLEM No. 13 
 
 To Verify Boyle’s Law. 
 
 Arrange the mercury tube in an upright 
 position (position 1 in the diagram), being 
 sure to have the sealed end at the top. Call 
 the end of the mercury column next the seal- 
 ed end A; the opposite end of the mercury 
 column B : and the sealed end D. 
 
 With a meter stick, measure carefully the 
 length of the air column AD in cms. As- 
 suming the coss-section of the tube is every- 
 where the same, the volume of the confined 
 air will always be proportional to its length ; 
 meters of mercury, to which the air V, is 
 subjected. Call this pressure P,. 
 
 Next measure the length of the mercury 
 column AB in cms. Read the barometer in 
 cms. This barometric height minus the 
 length AB is the pressure, measured in centi- 
 meters of mercury, to which the air VI is 
 subjected. Call this pressure PI. 
 
 Turn the tube slowly to position II of the 
 diagram. Now measure the heights of the 
 points A and B above the table and subtract 
 the difference between these two distances 
 (representing the vertical height of the mer- 
 cury column) from the barometric reading, j 
 thus obtaining P.„ the pressure correspond- 
 ing to volume V... 
 
 Place the tube successively in positions 
 ITT, IV and V, calling the volumes V., V,, 
 etc. Measure each case the heights of A 
 and B from the table and compute the cor- 
 responding pressure P.,, P„ etc. 
 
 (Remember that the pressure on the con- 
 fined air is less than the barometric pressure 
 if the open end of the tube is lower than the 
 closed end, and greater than the barometric 
 pressure if the open end is higher than the 
 closed end.) 
 
Record as indicated by the instructor. 
 
 1. What is Boyle’s Law? 
 
 2. If the pressure of the atmosphere is 15 
 lbs. to the square inch, how many times the 
 capacity of an auto tire may be pumped into 
 it when the pressure gauge indicates a pres- 
 sure of 70 lbs. per sq. inch, supposing, of 
 course, that the fabric of the tire does not 
 allow it to expand? 
 
PROBLEM No. 14 
 
 A. To Find the Pressure of the City Gas; B. To Find the Pressuie of \oui Lungs 
 
 Both by Means of Open Manometers. 
 
 A. Arrange two open manometers, one con- 
 taining water and the other alcohol, so that 
 the gas pressure may force the liquids up 
 in the long arm of the manometers. Turn 
 on the gas and carefully measure the 
 height of the liquid in the long tube above 
 that in the short tube in each manometer 
 in inches. When gas pressure is spoken 
 of in this country, it is usually expressed 
 in inches of water that it will hold up rath- 
 er than in lbs. per sq. inch, as the water 
 pressure is. 
 
 1. From your experiment, how many 
 inches of water does the gas support and 
 how many inches of alcohol? 
 
 2. Which liquid is higher and why? 
 
 3. What pressure is the gas balancing, 
 other than that of the liquids? 
 
 4. Compute from the reading of each 
 liquid, the pressure of the gas in lbs. per sq. 
 inch. 
 
 B. Mercury is used in this part of the ex- 
 periment because water would be too light 
 for the length of the manometers we use. 
 
 1. If the atmospheric pressure supports a 
 column of mercury 30 inches high, how high 
 a column of water will it support? 
 
 2. Why are water barometers not com- 
 monly used? 
 
 Take the open manometer containing mer- 
 cury and attach to the short arm, a rubber 
 tube. Before exerting pressure, get a glass 
 mouthpiece which has been sterilized. This 
 should be sterilized before being used again 
 by another person. (Put in boiling water to 
 sterilize, or wrap a piece of paper around the 
 mouthpiece.) When the mouthpiece has 
 been attached, blow steadily, reading the 
 position of the mercury in the two tubes. 
 
 (Do not read the points to which it jumps 
 but the points at which you can hold it for 
 an appreciable length of time.) As the mer- 
 
cury ascends in the long tube, it descends in 
 the short tube, so to get the height of the 
 mercury which you really held up, subtract 
 the two readings. Tabulate all measure- 
 ments and compute the pressure in lbs. per 
 sq. inch. 
 
 3. How high a column of water could you 
 have supported? 
 
 4. What was the pressure of your lungs in 
 lbs. per sq. inch? 
 
 5. How much is this above normal atmos- 
 pheric pressure? 
 
PROBLEM No. 15 
 
 To Determine the Relation Between the Buuoyant Force Exerted Upon a Metal Cylinder 
 Immersed in Water and the Weight of the Displaced Water. 
 
 Directions: Weigh the cylinder accurate- 
 ly in air. Suspend the cylinder by means of 
 a thread and weigh it accurately when en- 
 tirely immersed in water. The difference of 
 these two weights is the buoyant force. 
 
 Next, find the weight of the water displaced 
 by method A, B or C. 
 
 Method (A): Measure the dimensions of 
 the cylinder carefully in centimeters and 
 compute its volume. This will numerically 
 equal the weight in grams of the water dis- 
 placed. (1) Why? Tabulate measurements. 
 
 Method (B): Fill an overflow can until 
 the water just begins to run out at the spout. 
 
 When the last drop has run out, lower the 
 cylinder in the can by means of the thread 
 and catch in the bucket the water that has 
 been forced out. Accurately obtain the 
 weight of this water. This should also 
 equal the buoyant force. 
 
 Method (C): Find the displacement of 
 the cylinder as follows : Support a burette in 
 a vertical position fitted with a rubber tube 
 and a pinchcock at the lower end. Put 
 enough water in the burette to come up to 
 where the marks are, and take the reading. 
 
 Tie cylinder to a thread and lower into the 
 burette. Take reading again. The differ-^ 
 ence between these two readings gives the 
 volume displacement of the cylinder, or, nu- 
 merically the weight of the displaced water. 
 
 (1) Why? If time permits, take three sets 
 of readings at different heights of the tube 
 and find the average of your results. 
 
 Record the results in the following form : 
 
 Weight of cylinder in air 
 
 Weight of cylinder in water 
 
 Buoyant force on cylinder 
 
 Weight of water displaced 
 
 Difference 
 
 Per cent of difference 
 
 (2) State Archimedes’ principle in your 
 own words. 
 
 (3) A cake of ice floating in water is 6 
 ft. square and 2 ft. thick. A man stepping 
 on it causes it to sink one inch. Find the 
 weight of the man. 
 

 
 
 
 
 
 ‘ 
 
 ’ 
 
 . 
 
 ‘ 
 
 ■ 
 
 ' 
 
PROBLEM No. 16 
 
 To Find the Specific Gravity of an Irregular Solid. Which Sinks in Water. 
 
 Suspend the solid from the specific gravity 
 balance and weigh it in air. Then accurately 
 weigh it entirely immersed in water. 
 
 1. What is the difference in the two weigh- 
 ings in grams? 
 
 2. According to Archimedes’ principle, to 
 what is the loss of weight in water equal? 
 
 3. How is the volume in C.C. obtained? 
 
 4. What is meant by the specific gravity 
 of a substance? 
 
 Record your results for several materials 
 as follows: 
 
 Kind of solid 
 
 Weight in air 
 
 Weight in water 
 
 Loss of weight in water 
 
 Weight of an equal volume of water 
 
 Specific gravity computed 
 
 Specific gravity accepted 
 
 5. A block of marble weighs 5000 g. in air 
 (sp. gr. 2.5) ; how much will it weigh in 
 water ? 
 
 6. Calculate the density of marble in (5). 
 
 (a) In metric units 
 
 (b) In English units 
 

 \ 
 
 v ' • 
 
 . 
 
 - 
 
 < i‘ n ■ iil uoL:'> r> 
 
 ) 
 
PROBLEM No. 17 
 
 To Determine the Specific Gravity of a Solid Lighter Than Water. 
 
 Attach a cord to the body and weigh it. 
 
 Then, with a sinker attached, weigh them 
 when the sinker is immersed in water and 
 the body is in the air. Next, weigh when 
 both body and sinker are immersed. Be 
 careful that the objects do not touch the 
 sides of the vessel containing the water, 
 otherwise the true weights will not be ob- 
 tained. The difference between the second 
 and third weighings is the buoyant effect of 
 the water on the body alone. 
 
 1. Show clearly why this last statement is 
 true. 
 
 From this data, calculate the specific grav- 
 ity of the body, making a complete tabula- 
 tion of your measurements. 
 
 2. What is the density of this body? 
 
 3. What is the distinction, if any, between 
 density and specific gravity? 
 
 4. Are they ever numerically equal to one 
 another? If so, when? 
 

 ' 
 
 
 
 . 
 
 I 
 
 
 
 
 * 
 
 
PROBLEM No. 18 
 
 To Find the Specific Gravity of a Liquid by the “Loss of Weight” Method. 
 
 Directions: (a) Weigh an irregular solid 
 in air; then in the liquid. The difference in 
 weight is the weight of the liquid displaced 
 by the solid. (1) Why? Next, weigh the 
 solid immersed in water. The difference be- 
 tween this weight in water and the weight in 
 air equals the weight of the water displaced. 
 
 (2) Why? From these two weights, deter- 
 mine the specific gravity of the liquid. 
 
 (3) How? 
 
 (b) Now apply the hydrometer to the 
 liquid and read the specific gravity, which 
 this gives. Compare this result with your 
 other determination. 
 
 (4) Which method, (a) or (b) is quicker? 
 
 (5) Which result would you judge to be 
 the more accurate of the two and why? 
 
 Tabulate your results in a convenient 
 form. 
 

 . 
 
 ■ 
 
 ' 
 
 V 
 
PROBLEM No. 19 
 
 To Find the Specific Gravity of a Liquid by Hare’s Method. 
 
 Arrange the apparatus 
 according to the diagram. 
 
 Raise the liquids in the 
 tubes to a height of 30 
 or 40 cm. by a partial 
 exhaustion of the air 
 through the rubber tube 
 at the top. Now close 
 the tube air-tight by 
 means of the clamp. 
 
 Watch the liquids in 
 the tubes for a few mo- 
 ments to see whether the 
 liquids fall or not. If 
 they do, it shows that the 
 tube is not air-tight. 
 
 Measure the vertical 
 height of the two columns above the sur- 
 . faces of the liquids in the tumblers. Make 
 three sets of readings, changing the height 
 each time. 
 
 (1) Why is water used as one of the 
 liquids ? 
 
 Divide the height of the water column by 
 the height of the liquid to be measured. This 
 will give you the specific gravity of the j 
 
 liquid. 
 
 Devise a suitable record in tabulated form 
 for your readings. 
 
„V; .. , ' ' , • l • — 
 
 >~Vi 
 
 V 
 
 . 
 
 - 
 
 I r^r-lv : 
 
 . 
 
 iri . .... :■ . .. s 
 
 > . . . 1 -. 
 
 . 
 
 ... 
 
 
 
 
 
PROBLEM No. 20 
 
 e 
 
 o 
 
 o 
 
 © 
 
 To Test Hooke’s Law on Stretching by Means of the Jolly Balance. 
 
 Adjust the spring and read the position of 
 the index before any load has been placed on 
 the pan and call this the “zero reading.” 
 
 Now place a weight of 1 or 0.5 g., (let the 
 instructor determine this for you) and adjust 
 the index. The difference between this read- 
 ing and the zero reading is the stretch of the 
 spring. Continue increasing the load on the 
 pan by 1 or 0.5 g. until ten trials have been 
 made and record the position of the index 
 for each load. Compute the stretch for each 
 trial. 
 
 Record in a tabulated form the results: 
 zero reading, position of index, load and 
 stretch. 
 
 1. What relation is there between the load 
 (stress) and the stretch (strain) ? 
 
 2. As directed, plot a graph to show the 
 relationship between the strain and stress. 
 
 If the graph is a straight line, it indicates 
 that one measurement is proportional to the 
 other. State Hooke’s Law as shown by the 
 graph. 
 
 3. Explain how you can weigh accurately 
 a small piece of any substance with a Jolly 
 balance and a single one gram weight. 
 
' 
 
 ! 
 
 
PROBLEM No. 21 
 
 To Find the Breaking Strength of Wires Composed of Different Materials. 
 
 Measure with a micrometer the diameter 
 of the copper wire in three places. After 
 determining its size with a standard wire 
 gauge, consult a wire table and compare the 
 two results. Fasten the proper length of 
 wire firmly to the tension balance and to the 
 crank shaft. Turn the crank handle slowly 
 until the wire breaks. Repeat this process 
 three times with each wire and using the 
 average values obtained, calculate the break- 
 ing strength in pounds for a wire made of 
 the same material, one square inch in cross 
 section. This number is called its tensile 
 strength and is useful to the engineer in 
 building bridges, etc. 
 
 Repeat the experiment using iron and 
 aluminum wire. 
 
 1. How many lbs. will it take to break a 
 No. 10 copper wire? 
 
 2. How many lbs. will it take to break a 
 No. 10 aluminum wire? 
 
 3. What relation exists between breaking 
 strength and area of cross section? 
 
. 
 
 • • ' 
 
 ■ ■■ 
 
 . 
 
 . 
 
 , 
 
 ■ 
 
 ... .. . 
 
 
PROBLEM No. 22 
 
 To Test the Freezing and Boiling Points of Water on a Centigrade Thermometer. 
 
 To test the freezing point, pack the bulb of , 
 the thermometer furnished in clean snow or 
 fme ice, with the zero mark far enough 
 above the snow for you to see it plainly. 
 
 When the mercury has sunk to 
 within one degree, begin to take 
 readings once a minute and con- 
 tinue until three successive read- 
 ings are the same to a tenth of a 
 degree. Record the last reading 
 as the freezing point, using a 
 small hand lens, if convenient, to 
 read to tenths of a degree. The 
 temperature of melting ice is 
 practically constant and is de- 
 noted by zero on the Centigrade 
 scale. Find the error in your 
 thermometer in degrees and 
 tenths of degrees. 
 
 To test the Boiling Point: 
 
 For purposes of testing the 
 boiling point, the thermometer is exposed to 
 the steam from boiling water. Suspend the 
 thermometer within the inner tube of the 
 hypsometer, passing the stem through a 
 cork at the top. If the cork fits the stem 
 loosely, slip a rubber band over the ther- 
 mometer just above the cork. The 100 de- 
 gree mark should project only one or two de- 
 grees above the cork at the top, so that as 
 much of the stem as possible is exposed to 
 the steam. Fill the boiler of the hypsometer 
 about a quarter full of water and heat it to 
 boiling over a suitable burner. After the 
 steam has escaped freely for several minutes, 
 read the thermometer to tenths of degrees 
 as before. 
 
 The boiling point should not be 100 degrees 
 because the pressure in the room is not 76 
 cm., so that you are not yet in a position to 
 judge the correctness of your thermometer 
 on this point. It has been found that the 
 boiling point is lowered .375 degrees by a fall 
 of one centimeter in the barometric reading. 
 
 Read the barometer and compute the correct 
 boiling point by subtracting from 100 de- 
 grees .375 degrees centigrade for each cm. 
 the barometer is less than 76 cm. [or by 
 
formula, true boiling point equals 100 — .0375 
 (760 Bar. reading)]. 
 
 Find the error in the boiling point recorded 
 on your thermometer, considering your work 
 to be accurate. 
 
 Record your results as follows : 
 
 Height of barometer 
 
 Number of thermometer 
 
 Thermometer reading in melting ice 
 
 Error of freezing point 
 
 Thermometer reading in steam 
 
 Correct temperature of steam 
 
 Error of boiling point 
 
 1. Draw a graph according to sample. 
 
 1 /PU r /> w 
 
 2. Would your thermometer give a correct 
 reading of the room temperature? Give 
 reasons for your answer. 
 
PROBLEM No. 23 
 
 To Determine the Effect of Pressure on the Boiling Point of a Liquid. 
 
 METHOD A 
 
 Set up the apparatus ac- 
 cording - to the diagram. Ob- 
 serve the temperature of the 
 boiling point when the mer- 
 cury levels in the manometer 
 are equal. Then allow the 
 steam to pass through the 
 tube and very gradually close 
 the pinchcock until the mer- 
 cury in the open arm of the 
 manometer is 3 or 4 cm. high- 
 er than in the closed arm. 
 
 This shows the pressure in 
 the boiler to be that many cm. above atmos- 
 pheric pressure. Record the exact number 
 of cm. and the reading of the thermometer. 
 
 The difference between the thermometer 
 reading now and the reading when the pres- 
 sure was room pressure is that caused by the 
 increase of pressure indicated by the man- 
 ometer. Find the difference caused by one 
 cm. and the per cent of error from .375 de- 
 grees, the accepted value. Tabulate your 
 results. 
 
 METHOD B. 
 
 Set up the apparatus according to the dia- 
 gram. Fill the flask one half full of water. 
 
 zl 
 
 Insert the thermometers and glass tube 
 through the holes in the stopper. Place the 
 stopper on the neck of the flask. Heat the 
 water until it boils freely. Notice the tem- 
 perature of the steam and then of the water. 
 Next remove the burner, place the free end 
 of the glass tube in the jar of water, and 
 press down slightly on the stopper. Notice 
 the temperature of the water in the flask. 
 
 
 L 
 
 Hi 
 
 rrf 
 
 y u 
 
Observe as closely as possible at how low a 
 temperature the water continues to boil. 
 Tabulate your results. 
 
 1. Why is the temperature of the steam 
 different from that of the boiling water? 
 
 2. While the water is heating, a slight 
 amount of air is leaving from the free end of 
 the glass tube. Why? 
 
 3. After the burner is removed and the 
 free end of the glass tube placed in the jar 
 of water, why does the water go up the tube 
 into the flask? 
 
 4. Why does the water boil when cold 
 
 water enters the flask? 
 
 5. State the effect of pressure on the boil- 
 ing point. 
 
PROBLEM No. 24 
 
 To Find the Coefficient of Linear Expansion of Some Metals. 
 
 The coefficient of linear expansion is the 
 amount that one centimeter of a rod expands 
 when heated one degree Centigrade, or (it is 
 the fraction of its length that a rod expands 
 when heated one degree Centrigrade) . 
 
 To find experimentally the coefficient of 
 linear expansion, say of steel, we have to take 
 a rod or tube of considerable length, heat it 
 through a certain number of degrees and 
 then measure how much it expands. To il- 
 lustrate, suppose that an iron tube whose 
 length is 60 cm. at a room temperature of 
 20 degrees C. is heated to 100 degrees C. and 
 as a result expands .0576 cm. To find how 
 much 1 cm. expanded for one degree, we 
 must take 1/60 times 1/80 of .0576 cm. 
 
 Working this out, it is found to be .000012, 
 which is the coefficient of linear expansion of 
 iron. 
 
 In the example given above, it was stated 
 that a rod 60 cm. long expanded .0576 cm. 
 when heated 80 degrees. This expansion is 
 so small that it cannot be measured directly 
 with sufficient accuracy. It is, therefore, 
 necessary to use some method that will give 
 the required degree of precision. There are 
 several methods that are in common use for 
 this purpose. The one that we shall employ 
 is applied in what is known as a Cowen linear 
 expansion apparatus. The construction and 
 working of the apparatus must be learned 
 from a study of the model that is set up for 
 that purpose in the laboratory. When you 
 understand the apparatus, proceed to per- 
 form the experiment. Take the measure- 
 ments and observations indicated in the ac- 
 companying table and make the required cal- 
 culations for the metal rods furnished by the 
 instructor. 
 
 Name of metal 
 
 1 2 3 Av. 
 
 Temperature 
 
 of room 
 
 Temperature 
 
 of steam 
 
 Rise of 
 
 temperature 
 
Number of degrees pointer turned 
 
 Diameter of dial axis. .'. . . .mm cm. 
 
 Circumference of dial axis cm. 
 
 Part of complete revolution made by dial 
 
 axis 
 
 Total expansion (indicate your work) 
 
 Length of tube between supports 
 
 Statement: cm. length of 
 
 tube has expanded cm. for a change 
 
 in temperature of degrees C. 
 
 Expansion for one cm. (equation) 
 
 Expansion for one cm. for one degree (equa- 
 tion) 
 
 The coefficient of linear expansion of 
 
 (metal) is 
 
 Problem: A surveyor’s steel tape is 100 
 feet long. Assuming the lowest temperature 
 in winter to be 40 degrees below zero C. and 
 the warmest in summer 40 degrees above 
 zero C., what is the maximum error per foot? 
 Express the amoun tof this error in per cent. 
 (Consider it correct at 0 degrees C.) 
 
PROBLEM No. 25 
 
 To Study the Law of Heat Exchange by the Method of Mixtures. 
 
 A. The law of heat exchange states that 
 when two substances at different tempera- 
 tures are mixed, the number of heat units 
 lost by one is equal to the number of heat 
 units gained by the other. The heat unit 
 used in the metric system is the calorie and 
 is defined as the amount of heat required to 
 raise one gram 1 degree C. In the English 
 system, it is known as the British Thermal 
 Unit (B.T.U.) and is the amount of heat nec- 
 essaary to raise one lb. of water 1 degree F. 
 
 Weigh a dry calorimeter and place about 
 200 cc. of water into it. After accurately 
 weighing the two, heat the water to 50 de- 
 grees C. Into another dish measure about 
 200 cc. of water whose temperature is about 
 10 degrees C. Now carefully take the tem- 
 peratures of the water in each receptacle 
 with separate thermometers and quickly 
 pour the cold into the hot water. Stir the 
 mixture with both thermomters with their 
 bulbs held together and take its temperature 
 near the top of the water and near the bot- 
 tom. If the readings differ, stir again and 
 then take the highest uniform temperature. 
 
 When you take the thermometers out of the 
 calorimeter, touch the bulbs to its side, that 
 the adhering water may be removed. Again 
 weigh the calorimeter with its contents. 
 
 From this data, calculate the following and 
 record your results in tabular form : 
 
 A B 
 
 1. Weight of dry 
 
 calorimeter 
 
 2. Weight of calorimeter 
 
 and warm water 
 
 3. Weight of warm 
 
 water 
 
 4. Weight of calorimeter 
 
 warm and cold water 
 
 5. Weight of cold water 
 
 6. Temperature of warm 
 
 water 
 
 7. Temperature of cold 
 
 water 
 
 8. Temperature of the 
 
 mixture 
 
 9. Calories of heat lost 
 
 by warm water 
 
10. Calories of heat lost 
 
 by the calorimeter 
 
 11. Total number of cal- 
 ories lost 
 
 12. Calories (calculated) 
 
 gained by cold water . . . : 
 
 13. Difference between 
 
 the last two items 
 
 If your results are not reasonably close, 
 repeat the experiment. 
 
 1. Define a calorie. 
 
 2. Is the resulting temperature an aver- 
 age of the two? If it is not, how do you ex- 
 plain the difference? 
 
 B. Repeat the experiment, using about 
 100 cc. of warm water and about 250 cc. of 
 cold water. 
 
 3. Since your results show that some heat 
 has been lost, could the calorimeter, the 
 thermometers or the air, account for the heat 
 wasted? Explain. 
 
PROBLEM No. 26 
 
 To Determine the Specific Heat of a Solid. 
 
 To raise the temperature of 1 gram of 
 water 1 degree C., requires a unit quantity 
 of heat called the calorie. It is shown by 
 experiment that this quantity of heat will 
 raise the temperature of 1 gram of almost 
 any substance more than 1 degree C. The 
 specific heat of any substance is the number 
 of calories necessary to raise the tempera- 
 ture of 1 gram of that substance through 1 
 degree C. 
 
 Suspend a metal coil in boiling water until 
 it has acquired the temperature of the water; 
 determine the temperature of the water by 
 means of a thermometer. Weigh a calori- 
 meter, fill it about two-thirds full of cold 
 water and weigh again. Determine the mass 
 of the water and also find its temperature. 
 
 The temperature of the water should be low- 
 er than that of the room. Remove the 
 metal quickly from the boiling water and 
 suspend it in the cold water so that it will 
 not touch the sides or the bottom of the 
 calorimeter. Stir the water with the ther- 
 mometer until its temperature ceases to rise, 
 and record the temperature of the water. 
 
 The mass of the cold water times its rise, 
 would be the number of calories imparted to 
 it and hence lost by the metal. From the re- 
 sults determine the specific heat of the metal, 
 which is the number of calories required to 
 raise the temperature of one gram of the 
 metal one degree. 
 
 Tabulations : 
 
 Kind of metal 
 
 Weight of calorimeter 
 
 Water equivalent (mass times sp. lit.) 
 
 Weight of water 
 
 Temperature of water at beginning 
 
 Mass of metal 
 
 Temperature of heated metal 
 
 Final temperature of mixture 
 
 Amount of heat exchanged (Indicate 
 
 your work) 
 
 Portion of the heat exchanged by one 
 gram of the metal changing one 
 
 degree C 
 
 Specific heat of the metal used 
 
 Question : Did you actually find the 
 amount of heat required to raise 1 gram 1 de- 
 gree C. or the amount given off by 1 gram in 
 cooling 1 degree C. ? 
 
' 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 ■ 
 
 ' 
 
 ■ 
 
 
 
 
PROBLEM No. 27 
 
 To Find the Dew Point of the Air in the Laboratory and to Determine Its Relative Hu- 
 midity. 
 
 The dew point is defined as the tempera- 
 ture to which the air must be cooled so that 
 condensation of water vapor may occur. This 
 temperature depends upon the relative 
 amount of water vapor in the air at that 
 time. 
 
 Method: (a) To find the dew point, put 
 some water in a calorimeter to about an inch 
 in depth. Have on hand a tumbler of water 
 and some finely crushed ice, or snow. Be 
 careful not to breathe on the bright surface 
 of the calorimeter, as the warm, moist breath 
 will produce an error in your results. Add 
 ice to the calorimeter, a very little at a time, 
 stirring constantly with the thermometer. 
 
 Watch closely for the first thin film of moist- 
 ure neay the bottom of the can and when it 
 does appear take the temperature of the 
 water. Wipe the dew off with a cloth or 
 your finger and observe whether the deposit 
 quickly gathers again. If it does, add a 
 small amount of warm water and find the 
 highest temperature at which the dew will 
 form. Take their average as the dew point 
 of the air in the laboratory at the time of the 
 experiment. Make three trials and record 
 each result obtained. 
 
 To determine the relative humidity, use is 
 made of the ratio of the pressure exerted by 
 the water vapor in the air at that time to the 
 pressure of the vapor were the air saturated 
 with it. To find the various pressures ex- 
 erted, turn to a Table of Constants of Satu- 
 rated Water Vapor, as will be found, for ex- 
 ample, on Page 171, Millikan and Gale. Sup- 
 pose the dew point were found to be 14 de- 
 grees C. when the room temperature was 26 
 degrees C. This indicates that the amount 
 of moisture in the air at 26 degrees C. would 
 saturate it at 14 degrees C. From the col- 
 umn P in the table, we find that at 14 degrees 
 C., the vapor exerts a pressure of 11.9 milli- 
 meters, and that had the air been saturated 
 at 26 degrees C., the vapor pressure would 
 have been 25 mm. Under these supposed 
 conditions, the air would have contained 11.9 
 divided by 25 or .476, the amount of moisture 
 that it would hold. So we say, the relative 
 humidity is 47.6%. In a like manner, calcu- 
 A late your result. 
 
Method: (b) Another means of obtaining 
 
 these results is by using the hygrodeik, which 
 employs wet and dry bulb thermometers, 
 thus applying the principle of cooling by 
 vaporization. If this principle is not clear to 
 you, review it in your text. 
 
 Fan the wet bulb until the reading be- 
 comes stationary. Observe the readings of 
 the wet and dry bulbs. Set the sliding point- | 
 er at the line on the wet bulb side of the 
 chart, corresponding with the degree reading 
 of the wet bulb tube; swing the arm to the 
 right, to the point of intersection with the 
 red line curving from the dry bulb side and 
 corresponding to the degree reading of the 
 dry bulb tube. At this intersection, the in- 
 dex hand will point to the relative humidity 
 on the scale at the bottom of the chart. 
 
 To find the dew point, observe the intersec- 
 tion as above, and follow the heavy black 
 line passing through it, which runs from the 
 top downward to the right to the point of 
 contact with the dry bulb scale. Compare 
 these results with those obtained in method 
 (a). 
 
 1. Of what practical use is the determina- 
 tion of the dew point? 
 
 2. How would you determine the relative 
 humidity out-of-doors on a very cold day ? 
 
PROBLEM No. 28 
 
 To Discover the Latent Heat of Fusion. 
 
 In general, when heat is applied to a solid, 
 its temperature rises until it reaches a point 
 where it begins to pass into the liquid form. 
 
 Any further supply of heat fails to produce 
 any rise in temperature while the melting is 
 in progress; the heat used goes to melt the 
 solid. The heat absorbed by the solid is 
 enrgy converted into the potential form in 
 the work of giving mobility to the molecules 
 and is said to become latent or hidden. As 
 soon as the solid is melted, a continued ap- 
 plication of heat causes the temperature to 
 rise. Converselv, when the temperature 
 falls, a stationary point is reached where the 
 solidification sets in and the heat rendered 
 latent on melting is set free again. Under 
 the same conditions of pressure, the two sta- 
 tionary temperatures, that of melting and 
 that of solidification coincide. The quantity 
 of heat required to convert one gram of a 
 substance from the solid to liquid is called 
 latent heat of fusion. 
 
 To determine the latent heat of fusion of 
 ice or snow: 
 
 Dry and weigh the calorimeter. Heat a 
 quantity of water to about 35 degrees in the 
 beaker. While the water is heating, prepare 
 pieces of clean ice and place them upon a 
 piece of cloth to absorb the water formed in 
 melting. Pour the water into the calori- 
 meter to within about three cm. of the top 
 and weigh to find the mass of water taken. 
 
 Stir the water with the thermometer and 
 when the temperature is about 30 degrees, 
 begin to add dry pieces of ice. 
 
 Take the temperature of the water at the 
 very instant before the first piece of ice is 
 dropped in. Add ice until the temperature 
 when all the ice is melted falls to about 10 
 degrees. Take the reading of the ther- 
 mometer, the very instant that the ice be- 
 comes all melted and weigh the calorimeter 
 and contents to find the mass of ice added. 
 
 By means of the data obtained, answer the 
 following questions : 
 
 1. How much heat has the warm water 
 given out in cooling? 
 
 2. What is the rise in temperature of the 
 melted ice? 
 
3. How much heat has the calorimeter giv- 
 en out in cooling? 
 
 Part of the heat has caused the ice to melt 
 and part has raised the temperature of the 
 melted ice. 
 
 4. How much of the heat given out by the 
 calorimeter and by the water has gone to 
 raise the temperature of the melted ice? 
 
 5. How much heat has been used in caus- 
 ing the ice to melt without change in tem- 
 perature ? 
 
 6. How much ice was used? 
 
 7. How much heat was needed to melt one 
 gram of ice without changing the tempera- 
 ture ? 
 
 8. What is the latent heat of fusion of ice ? 
 
 The results may be tabulated as follows: 
 
 Mass of calorimeter, Water equivalent of 
 calorimeter, Mass of water taken, Mass of ice 
 added, Initial temperature of the water and 
 calorimeter, Final temperature after mixing, 
 Latent heat of fusion of ice, Percentage of 
 error. 
 
PROBLEM No. 29 
 
 To Determine the Heat of Vaporization of Water. 
 
 The amount of heat required to vaporize 
 one gram of a substance without changing 
 its temperature is called the heat of vapor- 
 ization of that substance. 
 
 Generate the steam the same way you have 
 before and place a trap between the steam 
 supply and the calorimeter to guard against 
 the introduction of hot water instead of 
 steam. (1) How should the trap be used to 
 prevent condensed steam from entering the 
 water? Weigh the calorimeter and put into 
 it about 450 grams of cold water at about 10 
 degrees C. Take the temperature of the 
 water. Place a screen around the calorimet- 
 er to prevent the loss of heat. When the 
 steam is given off freely from the boiler, in- 
 troduce sufficient steam to raise the temper- 
 ature of the water to about 30 degrees centi- 
 grade, at the same time constantly stirring 
 the water with the thermometer. Before 
 turning off the burner, remove the calori- 
 meter, stir and take the final temperature. 
 Again weigh to find the amount of steam 
 condensed in the water. 
 
 A. The number of calories of heat taken 
 up by the cold water is evidently the weight 
 of the water times its rise in temperature. 
 (To the amount of water must be added the 
 water equivalent of the calorimeter.) Part 
 of this heat was given up by the steam in con- 
 densing and part by the condensed steam in 
 being lowered to the final temperature. 
 
 B. The number of calories of heat given 
 up by the steam after it has condensed is 
 equal to the weight of the steam multiplied 
 by its fall in temperature. 
 
 Subtracting B from A, we have the amount 
 of heat given up by the steam in condensing. 
 This divided by the weight of the steam 
 gives the amount of heat given up by one 
 gram of steam in condensing. 
 
 Tabulations: 
 
 1. Temperature of cold water 
 
 2. Temperature of steam 
 
 3. Temperature of mixture 
 
 4. Weight of cold water 
 
 (A) Amount of heat taken up by the water 
 
 and calorimeter 
 
 5. Weight of steam 
 
 (B) Amount of heat given up to the water 
 
 by condensed steam 
 
6. Amount of heat give up by the steam in 
 
 being condensed (A-B) 
 
 7. Amount of heat give up by one gram of 
 
 steam in being condensed 
 
 The result just obtained is, of course, the 
 same as the amount of heat required to 
 change one gram of boiling water into steam 
 without changing its temperature. This is 
 known as the Heat of Vaporzation of water. 
 
 2. Explain how a steam heating plant 
 operates. 
 
 3. Why are burns from steam more pain- 
 ful and injurious than those from boiling 
 water? 
 
 4. In hot, dry countries, water is cooled by 
 placing it in porous earthenware jars or can- 
 vas bags. Explain. 
 
PROBLEM No. 30 
 
 To Find the Image of a point in a Plane Mirror and to Determine the Relation Between 
 the Angle of Incidence and the Angle of Reflection. 
 
 In these experiments, use only a sharp 
 pointed pencil and draw all lines very fine. 
 
 Designate the path of all rays of light by us- 
 ing arrow heads. Make all construction lines 
 dotted and all lines representing rays con- 
 tinuous. 
 
 Draw a line across the middle of a sheet of 
 paper and stand the mirror with its reflect- 
 ing surface exactly on it. Stick a pin (mark 
 it O, Object) vertically about ten centimeters 
 in front of the middle of this mirror. At 
 least lb cm. on either side of the pin. place 
 the edge of a rule in line with the image of 
 the pin, sighting along the rule with one eye. 
 
 Without moving the rule, draw a fine line 
 along its edge in each of these positions. 
 
 (Should you have difficulty in accurately lo- 
 cating these sight lines by this method, place 
 the eye in the same plane as the table top 
 where a good image of the object may be 
 seen. Then place a pin near the mirror and 
 another one about 8 or 10 centimeters dis- 
 tance, so that the image of the pin, 0. and 
 these two sight pins appear to be in the same 
 straight line. Align these pins very care- 
 fully, looking at their base, and keep them 
 perpendicular to the board. Remove the 
 sight pins and through these holes draw 
 lines.) After removing the mirror, continue 
 the two sight lines till they meet back of the 
 mirror. This intersection locates the image 
 of the point represented by the pin. 
 
 Mark this point I (Image) . Draw OR cut- 
 ting the mirror line at A. Find the differ- 
 ence in length between OA and IA. Mark 
 the point where- the left sight line cuts the 
 mirror line, B, and where the right one cuts 
 it, C. Call these two lines BF and CH re- 
 spectively. 1. What do these lines repre- 
 sent? At C and B, erect perpendiculars to 
 the mirror line on its object side and mark 
 them CE and BD. Draw OB and OC. 
 
 2. What do these lines represent? OBD and 
 OCE are called angles of incidence and DBF 
 and ECU are called angles of reflection. With 
 a protractor measure the value of each angle. 
 
 Find the difference between the angles of in- 
 cidence and reflection for each observation. 
 
Record results in the following form: — 
 
 Length OA 
 
 Length IA 
 
 Difference 
 
 Angle ODD 
 
 Angle DBF 
 
 Difference 
 
 Angle OCE 
 
 Angle ECH 
 
 Difference 
 
 3. What does the intersection of the two 
 lines back of the mirror mark? Why? 
 
 4. What angle does the line 01 form with 
 the mirror? 
 
 5. Describe accurately the position of the 
 image. 
 
 6. Show by construction how long a wall 
 mirror must be in order to see the full image 
 of your body. 
 
 7. Design the path of the rays of light in 
 a tailor mirror of three parts showing the 
 image of your back. 
 
PROBLEM No. 31 
 
 To Verify the Formula Connecting the Position of an Object and Its Image in a Spher- 
 ical Mirror, and to Find the Radius of the Mirror. 
 
 The mirror formula: Let 0 be the lumin- 
 ous point on the principal axis of a concave 
 mirror, OM a ray from 0 meeting the mirror 
 at M. Join OM and draw MI making angle 
 CMO equal to CMI. Then the intersection I 
 of MI and NO is the conjugate focus of 0. 
 Since the angle is bisected by MC, by geom- 
 etry OM:MI: :OC:CI. 
 
 Let NF equal f, OM equal Do, MI equal Di. 
 If the aperture is small ON equals Do and IN 
 equals Di very nearly. OC equals Do minus 
 2f, IC equals 2f minus Di. Substituting 1 
 these values in the above proportion, we have 
 Do:Di:Do — 2f:2f — Di. Equating the prod- 
 ucts of the means and extremes 
 2Dof — DoDi=DoDi — 2Dif. 
 
 Dif+Dof=DoDi. Dividing by DoDif we have 
 I/Do+I/Di=I/f. 
 
 Support the concave mirror on a holder at 
 the end of the optical bench so that its prin- 
 cipal axis shall be parallel to the scale of the 
 bench. Attach the source of light to a 
 bracket sliding on the optical bench. Ar- 
 range so that the light shall be on a level 
 with the center of the mirror. On another 
 bracket, fix a small screen and adjust the 
 height of the screen so that it is also on a 
 level with the center of the mirror. 
 
 Place the light 100 cm. from the mirror, 
 and adjust the screen so that the image 
 formed on it may be as distinctly focussed as 
 possible. 1. Note whether the image is 
 erect or inverted, whether it is larger or 
 smaller than the object. 2. Is the image 
 real or virtual? Carefully measure the dis- 
 tance Do between the object and the mirror 
 and the distance Di between the image and 
 the mirror. Move the light nearer to the 
 mirror, again adjust the screen and measure 
 Do and Di. 
 
Proceed thus to find a set of corresponding 1 
 values of Do and Di, making five trials in all. 
 
 3. Note as the light is moved toward the mir- 
 ror, the direction in which the image moves. 
 
 4. Note also changes as to size, etc., of the 
 image. Find a position in which both object 
 and image are at the same distance from the 
 mirror. In this case, Do equals Di. 5. When 
 the object is at the center of curvature of the 
 mirror, what is the character of the image 
 formed? Verify the result by the parallax 
 method as follows: 
 
 Set up a pin so that its head is about op- 
 posite the middle of the mirror. Place the 
 eye so as to see both image and object and ; 
 move the head from side to side, observing 
 that the more remote object moves with the 
 head. Shift the pin until the pin and image 
 are at the same point. Measure the distance 
 to the mirror and compare with the value for 
 r (with that found by first method). 
 
 6. What results do you obtain? 
 
 Tabulate as follows: 
 
 Do Di f r 
 
PROBLEM No. 32 
 
 To Determine the Ratio of the Speed of Light in Air to Its Speed in Llass. 
 
 The index of refraction of glass is the ra- 
 tio of the sine of the angle of incidence in air 
 to the sine of the angle of refraction in glass. 
 
 Draw a straight line on a piece of paper 
 and place a plate glass with parallel edges 
 flat on the paper with one edge exactly along 
 this line. Place a pin at some point A, Fig. 
 (1), and another at a point B. With a ruler 
 sight through the glass from B to the image 
 of A and draw a line C on the paper along 
 the edge of the ruler. Be sure that your 
 pencil is sharp. A blunt pencil will spoil the 
 experiment. 
 
 Remove the glass plate and draw a line 
 BA, and a line MBN perpendicular to the 
 plate line at the point B. Draw a circle with 
 B as center and draw the lines GK and FTI 
 perpendicular to MBN. 
 
 The image of the pin A is seen in the glass 
 because light starting from A passes through 
 the glass to B and then through the air to 
 the eye at G. The image of A in the glass is 
 in a new position. The reason for this is 
 that the light which travels from A through 
 the glass to B is bent away from the per- 
 pendicular MBN when it enters the air at B. 
 THe light when in glass makes an angle r 
 with the perpendicular MBN and when in air 
 makes the larger angle i. To prevent con- 
 fusion, the angle i in air is always called the 
 angle of incidence and the angle r in the oth- 
 er medium (in this case, glass) is always 
 called the angle of refraction. The index of re- 
 
fraction of glass is sine i ^sine r. Sine i=GK 
 
 GB 
 
 and sine r=FH, but since GB=FB (radii of 
 FB 
 
 the same circle) sine i GK 
 
 ■=■ ^index of refrac- 
 
 sine r FH 
 
 tion. 
 
 Measure GK and FH carefully and calcu- 
 late the index of refraction of glass or the 
 ratio of the speed of light in air to its speed 
 in glass. 
 
 Tabulate your results as follows: 
 
 Length of GK— cm. 
 
 Length of FH= cm. 
 
 Index of refraction of g!ass= 
 
 Repeat the work for three trials. 
 
 1. Why does an oar appear bent when 
 placed in the water? Draw a diagram. 
 
 2. Why does the sun appear larger in the 
 morning and evening than at noon? 
 
 3. From your experimental results (aver- 
 age) find the speed of light in glass. 
 
PROBLEM No. 33 
 
 To Find the Focal Length of a Convex Lens by the Method of Conjugate Foci. 
 
 Set up the apparatus according to the 
 model, using a luminous gas flame or other 
 light source in front of a cardboard screen. 
 
 An L shaped opening may be cut in the card- 
 board to serve as the object. 
 
 Line up the L, the lens and the screen so 
 that their centers shall all be in a line paral- 
 lel to the optical bench. 
 
 Making the distance from the L to the 
 screen, say about 100 cms. for the first trial, 
 move the lens back and forth between the 
 cardboards until a sharp image of the L is 
 formed on the screen. Adjust carefully and 
 try to get the place where the image is most 
 distinct. 
 
 Measure the distance from the L to the op- 
 tical center of the lens and call this Do. 
 
 The distance between the lens and the 
 image is Di. 
 
 Calculate the value of focal length (f) 
 from the formula 111 
 
 Do Di f 
 
 from which f=DoXDi 
 
 Do+Di 
 
 1. Describe the image as regards size, j 
 kind, position, etc. 
 
 Without changing the relative position of 
 the two screens, move the lens away from the 
 L until you secure another distinct image on 
 the screen. Measure object and image dis- 
 tance as before. 
 
 2. How does Do in trial 2 compare with 
 Di in trial 1 ? Explain. 
 
 Change the distance between the two 
 screens by 10 cms. and repeat the two cases. 
 Make a third trial at another distance. 
 
 Tabulate all results in a neat form. 
 
 The case in which the image is larger than 
 the object illustrates the projection of pic- 
 tures by a magic lantern, stereopticon or 
 moving picture machine. 
 
 The case in which the image is smaller 
 than the object illustrates the formation of 
 an image in the camera. 
 
 3. What is the advantage of an L shaped 
 cut? 
 
 4. Diagram the image formed by T shaped 
 cut. 
 

 
 ■ . 
 
 
 
 
 
 
 
 
 ■ 
 
 
 
 
 * 
 
 . 
 
 
 
 
 
 
PROBLEM No. 34 
 
 To Find the Images Formed by a Converging Lens, When the Lens Is at Different Dis 
 
 tanccs From the Object. 
 
 Make a drawing showing lens, screen and 
 image in each case. Use your optical bench 
 with a source of light at one end and direct- 
 ly in front of it a screen with an L-shaped 
 window cut in it to serve as the object. 
 
 (A) Now set your lens at its focal length 
 (known from previous Experiment) from the 
 illuminated screen. The object is now at the 
 principal focus of the lens. Move the opaque 
 screen on the other side of the lens and see 
 whether an image is formed on the screen. 
 
 The formation of an image means that the 
 rays of light leaving the lens converge. If 
 an image is not formed, the rays leaving the 
 lens are either parallel or divergent. (1) 
 
 When the object is at the principal focus, 
 what is the direction of the rays of light 
 leaving the lens? 
 
 (B) Move the lens nearer the object. 
 
 The object is now within the principle focus. 
 
 Move the screen to find out whether an 
 image is formed. Look through the lens 
 and describe its appearance. (2) In this 
 case, what do you think is the direction of 
 the rays leaving the lens ? Explain by means 
 of a diagram. 
 
 (C) Place the lens so that the object is 
 at twice the focal length. Place the screen 
 at an equal distance on the other side of the 
 lens. (3) Is the image on the screen erect 
 or inverted? (4) Is it larger or smaller 
 than object? (5) Compare the relative dis- 
 tances from the lens of object and image. 
 
 (6) At what distance from a camera lens 
 would you place a drawing in order to ob- 
 tain a photographic copy of the same size? 
 
 (D) Move the lens in a little toward the 
 object, so that it is at a distance greater 
 than the focal length but less than twice the 
 focal length. Move the screen till a sharp 
 image is formed. Now measure! the dis- 
 tance between the lens and the image. Also 
 measure the object distance. (7) Compare 
 the image distance with twice the focal 
 length. (8) Note the relative size of ob- 
 ject and image. 
 
(E) Move the lens to a point whose dis- 
 tance from the object is equal to the image 
 distance obtained in (D). The object dis- 
 tance is now greater than twice the focal 
 length. Move the screen till a sharp image 
 is formed. (9) Note relative size of object j 
 and image. Measure the object distance 
 and image distance as in (D). (10) com- 
 
 pare the image distance in this case with the 
 focal length and twice the focal length. 
 
 (11) State the two cases of conjugate foci 
 shown in these experiments. 
 
 Tabulate your focal length and distance 
 for each of three trials. Then average re- 
 sults. 
 
 (12) Where will the screen for a stereop- 
 ticon be located with reference to the focal 
 length of the objective lens? 
 
 (13) Where will the lantern slide be lo- 
 cated ? 
 
 (14) What is the least distance from a 
 cgn verging lens at which an object can be 
 placed in order that a real image may be 
 formed ? 
 
 (15) State a general relation between the 
 size of the object and image and their re- 
 spective distances from the lens. 
 
PROBLEM No. 35 
 
 To Construct an Astronomical Telescope and Find Its Magnifying Power. 
 
 1. Set up a convex lens of fairly long 
 focal length across the room from the win- 
 dow. Adjust a screen until you get a clear 
 real image of the wire netting on the win- 
 dow. Set up another convex lens of small- 
 er focal length in. a line with the first, the 
 screen on which the first image was formed 
 being at the focal length of the second one. 
 Remove the screen and look thru both of 
 the lenses at the wire netting. If the work 
 is done correctly, the netting will be seen 
 clearly, enlarged and inverted. The first 
 lens, called the objective, forms a real in- 
 verted image at a distance approximately its 
 focal length. This image is smaller than 
 the object. The second lens, called the eye 
 piece, takes this image as an object and 
 forms a virtual, erect and enlarged image. 
 Even though the image formed by the ob- 
 jective is smaller than the object, the sec- 
 ond lens magnifies enough so as to produce 
 on the retina of the eye an image larger than 
 that obtained by the eye alone, looking at 
 the object without the aid of the two lenses. 
 Notice that the image formed by the objec- 
 tive is inverted, while that formed by the 
 eye piece is erect, hence the object as seen 
 through the telescope is inverted. As the 
 'astronomical telescope is used to view only 
 heavenly bodies, it is not important that an 
 erect image be obtained. 
 
 2. To find the magnifying power of the 
 telescope, draw two lines on the black-board, 
 four or five inches apart and set up the tel- 
 escope at as great a distance as possible so 
 that they may be clearly seen through the 
 telescope. Then looking at the lines, one eye 
 through the telescope, the other half shut di- 
 rectly at the board, have a fellow student in- 
 dicate by dots on the board the apparent posi- 
 tion of the lines as seen through the tele- 
 scope. In this work, be careful to have the 
 best focus with the eye which is looking 
 through the telescope. The magnifying pow- 
 er is the apparent distance between the lines 
 
as shown by the distance between the dots 
 divided by the real distance between the lines 
 themselves. Measure the distances and find 
 the ratio. The theoretical magnifying power 
 of an astronomical telescope is the focal 
 length of the objective divided by the focal 
 length of the eye piece. 
 
 Make a tabulation showing the focal 
 lengths, distance between lenses, real dis- 
 tance between lines and apparent distance 
 between lines, magnifying power as comput- 
 ed and theoretical magnifying power. 
 
 a. Draw a diagram of the astronomical 
 telescope, showing the lenses and position of 
 images formed by each. 
 
PROBLEM No. 36 
 
 To Construct an Erecting Telescope Using a Convex and Concave Lens. 
 
 A 
 
 Whenever an object is seen by the eye, the 
 lens of the eye forms a real, inverted, small 
 image of the object on the retina of the eye, 
 the retina serving as a screen. The nerve 
 endings of the optic nerve compose the reti- 
 na. The impression produced by the image 
 is transmitted by the optic nerve to the 
 brain. Even though the image on the retina 
 is inverted, the brain by some means makes 
 us see the object erect. In the case of the 
 erecting telescope, the convex lens, which is 
 used as an objective, forms a real image. 
 The concave lens, used as an eye piece, is 
 placed so as to diverge the rays which have 
 come from the objective sufficiently to have 
 the image produced by the objective formed 
 on the retina of the eye. The image is larg- 
 er than it would be without the telescope. 
 The effect of the eye piece is to neutralize 
 the crystalline lens of the eye. which is a 
 convex lens, the magnification being due to 
 the objective. 
 
 Set up a convex and a concave lens, using 
 the convex as the objective and the concave 
 as an eye piece and view some object at a 
 considerable distance, e.g.. the screens on 
 the windows from across the room. Have 
 the concave lens near the eye and move the 
 convex lens away from you until you obtain 
 a clear image. Estimate the magnification 
 produced. Draw a diagram showing the re- 
 lative position of the objective, eve piece and 
 the eye. and the position of object and image. 
 Record the estimated magnifying power. 
 
 1. Look at the object through the concave 
 lens alone. What kind of an image is form- 
 ed, real or virtual, inverted or erect, large or 
 small ? 
 

 
 
 
 
 - 
 
 
 . 
 
 . 
 
 ( 
 
 ' 
 
 
PROBLEM No. 37 
 
 To Find the Rate of Vibration of a Tuning Fork by the Siren Disc Method. 
 
 Use the motor rotator with the siren disc 
 attached and a tuning fork whose vibration 
 rate you wish to find. Rotate the disc and 
 by means of a rubber tube and a glass tube 
 drawn to a point, produce a musical tone by 
 blowing a jet of air across one of the rows 
 of holes on the disc. (For regulating the 
 speed of the motor, there is a rheostat built 
 into the base of the motor. By moving the 
 lever, the motor is made to rotate faster or 
 slower.) Sound the tuning fork by striking 
 with the rubber mallet given you and place 
 the stem of the fork on some wooden surface 
 so as to make the sound of the fork readily 
 heard. Change the speed of the motor until 
 the tone given by the jet blown across the 
 row of holes is the same as that of the fork. 
 
 This will be difficult to do, as the motor will 
 probably vary in speed. Make trials until 
 the tone seems constant and then, by means 
 of the speed indicator and a watch, count the 
 number of revolutions made by the rotator 
 in one minute. This will require consider- 
 able practice. One person may keep time, 
 giving the signals to begin and stop count- 
 ing, and the other may do the counting. 
 
 Count by tens. It is evident that the speed 
 indicator may not be registering on a ten 
 when the first signal is given, nor when the 
 second signal is given, so it will take a little 
 thought and skill to get a correct count. Hav- 
 ing the number of revolutions in one minute, 
 compute the number per second, and find the 
 vibration rate of the tone given, by multiply- 
 ing by the number of holes in the row. (The 
 tone is produced by the stopping of the air 
 current, thus causing condensations and 
 rarefactions to be sent through the air.) 
 
 The result of your readings and computations 
 is to be compared with the number on the 
 fork. 
 
 Make a tabulated record containing the 
 number of revolutions per minute, the num- 
 ber of revolutions per second, the number of 
 holes in the row, the computed number of 
 impulses per second causing the tone, the 
 number of vibrations per second as regis- 
 tered on the fork and the difference. 
 
 1. Which row of holes on the disc would 
 give the highest tone and why? 
 
 2. Do you think the number on the fork is 
 exactly correct? Give reasons. 
 

 
 
 ' 
 
 * 
 
 iJH <*f! ' 
 
 
 
 
PROBLEM No. 38 
 
 To Find the Vibration Rate of Any Vibrating Body by the Resonator Method. 
 
 The variable resonator used in this experi- 
 ment may consist of either a large glass tube 
 with a close fitting piston, or a cylindrical 
 jar or tube into which water may run. Strike 
 the body with a wooden or rubber mallet to 
 set it vibrating and place it at the opening of 
 the I'esonator, parallel to the reflecting sur- 
 face. While it is sounding, slowly draw the 
 & piston back, (or introduce water) till the 
 first position of maximum sound is reached. 
 Be sure that the true position of resonance 
 has been accurately located and mark it by 
 means of a small rubber band. Carefully 
 measure the length of this air column, which 
 is one-fourth of a wave length of the tone 
 given by the vibrating body, with a correc- 
 tion dependent on the diameter. It is found 
 that the size of the glass affects the length 
 of this air column in such a way that if two- 
 fifths of its diameter be added, the length of 
 the air column for a given vibrating body 
 will be the same for all sizes of resonators 
 used. Add this correction and obtain the 
 full wave length. Calculate the velocity of 
 sound at the room temperature by adding to 
 the velocity at zero degrees C. (331 3M) o.G 
 m. for each degree above that mark. With 
 these corrected values, calculate the number 
 of vibrations of the sounding body bv the 
 formula V=NL, or N=V/L. 
 
 Record your results as follows: 
 
 Temperature of room 
 
 Velocity of sound at this temperature 
 
 Number of vibrating body 
 
 Diameter of resonator tube 
 
 One-fourth wave length. Trial 1 
 
 Trial 2 Average 
 
 Full wave length. (Corrected) 
 
 Vibration rate 
 
 If the resonator is long enough, locate in 
 the same way a second position of resonance 
 with a longer air column. Mark this place 
 with a second rubber band. The distance be- 
 tween the two positions of maximum reso- 
 nance is one-half a wave length of the note 
 sent forth by the vibrating body. 1. Why 
 isn’t it necessary to make a diameter correc- 
 tion this time? Measure this distance and 
 from this data, obtain the full wave length. 
 Compare it with the value obtained above. 
 
Again using the formula N=V/L, compute 
 the vibration rate. Repeat the experiment 
 with different bodies and take two trials for 
 each one. 
 
 Compare the results obtained here with 
 those obtained in the first part of this ex- 
 periment. 
 
PROBLEM No. 39 
 
 To Find the Velocity of Sound in Air by the Resonance Method. 
 
 Loudest resonance is secured when the air 
 column is one- fourth of a wave length of the 
 tone given by the fork. Resonance is also 
 produced when the air column is three- 
 fourths of a wave length or any odd number 
 of fourths of a wave length of the tone given 
 by the fork. It is evident that for each 
 longer distance the resonance will not be so 
 good, for the sound in traveling the longer . 
 distance has lost some of its intensity and 
 will not, therefore, be so loud. If the best 
 resonance is secured with the shortest reso- 
 nating air column (one-fourth of a wave 
 length) and resonance again with the air 
 column three-fourths of a wave length, the 
 difference is one-half of a wave length, and 
 the diameter of the tube does not have to be 
 considered. 
 
 Run the piston forward into the resonator 
 tube until it is within three or four inches of 
 the open end. Set the tuning fork in vibra- 
 tion by bringing it down against the striking 
 board with moderate force and then hold it 
 opposite the open end of the tube. As the 
 fork vibrates, draw the piston slowly back 
 into the tube until the first point of resonance 
 is found. Move the piston back and forth 
 several times while the fork is vibrating, 
 till, judging by the loudness and clearness 
 of the sound, you are certain the correct posi- 
 tion for best resonance has been found. Place 
 one of the wire markers at this point on the 
 tube. 
 
 Since the next position of resonance will 
 be three-quarters of a wave length from the 
 mouth of the tube, draw the piston back 
 into the tube approximately three times the 
 length of the first resonance column. Then, 
 sounding the fork as before, move the piston 
 back and forth until the second point of best 
 resonance has been located as accurately as 
 possible. Set the second marker at this 
 place. (The sound produced at this second 
 position of resonance will not be as loud as at 
 the first position because in the second case 
 the fork has to set three times as much air 
 in vibration.) Measure the distance be- 
 tween the markers and record it as “trial 1.” 
 
 Next displace the markers and proceed to 
 
locate the same points of resonance two 
 times more. When this lias been done, find 
 the average of the three values thus obtain- 
 ed.' Repeat the experiment with as many 
 forks as you have time to use. Calculate the 
 value of 1 and record the number of vibra- 
 tions stamped on the fork. Letting V stand 
 for the velocity of sound at the temperature 
 of the experiment, we have 
 V= nl 
 
 Substitute the values of n and 1 in this and 
 find V. Since we cannot compare this with 
 the velocity at zero degrees, we must reduce 
 V to the velocity at zero. As the velocity 
 changes .6 m per degree change in tempera- 
 ture, we have 
 
 Vo=V — .6t 
 
 in which Vo stands for the velocity at zero 
 degrees Centigrade. Compare this result 
 with 331.3 m., the standard value for the 
 velocity of sound If your result is more 
 than 2 r /r off from the standard, do the ex- 
 periment over again. 
 
 Neatly tabulate the results for all trials. 
 Include in the tabulated record: (1) Half 
 a wave length, (2) Wave length, (3) Num- 
 ber on fork, (4) Velocity as computed from 
 experiment at room temperature, (5) Tem- 
 perature of room, (6) Computed velocity at 
 0 degrees and (7) Difference. 
 
PROBLEM No. 40 
 
 To Test the Law of Lengths of Vibrating Strings. 
 
 The law says that the vibration rate of a 
 string is inversely proportional to the length, 
 i.e., the longer the string, the lower the rate 
 of vibration in exact proportion. A string 
 two times as long vibrates one-half as fast; 
 a string three times as long vibrates one- 
 third as fast ; etc. 
 
 Use a sonometer and by means of the mov- 
 able bridge obtain the length of wire which, 
 when made to vibrate, will give the same 
 tone as the lowest of the eight forks of the 
 diatonic scale, C. Change the length of the 
 string after having recorded the measure- 
 ment and make two more trials for the same 
 fork. Average the lengths obtained. Re- 
 peat for three trials for the D fork and aver- 
 age as before. Use the proportion, 256 (C) 
 is to 288 (D) as the average of the lengths 
 for D is to the average of the lengths for C, 
 
 (inverse proportion) and find the product of 
 the means and the product of the extremes. 
 
 Their equality will determine the truth of 
 the law, considering your work to be abso- 
 lutely exact in every respect, and the num- 
 ber on the forks to be correct. If your prod- 
 ucts are cloqn enough, considering errors, to 
 warrant your drawing the conclusion that 
 the law holds in the experiment, you may 
 continue. 
 
 It is evident that if all the forks of the 
 diatonic scale be used in a similar manner, 
 the product of the means and extremes as 
 you use them is really the product of the vi- 
 bration rate in each case and the correspond- 
 ing length. (Did you not, in the first pro- 
 portion, multiply 288 by the length of the 
 string you obtained for that vibration rate, 
 and 256 by its corresponding leng-th?) If 
 you realize this, it will not be necessary to 
 write out the proportion in full each time. 
 
 Instead, you may merely multiply each vi- 
 bration rate (number on the fork) by the 
 length you obtain for that fork, and compare 
 those products. 
 
 Use all the forks of the scale E, F, G, A, 
 
 B, C, as indicated and record all measure- 
 
ments, etc., in a neat tabulated form, show- 
 ing all products of means and extremes. 
 
 1. What is a sonometer? Draw a figure. 
 
 2. Do you consider that your products are 
 near enough alike to prove the law in the ex- 
 periment as performed? 
 
 3. Is there any other way of testing a pro- 
 portion than equating the product of means 
 and extremes? 
 
 4. Why do small errors show up large in 
 equating the products of the means and ex- 
 tremes ? 
 
PROBLEM No. 41 
 
 To Test the Law of Tensions of Vibrating Strings. 
 
 The law says that the vibration rate of a 
 vibrating string is directly proportional to 
 the square root of the tension or stretching 
 weight, e.g., if the tension in one case is four 
 times as much as it is in another case, the 
 vibration rate is two times as much, or if 
 the tension is nine times as much, the vibra- 
 tion rate is three times as much. 
 
 Use a sonometer, and three sets of two 
 forks each, the vibration ratios of the forks 
 in each set being 2 to 3, e.g., C and G, F and 
 c, and G and d. Put a tension of four lbs. or 
 four kg. on the wire, and move the sliding 
 bridge until the vibration rate of the wire 
 when bowed or plucked is the same as that 
 of the lower fork in set number one, as near- 
 ly as you can judge. Make the trial very 
 carefully to be sure you have the tone as 
 nearly like that of the fork as you can make 
 it. Then increase the tension to 9 lbs. or 
 kg. (use the same weight unit as before) 
 and compare the tone produced by it when 
 vibrating with the second fork of set num- 
 ber one. 
 
 1. How does the vibration rate of the wire 
 under the second tension compare with that 
 of the second fork? 
 
 2. What is the ratio of the square roots of 
 the two tensions used? 
 
 Again put a tension of 4 units of weight 
 on the wire and move the bridge until the 
 wire vibrates with the rate of that of the 
 first fork in set number two, F. Letting 
 the bridge remain stationary, put on the wire 
 a tension of 9 units, and compare the vibra- 
 tion rate under this tension with that of the 
 second fork in the set, c. 
 
 3. How does it compare? 
 
 Repeat the process with the third set of 
 forks. 
 
 4. How does the vibration rate of the sec- 
 ond fork in set number three compare with 
 that of the wire under the second tension? 
 
 5. Do you feel that the results of the ex- 
 
 periment justify you in saying that the law 
 holds true under the conditions you have 
 taKen . f 
 
6. Use the law and solve: — If a wire un- 
 der 36 oz. tension vibrates 384 times a sec- 
 ond, how fast will it vibrate if the tension is 
 increased to 64 oz. ? Show the numbers sub- 
 stituted in the proportion. What note will 
 this be if 384 is G? 
 
PROBLEM No. 42 
 
 To Compare the Speed of Sound in Brass With That in Air 
 
 Use the apparatus furnished. See that 
 the rod is clamped at its middle point. Dis- 
 tribute the cork dust evenly along the tube 
 from one disc to the other. Now produce a 
 clear musical tone by rubbing the rod with the 
 cloth on which a little powdered rosin has 
 been sprinkled. A slight, steady pressure will 
 be found most effective. After rubbing once 
 or twice, adjust the movable disc, moving it 
 a very short distance, say two or three milli- 
 meters. Continue to excite the rod and ad- 
 just the length of the air column alternately, 
 moving the disc always in the same direction, 
 until the dust begins to be agitated. After 
 this, only a slight further adjustment will be 
 needed to produce fairly sharp nodes. The 
 final adjustment is most easily made by ro- 
 tating the tube a little on its supports, so that 
 the dust lying in the disturbed regions be- 
 tween the nodes will slide down, while the 
 nodes themselves will be marked by fairly 
 sharp peaks of dust projecting up the sides of 
 the tube. With a meter stick, determine the 
 length of the brass rod .and from this length 
 and a study of the diagram, determine the 
 half wave length of sound in brass. 
 
 Find the distance between the successive 
 nodes in the air column. Do not measure 
 single segments, but select two nodes as far 
 apart as possible and measure the distance 
 between these. Divide the distance by the 
 number of intervening segments and thus ob- 
 tain the average length of one segment. 
 
 (Repeat the readings until you have a set of 
 average readings.) 
 
 The ratio of the wave lengths in brass and 
 air is the ratio of their velocities. Compute 
 the velocity in brass from the velocity in air 
 at room temperature. 
 
 Tabulate all readings, computations and 
 results. 
 
 Questions : 
 
 1. Could you determine the speed of sound 
 in any substance that could be put in the 
 form of a rod? 
 
 2. Give all the steps in the process of rea- 
 
A\uvyj = 
 
 soning used in reaching your conclusion. 
 
 _vVAyf_ j, 
 
PROBLEM No. 43 
 
 To Map the Magnetic Fields of (1) a Bar Magnet, (2) a Horse-shoe Magnet, (3) Two 
 Magnetic Poles of Unlike Kind, (4) Two Magnetic Poles of Like Kind, (5) Two Un- 
 | like Poles With a “Keeper” Placed Mid-way Between Them. 
 
 Directions: 
 
 1. Use a compass and locate the poles of a 
 bar magnet. Place a sheet of paper above 
 a bar magnet and sift iron filings over it, 
 tapping the paper gently to facilitate the ar- 
 rangement of the filings. Draw a figure to 
 show the arrangement. Always place ar- 
 rows on drawings to show the direction of 
 the lines of force. 
 
 2. Use a horse-shoe magnet similarly. 
 
 3. Place the ends of] two bar magnets' 
 about an inch apart so that opposite poles 
 face each other, and proceed as before. 
 
 4. Arrange two bar magnets as in 3, but 
 with like poles facing each other. 
 
 5. Place a piece of soft iron between the 
 unlike poles as in 3, but not touching either 
 pole and note the arrangement of the filings, 
 making a drawing as before. 
 
 Questions : 
 
 1. Which cases indicate attraction and 
 which repulsion? 
 
 2. What is a compass? Explain its ac- 
 tion. 
 
 3. What is declination? 
 

 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ' 
 
 ■ 
 
 
 
 
 
 
 
 
 
PROBLEM No. 44 
 
 To Determine How Bodies May Be Magnetized and Study the Theory of Magnetism. 
 
 Directions: 
 
 a. Stroke a knitting needle or other thin 
 piece of steel with a bar magnet, being sure 
 to stroke from the center of the needle to- ! 
 ward one end with the N. pole of the magnet 
 and toward the other end with the S. pole. 
 
 Test the polarity of the needle and state how 
 it became a magnet-. 
 
 b. After testing carefully the polarity of 
 the magnetized knitting needle, cut it in two 
 nearly equal pieces with a wire cutter and 
 test the polarity of each half. Cut each half 
 into smaller pieces, testing the polarity each 
 time. 1. How long might this process be 
 continued? 2. What may be considered the 
 units of which a bar magnet is composed? 
 
 c. Bring the N. pole of a bar magnet near 
 the head of a small nail or brad but not in 
 contact with it. (Place a small piece of pa- 
 per or glass between the magnet and nail to 
 keep them apart if necessary.) 3. Does the 
 magnet attract the nail? Remove the mag- 
 net. Hold the point of the first nail near the 
 head of a second nail. 4. Does it attract the 
 second nail? Now bring the N. pole of the 
 bar magnet near the head of the first nail. 
 
 5. Does the first nail receive the power to at- 
 tract the second? 6. By what method were 
 the nails magnetized? Determine and mark 
 the polarity of the ends of the two nails. 
 
 d. Take an ordinary soft iron rod, about 3 
 inches long, wind ordinary insulated copper 
 wire about ten times around it and pass an 
 electric current through the wire. Test the 
 ends of the rod for polarity while the current 
 is passing through the wire. 7. Is the rod 
 magnetized? By what means? Repeat the 
 experiment with about twenty turns of wire. 
 
 8. What difference do you observe? 
 
 e. Hold a demagnetized rod in a north and 
 south plane with the north end dipping 
 downward at an angle of about 70 degrees 
 with the horizontal. With the rod held in 
 this position, strike it near each end several 
 smart blows with a hammer. Test the pol- 
 arity of the rod. Reverse the rod, tap again 
 and test for its polarity. Now hold the rod 
 
in a horizontal position with its ends point- 
 ing east and west and strike again. Test 
 its polarity and state clearly the results in 
 each case. 9. How was the rod magnetized 
 in this case? 10. State the theory of mag- 
 netism. 
 
PROBLEM No. 45 
 
 To Determine the Distribution of Magnetism in a Bar Magnet. 
 
 The working principle of this experiment 
 is to measure the amount of force (express- 
 ed in centimeters stretch of the spring) nec- 
 essary to pull a small piece of soft iron away 
 from the magnet at different points along its 
 length. In order to do this, suspend a small 
 piece of soft iron from the balance spring in 
 place of the scale pans. Make such neces- 
 sary adjustments that the scale is balanced 
 so the index swings free. When in this posi- 
 tion, take the reading at the middle or point- 
 er as the zero reading, and record it. Insert 
 the bar magnet into the holder so that each 
 end is the same distance from the zero or 
 center scale of the slide. Next, place the 
 holder on the shelf. Adjust the holder so 
 that the soft iron piece suspended from the 
 spring will drop into the hole on the holder 
 when the spring is lowered. Finally, the 
 shelf must be pushed up to a height so that 
 the soft iron just touches the magnet. 
 
 Starting with the N. pole of the magnet, 
 make the necessary adjustments so that 
 when the test piece is lowered, it will make 
 contact with the magnet as near the end as 
 possible. Then either lower the shelf or raise 
 the spring as directed until the test piece is 
 drawn away from the magnet. The differ- 
 ence between this reading and the zero read- 
 ing is the stretch. Make three trials and 
 average. In a like manner, take three trial 
 readings for points one centimeter apart on 
 the magnet. .If one of the trial readings 
 should differ too much from the other two, 
 discard it and take another reading to obtain 
 a value more nearly in ag'reement with the 
 other two. 
 
 Plot a graph to show the relation between 
 the distances on the magnet and the stretch 
 of the spring, that is, to represent the dis- 
 tribution of the magnetism in the bar. To 
 do this, lay off on the bottom of the cross 
 section paper, spaces to represent the total 
 lengh of the bar. On the left hand side of 
 the sheet, lay off spaces to represent the cen- 
 timeters of stretch of the spring. Begin 
 with 0 at the bottom and number towards 
 the top, choosing a unit that will bring the 
 
largest stretch near the top of the sheet. 
 Locate all reference points and then draw a 
 smooth curve taking the general direction of 
 the dots. 
 
 (1) How do you find the magnetism to be 
 distributed in the bar? 
 
PROBLEM No. 46 
 
 To Study the Nature of Electric Charges by Means of an Electroscope. 
 
 Rub a piece of ebonite with flannel. Stroke 
 it with a proof plane and apply directly to 
 the knob of an electroscope. The electro- 
 scope is now charged negatively. 1. What 
 happened to the leaves? 
 
 Discharge the electroscope by touching it 
 with the finger and then recharge it positive- 
 ly from the glass rod rubbed with silk. 2. 
 
 Why do the leaves behave the same as when 
 charged negatively? While the electroscope 
 is still charged, touch the knob with a dis- 
 charged ebonite rod, a meter stick, and fin- 
 ally your finger. 3. Which is the better 
 conductor ? 
 
 An electroscope may be charged also by 
 induction. To do this, charge a glass rod by 
 rubbing it with silk and by bringing it near 
 the knob of the electroscope, but not in con- 
 tact with it, till the leaves diverge about one 
 inch. There is now an induced negative 
 charge on the knob and an induced positive 
 charge on the leaves. While still holding the 
 glass rod there, touch the knob with the fin- 
 ger. 4. What happens to the leaves? 5. 
 
 What charge must have passed off through 
 the finger? Remove the finger first and 
 then the rod. The negative charge that now 
 spreads to the leaves was “bound” by the in- 
 ducing glass rod and could not get away even 
 when “grounded” by the finger. 
 
 To prove that the elecroscope is now 
 charged negatively, rub an ebonite rod with 
 flannel and bring near the knob. The great- 
 er divergence of the leaves shows that the 
 leaves and the rod are alike, for like charges 
 repel. 6. Why would not the convergence of 
 leaves be a test for positive charges ? 7. How 
 is an insulated conductor charged permanent- 
 ly by induction? 8. Why must the body be 
 insulated? 9. What must be the condition 
 of an electroscope before it is useful in deter- 
 mining the kind of charge on a body? 
 
 Charge the electroscope by induction. 
 
 Take the following substances: (a) glass, 1 
 (b) ebonite, (c) sealing wax, (d) shellac and 
 (e) wood. Rub each one in succession with 
 fur, flannel and silk. By means of an elec- 
 troscope, determine the sign of a charge on 
 each after each rubbing. Test for both neapa- 
 
tive and positive charges. Tabulate as fol- 
 lows : 
 
 I II III IV 
 
 Substances Charge on Behavior Sign of 
 rubbed to- electroscope of leaves charge 
 gether 
 
 Glass with 
 fur 
 
 Etc 
 
PROBLEM No. 47 
 
 To Show That Static Electricity Is Directly Connected With Current Electricity and 
 
 Magnetism. 
 
 ~l 
 
 q m s x ► 
 
 & 
 
 Place a sensitive compass needle on the 
 table and allow it to come to rest. Now, 
 place a knitting needle, as shown in the dia- 
 gram, exactly at right angles with one end 
 of the compass needle. Connect the termin- 
 als of the coil on the glass tube with a charg- 
 ed Leyden jar. When the spark passes, 
 watch the compass needle- for attraction or 
 repulsion. Repeat the experiment with the 
 Leyden jar charged from the other terminal 
 of a static machine. 
 
 (1) Does a current flow through the wire? 
 
 Using an electroscope and a proof plane, 
 
 determine the sign of the charge used in each 
 trial. Charge the electroscope positively by 
 induction, then touch the proof plane to the 
 terminal of the static machine. Now, move 
 the proof plane near the charged electroscope 
 and decide the sign of the charge from the 
 behavior of the leaves on the electroscope. 
 
 (2) Have you established a connection as 
 stated in the problem ? 
 

 
 
 ■ 
 
 
 
 
 
PROBLEM No. 48 
 
 To Study the Magnetic Effects of an Electric Current. 
 
 I. Use about two meters of ordinary bell 
 wire and a magnetic needle. Connect up the 
 wire in circuit with a source of current and 
 contact key or circuit breaker. Determine 
 the direction of the current in circuit under 
 the directions of the instructor. 
 
 Let the magnetic needle come to rest point- 
 ing north. Place the wire above the needle 
 with the current flowing north. Complete 
 the circuit and note the deflection of the N. 
 pole of the needle, whether east or west. 
 
 (The direction of the deflection of the N. pole 
 indicates the assumed direction of the lines 
 of force.) 
 
 Repeat the experiment with the current 
 flowing North below the needle. 
 
 (1) With a north flowing current, what is 
 the direction of the magnetic field below the 
 wire and above the wire? (East or west in 
 each case.) 
 
 (2) The magnetic field is continuous 
 around the wire. If you were facing the 
 north and looking down on the wire under 
 the above conditions, what is the direction of 
 the lines of force at the right and left of the 
 wire respectively, up or down. 
 
 The thumb rule says that if a wire is 
 grasped with the right hand with the thumb 
 pointing in the direction of the current, the 
 fingers will encircle the wire in the direction 
 of the lines of force. 
 
 Grasp the wire according to the thumb 
 rule, thumb pointing north. 
 
 (3) Indicate the direction of the lines of 
 force below and above the wire respectively. 
 
 (4) Do these directions correspond to the 
 answers to question 1 ? 
 
 Repeat the experiment with the current 
 flowing south above and below the needle. 
 
 (5) Indicate the direction of the lines of 
 force below and above the wire, whether east 
 or west. 
 
 (6) What are the directions of the lines of 
 force in this case compared with those when 
 the current was flowing north? 
 
 Fry the thumb rule with the thumb point- 
 ing south. (7) Does the indicated direction 
 
of the lines of force correspond to the experi- 
 mental results? 
 
 Make a loop with one turn, the current 
 flowing- north above and south below the 
 magnetic needle and note the amount of de- 
 flection compared with one wire alone. Make 
 a similar loop with two turns and again with 
 several turns and observe the action of the 
 needle. 
 
 (8) What is the general .effect of increas- 
 ing the number of turns? 
 
 (9) Do you think a small current in a loop 
 of a hundred turns would affect a sensitive 
 magnetic needle? 
 
 (10) What is the difference between mov- 
 able magnet and movable coil galvanometers ? 
 
 II (1) State the converse of the thumb I 
 rule, showing how to find the direction of the 
 current in a wire when the direction of the 
 magnetic field around the wire is known. 
 
 Test and determine the direction of a cur- | 
 rent indicated by the instructor by using a 
 magnetic needle and above rule and have it 
 verified. 
 
 (2) Were your conclusions correct at the 
 first trial? 
 
 III. Wind a coil of 50 or 60 turns of wire 
 around an iron rod. Remove the rod and, 
 passing a current through the coil, present 
 its ends in turn to the N. pole of the magnetic 
 needle. (Press the turns closely together so 
 as to make the force noticeable.) 
 
 (1) What effects do you observe? 
 
 Thrust the iron rod into the coil and re- 
 peat the experiment. 
 
 (2) What effect has the iron core? 
 
 Determine the direction of the current 
 
 around the coil and apply what is known as 
 the thumb rule for coils to discover the N. 
 pole. Test your results by presenting this 
 pole to the N. pole of the magnetic needle. 
 
 (3) What happened? 
 
 (4) Write out the thumb rule for coils. 
 
 (5) Name five electrical devices having 
 electro magnets as essential parts. 
 
PROBLEM No. 49 
 
 To Learn How to Connect Up Various Bell Circuits. 
 
 In connecting up the following bell circuits, 
 practice economy in the use of wires; that is, 
 use the least number that will accomplish the 
 purpose. Make a neat diagram to show how 
 the connections are made in each case. In 
 doing this, represent the wires by straight 
 lines and have them turn square corners. 
 
 Use a small square to represent the bell and 
 a small circle for the key or push button. 
 
 1. Place two buzzers, (or bells) on one side 
 of the table and one key on the other. Con- 
 nect these into a circuit with electric current 
 so that upon closing the key both buzzers will 
 sound. Be sure that the circuit is so con- 
 nected that should one buzzer be out of order 
 and so not sound, the other would give a sig- 
 nal. While only two buzzers are used in this 
 experiment, the connection must be such that 
 more might be sounded from the same key. 
 
 An illustration of this kind of a circuit is the 
 bell system in the school where the master 
 clock in the office closes one key and rings all 
 the bells. 
 
 2. Place one buzzer on one side of the table | 
 and two keys on the other side. Connect : 
 these into a circuit so that the buzzer may be 
 sounded from either key. A good example of 
 this kind of connection is to be found in the 
 street cars, where the bell for signaling the 
 motorman may be rung from a great many 
 buttons. 
 
 3. Place one buzzer and one key at one side 
 of the table and another buzzer and key at 
 the other side. Connect these in a circuit so 
 that both buzzers will sound from either key. 
 
 Such a system is used where two different 
 rooms or places are to be signaled at the same 
 time from two or more points. 
 
 4. Place the buzzers and keys the same as 
 in (3), connect them in such a circuit that 
 pressing the key at either side of the table 
 will ring the bell at the other side. An ex- 
 ample of this kind of circuit is in the home 
 where the push button, at the front door 
 operates the bell and the one at the rear door 
 operates the buzzer. 
 
 5. Diagram the wiring for the following ; 
 
arrangement, using the least number of wires 
 possible. A 4-family flat, of which each 
 apartment is equipped with a bell for the 
 front door signal and a buzzer for the rear. 
 The current is supplied either by a common 
 battery, or by a bell ringing transformer, 
 which is located in the basement. 
 
 6. Make a large diagram showing the 
 course of the current through an electric 
 bell. Use arrows to trace the current. Ex- 
 plain its action. 
 
PROBLEM No. 50 
 
 To Determine the Effect Produced on the E. M. F. of a Battery by Connecting the Cells 
 in Series and in Parallel. 
 
 The voltmeter readings must be taken as 
 accurately as possible. First see if the 
 needle is at zero. If not, make an allowance 
 for this with every reading taken. 
 
 Connect the voltmeter to each of the cells 
 separately to see if all are good. Dry cells 
 in good condition should register about 1.5 
 volts. Edison primary batteries have an E. 
 
 M. F. of from .66 to .7 volts. If any cell 
 shows a voltage less than it should, see that 
 it is replaced by a good one before going 
 ahead with the experiment. Next, join any 
 two of the dry cells in series and obtain their 
 combined voltage. Then join three dry cells 
 in series and obtain their combined voltage. 
 
 Next, connect two of them in parallel and fin- 
 ally three in parallel and take the voltmeter 
 indication for both. Proceed in the same 
 manner to find the E. M. F.s of the same 
 combination of Edison primary cells. Record 
 all these observations in the tabular form 
 shown below. 
 
 1. What effect do you find that series con- 
 nection has on the E. M. F. of a battery? 
 
 2. What effect is produced by parallel con- 
 nection ? 
 
 1 
 
 1 Cell connections | E. M. F. 
 
 j 1 
 
 1 
 
 1 1 
 
 ! 1 
 
 ! Av. of all 
 
 1 L 
 
 1 Two in S 
 
 
 1 Three in S 
 
 
 ! Two in P 
 
 
 1 Three in P 
 
 i 
 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
PROBLEM No. 51 
 
 To Show the Relation That Exists in a Circuit Between the Resistance and the Fall of Po- 
 tential or Pressure. 
 
 Set up the apparatus as shown in the dia- 
 gram, using a meter of high resistance wire, 
 e. g., German silver, or nichrome, stretched 
 along a meter stick. V is a voltmeter. The im- 
 portant thing in connecting up this circuit is 
 to have the wires from the plus terminals of 
 the battery and the plus binding post of the 
 voltmeter joined to the same end of the re- 
 sistance wire. 
 
 Having connected the apparatus as shown 
 in the figure, set the sliding key at a point 20 
 centimeters from the end A. Press the key 
 and take the voltmeter reading as accurately 
 as possible, allowing for any error at the zero 
 and estimating all readings to tenths of the 
 smallest scale division. Proceed in this way 
 to obtain the P.D. across the other lengths 
 called for in the tabular form. Obtain the 
 resistances of each of the lengths. (Resist- 
 ance of 1 mil ft. of German silver is 114 ohms 
 and of nichrome wire 660 ohms.) 
 
 (1) What relation do you find to exist be- 
 tween the resistances of parts of a circuit 
 and the P.D.s across the respective parts ? 
 
 Length 
 of wire 
 
 Resistance 
 
 P. D. 
 
 | 
 
 |(P. D. : R) 
 
 L 1 =20 
 
 R = 
 
 E,= 
 
 
 L„=40 
 
 R 2 = 
 
 E,= 
 
 
 L 3 =60 
 
 R,= 
 
 e 3 = 
 
 1 
 
 L 4 =70 
 
 r 4 = 
 
 K= 
 
 1 
 
 L,=90 
 
 R,= 
 
 E,= 
 
 1 
 

 
 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 ■ ■ ; 
 
 . 
 
 ' 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PROBLEM No. 52 
 
 To Study the Laws of Induced Currents. 
 
 Since the galvanometer is to be used to de- 
 termine the direction, as well as the existence 
 of induced currents, it is first necessary to 
 observe the direction of the deflection of al 
 current whose direction is known. 1. Dia- 
 gram the arrangement of the apparatus 
 showing the deflection of the galvanometer 
 needle bv a known current. 
 
 Use a single cell, or a current designated by 
 the instructor, for this purpose, but before 
 closing the circuit, connect the terminals of 
 the galvanometer with a short wire, which 
 will act as a shunt to the galvanometer, per- 
 mitting only a small fraction of the current 
 to pass through it. (This is a necessary pre- 
 caution with a sensitive instrument.) Ob- 
 serve the direction of the current from the 
 battery connections, also the direction of the 
 deflection. A current entering by the other 
 terminal would cause a deflection in the oppo- 
 site direction. Hence, in the experiments, 
 the direction of the deflection will indicate by 
 which terminal the current enters the gal- 
 vanometer; and from this, the direction of 
 the current can be traced through the entire 
 circuit. 
 
 Connect the galvanometer with the large 
 coil of wire called the secondary coil. The 
 connections must be a meter or more long. 
 2. Why? The circuit consists of the coil, the 
 galvanometer and the connecting wires. 
 
 Thrust the north pole of a magnet sudden- 
 ly up to the coil while observing the galvan- 
 ometer. Note the direction of the deflection. 
 Observe the effect of removing the magnet. 
 Repeat until you are sure of the results. 
 From the direction of the deflection deter- 
 mine whether the induced current makes the 
 end of the coil near the magnet a north pole 
 or a south pole. Apply the right hand rule. 
 
 3. Draw diagrams showing the polarity of 
 
the magnet and the direction in which it 
 moves, and the resulting polarity of the coil 
 and the direction around it. 
 
 4. Is there a current when the magnet is 
 at rest? 5. What is the experimental evi- 
 dence? Study the effect when the south 
 pole is used instead of the north pole. 
 
 Repeat your experiments with an electro- 
 magnet in place of the bar magnet and com- 
 pare results. 6. Draw diagrams to indicate 
 results. 
 
 7. What effect does the rate of change in 
 the magnetic lines of force have upon the 
 strength of the induced currents? Review 
 the subject of magnetic induction in your 
 laboratory experiments with the laws of 
 magnetic induction. 
 
PROBLEM No. 53 
 
 — ^ 
 
 To Test Fuses. 
 
 Every electric circuit should be provided 
 with some form of protective apparatus, so 
 that when a “short” is formed on the line, the 
 great current which would flow, would neith- 
 er burn-out or damage the instruments. One 
 other great danger is that a shorted line may 
 cause a fire through a heated wire. It is for 
 this reason that the insurance companies re- 
 quire that all lighting and power circuits be 
 protected by a fuse. There is one other 
 protective device, called a circuit breaker. 
 Instead of fusing, this automatically cuts off 
 the source of energy. These circuit break- 
 ers are used on heavy lines where a large 
 current is carried and where there is a ten- 
 dency for overloading the line. They occupy 
 more space than do fuses, are more costly, 
 but on the other hand, are more convenient. 
 
 Fuses are made of strips of fusible metal, 
 generally lead alloyed with a small amount of 
 tin. Sometimes zinc is used and at times 
 copper or aluminum forms the fusible metal. 
 Such metal is chosen as will fuse or “blow” 
 before any excess current can flow through 
 the line. Copper is not used to such a great 
 extent because it heats and does not fuse at 
 a low temperature. In all the fuses the heat 
 must be generated faster than it can be radi- 
 ated, or the metal would not melt. After it 
 has fused, there is a tendency of the metallic 
 vapor to cause an arc. This would make a 
 fuse too slow acting. To remedy this, the 
 cartridge (or enclosed) type has a non-in- 
 flammable substance packed about the metal, 
 which immediately condenses and absorbs the 
 vapor formed. One other form (expulsion 
 type) provides a means for expelling the hot 
 vapor. A third type (the open fuse) is not 
 very dependable because of the air currents 
 and the chemical action of the elements on 
 them. Then if the fuse is open and large, 
 the discharge of the molten metal is dan- 
 gerous. 
 
 Wherever a valuable apparatus is used, it 
 is a wise policy to use a fuse in the circuit, 
 for the fuse will cost ten cents or less, where- 
 as the instrument will probably cost twice 
 that many dollars. 
 
 PROBLEM: To test various fuse wires; to 
 
show how circuits are protected by fuses. 
 
 Directions: A. Connect the fuse block 
 with a suitable resistance and have a 20 amp- 
 Ammeter in series with it. 
 
 Put a one amp. fuse wire in the block and 
 slowly cut out resistance until the fuse is 
 blown. Record the reading of the ammeter 
 just as it is blown. 
 
 Repeat this, using such other fuse wire as 
 the instructor directs. 
 
 1. Under your discussion, tell briefly what 
 was done, sketch the connections and put 
 down your results in tabular form. Then 
 answer the following: 
 
 2. The one amp. fuse was found to carry 
 % overload before blowing. 
 
 3. Do the same for the other wires tested 
 if you know their listed capacity. 
 
 4. How many different kinds of fuse plugs 
 do you know of? Name them. 
 
 5. Did the fuse explode? 
 
 6. Why should it be enclosed? 
 
 7. How does a fuse protect a circuit? 
 
 B. Measure the diameter of the fuse wires 
 and see if there is any ratio between the 
 cross section area and the amount of current 
 rent that fuses it. 
 
 C. In place of regular fuse wire, insert 
 copper wires and get their maximum carry- 
 ing capacity. 
 
 1. Notice if their action of fusing differs 
 from the first wires used and if so, in what 
 respect. 2. Compare the cross sections with 
 the load they carry. 3. Do you think the 
 length of the wire used would vary its carry- 
 ing capacity? 4. If so, how? 
 
PROBLEM No. 54 
 
 To Find Resistance of 1 Lamp and Compare With “n” Lamps in Series. 
 
 Directions: Arrange five lamps in series 
 or so that the current has but one path thru 
 all the lamps. 
 
 Connect the instruments as shown in dia- 
 gram of connections, making sure that the 
 ammeter is in series with the lamps while 
 the voltmeter has one free wire or traveler. 
 Before closing the switch, have the circuit 
 examined by your instructor. 
 
 Read the ammeter and take a set of read- 
 ings with voltmeter, getting first the differ- 
 ence of potential for 1 lamp, then 2, 3, 4 and 
 5 together by moving the traveler along the 
 row of lamps and connecting at 1, 2, 3, 4 and 
 5 respectively. Disconnect the voltmeter and 
 find the P.D. across each lamp singly. 
 
 Arrange the circuit with only four lamps 
 in series. 
 
 Take another set of ammeter and voltmet- 
 er readings. Continue this method of cut- 
 ting out one lamp at a time until only one 
 lamp is left. 
 
 Record all readings in tabular form. Cal- 
 culate the ohms by means of Ohm’s law 
 P.D. 
 
 (R r = ) for each lamp, two, three, four, 
 
 I 
 
 etc., lamps in series respectively. 
 
 Find average resistance of all single lamps. 
 
 1. Do the volts increase or decrease as you 
 increase number of lamps between the volt- 
 meter wires ? 
 
 2. How does the resistance of 2, 3, 4, etc., 
 lamps compare with that of one? (Average.) 
 
 3. Explain the brightening up of the lamps 
 as first one and then another is cut out. 
 
 4. What is meant by “fall of potential?” 
 
 5. What method was used in determining 
 resistances in above problem? 
 

 
 
 
 
 
 
 . 
 
 
 
 
 >, • *v 
 
PROBLEM No. 55 
 
 To Find the Resistance of “n” Lamps in Parallel and Compare With That of One Alone. 
 
 Connect the lamps with ammeter and volt- 
 meter as shown in diagram. Observe care- 
 fully directions previously given for connect- 
 ing the instruments into the circuit. Have 
 the circuit examined by the instructor before 
 closing the switch. 
 
 Lamps may be cut out of circuit by simply 
 unscrewing from the sockets. 
 
 Take a set of readings on each lamp by it- 
 self, recording amperes and volts. Then 
 commence at the same end, leaving lamps on, 
 and take readings for 2, 3, 4, etc., together. 
 
 Tabulate as follows: 
 
 Let r=average resistance of single lamp 
 readings. 
 
 n=number of lamps. 
 
 I 
 
 1 
 
 E 
 
 E 
 
 R= — 
 
 I 
 
 r | 
 R— — 
 n 
 
 1 
 
 1 
 
 
 
 
 1. How c 
 
 oes the resistance of 2, 3, 4, etc., 
 
 lamps in parallel compare with that of one 
 alone? (Average.) 
 
 2. Why is there no dimming of the lamps 
 as in the series experiment? 
 

 
 
 
 
 
 
 
 
 ' 
 
 
 
 
 
 
 
 
 
 
 
 ■* 
 
 
 
 
PROBLEM No. 56 
 
 A. To Measure Resistances by Means of a Slide Wire Bridge. 
 
 Directions: Set up the apparatus accord- 
 ing - to the diagram, using a D’Arsonval or 
 other sensitive galvanometer. Use the cur- 
 rent furnished. 
 
 The difficulty is to get the movable contact 
 key in such a position that there will be no 
 deflection of the galvanometer needle, when 
 contact is made. First of all, test the gal- 
 vanometer to see that it is in good working 
 condition. Handle it carefully for it is a 
 delicate instrument. Every galvanometer 
 has some arrangement whereby the coil is 
 held stationary when not in use. If you 
 move the galvanometer when the coil is 
 swinging freely, you will probably break the 
 supporting ribbon. Always see that the coil 
 is made secure before leaving it or when 
 moving the instrument. 
 
 After seeing that you have a galvanometer 
 in good working condition, move the contact ! 
 key to one end of the bridge and, making 
 contact, notice the direction of the deflection 
 of the galvanometer. Move the contact key 
 to the other end of the bridge and make con- 
 tact. The direction of the deflection of the 
 needle should be opposite. If not, either of 
 the sets of connections at the ends of the 
 bridge are broken. Trace connections and 
 make trials until the deflection when contact 
 is made at one end of the bridge is opposite 
 to that at the other end. 
 
 By trials, find a point where with contact 
 of key, no deflection of the galvanometer 
 needle is obtained. Then the ratio of the un- 
 known resistance to the known resistance 
 will be equal to the ratio of the lengths of 
 the high resistance wire on the correspond- 
 ing sides. R:X::D:D2. Use the proportion 
 and compute the value of the unknown re- 
 sistance. 
 
 (Wheatstone.) 
 
Find in this way three unknown resist- j 
 ances furnished by the instructor, making- 
 three trials for each, with two, five and ten 
 ohm coils respectively. Make a neat tabula- 
 tion showing known resistance used, length 
 of high resistance wire, proportions and val- 
 ues for the unknown resistance, indicating j 
 what the unknown resistance is. 
 
 B. To Measure the Resistance of the Three 
 Resistances Just Found, (1) In Series, (2) 
 In Parallel. 
 
 Connect the three resistances you have 
 just found in series and find by the same ' 
 method their resistance for three trials, us- 5 
 ing as before two, five and ten ohm known 
 coils. Compare the average result of these 
 trials with the sum of the separate resistance 
 as found above. Tabulate completely your 
 results. 
 
 Connect the three resistances in parallel 
 and similarly, for three trials, find the resist- 
 ance. Compare the average with the resist- 
 ance as found bv the formula 
 
 111 
 
 R r, r, 
 
 where R is the total resistance in parallel, r, 
 the resistance of one coil alone, and r 2 the 
 resistance of the other alone. Tabulate com- 
 pletely the results. 
 
PROBLEM No. 57 
 
 To Measure the Amount of Electrical Energy Expended in Any Part of a Circuit and to 
 Learn to Calculate the Energy in Watts When Any of the Following Pair of Values 
 
 Is Given: 
 
 1. Current and P.D. 
 
 2. Resistance and P.D. 
 
 3. Current and Resistance. 
 
 The number of watts of energy expended 
 in any part of a circuit is found by multiply- 
 ing the current in amperes flowing through 
 that part of the circuit by the difference in 
 potential between its terminals; that is, 
 W=IE. Using Ohm’s law equation, two 
 more formulas may be derived to express the 
 energy of an electric circuit ; namely, 
 W=E 2 /R and W=T 2 R. To obtain experi- 
 mentally the energy expended in an arc lamp 
 circuit, connect it up through a rheostat to 
 such a line voltage as the instructor indi- 
 cates. Also connect a suitable ammeter and 
 a voltmeter to this circuit under the direc- 
 tions of the instructor. When the resistance 
 in the rheostat is about half on, close the cir- 
 cuit and read the instruments to the smallest 
 possible division. Record all values in a 
 suitable table. 
 
 Move the regulator on the rheostat so that 
 you will have less resistance in the circuit. 
 Note the effect of this on the reading of the 
 instruments and record these results. 
 
 1. What would happen if the carbons were 
 left in contact? 
 
 2. Is alternating current ever used on an 
 arc lamp? 
 
 3. What arrangement is made to feed the 
 carbons on a street arc lamp? 
 
 4. How could you determine which carbon 
 is positive? 
 
 From a previous experiment, enter in the 
 
tabular form the values called for opposite 
 each of the lamps or lamp combinations nam- 
 ed. By means of the three formulas, calcu- 
 late the energy consumed in each case. 
 
 5. Do all three results agree? 
 
 Circuit in which thel 
 
 
 
 1 E 5 I 
 
 energy is to be meas-l I 
 
 E 
 
 R 
 
 W=IE|\V=— W = I 2 I 
 
 ured 1 
 
 
 
 1 R 1 
 
 Arc Lamp, 1st Trial.... 
 
 
 
 
 
 
 Arc Lamp, 2nd Trial.. 
 
 
 
 
 
 
 
 One incandescent lamp 
 
 
 
 
 
 
 
 Two lamps in parallel.. 
 
 
 
 
 
 
 
 Three lamps in parallel 
 
 
 
 
 
 
 
PROBLEM No. 58 
 
 To Determine the Cost Per Hour of Operating an Electric Flatiron and an Electric 
 
 Toaster, Etc. 
 
 Directions: In connecting a voltmeter 
 and an ammeter to a 110-volt service line of 
 direct current, care must be taken that the 
 positive terminals of the instruments be con- 
 nected to the positive side of the line. To 
 find the direction of the flow of current in 
 the line, use a voltmeter with the binding 
 posts marked plus and minus, or connect the 
 wires to a lamp (110 volt), (never touch the 
 wires together), recall the thumb rule and, 
 with the aid of a compass, apply the rule. 
 
 Having determined the positive side of the 
 service line, connect the flatiron in series with 
 the ammeter and connect the voltmeter 
 across the line. While making the connec- 
 tions, be sure that the current is “off.” Do 
 not turn the current on until the instructor 
 has O.K.d the connections. 
 
 Take readings of the two instruments 
 every minute for five minutes, average and 
 place them in the following chart. Use cur- 
 rent rates in computing the cost. 
 
 Next, put the toaster in the circuit and do 
 a? directed for the iron. 
 
 Device 
 
 Volts 
 
 Watts 
 
 Rate 
 
 Per Kw. 
 
 Cost per Hour 
 of Operation 
 
 Toaster 
 
 
 
 
 
 Flatiron 
 
 
 
 
 
 1. Sketch the connections and tell what 
 was done. 
 
 2. I find the cost to operate an electric flat- 
 iron each hour to be cents and a 
 
 toaster to be cents. 
 
 8. What would it cost to burn the lights 
 in your laboratory for a double period, using 
 the Minneapolis rate as a basis? 
 
 4. What would it cost to operate a 110 volt 
 1.5 ampere Christmas tree lighting outfit one 
 week, three hours per night, if electricity 
 costs 10 cents per Kw. hour? 
 
 5. If it takes 40 amperes of current at 110 
 volts to run the light of a moving picture 
 machine, what would it cost to keep this light 
 burning for two hours at current rates? 
 
) 
 
PROBLEM No. 59 
 
 i 
 
 To Find the Per Cent of Total Electrical Energy Which Is Turned Into Useful Heat En- 
 ergy in an Electric Disc Stove, Immersion Heater, Etc. 
 
 3 
 
 Connect up a disc stove in circuit with a 
 suitable ammeter in series and a voltmeter 
 across the circuit. Let the water in the fau- 
 cet run until it is as cold as it will get, then 
 carefully weigh out about 500 grams of it in 
 a calorimeter. Place the calorimeter on the 
 stove and take the temperature of the water. 
 
 Also note the room temperature and calcu- 
 late the temperature to which the water must ; 
 be heated so that it will finally be as much 
 above the room temperature as it is below in 
 the beginning. 
 
 After the circuit has been O.K.d by the in- 
 structor, note the time to seconds, close the 
 circuit, read the ammeter and voltmeter and 
 constantly stirring the water with the ther- 
 mometer, let it attain the desired tempera- 
 ture above the room temperature as comput- 
 ed above. If there is considerable variation 
 in the ammeter and voltmeter readings as 
 the heating continues, read them every min- 
 ute and finally average the separate readings. 
 
 Take the time when the water reaches the 
 desired temperature, and calculate the time 
 elapsed in seconds. 
 
 Compute the resistance of the stove and 
 the calories of heat gained by the water. Al- 
 so compute the total energy supplied by the 
 formula, H equals .24 I 2 Rt, as explained in 
 your text. Divide the calories gained by the 
 water by the calories given off by the current. 
 
 Repeat the experiment with an immersion 
 heater and any other heating device you may 
 have time for. Record all readings and com- 
 putations in a neat form. 
 
 1. What becomes of the wasted energy? 
 
 2. Which device has the highest efficiency 
 and why? 
 
 3. Give any conditions under which less 
 energy would be wasted in any of the cases. 
 
PROBLEM No. 60 
 
 To Find the Cost of Bringing a Pint of Water to the Boiling Point On An Ordinary Gas 
 Burner and Compute the Efficiency of the Burner and Kettle. 
 
 Weigh carefully a pound of water in an or- 
 dinary pan or kettle. (A pound is practic- 
 ally a pint.) Use the gas meter provided un- 
 der the directions of the instructor. Take 
 the temperature of the water, then place it 
 upon the gas burner covered, turn on the gas 
 from the meter, and allow the gas to burn 
 until the water begins to boil. (Do not mis- 
 take air bubbles for boiling.) Read the dial 
 on the scale when boiling begins and take the 
 temperature of the water. At current price, 
 compute the cost of that used in boiling the 
 pint of water. 
 
 To compute the efficiency, find the number 
 of B.T.U. used up by allowing 600 B.T.U. for 
 each cu. ft. of gas; find the B.T.U. turned 
 into useful heat by multiplying the weight of 
 the water by the number of degrees Fahren- 
 heit raised. Divide the number of B.T.U., 
 which are useful, by the total number used 
 and the result is efficiency. 
 
 1. Mention ways in which the efficiency 
 could be increased. 
 
 , 2. At this rate, calculate the cost of heat- 
 ing the water in a 20 gallon tank from 50° F. 
 to 212° F. 
 
 3. How does the efficiency of electric burn- 
 ers compare with that of gas burners? 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ■ 
 
 
 
 
 
 
 
 
 
 ■ 
 
 
 
 
 
 
 
 
 
 
 ■ 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 ' 
 
 “ . , , 
 
 ( 
 
 
PROBLEM No. 61 
 
 To Diagram and Explain the Acttion of a Telephone. 
 
 I. Trace out the complete circuit of a tele- 
 phone line between two points. Represent 
 your findings in a diagram with a complete 
 instrument at each point. 
 
 1. Will the bell ring when the receiver is 
 on the hook? Trace the circuit on your dia- 
 gram to answer this question. 
 
 With the receiver off the hook, trace the 
 circuit through the transmitter and the pri- 
 mary coil.. .2. Is it complete? 
 
 3. What change takes place in the connec- 
 tions with the receiver on and off the hook? 
 
 4. Explain the action of a telephone in 
 transmitting sounds. 
 
 II. Study the construction and action of 
 a microphone. This instrument was very 
 closely connected with the early history of 
 the telephone. Listen to the sounds of a 
 watch lying on the microphone. 1. Is the 
 reproduction like the original sound in char- 
 acter? 2. Is it louder? 3. Diagram the 
 microphone circuit. 
 

 
 
 
 
 
 
 >V. ' , 
 
 
 
 
 
 
 
 ' • 
 
 
 
 
 
 
PROBLEM No. 62 
 
 To Study the Motor and Dynamo. 
 
 Simple motors and dynamos are alike in j 
 construction. Some companies make a ma- j 
 chine which can be used either as a motor or j 
 as a dynamo. If a machine is to be used as 
 a dynamo, the core of the field magnets must 
 be made of hard enough iron to retain suffi- 
 cient magnetism to furnish a weak field ; oth- 
 erwise there will be no currnt produced when 
 the armature is rotated. With the machine 
 used as a motor, on the other hand, it is not 
 necessary that the cores of the field magnets 
 retain any magnetism. The St. Louis mo- 
 tors to be used in this experiment will, with 
 the bar magnets, serve either as motors or 
 dynamos, though there is no arrangement 
 made to rotate the armature as a dynamo. 
 
 Use a St. Louis motor with the bar mag- ! 
 nets in position. (Be sure that opposite 
 poles are on either side of the armature.) In 
 order to produce rotation when the current is 
 turned on, the brushes must be in the proper 
 position. Study the instrument and deter- 
 mine where the armature should be when 
 the current changes direction so that it con- 
 tinue its rotation. Adjust the brushes so 
 that the change of current will come at the 
 proper place. Connect the instrument with 
 a dry cell. If the brushes have been ad- 
 justed correctly, the armature will rotate. 
 
 1. In what position did you determine the 
 aramture should be when the current 
 changes direction? 
 
 2. Did you have any trouble in making the 
 armature rotate? If so, what? Attach the 
 wires so that the current will flow through 
 the motor in an opposite direction and note 
 the direction of rotation as compared with 
 that at first. 
 
 3. What is the direction of rotation as 
 compared with that at first? 
 
 (Reverse the magnets.) 
 
 4. What is the effect on the direction of 
 rotation ? 
 
 With the armature rotating, gradually 
 move the bar magnets away. 
 
 5. What is the effect? Why? 
 
 Attach the electromagnet so that the cur- 
 rent on entering will divide, part passing 
 through the armature and part through the 
 
field magnets. This is called a shunt wind- I 
 ing. See that the brushes are adjusted cor- 
 rectly so that the motor will operate. 
 
 6. Does the speed of rotation seem to be 
 slower or faster than with the bar magnets ? 
 
 (Change the direction of the current 
 through the instrument.) 
 
 7. Does this affect the direction of rota- 
 tion? 
 
 (Arrange the wires so that all the current 
 must pass through both the field magnet and 
 armature. This is called a series winding.) 
 
 8. Does the armature rotate as fast as 
 with the shunt winding? 
 
 (Reverse the current.) 
 
 9. Does this affect the direction of rota- 
 tion? 
 
 Disconnect the dry cell and replace the bar 
 magnets. Attach a D’Arsonva! galvanomet- 
 er to the terminals and spin the armature, 
 first in one direction and then in the opposite 
 direction. 
 
 10. What was the direction of the current 
 in the second case as compared with the first, 
 as shown by the galvanometer needle? 
 
 11. What factors affect the voltage of a 
 dynamo ? 
 
PROBLEM No. 63 
 
 To Find the Efficiency of an Electric Motor. 
 
 Use the electric motor furnished. Con- 
 nect a suitable voltmeter across the terminals 
 of the motor and a suitable ammeter in series 
 with it. See diagram or model. (Be sure 
 you are using the proper instruments and 
 that they are connected up correctly by con- 
 sulting with the instructor before the current 
 is turned on.) The motor should be “braked” 
 by a small belt or cord fastened to two draw 
 scales, as shown in the diagram or model. 
 
 With the motor running, read the volt- 
 meter and ammeter and the two spring bal- 
 ances. Also, using a speed counter, count 
 the number of revolutions per minute, and 
 measure the circumference of the pulley 
 around which the cord passes. Watts put-in 
 equals volts times amperes, and watts got- 
 ten out equals load in kg. (difference in the 
 reading of balances) times circumference of 
 pulley, times revolutions per minute divided 
 by 6.12 (the number of kg. meters per min- 
 ute equivalent to 1 watt). 
 
 Make three trials with different brake 
 loads and tabulate as follows: 
 
 12 3 
 
 1. Reading of voltmeter 
 
 2. Reading of ammeter 
 
 3. Watts (input) VxA — 
 
 4. Reading of Balance A 
 
 5. Reading of Balance B 
 
 6. Load in Kg. (differ- 
 
 ence in readings) 
 
 7. Circumference of 
 
 pulley 
 
 8. Rotations per minute 
 
 9. Work done per 
 
 minute 
 
 10. Watts — Output 
 
 11. Efficiency (Output^ - 
 
 Input) 
 
 A. Find the H.P. of the motor above. 
 
 B. How does the load affect the efficiency? 
 
 C. How does the load affect the current 
 needed to run the motor? 
 
 D. Find the cost at $0.10 per K.W. hour to 
 run the motor under the heaviest load for 10 
 hours. 
 
PROBLEM No. 64 
 
 To Construct a Small Step-Down Transformer. 
 
 The core design offered will be found serv- 
 iceable up to about 600 watts. By the use of 
 this design, form wound coils may be used. 
 
 This form, which is actual size, should be 
 built up to 7.6 cm. in thickness. The sheets 
 should be about .3 mm. thick. 600 watts on 
 120 volts would require 5 amperes primary. 
 600 watts at 40 volts would require 15 am- 
 peres secondary. (Consider the efficiency 
 100 per cent.) 
 
 Wire table shows No. 20 copper will carry 
 5.7 amperes; No. 14 copper will carry 16.2 
 amperes. Hence, these sizes should be used. 
 By Ohm’s law, we should have 24 ohms re- 
 sistance in the primary. It has been found 
 that 2.58 ohms are all that is required. The 
 balance is made up by the A.C. effect known 
 as impedance. By consulting the wire table, 
 we find that 250 ft. No. 20 will give 2.58 
 ohms. The number of turns this makes 
 when wound into the primary coil enables us 
 to determine the number of turns required 
 in the secondary. Vp:Vs: :Tp:Ts, or 
 120 :40 : :360 :120. In case different voltages 
 are required, taps should be brought out of 
 the secondary at points so that the ratio of 
 the number of turns in the primary and sec- 
 ondary will conform to the voltage ratio. 
 
 The insulation between the primary and 
 core must be constructed with great care. 
 Use mica wherever possible and empire cloth 
 on the sharp angles. 
 
 When the insulation has been completed, 
 the transformer should be tested for voltage j 
 
breakdown by subjecting it to 100% over- 
 voltage for a few moments. Then run it for 
 an hour or more at normal voltage and note 
 whether it heats to excess. If these tests 
 are satisfactory, the transformer should be 
 mounted in a sheet iron box a little larger 
 than the transformer itself. The space 
 should then be filled with insulating com- 
 pound, which protects the coils from mechan- 
 ical injury and the insulation from moisture. 
 The primary and secondary leads should have 
 hard rubber bushings where they pass 
 through the iron case. 
 
 By using a large number of turns of very 
 fine wire in the secondary, this transformer 
 may be used as a step-up for high voltage.