A PRACTICAL APPLICATION OF PHYSICS 
 
HOUSEHOLD PHYSICS 
 
 BY 
 C. H. BRECHNER 
 
 FORMERLY TEACHER OF PHYSICS IN THE 
 
 EAST TECHNICAL HIGH SCHOOL 
 
 CLEVELAND 
 
 ALLYN AND BACON 
 
 BOSTON NEW YORK CHICAGO 
 
 ATLANTA S^ N FRANCISCO 
 

 
 _ 
 
 COPYRIGHT, 1919 
 BY C. H. BRECHNER 
 
 J. 8. Gushing Co. Berwick & Smith Co, 
 Norwood, Mass., U.S.A. 
 
PREFACE 
 
 Household Physics was written primarily for girls. The 
 principles of physics in such a book are of course the same 
 as in a text-book for boys or for mixed classes. But in 
 Household Physics these principles are applied in such a 
 way as to interest girls, by using examples and references 
 with which they are thoroughly familiar. 
 
 The work was developed in the classroom. At first the 
 author used an outline, filling in with applications and 
 drawings in the recitation periods. The following year the 
 text was written and mimeograph copies put into the hands 
 of the students. After this material had been carefully 
 worked over with various classes, it was revised into the 
 present book. 
 
 The subject of Heat is taken up first, since it is one which 
 has many applications of vital importance to the household. 
 Thus the girl becomes interested in physics from the first, 
 and looks forward to recitations with pleasure. 
 
 The language of the book has been kept as simple as 
 possible throughout. The topics are carefully explained 
 and these explanations are illustrated by a wealth of line 
 drawings and photographs. The problems are especially 
 easy and practical. 
 
 The author wishes to take this opportunity to thank the 
 several industrial concerns which supplied many of the 
 photographs, and also those teachers and pupils who so 
 kindly assisted him in bringing the work to completion. 
 
 C. H. B. 
 
 AUGUST, 1919. 
 
 v 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. HEAT AND HEAT MEASUREMENT 1 
 
 II. EXPANSION 29 
 
 III. HEAT TRANSFERENCE 41 
 
 IV. SOURCES OF HEAT 60 
 
 V. WAVE MOTION 67 
 
 VI. SOUND 74 
 
 VII. BASIS FOR Music . . . . f. . .85 
 
 VIII. LIGHT . 91 
 
 IX. REFLECTION AND MIRRORS 96 
 
 X. REFRACTION AND LENSES 103 
 
 XI. ILLUMINATION AND CANDLE POWER .... 127 
 
 XII. COLOR 132 
 
 XIII. MAGNETISM 146 
 
 XIV. ELECTRICITY. 153 
 
 XV. MAGNETIC EFFECT OF AN ELECTRICAL CURRENT . 163 
 
 XVI. HEATING EFFECT OF AN ELECTRIC CURRENT . .173 
 
 XVII. MOTION-PRODUCING EFFECT OF AN ELECTRIC CURRENT 184 
 
 XVIII. INDUCTION 200 
 
 XIX. CHEMICAL RELATION OF AN ELECTRICAL CURRENT . 212 
 
 XX. BATTERIES 217 
 
 XXI. MECHANICS OF SOLIDS 227 
 
 XXII. MACHINES 23^ 
 
 XXIII. DYNAMICS 248 
 
 XXIV. MECHANICS OF FLUIDS 259 
 
 Appendix 285 
 
 Index . . 297 
 
 vii 
 
HOUSEHOLD PHYSICS 
 
 CHAPTER I 
 HEAT AND HEAT MEASUREMENT 
 
 1. Nature of Heat. Physics is a study of the e very-day 
 events of life. It is defined as the science of matter and energy. 
 
 TAKING THE TEMPERATURE OF MELTING ICE TO DETERMINE THE MELTING 
 OR FREEZING POINT. 
 
 Matter is anything which occupies space ; e.g. air, water, 
 wood, iron, etc. Energy is ability to do work. 
 
 1 
 
2 HF^T : AND HEAT MEASUREMENT 
 
 The student of domestic science must often wonder why 
 some of the remarkable things in cooking, freezing, and melt- 
 ing happen as they do. One of the divisions of physics, the 
 subject of heat, touches closely many of the things done in 
 domestic science work ; and it has a vital relation to the coal 
 and gas bills at home. 
 
 Heat is one form of energy, and is of two kinds, molecular 
 heat and radiant heat; sometimes called sensible heat and 
 insensible heat. Sensible heat can be detected by the senses, 
 while insensible heat cannot. 
 
 All substances are composed of molecules, or very small particles of 
 matter, which are never at rest but are always vibrating with great 
 rapidity. In a hot body they vibrate faster than in a cold body. 
 When you heat a flat-iron you make the molecules jump faster. If 
 you rub your hands together, they are warmed by the increased vibra- 
 tion of the molecules. 
 
 The energy which molecules possess, due to their vibra- 
 tion, is called sensible heat or molecular heat. Put your 
 hand on anything hot and you will see how easily sensible 
 heat can be detected by the sense of touch. 
 
 Heat comes all the way from the sun to the earth. It 
 travels through air or clear glass without warming it ; but 
 when it strikes any object not transparent it is absorbed 
 and warms the object. Heat passing through the air is 
 insensible or radiant heat, but when it strikes a non- 
 transparent object it is changed to sensible or molecular 
 heat. 
 
 If you touch a window pane when the sun is shining 
 through, the glass feels cold. If you touch a piece of black 
 cloth lying in sunshine, it feels warm. 
 
 Either form of heat may be changed into the other. Sen- 
 sible heat in the glowing coals of the fireplace starts out as 
 
TEMPERATURE 3 
 
 radiant heat, or vibration in ether ; but when it strikes you, 
 it is changed back to sensible heat. 
 
 2. Hot and Cold. Hot and cold are common words used 
 to denote how a body feels to the touch. They are only rela- 
 tive terms, and are not very definite. The term cold is 
 
 TAKING THE TEMPERATURE OF STEAM OVER BOILING WATER TO 
 DETERMINE THE BOILING POINT. 
 
 negative in meaning, and refers to the absence of heat. Cold 
 does not come into your house ; but heat goes out, leaving it 
 cold, or without heat. 
 
 3. Temperature. Since the terms hot and cold do not 
 give us a definite means of expressing the heat condition of 
 bodies, we use another word, temperature. Temperature is 
 
4 HEAT AND HEAT MEASUREMENT 
 
 the measurement of the speed of vibration of the molecules of an 
 object; that is, it is a means of expressing the hotness of a 
 body. 
 
 4. Thermometers. Since the sense of feeling is inac- 
 curate, we must have some definite means of measuring 
 temperature ; and for such purpose we use the thermometer. 
 
 Most substances expand when heated, and contract 
 when heat is removed. This expansion is used to measure 
 temperature. Mercury expands or contracts rapidly and 
 evenly, and therefore is commonly used as the expanding 
 substance in thermometers. Sometimes alcohol containing 
 red dye takes its place. There are several kinds of ther- 
 mometers, and we must be familiar with two, Centi- 
 grade and Fahrenheit. 
 
 The best way to understand these is to learn how they 
 are made, and how the scales are placed on them. First 
 we must have some fixed point, that is, some point which will 
 mean the same temperature everywhere in the world. Pure 
 water furnishes such a point, as it freezes (changes from liquid 
 to solid), or melts (changes from solid to liquid), always at 
 the same temperature, under uniform atmospheric condi- 
 tions. For another fixed point the boiling temperature of 
 pure water, under uniform atmospheric conditions, is taken. 
 
 In making a thermometer, take a glass tube of small 
 uniform bore with a bulb at one end. Fill it partly with 
 mercury, removing all air; then seal it. To put on the 
 Centigrade scale, place the bulb in cracked ice and when 
 the mercury stops falling make a scratch on the glass op- 
 posite this point and mark it " 0." 
 
 Next place the bulb in the steam just above water boil- 
 ing under normal pressure. When the mercury stops rising, 
 mark this point " 100." Divide the space between into 
 
CHANGING FROM ONE SCALE TO THE OTHER 5 
 
 100 equal divisions, each one representing one degree change 
 of temperature. This is the most convenient thermometer 
 scale we have, since one of the fixed points is at 0. How- 
 ever, since the Fahrenheit scale is more commonly used in 
 this country, we must learn 
 
 Jjl /~1 
 
 that also and how to change 
 
 from one to the other. 212 -hi 100 
 
 On the Fahrenheit scale the 
 
 freezing temperature of water 180 Degrees \ 100 Degrees 
 is marked " 32," and the boil- . 
 ing point "212." The space 32 
 
 between is then divided into 
 180 equal parts, each called a FlGURE ! FIXED POINTS ON 
 
 j THE CENTIGRADE AND FAHREN- 
 
 aegree. HEIT ScALES> 
 
 5. Relation of the Two 
 
 Scales. The relation between the two scales is shown in 
 Figure 1. A little study of this figure will show you that 
 an equal space is divided into 100 Centigrade degrees and 
 180 Fahrenheit degrees. This means that the C. degree is 
 almost twice as large as the F. degree. 
 
 F 
 
 C 
 
 212 
 
 100 
 
 68 F 
 
 
 
 ?-20C 
 
 36 
 
 
 
 ?-ao 
 
 32 
 
 
 
 
 
 F C 
 
 That is, 100 C. = 180 F., or 
 
 
 
 1C. -WF.,or* F.;and 
 
 212 
 
 100 
 
 
 
 
 180 F. = 100 C., or 
 
 
 
 1 F. = iSS F., or | F. 
 
 >8F 
 
 | 20 C 
 
 
 
 
 
 
 6. Changing from 
 
 =36 < 
 
 
 
 20 
 
 One Scale to the Other. 
 
 32 
 
 
 
 
 
 -To change from one 
 
 
 
 scale to the other al- 
 
 
 r 
 
 ways make a sketch as 
 
 FIGURE 2. -SHOWING HOW TO CHANGE FROM shown in Fi S ure 2 > and 
 ONE SCALE TO THE OTHER. solve as follows : 
 
6 HEAT AND HEAT MEASUREMENT 
 
 Problem : Change 68 F. to the corresponding Centigrade reading. 
 68 F. - 32 F. = 36 F. above the fixed freezing point, (a) Figure 2. 
 36 X i = 20 C. above the freezing point on the Centigrade scale. 
 Therefore 20 C. is the corresponding Centigrade reading. 
 
 Problem : Change 20 C. to F. reading. 
 
 20 C. is 20 above freezing point. 
 
 20 X = 36 F. above freezing point on F. scale, (6) Figure 2. 
 
 But freezing on F. scale is 32 F. 
 
 Therefore 32 F. + 36 F. = 68 F., Fahrenheit reading. 
 
 Problems 
 
 1. Change from Centigrade to Fahrenheit readings : 40 C., 10 
 C., - 40 C. 
 
 2. Change from Fahrenheit to Centigrade readings : 60 F., 22 F., 
 40 F. 
 
 3. A change of temperature of 28 C. equals what change of tem- 
 perature on the Fahrenheit scale? 
 
 4. A range of 48 F. equals what range on the Centigrade scale ? 
 
 7. Freezing and Boiling Points. We have already said 
 that the freezing point is the temperature at which a liquid 
 changes to a solid. Such substances as iron, lead, gold, 
 paraffine, and mercury have a freezing or melting point, 
 each differing from the others. As you have already learned, 
 in making ices and ice cream, putting salt on the ice lowers 
 its melting temperature ; that is, a salt solution has a lower 
 freezing point than pure water. 
 
 When you put a kettle of water on the stove to boil, how 
 do you know when it is boiling? It is not when the vapor 
 begins to come from the kettle, but when it bubbles freely. 
 
 If a thermometer is placed in a pan of cold water over a 
 flame, the mercury gradually rises. When bubbles begin 
 to come out of the water, the mercury becomes stationary 
 and will never rise higher, no matter how long or how rapidly 
 
FREEZING AND BOILING POINTS 7 
 
 you heat the water, if the bubbles are free to escape. If 
 you examine the escaping bubbles at such a time, you will 
 find that they form at the bottom of the vessel, where the 
 heat is applied, rise to the top, and break. They are not 
 bubbles of air, but are bubbles of steam, able to push the 
 air and water back and thus get out of the water. These 
 steam bubbles have the same kind of molecules as liquid 
 water, but the molecules are so far apart that they form a 
 gas instead of a liquid. 
 
 The boiling point is that temperature at which the vapor 
 tension is equal to the applied pressure. The vapor tension 
 is the pressure exerted by the molecules of the vapor trying 
 to escape. The applied pressure is the pressure of the sur- 
 rounding element. 
 
 Freezing and Boiling Points of Some Common Substances 
 Under Normal Atmospheric Pressure 
 
 SUBSTANCE 
 
 FREEZING PT. 
 
 BOILING PT. 
 
 Oxvffcn 
 
 Centigrade 
 
 - 235 
 
 Centigrade 
 
 - 182 
 
 Ammonia 
 
 - 75 
 
 - 39 
 
 Ether . . .... 
 
 - 113 
 
 35 
 
 Methylic Alcohol 
 
 - 112 
 
 66 
 
 Distilled Water ....... 
 
 
 
 100 
 
 Acetic Acid 
 
 17 
 
 117 
 
 Turpentine 
 
 - 27 
 
 157 
 
 Fat, Oil etc. 
 
 - 33 
 
 210 
 
 Mercury 
 
 - 38.8 
 
 357 
 
 
 
 
 Hardly any two substances have the same freezing or 
 boiling points and some are used for specific purposes be- 
 cause of this. Mercury, for example, is used in the ther- 
 
8 HEAT AND- HEAT MEASUREMENT 
 
 mometer because its freezing point is low and its boiling 
 point is high. Ammonia is used in the manufacture of 
 artificial ice because its boiling point is low. Doughnuts 
 are dropped into hot fat instead of water because fat 
 boils at about 400 F. and so can be made hotter than 
 water. 
 
 8. Effect of Pressure on Freezing and Boiling Points. 
 When water is placed under a pressure it becomes more 
 difficult to freeze; that is, its freezing point is lowered. 
 Under normal atmospheric pressure water freezes at C. 
 or 32 F., but if it is put under a higher pressure it must 
 be cooled to a temperature lower than C. or 32 F. before 
 it will freeze. 
 
 An example of this is to be had in pressing a snow-ball. 
 A good time for snow-balling is when the snow is damp, 
 that is, when it is at the freezing point. The loose snow 
 is taken in the hands and pressed. This increased pressure 
 lowers the freezing point below the temperature of the 
 snow, and part of it melts. Then when the pressure is 
 removed the freezing point again goes up to C., and the 
 melted snow freezes again, making the ball hard. 
 
 If water were put into a strong vessel and sufficient 
 pressure were applied, the water would stay a liquid, even 
 in our coldest weather. 
 
 The effect of pressure on the boiling point is just the 
 opposite of what it is on the freezing point ; that is, pressure 
 raises the boiling point. Instead of boiling at 100 C. or 
 212 F., the water must be made hotter when a pressure 
 above that of the normal atmosphere is put on it. Water 
 in the boiler of a locomotive under a pressure of 200 pounds 
 per square inch boils at 380 F. instead of at 212 F. On 
 the other hand, water under a pressure less than normal 
 
EFFECT OF PRESSURE ON THE BOILING POINT 9 
 
 FIGURE 3. A 
 PRESSURE KETTLE. 
 
 atmospheric pressure boils at a lower temperature than 
 212 F. / 
 
 9. Application of Effect of Pressure on the Boiling Point. 
 - Water in an open kettle boils at a comparatively low tem- 
 perature on the top of a high mountain 
 because the pressure of the air is much 
 less than at the sea level. Sometimes 
 this temperature is lower than the cook- 
 ing temperature of starch ; and so at high 
 elevations it is possible to put potatoes 
 into an open kettle and boil the water 
 freely, without cooking the potatoes. In 
 the mountains this difficulty is sometimes 
 overcome by using a pressure kettle (Figure 3), that is, 
 a kettle with a lid screwed on, making it air-tight. This 
 lid holds the steam in the kettle and increases the pressure, 
 thereby raising the boiling point above the cooking tem- 
 perature. 
 
 Gelatin is a product which comes from the bones of 
 animals. To extract it from the bones a temperature 
 
 higher than 100 C. is necessary. 
 To get this higher temperature 
 the bones are cooked in a closed 
 vessel, under pressure. (Figure 3.) 
 In the manufacture of sugar the 
 principal thing is to evaporate the 
 water from the juice of the sugar 
 cane or sugar beet. This is done 
 by boiling, but when the syrup 
 begins to get thick, it is easily 
 burned; so it is put into vacuum pans (Figure 4) which 
 are closed, and part of the air and steam is pumped out, 
 
 Suction 
 
 FIGURE 4. A VACUUM PAN. 
 
10 HEAT AND HEAT MEASUREMENT 
 
 making the pressure inside lower than that of the atmos- 
 phere. This causes the syrup to boil at a lower temperature, 
 and so prevents scorching of the sugar. 
 
 10. Quantity of Heat. Temperature and quantity of heat 
 mean very different things. The water in a tea-kettle may 
 be at the same temperature as the water in a lake ; yet the 
 lake would have much more heat. Even if the water in 
 the tea-kettle were boiling, the lake would have more heat, 
 though the water in it might be ice-cold. 
 
 The term quantity of heat does not refer to the tempera- 
 ture of the body, but denotes the amount of energy in the 
 vibration of its molecules. 
 
 11. Heat Units. The quantity of heat can be measured, 
 but not by our familiar units of pound, gallon, foot, etc. 
 Other kinds of units must be used, and these are based 
 on the effect produced upon water when heat is applied. 
 They are B. T. U. (British Thermal Unit), calory, and great 
 calory. 
 
 The B. T. U. is the amount of heat required to raise 
 the temperature of 1 pound of water 1 F. The calory is 
 the amount of heat required to raise the temperature of 1 
 gram of water 1 C. The great calory is 1000 calories. 
 
 In these definitions we see that no certain degree is men- 
 tioned. This is because it takes approximately the same 
 amount of heat to raise the temperature of a certain amount 
 of water any one degree as to raise it any other degree. 
 
 Although the calory and B. T. U. are units of two distinct 
 systems, there is a definite relation between them. For 
 all practical purposes, 1 B. T. U. equals 250 calories, or 1 
 great calory equals 4 B. T. U.'s. 
 
 12. Heat of Fusion. If a piece of ice is placed in a pan 
 on the stove, the ice begins to melt; but the temperature 
 
HEAT OF FUSION 11 
 
 of the water does not rise. Both the ice and the water 
 stay at C. or 32 F. until all the ice is melted. After 
 that, the water begins to get warmer. The question is : 
 Where did all the heat go while the ice was melting? It 
 was used to melt the ice. 
 
 As we have learned, everything that occupies space is 
 made up of small particles, called molecules. When the 
 water is frozen solid, these molecules are drawn together 
 by a force called cohesion ; and this force keeps them in 
 place. When the ice melts, the molecules are torn apart, 
 and slip past one another, making it possible to pour the 
 water. To tear these molecules apart requires energy; 
 and this energy is the heat which melts the ice. 
 
 In other words, we can say: While the ice is melting, 
 the heat supplied is used to tear the molecules apart, chang- 
 ing the solid to a liquid. 
 
 Some substances require more energy to tear the mole- 
 cules apart than others ; so in order to melt some substances 
 more heat is required than to melt others. The heat required 
 to change a unit mass of a substance from a solid to a liquid 
 is called the heat of fusion of that substance. 
 
 If a pound of ice at 32 F. were put on the stove and 
 heated, it would have to take up 144 B. T. U.'s before it 
 would be all melted. If a gram of ice were used instead of 
 a pound, 80 calories would be required to melt it. 
 
 The heat of fusion of ice is the amount of heat required to 
 melt 1 pound of ice without changing its temperature. This 
 has been found to be 144 B. T. U.'s. (English system.) 
 
 Or, the heat of fusion of ice is the amount of heat required 
 to melt 1 gram of ice without changing its temperature. This 
 has been found to be 80 calories. (Metric system.) 
 
 On the other hand, when water freezes, it gives out as 
 
12 
 
 HEAT AND HEAT MEASUREMENT 
 
 much heat as it takes in when the same weight of ice melts ; 
 
 that is, when 1 pound of water freezes, it gives off 144 
 
 B. T. U.'s; and when 1 gram of water freezes, it gives off 
 
 80 calories. 
 
 13. The Refrigerator. Every one is familiar with the 
 
 refrigerator. It is a box with special walls so constructed 
 
 that heat cannot 
 easily get through 
 them. A com- 
 partment is made 
 to put ice in, and 
 at least one other 
 compartment is 
 made to hold the 
 butter, meat, fruit 
 or any article one 
 wishes to keep 
 cold. Later a 
 more thorough 
 study will be made 
 of the construc- 
 tion of the refrig- 
 erator. All we 
 are interested in 
 now is that it is 
 a box in which to 
 
 place ice to keep articles cool so they will remain fresh. 
 The ice, when placed in the refrigerator, begins to melt; 
 
 but, to melt, it must have heat. It takes the heat from 
 
 the other things in the refrigerator; and thus keeps them 
 
 cool. For every pound of ice that melts, 144 B. T. U.'s 
 
 must be used up. 
 
 FIGURE 5. A REFRIGERATOR. 
 
FREEZING ICE CREAM 
 
 13 
 
 FIGURE 6. LINE DRAWING OF AN 
 ICE CREAM FREEZER. 
 
 Two refrigerators can be tested as follows : Place equal 
 
 weights of ice in the two empty refrigerators. Close the 
 
 doors, and note the time re- 
 quired for the ice to melt in 
 
 each. The one in which the 
 
 ice melts first lets in the more 
 
 heat, and hence is not so 
 
 good as the one in which the 
 
 ice lasts longer. 
 
 14. Freezing Ice Cream. 
 
 The freezer in which ice cream 
 
 and ices are frozen is made up 
 
 of two compartments ; one, a 
 
 can, which fits very loosely 
 
 into the other, a wooden pail. 
 
 (Figure 6.) 
 The cream, with its other ingredients, is placed in the 
 
 inner can, which, in turn, is placed in the wooden pail. 
 
 Cracked ice, mixed with salt, is packed firmly around the 
 
 can. Then the can is 
 kept turning, so that 
 the cream will not freeze 
 in lumps. But what 
 makes the cream freeze 
 at all? When the ice 
 begins to melt it takes 
 the heat from the cream, 
 thus reducing its tem- 
 perature. 
 
 But the cream would 
 never freeze if salt had 
 not been put on the ice. 
 
 FIGURE 7. PHOTOGRAPH OF AN ICE 
 CREAM FREEZER. 
 
14 HEAT AND HEAT MEASUREMENT 
 
 When pure ice melts, its temperature is C. or 32 F., a 
 temperature at which cream will not freeze. But when 
 salt is mixed with the ice, the freezing point is lowered 
 until the temperature has been reduced several degrees 
 below C. or 32 F. This low temperature causes the 
 cream to freeze. 
 
 Salt is also used to melt the ice on a sidewalk in the winter 
 time. The salt reduces the freezing point of the ice to a 
 point below the temperature of the air, and so it melts, 
 even though the water is still freezing in the gutter. 
 
 15. Getting Heat from Freezing Water. Sometimes 
 when the weather is likely to be cold enough to freeze the 
 vegetables and fruits in the cellar, farmers put tubs of 
 water in the cellar to protect them. If water is in the 
 cellar, it will begin to freeze just as soon as the temperature 
 gets as low as C. or 32 F. The vegetables and fruits 
 will not freeze at this temperature, because they contain 
 solutions of sugar. As the heat leaks out of the cellar, 
 more water freezes, giving up its 144 B. T. U.'s per pound, 
 and keeping the temperature up to C. or 32 F. 
 
 This goes on as long as there is any water left unfrozen; 
 and so protects the vegetables and fruits. Should all the 
 water freeze, then the temperature may fall low enough for 
 these things to freeze also ; therefore, large tubs are used. 
 
 16. Effect of Heat of Fusion on Climate. In regions 
 near large bodies of water the climate is affected by the 
 high heat of fusion of water. The general effect is to make 
 both fall and spring come later. 
 
 At the end of summer, as the weather gets colder and 
 colder, the water begins to freeze. As it freezes, it gives off 
 144 B. T. U.'s per pound, and thus keeps the temperature 
 up to C. or 32 F. ; just as putting water in the cellar 
 
HEAT OF VAPORIZATION 15 
 
 to keep the vegetables from freezing kept the temperature 
 of the cellar up to C. or 32 F. This, then, causes the 
 fall to be late. 
 
 Again, at the end of winter, when the weather gets warmer, 
 the ice begins to melt. In melting, it takes in 144 B. T. U.'s 
 for every pound; and so keeps the temperature down to 
 C. or 32 F. ; just as putting ice in the refrigerator keeps 
 the things in it cold. Thus, the spring is also late. 
 
 This fact has much to do with fruit-raising. More fruit 
 is destroyed by changeable weather in the spring than by 
 anything else. If a few warm days come the last of March 
 or the first of April, the buds on the fruit trees start. Then, 
 if a cold snap comes, the buds are frozen, and the fruit is 
 ruined. Near a large body of water the melting ice may 
 prevent a warm period early in the season, so that the buds 
 do not start until there is no danger of frosts. 
 
 Problems 
 
 1. How many B. T. U.'s are required to melt 50 Ib. of ice in a re- 
 frigerator ? Where does the heat come from ? 
 
 2. When a tub of water, weighing 60 Ib., is placed in the cellar, 
 and it all freezes, how much heat is given up? Where does the heat 
 go? 
 
 3. How many calories are required to melt 25 grams of ice at C. 
 and raise its temperature to boiling? 
 
 4. If 100 grams of ice at C. are placed in 400 grams of water at 
 30 C., and if, after all the ice is melted, the temperature is 8 C., how 
 much heat was given up by each gram of ice in melting? 
 
 17. Heat of Vaporization. If a pan of water is placed 
 on the stove and heated, its temperature gradually rises 
 until the water begins to boil. After that, the temperature 
 remains constant until all the water is boiled away, just 
 as in the preceding experiment the temperature remained 
 
16 HEAT AND HEAT MEASUREMENT 
 
 constant until all the ice was melted. While the water is 
 boiling, the heat supplied goes to change the liquid to a gas. 
 
 We have seen that it takes heat to change ice to a liquid 
 and that the heat is used to tear the molecules apart. The 
 same thing happens when a liquid is changed to a gas. In 
 the form of a liquid, water still has the force of cohesion, 
 the force of holding its molecules together, so that the water 
 stays in a body and remains in the bottom of a vessel. 
 
 When the liquid changes to a gas or vapor, the molecules, 
 being much farther apart, do not attract one another per- 
 ceptibly, but fly as far apart as the containing vessel allows 
 them to go. The energy needed to tear them apart is the 
 heat we supply in boiling the water. 
 
 The amount of heat necessary to change a unit weight of a 
 liquid to a gas without changing its temperature is called its 
 heat of vaporization. 
 
 The heat of vaporization of water is the amount of heat 
 necessary to change 1 pound of water to steam without chang- 
 ing its temperature. This has been found to be 966 B. T. 
 U.'s per pound. (English system.) 
 
 Or, the heat of vaporization of water is the amount of heat 
 necessary to change 1 gram of water to steam without chang- 
 ing its temperature. This has been found to be 537 calories 
 per gram. (Metric system.) 
 
 When water vapor or steam condenses, it gives up the 
 same amount of heat as was taken in to vaporize it, that is, 
 537 calories per gram, or 966 B. T. U.'s per pound. 
 
 The heat of vaporization has many applications in steam 
 heating of houses, effect on climate near a large body of 
 water, steam cookers, double boilers, etc. 
 
 18. Steam Heating of Houses. Due to the great heat 
 t>f vaporization of water, steam is very commonly used for 
 
STEAM HEATING OF HOUSES 
 
 17 
 
 heating buildings. The steam is sent through radiators in 
 the rooms, and the 966 B. T. U.'s per pound, absorbed when 
 the water was changed to steam, is given to the air of the 
 room when the steam condenses in the radiators. 
 
 FIGURE 8. A STEAM-HEATING SYSTEM. 
 
 There are several systems of steam-heating. Figure 8 
 shows one of them. This is called the one-pipe system. 
 The steam is led out of the top of the boiler in the basement 
 to the radiators in the different rooms. Here it condenses, 
 
18 
 
 HEAT AND HEAT MEASUREMENT 
 
 gives off its heat, and the condensed water runs back down 
 the same pipe. 
 
 To get the steam into the radiator at the start, the little 
 stop-cock at the top of the radiator should be opened in 
 order that the air may get out and the steam take its place. 
 After the radiator is full of steam the valve can be closed, 
 and as fast as the steam condenses new steam will flow up 
 and take its place. Some radiators have stop-cocks which 
 
 are open when the radi- 
 ators are cold, but close 
 automatically when 
 heated by the steam. 
 19. The Steam Cooker. 
 - The steam cooker 
 (Figure 9) is a closed 
 box with shelves. It is 
 partly filled with water 
 and set on the stove, or 
 directly attached to a 
 stove with a separate 
 burner. When the water 
 boils, the steam fills the 
 space about the food on 
 the shelves. This hot 
 steam cooks the food, 
 
 without danger of burning. The steam cooker is well 
 adapted for cooking puddings, custards, etc. 
 
 20. The Double Boiler. The double boiler is a com- 
 bination of two vessels. (Figure 10.) 
 
 The smaller, containing the food to be cooked, is set in- 
 side a larger vessel, partly filled with water. The food can 
 be cooked for a long time and cannot burn as long as there 
 
 FIGURE 9. A SIMPLE STEAM COOKER. 
 
DISTILLATION 
 
 19 
 
 FIGURE 10. LINE DRAWING OF A 
 DOUBLE BOILER. 
 
 is water in the outer vessel. The temperature never rises 
 above 100 C. or 212 F. 
 
 21. Distillation. Theques- 
 tion of pure drinking water is 
 of vital importance, especially 
 in large cities. Sometimes 
 chlorine is put into the water 
 to kill the germs. As chlorine 
 is very distasteful to some 
 people, they prefer to buy, or 
 prepare, distilled water. 
 
 The process consists of boil- 
 ing the water, converting it 
 into steam, and then con- 
 densing this steam, thus pro- 
 curing pure water. Figure 12 shows the principle used 
 even in large establishments. 
 
 Water is heated in a boiler (B), and the steam is conducted 
 through a pipe to a coil (C), in a tank of running cold water. 
 
 The cold water is supplied 
 by a hose from the city 
 water main to the point a, 
 and when warmed flows 
 out of the opening b into 
 the sewer or into a tank. 
 The steam, passing through 
 the coil, is condensed, giving 
 up its 966 B. T. U.'s per 
 pound to the cold water, 
 and then runs out of the coil 
 as pure water. It is pure because only the water will evapo- 
 rate ; hence only pure water vapor is in the coil to condense. 
 
 FIGURE 11. PHOTOGRAPH OF AN 
 ALUMINUM DOUBLE BOILER. 
 
20 
 
 HEAT AND HEAT MEASUREMENT 
 
 Distillation is used to refine other substances, such as 
 alcohol and turpentine. But in these cases the substance 
 has to be distilled several times, and the process is then 
 called fractional distillation. 
 
 In the case of alcohol, the liquid which contains the 
 alcohol is placed in a boiler and heated, the temperature 
 
 FIGURE 12. DIAGRAM OF A SIMPLE DISTILLATION SYSTEM. 
 
 being kept at the boiling point of alcohol, which is below 
 the boiling point of water. The alcohol vapor is driven off, 
 but with it a little water evaporates. When this is con- 
 densed again, it still contains some water. This new liquid 
 is again distilled, yielding a product more nearly pure alcohol. 
 This process is kept up until the liquid is as nearly pure as 
 desired. 
 
ARTIFICIAL ICE PLANT 21 
 
 22. Other Applications of Heat of Vaporization. In 
 
 the summer time, regions far inland get very warm. But 
 near a large body of water the heat is less intense because, 
 in evaporating, the water takes up 966 B. T. U.'s for every 
 pound evaporated; and thus keeps the temperature lower 
 than it would otherwise be. 
 
 You have probably noticed that the air gets cooler after 
 you have sprinkled the street or lawn. The water on the 
 ground begins to evaporate, taking heat from the ground 
 and air, thus lowering the temperature. The same thing 
 occurs after a rain. 
 
 Nature uses the same principle to keep your body cool. 
 When you exert yourself strenuously, or when the day is 
 warm, perspiration is thrown out to the surface by the skin. 
 This perspiration evaporates, taking the heat from the 
 body to do it. Would you get as cool if you removed the 
 drops with your handkerchief? 
 
 23. Artificial Ice Plant. In making artificial ice, the 
 same principles apply as in natural evaporation and freezing. 
 The ice freezes as naturally as the ice on a lake. The only 
 artificial part is the producing of the low temperature. 
 Nature does the rest. 
 
 The artificial ice plant (Figure 13) consists of four prin- 
 cipal parts: a cooling coil (A) for the ammonia gas; a 
 force pump (P) for compressing the ammonia gas; an 
 expansion coil (B) where the brine cools; and a freezing 
 tank (C) where the ice is frozen. 
 
 The operation of the plant is as follows : the force pump 
 P draws the ammonia gas through the valve d and forces 
 it through the valve e, under high pressure. From here it 
 is led through the coils in the tank (^4), where it is cooled 
 by running cold water. 
 
22 
 
 HEAT AND HEAT MEASUREMENT 
 
 As the gas, under high pressure, becomes cool, it con- 
 denses and is led out of the coil at the bottom as liquid 
 ammonia. At the stop-cock / the liquid is allowed to flow 
 through slowly, and there it turns to a gas and expands sud- 
 denly. This evaporation and expansion require a great 
 amount of heat. 
 
 As this evaporation and expansion take place in the coil 
 in the tank (B), the heat is taken from the brine in tank (B), 
 
 FIGURE 13. DIAGRAM OF A SIMPLE ARTIFICIAL ICE PLANT. 
 
 thus reducing its temperature several degrees below C. 
 or 32 F. The ammonia gas then passes on up to the force 
 pump, to be again compressed and used over. The cold 
 brine is pumped from tank (B) to tank (C). In (C) are 
 placed the molds containing pure water. The heat passes 
 from the water to the brine, and thus the water freezes. 
 
 In iceless refrigerators cold brine is pumped through 
 coils just as in the artificial ice plant. Modern meat mar- 
 kets use this method. 
 
 The ice in artificial ice skating rinks is frozen by the n ethod 
 
WATER VAPOR IN THE AIR 23 
 
 above. Coils of pipe are placed on the bottom of the 
 floor, and then enough water is run over it to cover these 
 pipes an inch or two. Brine is pumped through the pipes, 
 which in turn freezes the water. In this way ice skating 
 can be had at any time of the year. 
 
 Problems 
 
 1. Find the heat required to evaporate two pounds of water with- 
 out changing its temperature. 
 
 2. Find the heat required to evaporate 1500 grams of water with- 
 out changing its temperature. 
 
 3. If, in making jelly, one half of the weight of the juice is boiled 
 away, how much heat is required to make one quart of jelly ? (Take 
 weight of juice as eight pounds per gallon, and starting temperature 
 as 62 F.) 
 
 4. When ten pounds of steam is condensed in your radiator, how 
 much heat is given to the room ? 
 
 24. Water Vapor in the Air. When water is boiled 
 away in a tea-kettle or a pan, or when it evaporates from 
 any body of water, the water seems to disappear; but it 
 does not go out of existence. It simply goes into the air 
 and is invisible. The molecules of water vapor mix with 
 the molecules of other substances in the air, of which they 
 become a part. 
 
 There is a limit to the amount of water vapor that the 
 air will hold, and this limit depends upon the temperature 
 of the air. The warmer the air, the more vapor it will 
 hold. 
 
 When the air contains all the water vapor it will hold, it 
 is said to be saturated, or to have reached the saturation 
 point. The saturation point depends upon the temperature. 
 
 The following table shows the vapor tension of water 
 under normal pressure at different temperatures. 
 
24 
 
 HEAT AND HEAT MEASUREMENT 
 
 TEMPERATURE 
 
 VAPOR TENSION 
 (cm. of mercury) 
 
 TEMPERATURE 
 
 VAPOR TENSION 
 (cm. of mercury) 
 
 oc. 
 
 0.460 
 
 21 C. 
 
 1.862 
 
 16 C. 
 
 1.362 
 
 22 C. 
 
 1.979 
 
 17 C. 
 
 1.440 
 
 23 C. 
 
 2.102 
 
 18 C. 
 
 1.546 
 
 24 C. 
 
 2.232 
 
 19 C. 
 
 1.645 
 
 25 C. 
 
 2.36- 
 
 20 C. 
 
 1.751 
 
 100 C. 
 
 76.000 
 
 25. The Hygrometer. An instrument used to measure 
 the amount of water vapor in the air is called a hygrometer. 
 Figure 14 shows a common form of the hygrometer. It 
 consists of a small spring, a pointer, and a scale. The scale 
 
 denotes the per cent 
 of water vapor in the 
 air, complete satura- 
 tion being 100 per 
 cent. For example, a 
 reading of 65 per cent 
 means that there is 
 65 per cent as much 
 water vapor in the air 
 as it would hold if 
 saturated. 
 
 By knowing the 
 weight of vapor re- 
 FIGURE 14. THE HYGROMETER. quired at a certain 
 
 temperature to satu- 
 rate the air, with the hygrometer reading it is easy to com- 
 pute the exact weight of vapor that is in the air. 
 
 If saturated air is heated to a higher temperature, it will 
 hold more vapor ; but if saturated air is cooled it will hold 
 less, and some of the vapor must condense. 
 
SNOW AND HAIL 25 
 
 26. Dew. If warm air comes in contact with a cold 
 object it may be cooled below the saturation point and some 
 of its water vapor may condense and appear as drops on 
 the cold object. These drops are called dew. You have 
 all seen a pitcher of ice water sweat in the summer time. 
 The pitcher does not really sweat, but merely has dew on it. 
 
 Dew also forms on grass and on the leaves of trees. During 
 the night small objects, such as the grass blades and leaves, 
 radiate their heat ; and thus become cooler than the surround- 
 ing objects. These grass blades and leaves then cool the 
 air that touches them, and dew forms when the air is moist. 
 
 27. Fog and Clouds. If a cool current of air strikes a 
 warm current, the warm air is cooled below the saturation 
 point, and the surplus water vapor condenses, in very small 
 particles, but large enough to be visible. If this condensa- 
 tion occurs near the surface of the earth, it is called fog. 
 If it occurs high in the air, it is called clouds. The greatest 
 fog region in the world is just off the banks of Newfound- 
 land, where the cold air from the north meets the warm 
 air from the Gulf Stream. 
 
 28. Mist and Rain. If, in the case of fog, the condensed 
 particles become sufficiently large to fall slowly, they are 
 called mist. If these particles become large enough to fall 
 rapidly, they become drops and are called rain. 
 
 29. Snow and Hail. When the water vapor is forced to 
 condense at a temperature below the freezing point, the 
 small particles freeze as they condense and form snow- 
 flakes. The flakes get larger and larger as they come into 
 contact with one another in the air. 
 
 The formation of hail is more complex than that of the 
 other forms of condensed water vapor we have noted. 
 Scientists are not entirely agreed as to the facts concerning 
 
26 HEAT AND HEAT MEASUREMENT 
 
 the process. The theory generally accepted is that a small 
 particle of water is condensed and frozen high up in the air. 
 It starts to fall and collects on its surface a layer of water ; 
 but before it hits the earth it is carried up again by an up- 
 ward current of air. This water freezes on its surface, 
 while at the high altitude, forming a new layer of ice. Again 
 it starts to fall, and collects a new layer of water, only to be 
 carried up again by another upward current. This process 
 is repeated until the hail stone becomes so heavy that it 
 cannot be carried up any more. 
 
 This theory of formation is based upon the structure of 
 a hailstone. When cut open, it is found to be made up of 
 distinct layers; some of clear ice and some of snow ice. 
 
 30. Heat Capacity. If you heat a five-pound flat-iron 
 to the boiling point, and place it in a pan of cold water, and 
 if you then pour five pounds of boiling water into another 
 pan containing an equal amount of equally cold water, you 
 will find that the five pounds of boiling water have made 
 the pan into which it was poured much warmer than the 
 flat-iron has made the pan in which it was placed. 
 
 What conclusion would you draw from this? Note that 
 the weights of the boiling water and the hot iron were the 
 same; that they were at the same temperature; and that 
 they were put into the same weights of water, which were 
 also at the same temperature. The answer is, the water 
 contained more heat than the iron. Different substances 
 hold different amounts of heat at the same temperature. 
 In other words, they have different capacities for heat. 
 
 The definitions of our heat units are based on the heat 
 capacity of water. We say that when 1 gram of water is 
 heated 1 C., a calory is put into it ; and that, if 1 pound of 
 water is heated 1 F., a B. T. U. is put into it. 
 
SPECIFIC HEAT 27 
 
 But if a gram of any substance other than water were to 
 be heated 1 C., it would not take exactly 1 calory, but a 
 certain fraction of a calory, depending upon the substance. 
 
 The heat capacity of a substance is the heat required to 
 raise a unit weight of the substance 1. If it is in the English 
 system, it is the number of E. T. U.'s required to raise 1 
 pound of the substance 1 F. ; if it is in the metric system, 
 it is the number of calories required to raise 1 gram of the sub- 
 stance 1 C. 
 
 31. Specific Heat. As the heat capacity of pure water 
 is uniform, substances having different heat capacities are 
 compared with water as a standard. From this comparison 
 we get the term specific heat. The specific heat of a sub- 
 stance is the ratio of the heat capacity of the substance to the 
 heat capacity of pure water. 
 
 Eliminating the idea of heat capacity, we can define specific 
 heat in this way : Specific heat is the ratio between the amount 
 of heat necessary to raise a certain weight of the substance 1 
 and the amount of heat necessary to raise the same weight of 
 pure water 1 ; or 
 
 Heat to raise substance 1 
 
 specific Heat = 77 ; ? . , , 73 
 
 Heat to raise equal weight oj water 1 
 
 Table of Specific Heats of Some of Our Most Common Substances 
 
 SUBSTANCE SPECIFIC HEAT 
 
 Aluminum 22 
 
 Brass 094 
 
 Copper 095 
 
 Iron 1138 
 
 Mercury 038 
 
 Lead 031 
 
 Ice 5 
 
 Air (at constant pressure) .2375 
 
 Hydrogen (at constant pressure) 3.4 
 
 Steam (at constant pressure) 48 
 
28 HEAT AND HEAT MEASUREMENT 
 
 32. Application of Specific Heat. The 
 high specific heat of water has a powerful 
 effect on the climate of regions near a large 
 body of water. This effect is the same as 
 that produced by the high heat of fusion. 
 The principle is slightly different, for the heat 
 is used to raise the temperature of the water, 
 instead of to melt the ice. (See 16.) The 
 effect is much greater than it would be if the 
 HO WATER body were mercury or alcohol or any substance 
 BOTTLE. whose specific heat is less than that of water. 
 
 The hot water bottle is an application of specific heat. 
 
 It is better than a hot flat-iron or other hot object, not only 
 
 because it is more convenient, but also because it holds more 
 
 heat. 
 
CHAPTER II 
 EXPANSION 
 
 33. Expansion. One effect of heat is to make the 
 molecules of a body vibrate faster. This increase in speed 
 causes the molecules to take up more space. The mole- 
 cules themselves do not get any larger, but they require 
 more free space in which to vibrate. 
 
 Suppose a number of people were to stand close together, 
 with a large rubber band stretched around the whole crowd. 
 If all stood perfectly still, they could get into a compara- 
 tively small space. But if every one began swaying and 
 elbowing his neighbor, each person would take up more 
 room, and consequently the space occupied would be larger, 
 and the rubber band would have to stretch. 
 
 This is what takes place when a body is heated ; and we 
 call it expansion. Expansion is the increase in length or 
 volume of a body. 
 
 34. Coefficient of Linear Expansion. All substances do 
 not expand at the same rate. For example, a bar of iron a 
 foot long would not expand as much as a bar of brass a 
 foot long, if both were heated through the same range of 
 temperature. In order to have a way of expressing how 
 much a substance expands we use the term coefficient of 
 linear expansion. 
 
 The coefficient of linear expansion of a substance is its 
 expansion per unit length per degree C. 
 
 29 
 
30 EXPANSION 
 
 Suppose a bar of aluminum, 60 cm. long at 25 C. (Figure 
 10), gets .1 cm. longer when heated to 100 C. The in- 
 crease in temperature 
 from 25 C. to 100 C. 
 
 -60 cmr 
 
 is 75 C. If the bar 
 h 1 cm. ex P an ds .1 cm. for 75 
 
 FIGURE 16.- EXPANSION OF A ROD. 
 
 C., it will expand ; 
 /o 
 
 cm. for 1 C. If 60 cm. expand 7 cm., then 1 cm. will 
 
 7o 
 
 expand ^ cm. or 7^ cm. - .000022 +cm. 
 
 / O A vJU -OUU 
 
 The number .000022 is called the coefficient of linear ex- 
 pansion of aluminum. 
 
 Table of Coefficients of Linear Expansion 
 SUBSTANCES COEFFICIENT 
 
 Aluminum 0000222 
 
 Brass 0000187 
 
 Copper 000017 
 
 Glass .0000083 
 
 Iron . . . . -. 0000112 
 
 Platinum 0000088 
 
 Steel 000013 (tempered) 
 
 Steel . . . 000011 (untempered) 
 
 If the range in temperature is given in F. degrees, then the above 
 coefficients must be multiplied by f . 
 
 35. The Thermostat. The thermostat which regulates 
 the heat of our rooms uses the principle of expansion. It 
 is constructed as shown in Figure 17. The pointer (P) is 
 made of a strip of steel (<S) and a strip of brass (B), laid 
 side by side and fastened so that they cannot slip on each 
 other. One end is fixed, and the other end is free. Electric 
 
THE THERMOSTAT 
 
 31 
 
 connections are made as shown in the figure. The battery 
 (Bat.) is placed in the circuit, together with two magnets 
 (Mi and M 2 ). 
 
 The thermostat is placed 
 in the room, the temperature 
 of which is to be regulated, 
 and the magnets (Mi and 
 M 2 ) are placed in the base- 
 ment. The wires lead from 
 the thermostat to the mag- 
 nets. When the room gets 
 
 FIGURE 17. DIAGRAM OF A THERMO- 
 STAT AND SYSTEM. 
 
 too warm, the two metals ex- 
 pand ; but the brass expands 
 the faster. This makes the 
 pointer bend and touch the 
 connection x, thus operating 
 magnet M 2 . Magnet M 2 re- 
 leases a spring which closes 
 the draft of the furnace, and 
 this allows the room to cool. 
 When it gets cool enough, 
 the two metals contract ; but 
 the brass one contracts the 
 more. This makes the pointer 
 bend in the other direction, 
 
 and it touches the contact point y. This operates magnet 
 MI, which releases a spring opening the draft. In this 
 
 FIGURE 18. PHOTOGRAPH OF THE 
 SENSITIVE PART OF A THERMOSTAT. 
 
32 
 
 EXPANSION 
 
 way a room may be automatically kept at an even 
 temperature. 
 
 36. Compensating Pendulum of a Clock. The pendulum 
 of a clock is the regulator which makes the clock run evenly. 
 If the pendulum is too short, the clock runs too fast; and 
 if it is too long, it runs too slowly. 
 
 Since metals expand when heated, a clock 
 will not run correctly at different tempera- 
 tures unless a special pendulum is arranged. 
 When a pendulum is so arranged that a 
 change in temperature does not affect it, it 
 is called a compensating pendulum. 
 
 One kind of compensating pendulum is 
 shown in Figure 20. The dark lines repre- 
 sent rods which are made of brass, while the 
 other ones represent 
 rods of steel. By 
 looking at the figure 
 you will see that the 
 steel rods make the 
 pendulum longer 
 when they expand, 
 and the brass rods 
 make it shorter when 
 they expand. The 
 lengths of brass and 
 steel are so calcu- 
 lated that whenever 
 the steel rods let the 
 bob down the brass 
 rods lift it up the 
 FIGURE 19. A THERMOSTAT INSTALLED. same amount. This 
 
BALANCE WHEEL OF A WATCH 
 
 33 
 
 keeps the pendulum at the same length, regardless of the 
 temperature. Another method of accomplishing the same 
 thing is shown in Figure 21. 
 
 The pendulum has a cup at the 
 bottom, containing mercury. As the 
 temperature rises, the rod of the 
 pendulum becomes longer ; but at 
 
 the same time 
 
 the mercury ex- 
 pands and rises 
 
 in the cup, thus 
 
 counteracting 
 
 the expansion 
 
 of the rod. 
 37. Balance 
 
 Wheel of a 
 
 Watch. Good 
 
 watches have to 
 
 Jbe so made that 
 
 change of tem- 
 perature will 
 
 not affect them. FIGURE 20. A COMPEN- 
 
 rpi i i SATING PENDULUM WITH 
 
 The balance BRASS AND STEEL RODS. 
 
 wheel is to the 
 
 watch what the pendulum is to a 
 clock. If the wheel gets larger, the 
 watch runs more slowly; and vice 
 versa. The rim of the wheel (Figure 
 22) is made of two metals, steel and 
 brass, just as is the pointer of the 
 thermostat. The brass is put on the 
 A MERCURY WELL. outside of the rim ; so that, whea 
 
34 
 
 EXPANSION 
 
 FIGURE 22. BALANCE WHEEL OF 
 A WATCH. 
 
 the temperature rises and the spoke gets longer, the brass 
 
 expands faster than the steel and makes the rim curve 
 
 more, tending to make the 
 wheel smaller. These two 
 effects exactly counterbalance 
 each other, and so the watch 
 keeps even time. 
 
 38. Hot Water Dangerous 
 to Glassware. Each of us 
 has probably broken glass- 
 ware by putting hot water 
 into it. Why does hot water 
 break the glass into which it 
 is poured? Unequal expan- 
 sion is the cause. 
 As the water goes into the glass the inside is heated first, 
 
 and so expands ; while the outside does not. This puts the 
 
 glass under a great stress, and so it breaks. 
 
 You feel the same effect in your teeth when you take a 
 
 bite of ice cream or drink ice water. The outside of the 
 
 teeth is cooled and contracts before the inside can cool off; 
 
 and so the nerves are squeezed under a high pressure. 
 
 If glasses are put into a pan of water and brought slowly 
 
 to a boil, they will not break; nor will a very thin glass 
 
 break as easily as a thick one when filled with hot water. 
 
 Explain. 
 
 When glass stoppers stick, they can often be gotten out 
 
 of bottles by applying a flame to the neck of the bottle for 
 
 a short time. This causes it to expand and so loosens the 
 
 stopper. 
 
 Thrusting the neck of the bottle into warm water will 
 
 produce the same result. 
 
EXPANSION EFFECTS WHEN WATER IS HEATED 35 
 
 39. Coefficient of Cubical Expansion. When a body is 
 heated, it gets larger in every direction. Therefore it has 
 more volume. This increasing in volume is called volume 
 expansion. The coefficient of volume expansion is the 
 increase in volume per degree C., per unit volume. 
 
 Since a body expands in three directions, its coefficient 
 of volume expansion is approximately three times its coeffi- 
 cient of linear expansion. 
 
 For example : What is the increase in volume of 1000 cubic centi- 
 meters of aluminum for a range of 50 C. ? 
 
 The coefficient of linear expansion for aluminum is .000022 ; so the 
 coefficient of volume expansion is .000022 X 3 = .000066. 
 Then 1000 X .000066 X 50 = 3.3 c.c. 
 Therefore the 1000 c.c. of aluminum will increase 3.3 c.c. ; 
 or will then contain 1000 + 3.3 = 1003.3 c.c. 
 
 Problems 
 
 1. Find the increase in length of an aluminum bar 60 cm. long 
 when it is heated from 22 C. to 100 C. 
 
 2. If an iron steam pipe leading from the boiler in the basement 
 to an upper story room is 120 ft. long, and 20 C., how much will it 
 expand when steam at 100 C. is passed through it? 
 
 3. Will the lids fit tighter when the stove is hot or when it is cold? 
 Why? 
 
 4. If the pointer of a thermostat is 2" long, and is made of brass 
 and steel, what is the difference in length of the brass and steel when 
 it is heated 10 C. ? 
 
 5. How much will a copper wire 10 ft. long expand in length if 
 heated from 60 F. to 180 F. ? 
 
 6. How much will 6000 c.c. of brass expand when heated from 
 32 F. to 212 F. ? 
 
 7. Will a glass flask hold more when hot or cold ? Why ? 
 
 40. Peculiar Expansion Effects when Water is Heated. 
 Nearly all of our common substances expand when heat is 
 
36 EXPANSION 
 
 applied, regardless of their state and temperature. For 
 example a piece of iron will expand when heated ; and when 
 it melts, it still expands; and when the molten metal is 
 heated, it still expands; and likewise when it is vaporized 
 and the gas is heated. Expansion takes place whenever 
 heat is applied. 
 
 But there is an exception to this rule. The exception is 
 when ice is melting, and when the water is heated from 
 C. to 4 C. 
 
 If a piece of ice at a temperature below C., say 10 C., 
 is heated, its temperature rises to C., and the ice increases 
 in volume. Then, if more heat is applied, the ice melts, the 
 temperature remaining at C. ; but the volume decreases. 
 After it is all melted, the temperature again rises ; and until 
 4 C. is reached, the water still contracts. After 4 C. is 
 reached, the temperature continues to rise to 100 C., but 
 the water expands. At 100 C. the water changes to steam, 
 the temperature remaining at 100 C. until it is all steam ; 
 and the volume increases to about 1650 times its former 
 volume. If, after the water is all steam, it is still heated 
 at constant pressure, the temperature increases, and the 
 gas expands. 
 
 The best way to remember all this is to keep in mind 
 that water is like all other common substances and expands 
 when heated, except when melting and being raised from 
 C. to 4 C. 
 
 41. Importance of 4 C., the Temperature at which Water 
 is Densest. Did you ever think why the rivers and 
 lakes freeze on top instead of at the bottom? The reason 
 is that water is densest, or, in other words, heaviest, 
 per cubic unit, at 4 C. 
 
 In the summer time the temperature of the water may 
 
WHY WATER PIPES BURST 37 
 
 reach 18 C. or 20 C. As the weather gets cooler in the 
 fall, the top layers of water are cooled by the air. They 
 are then heavier than the layers below them ; so they sink 
 until they come to water as cool as, or cooler than, they are. 
 This leaves exposed to the air a new layer which in turn 
 cools and sinks. 
 
 This displacement is kept up until the whole body of 
 water is cooled to 4 C. Then, when the top layer gets 
 colder than 4 C. it expands, and becomes lighter than the 
 water below it ; therefore, it remains on top, continuing to 
 get colder and lighter. When it reaches C., it freezes 
 and expands still more. This ice layer protects the un- 
 frozen water, which remains at 4 C., except for the layers 
 next the ice. 
 
 If water were like mercury and continued to contract as it 
 cooled, large bodies of water would freeze solid in cold 
 weather. The water would cool at the top and sink, letting 
 the warmer water come to the surface. This would con- 
 tinue till all the water was at the freezing point, when the 
 top would begin to freeze. Then the ice would sink ; and 
 the lakes and rivers would be frozen from the bottom up. 
 In a cold winter they would be a mass of solid ice. 
 
 Then in the summer the ice would melt only on top, 
 leaving the lake almost a solid cake of ice. The result 
 would be a climate too cold for vegetable life. 
 
 42. Why Water Pipes Burst. When water is allowed 
 to remain in the water pipes in very cold weather, it freezes 
 and expands, thus breaking the pipes. The ice acts as a 
 plug in the pipe, otherwise the expansion would force the 
 water back into the water mains, in which case the pipes 
 would not break. It is because the water is imprisoned in 
 the pipe behind the ice plug that the pipe must give way. 
 
38 EXPANSION 
 
 43. Expansion of Gases. We found, from our study of 
 expansion of liquids and solids, that they all expand at a 
 different rate, making it necessary to have a table of 
 coefficients of expansion. In the case of gases this 
 is different,, all gases expanding at the same rate. There- 
 fore there is only one coefficient of expansion for all 
 gases. 
 
 If a certain volume of gas be heated 1 C., it will expand 
 -2T3 of its volume at C., if kept at the same pressure. 
 This fraction, g-fs-, or -00366, is the coefficient of expansion 
 of gases. 
 
 If 273 c.c. of oxygen, hydrogen, air, or any other gas, 
 were heated from C. to 1 C., the gas would expand 
 JT-J of 273 c.c. = 1 c.c. Therefore the same amount of 
 gas would fill a vessel of 274 c.c. at the new temperature, 
 the pressure remaining the same. 
 
 44. Absolute Zero. Gases, like all substances, are com- 
 posed of molecules; but under normal pressure and tem- 
 perature the molecules are comparatively far apart. It 
 has been said that if the molecules of a gas, such as ordinary 
 air, were magnified until they were the size of an orange, 
 each molecule would be surrounded by a space equal to a 
 cubic yard. If this is true, the space actually taken up 
 by the molecules is very small, and the empty space about 
 them is large. 
 
 When heat is applied, each molecule flies faster than 
 usual, bumping its neighbors farther apart, thus making 
 the space about it larger. If the gas is cooled, the mole- 
 cules move more slowly than usual ; and consequently come 
 closer together. The more the gas is cooled, the more slowly 
 the molecules move, until, theoretically, they come to rest. 
 There is then absolutely no heat in the gas. When at rest 
 
SOME APPLICATIONS OF CHARLES' LAW 39 
 
 they occupy so little space that it is not counted at all; 
 and the gas is said to have no volume. 
 
 The temperature at which a gas has no volume is 
 273 C. This temperature is then called absolute zero, 
 because it means total absence of heat. 
 
 45. Charles' Law. A man by the name of Charles 
 formulated a law about the expansion of gases. This is 
 called Charles' Law : 
 
 " The volume of a gas at constant pressure is proportional 
 to its absolute temperature" 
 
 Example : What is the volume of a gas at 70 C., if it 
 occupies 800 c.c. at 20 C. ? 
 
 Solution : The original absolute temperature is 20 + 273 = 293 ; and 
 the final absolute temperature is 70 + 273 = 343. Since, by Charles' 
 Law, the volume of a gas is proportional to its absolute temperature, 
 the new volume is ftf of 800 c.c. = 936.5 + c.c., or 
 
 7 new absolute temperature ^, . . , , 
 
 new volume = X original volume. 
 
 old absolut : temperature 
 
 46. Some Applications of Charles' Law. The expansion 
 of gases has much to do with the baking of bread, cake, or 
 pie. 
 
 To make bread, yeast is used to produce the rising. The 
 dough is mixed and allowed to stand in a warm place. The 
 yeast plants grow and, in growing, give up carbon dioxide 
 gas. The dough does not allow this gas to escape; so it 
 forms bubbles in the dough, causing it to rise. The dough 
 is then " worked down," and again allowed to rise in the 
 same way. Usually it is " worked down " a second time 
 and again allowed to rise. When it has risen properly, it 
 is placed in a hot oven and baked. 
 
 Up to this time the rising has been caused by the growing 
 yeast plants. But when it is put into the oven, the heat 
 
40 EXPANSION 
 
 kills the yeast plants; so the rising after that is due to 
 something else. The carbon dioxide bubbles in the dough 
 are heated. According to Charles' Law, they expand -^TS 
 of their volume at 0C. for every degree Centigrade they are 
 raised in temperature. This makes the bread rise while 
 it is baking. 
 
 In baking biscuits and cakes, baking powder is used 
 instead of yeast. But the action is the same. Baking 
 powder, when wet, gives off carbon dioxide. The rising 
 takes place as in the case of the yeast. Expansion also 
 takes place when the cake or biscuits are placed in the oven. 
 
 In making pie crust there is usually nothing put into the 
 dough to make it rise. But the crust must rise a little ; or 
 else it will be tough, instead of brittle and flaky. The ex- 
 pansion of gases is used to produce this rise. In mixing, 
 the dough should be worked very lightly and the flour 
 should be sifted in. Doing this gets air into the dough, 
 and the light working leaves it there. Then if the dough 
 is chilled by placing it in the refrigerator, the open spaces 
 will fill up with cold air. This cold air will expand when 
 the pie is baked, producing a brittle, flaky crust. 
 
 On the other hand, in clay modeling care is taken to work 
 all the air out. The clay is kneaded and pounded and 
 squeezed so that no air is left in it. If the air is not all out, 
 when the piece is fired in the kiln these bubbles expand and 
 break the piece of pottery. 
 
 Other applications of the expansion of gases, which will 
 be studied under another topic, are : the draft in a stove, 
 grate, furnace, chimney, range; hot-air heating; and 
 ventilation. 
 
CHAPTER III 
 HEAT TRANSFERENCE 
 
 47. Transference of Heat. Heat is transferred from 
 one place to another by three methods, conduction, convection, 
 and radiation. Each of these will be taken up in detail. 
 
 48. Conduction. If heat is applied to one part of a 
 body, the molecules will be set into rapid vibration at that 
 point. These molecules strike their neighbor molecules 
 and set them in vibration. These in turn set the next ones 
 going, and the heat travels along the body by conduction. 
 
 If one end of a poker is placed in the fire, that end gets 
 hot, and all the rest of the poker is warmed. But the 
 temperature is lower, the farther away from the end in the 
 fire. 
 
 Different materials conduct heat at different rates. Those 
 that conduct it very readily are called good conductors. 
 Those that do not conduct heat readily are poor conductors, 
 or are good insulators. Silver, copper, gold, aluminum, iron, 
 and nearly all other metals are good conductors. Among 
 the poor conductors, or good insulators, are asbestos, a 
 vacuum, air space, water, paper, wood, glass, cloth, por- 
 celain, horn, and ivory. 
 
 49. Non-conducting Handles for Cooking Utensils. - 
 Figures 23, 24, 25, and 26 show different methods used to 
 keep the handles of cooking utensils cool. The teakettle 
 is made of metal, all except the handle, and that is made 
 
 41 
 
42 
 
 HEAT TRANSFERENCE 
 
 of wood. The metal becomes hot by conduction, but the 
 wood does not let the heat through. 
 
 The coffee-pot and the percolator have handles of wood, 
 porcelain, horn, or ivory, for the same reason. The stove- 
 
 poker has a metal Porcelain, Horn or Ivory 
 
 handle, but it consists 
 
 Wood 
 
 FIGURE 23. WOOD HANDLES 
 ON A TEA-KETTLE. 
 
 FIGURE 24. THE HANDLES OF THE 
 COFFEE-POT ARE INSULATED. 
 
 Horn or Wood 
 
 lass 
 
 of a wire wound in a coil about the end of the poker. 
 
 This allows air space between the poker and the wire 
 
 handle, and this air space is a good insulator. 
 
 50. Good Conductor Bottoms on Utensils. The bottoms 
 
 of coffee-pots, tea-kettles, wash-boilers, etc., are usually of 
 
 copper. This is for two 
 reasons. First, copper will 
 not corrode as readily as iron 
 or tin; and therefore will 
 keep cleaner and last longer. 
 Second, copper is a good con- 
 ductor, so that the heat is 
 readily conducted from the gas 
 
 FIGURE 25. - INSULATION FOR THE flame or from the stove to P to 
 HANDLES OF A PERCOLATOR. the contents of the utensil. 
 
THE FIRELESS COOKER 
 
 43 
 
 Coil of Wire 
 
 51. The Fireless Cooker. The fireless cooker is a box 
 arrangement with non-conducting walls. Figure 27 shows 
 how it is constructed. 
 On the inside are pails in 
 which the food is placed. 
 Around the pails is the 
 non-conducting wall. The 
 food is first heated to the 
 boiling point, and at the 
 same time slabs of soap 
 stone or iron are heated. 
 When these are hot 
 enough, the hot food is placed in the pails between the 
 hot slabs ; then the whole box is closed up tight. 
 
 The non-conducting walls keep the heat in, so that the 
 food stays up close to the boiling temperature without 
 
 FIGURE 26. -COILED WIRE HANDLE ON 
 A STOVE-POKER. 
 
 Pail of Food 
 
 Hot Plate 
 Wood 
 
 Felt 
 
 Asbestos 
 
 ' Enamel Ware 
 ^Mineral Wool 
 
 FIGURE 27. THE FIRELESS COOKER. 
 
 being supplied with more heat. This makes it necessary 
 to use the fire only long enough to get the food and heating 
 slabs hot. 
 
44 
 
 HEAT TRANSFERENCE 
 
 The non-conducting material used may be wool, felt, 
 mineral wool, asbestos, leather, paper, straw, shavings, or 
 sawdust. 
 
 52. The Refrigerator. The refrigerator (Figure 28) 
 uses non-conducting substances for its walls. On the out- 
 side is usually wood ; next is an insulating layer of paper ; 
 then another of wood ; then a layer of asbestos or felt ; 
 and then an air space. The inside material is usually glass, 
 
 Glass or 
 Enamel Ware- 
 
 Air Space 
 
 7 
 
 Mineral Wool or Felt 
 
 Rough Wood 
 Paper or Asbestos 
 Finished Wood 
 
 FIGURE 28. THE CROSS SECTION OF A REFRIGERATOR WALL. 
 
 enamel, or zinc. The ice is put into the top of the refrigera- 
 tor, and the things to be kept cool on the shelves below or 
 beside it. The insulating walls allow little heat to come 
 in from the outside ; so that most of the heat used to melt 
 the ice comes from the articles put in to be cooled. 
 
 53. The Thermos Bottle. The thermos bottle consists 
 of a double glass flask with the outside silver-coated (Figure 
 29). The space between the walls of the flask is a vacuum, 
 the air having been pumped out. The flask is then placed 
 inside of an outer cover, which is either silver or nickel 
 
WALLS OF HOUSES 
 
 45 
 
 plated. An air space is left between the outer cover and 
 the glass flask. 
 
 The bottle is used to keep liquids either cold or hot. 
 When cold liquids are placed in it, the heat is kept out by 
 the insulating walls; and if hot liquids are placed in it, 
 the insulating walls keep the heat in. 
 
 The reasons for this are apparent. First, the glass walls 
 of the flask are non-conductors, and do not permit heat to 
 pass through them 
 easily. Then, the 
 vacuum is the best 
 non-conductor there 
 is. Also, the air 
 space between the 
 outside cover and the 
 
 Air Space 
 Metal Case 
 
 Screw Cap 
 Cork 
 
 Glass Flask 
 -Vacuum 
 
 -Contents 
 
 ^Where Glass Flask 
 Was Sealed 
 
 FIGURE 29. CROSS SECTION OF A 
 THERMOS BOTTLE. 
 
 flask helps the in- 
 sulation. Finally, 
 the silvered and nickled surfaces have special uses, which 
 will be discussed under the subject of Radiation. 
 
 Good thermos bottles will keep coffee too hot to drink 
 for fifteen hours. Care must be taken to have the liquid 
 hot when it is placed in the bottle. 
 
 54. Walls of Houses. Walls of houses are so con- 
 structed that they do not allow the heat to pass through 
 them readily. Either brick, stone or lumber is used. 
 The lumber-made house is constructed as shown in 
 Figure 30. 
 
 First is put up studding, which is about two inches by 
 four inches. On the outside of this is nailed rough lumber, 
 called sheathing. Over this is usually tacked heavy paper, 
 and then the siding or weather-board. Inside the studding 
 the plaster lath is nailed, and then the plaster is spread 
 
46 
 
 HEAT TRANSFERENCE 
 
 over this. This constitutes the complete wall, except for 
 the wall paper usually placed on the inside. 
 
 Naming the insulating layers from the outside inward, 
 they are, weather-board, heavy paper, sheathing, air space, 
 plaster lath, plaster, and wall paper. 
 
 Studding 
 
 Lath 
 
 Plaster 
 
 Wall Paper 
 
 Sheathing 
 
 Paper 
 
 Siding 
 
 FIGURE 30. CROSS SECTION OF THE WALL OF A HOUSE. 
 
 Sometimes in cold countries an extra set of lath and 
 plaster is put in between the studding, making also an extra 
 air space. 
 
 55. Clothes. Winter clothing is usually made of non- 
 conductors. We wear light cotton clothes in summer and 
 heavy woolens in winter. Why? The cotton is compact 
 and conducts heat readily, while the wool is loose in con- 
 struction, containing many air spaces, which act as insulators. 
 You can easily tell the difference between cotton and wool 
 by dampening the thumb and finger and rolling a thread 
 of each between them. The cotton will pack closely to- 
 
CONVECTION 
 
 47 
 
 gether, while the wool will spring back to its original loose- 
 ness. 
 
 56. Convection. Convection is the second method of 
 transferring heat. In conduction we learned that it was 
 the heat energy only that moved along. In convection, 
 the heat passes from one place to another by means of 
 material bodies carrying it. 
 
 Convection can best be understood by studying the 
 following drawing. Figure 31 shows a section of air divided 
 into columns. If a r , 
 
 1 
 
 I 
 
 A 
 
 T I 
 t t 
 
 t t 
 
 B 
 
 1 
 1 
 
 
 
 
 1 t 
 
 
 1 
 
 U" 
 
 - 
 
 
 -> > 
 
 ; t 
 
 
 
 FIGURE 31. DIAGRAM SHOWING HOW CON 
 VECTION CURRENTS ARE SET UP. 
 
 fire were built under 
 column A BCD, the 
 air would be heated 
 and would conse- 
 quently expand. As 
 the air cannot push 
 sidewise, on account 
 of the other columns 
 of air, when it ex- 
 pands it must push 
 upward. This makes this column higher than the others; 
 so the air flows outward over the other air columns at the 
 top, as indicated by the arrows. 
 
 Now this makes the columns at the side heavier than 
 the middle one ; so they crowd down, forcing some of the 
 cold air under the middle column, as indicated by the ar- 
 rows. This air will then be heated, will expand, and be 
 pushed up by more cold air. 
 
 So the process goes on; the cold air flowing towards the 
 warm area at the bottom, and the warm air flowing away 
 from the warm area at the top. Over the warm area the 
 air moves upward, while over the cold area the air moves 
 
48 
 
 HEAT TRANSFERENCE 
 
 downward. These movements are called convection cur- 
 rents. 
 
 Convection currents take place in liquids as well as in 
 gases, but cannot take place in solids. 
 
 57. Drafts in Chimneys. Drafts in chimneys are due 
 to convection currents. A fire is started in the fire-box of 
 
 the furnace. (Figure 32.) This warms 
 the air, and causes it to expand and 
 become lighter than the surrounding air. 
 The cold air then pushes the warm air 
 up the chimney and takes its place in 
 the fire-box. This air is then heated, 
 and the process is repeated, or rather it 
 takes place continuously. The higher 
 
 the chimney, the greater the draft. 
 FIGURE 32. DRAFT 
 
 IN A CHIMNEY. Suppose the chimney (Figure 32) were 4 ft. 
 
 square and 100 ft. high; and suppose the air 
 raised from C. to 273 C., when the fire started. 
 
 4 X 4 X 100 = 1600 cu. ft. = volume of the chimney. 
 Now, air at C. weighs .08 Ib. per cu. ft. 
 
 1600 X .08 = 128.00 Ib. = wt. of air in the chimney, when air is 
 cold. 
 
 Since a gas expands 2 fj of its volume at C. when heated 1 C., 
 it will double its volume when heated to 273 C. 
 
 Therefore, since the chimney will contain only 1600 cu. ft., of the 
 air must flow out. 
 
 | of 128 Ib. = 64 Ib., wt. of air which remains in the chimney. 
 Now, since an equal volume of air on the outside weighs 128 Ib., 
 ancl inside it weighs 64 Ib., the cold air outside pushes up on the 
 warm air inside with a force of 64 Ib. This shows definitely why the 
 air rises in the chimney, or explains the draft. 
 
 58. Draft in a Kitchen Range. Figure 33 shows the 
 ordinary kitchen range. The air enters at the front and 
 
DRAFT IN A KITCHEN RANGE 
 
 1 Damper 
 
 49 
 
 FIGURE 33. DRAFT IN A KITCHEN RANGE. 
 
 goes up to the fire-box. Here it becomes hot and, with the 
 smoke, passes up over the oven, down at the end and under 
 
 FIGURE 34. DIAGRAM OF A HOT-AIR HEATING SYSTEM. 
 
50 
 
 HEAT TRANSFERENCE 
 
 the hot-water reservoir, then under the oven, and finally 
 up, at the back of the oven, to the stove pipe. Thus we 
 see the hot gases pass completely around the oven, except 
 in front, where the door is located. 
 
 If the oven is not to be used, the damper is closed, which 
 shuts the current off from the oven and lets the hot gases 
 circulate only under the top of the stove and the reservoir. 
 
 59. Hot-air Heating. 
 Figure 34 shows a 
 diagram of the modern 
 hot-air heating system. 
 The furnace located in 
 the basement consists 
 of two parts, a fire- 
 box, and a sheet-iron 
 jacket, the two being 
 separated by an air 
 space. 
 
 The air that feeds the 
 fire in the fire-box goes 
 in through a hearth, and 
 
 FIGURE 35. A HOT-AIR FURNACE, 
 
 the smoke and gases 
 pass on up the chimney. 
 
 This air and other gases never reach the rooms, nor are they 
 even in contact with the air that goes to the rooms. The 
 latter comes in through the cold-air shaft (from outside or 
 from the basement itself) ; is heated as it passes between 
 the sheet-iron jacket and the wall of the fire-box; then is 
 carried in convection currents through pipes that lead to 
 the separate rooms. 
 
 60. Hot-water Tank. Convection currents take place 
 in liquids as well as in gases. Use is made of this in the 
 
HOT-WATER HEATING SYSTEM 
 
 51 
 
 hot-water tank. Figure 36 shows a hot- water tank designed 
 to be heated by a separate heater, or by the furnace itself. 
 
 The water comes into the storage tank (.4) through 
 pipe (/). A pipe (g) comes out of the storage tank at the 
 bottom and passes up through a pipe (i), around which is 
 the heater (B). This pipe then returns to the top of the 
 tank through (h). The pipe (c) is for drawing off the hot 
 water to the places 
 where it is needed. 
 
 A fire is started in 
 the heater (J5), causing 
 the water in pipe (i) to 
 expand. Convection 
 currents are then set 
 up, and the warm 
 water flows over into 
 the top of the tank, 
 cold water coming in 
 all the time at pipe (g). If the furnace (C) is going, the 
 heater (B) is not needed, as the convection currents are 
 set up through the coils in the furnace. When water is 
 drawn off through (c), more water is supplied through the 
 inlet, from the water main. 
 
 If the water is allowed to get too hot, steam is generated, 
 which may force the water back into the main, thus en- 
 dangering the water meter. 
 
 61. Hot- water Heating System. Figure 38 shows a 
 modem hot- water heating system. The furnace is located 
 in the basement, and has a boiler above the fire-box. From 
 the top of the boiler, pipes are led off to the radiators in the 
 different rooms. Returning from the other end of the 
 radiators are pipes to bring the water back to the bottom 
 
 FIGURE 36. DIAGRAM OF THE HEATING 
 SYSTEM OF A HOT-WATER TANK. 
 
52 
 
 HEAT TRANSFERENCE 
 
 of the boiler. The pipes going up to the radiators are 
 called " risers," while those coming down are called " return 
 pipes." Connected in the system is a pipe which goes up 
 to the expansion tank, usually placed in the attic. 
 
 FIGURE 37. A KEROSENE HEATER USED IN CONNECTION WITH THE 
 HOT-WATER TANK. 
 
HOT-WATER HEATING SYSTEM 
 
 53 
 
 Before the furnace is started, water is let in from the city 
 main until the whole system is full and water rises into the 
 expansion tank. Then the stop-cock is closed, so that no 
 more water can get in or out. When the fire is started, 
 
 FIGURE 38. DIAGRAM OF A HOT-WATER HEATING SYSTEM. 
 
 convection currents are set up through the pipes, causing 
 hot water to flow through the radiators. 
 
 The expansion tank is to protect the pipes from bursting. 
 If there were no place for the water to go when the fire is 
 started, the expansion would burst the boiler or the pipes. 
 This sometimes happens if the pipe to the expansion tank 
 in the attic freezes. 
 
54 
 
 HEAT TRANSFERENCE 
 
 FIGURE 39. A HOT-WATER HEATING SYSTEM INSTALLED. 
 
 62. Ventilation. Ventilation is the supplying of pure 
 air and the removing of impure air from rooms and 
 buildings. 
 
VENTILATION 
 
 55 
 
 It is estimated that every person should have 3000 cubic 
 feet of pure air per hour. There are two distinct types of 
 ventilation the natural systems and the forced systems. 
 
 In the natural systems convection currents are depended 
 upon to change the air. In many dwelling houses no special 
 means are used for ventilation ; open windows, doors, or 
 crevices are depended upon entirely to supply pure air. 
 
 If a window is opened both 
 at the top and bottom, as is 
 shown by Figure 40, and a 
 lighted candle is held, first 
 at the bottom, and then at 
 the top, of the window, the 
 candle flame will blow to- 
 wards the room in the former 
 position, but will blow out- 
 wards when held at the top, 
 showing that air enters at the 
 bottom and leaves at the top. 
 This is explained by convec- 
 tion currents. Opening win- 
 dows is a quick means of 
 
 Outside 
 
 Inside 
 
 but it 
 
 FIGURE 40. -VENTILATION BY MEANS 
 OF THE OPEN WINDOW. 
 
 getting ventilation, 
 produces drafts. 
 
 Even when the windows or doors are dosed, air comes 
 in around the frames, where there is not a perfect fit. This 
 supplies pure air and is sufficient in many cases where very 
 few people use the rooms. Wind coming from one side of 
 the house often helps ventilate it, blowing pure air in on 
 one side and forcing impure air out on the other. 
 
 A grate or fireplace is a good ventilator. Why? 
 
 One of the simplest methods for special ventilation is 
 
56 
 
 HEAT TRANSFERENCE 
 
 shown in Figure 41. A cold air vent is made just below the 
 radiator. As the cold air comes in, it is heated by the radia- 
 tor and made to flow to all parts of the room by means of 
 convection currents. The impure air 
 leaves by way of crevices. 
 
 Another of the natural systems is 
 shown in Figure 42. Here the air 
 comes in from the outside, passes 
 around a special heating device in the 
 floor, and then is distributed by con- 
 vection currents. 
 Forced ventilation is used in large 
 
 If 
 
 Air DucT to OuTside 
 
 \ 
 
 Air-duct to Outside 
 
 FIGURE 41. ANOTHER 
 METHOD OF VENTILA- 
 TION. 
 
 is used in 
 
 buildings, such as schools, apartment houses, department 
 stores, and theaters. In such buildings there are great 
 numbers of people, and the ordinary method of ventilation 
 is not sufficient to supply the required 3000 cubic feet, per 
 hour, for each person. 
 
 Forced- ventilation 
 systems use fans to 
 make the air move. 
 One way is to draw the 
 impure air out by means 
 of fans, allowing the 
 pure air to flow in to 
 take its place. Other 
 methods force the pure 
 air in, driving the im- 
 pure air out. 
 
 Figure 43 shows a 
 forced-ventilating system in which the air is washed before 
 it passes through the rooms. Pure air, forced in by the fan, 
 enters the washing room. The washing room consists of a 
 
 Coils in Floor 
 
 Heating Pipe 
 
 FIGURE 42. VENTILATION WITH HEATING 
 DEVICE IN THE FLOOR. 
 
RADIATION 
 
 57 
 
 Heating Room 
 
 closed space in which water is kept spraying. Here the air 
 has most of the dust and impurities removed. Then it is 
 forced up the pipes to the heating space, and from there 
 it goes to the places where it is needed. 
 
 63. Radiation. Conduction and convection, the two 
 methods of transference of heat which we have just studied, 
 are easily understood; but the third method, radiation, is 
 much more difficult to 
 explain. We know that 
 heat travels from the 
 sun to the earth, and 
 that it comes through 
 space in the form of 
 waves in the ether. 
 
 No one knows just 
 what the ether is, but 
 there are many facts 
 which prove its exist- 
 ence. Whatever it is, 
 it has no weight or 
 body, but it fills the 
 whole universe. 
 
 Heat in the form of 
 waves in the ether is 
 insensible, for sensible heat is 
 molecules. 
 
 When heat waves strike opaque objects, they are partly 
 changed to sensible heat and partly reflected back as waves. 
 When they strike transparent objects, such as air, glass, clear 
 water, etc., they pass through without heating the object. 
 
 Radiation is the transference of heat by means of waves in 
 the ether. 
 
 Washing Room 
 
 FIGURE 43. DIAGRAM OF A FORCED- 
 VENTILATING SYSTEM. 
 
 due to the vibration of 
 
58 HEAT TRANSFERENCE 
 
 64. Radiators. We must not get the idea that the sun 
 is the only thing that sends out these heat waves, for all 
 hot bodies do this, more or less. Any body that sends out 
 heat waves is called a radiator. 
 
 All bodies at the same temperature do not radiate their 
 heat at the same rate. It is found that rough, black bodies 
 are the best radiators, while smooth, white, or shiny objects 
 radiate heat very slowly. 
 
 65. Absorbers. Heat waves striking opaque objects 
 are changed to sensible heat. These objects are said to 
 absorb the heat waves. Bodies which are good radiators, 
 namely, rough black ones, are also good absorbers. A 
 rough, black piece of iron will cool off quickly after it is 
 heated, because it is a good radiator; and, on the other 
 hand, it will become warm quickly if placed where heat 
 waves fall on it, because it is a good absorber. 
 
 66. Reflectors. Why are rough, black objects good 
 radiators and good absorbers, while smooth, white, or shiny 
 objects are poor ones? The answer is that smooth, white, 
 or shiny objects are good reflectors. The heat waves fall on 
 them and are reflected back, just as light is reflected by a 
 mirror. On the other hand, when the heat waves start to 
 leave the objects, the shiny surface turns them back again. 
 
 67. Applications. In the thermos bottle ( 53) the 
 glass and the vacuum stop conduction and convection, but 
 cannot stop the heat from radiating into or out of the bottle. 
 This is stopped by the silver surfaces. As they are smooth, 
 and shiny, any heat trying to radiate into the bottle is re- 
 flected out again; and any heat trying to radiate out is 
 reflected in again. Therefore all three avenues for the 
 transference of heat are stopped, so that either hot or cold 
 liquids put into the bottle remain hot or cold. 
 
APPLICATIONS 59 
 
 A black, rough stove would be more serviceable than a 
 bright, shiny one. Why? What kind of clothes would 
 you wear in hot weather or in a warm climate? In a cold 
 climate? Why? 
 
 Greenhouses trap the heat of the sun and do not let it 
 out. The heat waves pass through the glass of the green- 
 house and strike the plants and soil and other objects, 
 which absorb the waves. In other words, the waves are 
 changed to sensible heat. The glass walls are poor con- 
 ductors ; so the sensible heat cannot get out. 
 
 Dirty snow does not melt evenly, but in holes and patches. 
 Soot and dirt, being black, absorb the sun's rays and thus 
 melt the snow under them, causing holes in the snow. Where 
 there is no dirt, the snow reflects the rays and therefore 
 melts more slowly. 
 
 On a sunny day, would the snow melt faster under a 
 black woolen blanket, or without the blanket? Would it 
 be the same by night, or if the day were cloudy ? 
 
CHAPTER IV 
 SOURCES OF HEAT 
 
 68. Fuels. We have studied the nature of heat, have 
 seen what it will do, and how it is transferred from one 
 place to another. Now comes the question, where do we 
 get heat? 
 
 The sun is the great source of heat, but the sun's heat 
 is so widely distributed and so little under our control, that 
 it serves mostly the processes of nature, and for specific 
 purposes of service we rely mainly on fuels. 
 
 Fuels are materials which will supply heat when burned. 
 Sixty years ago the most common fuel was wood. What 
 fuel do you use at home to keep warm and to do your cook- 
 ing? Most of you will say gas, or coal. 
 
 There are two distinct kinds of gas natural gas and 
 artificial gas. The natural gas is forced directly from the 
 gas well to your home. The artificial gas does not come 
 from wells at all, but is made by baking soft coal and treat- 
 ing it in certain ways. 
 
 Natural gas is much better for heating purposes than 
 artificial gas, since the natural gives 1200 B. T. U.'s per cubic 
 foot, while artificial gas gives only half as much, or 600 
 B. T. U.'s per cubic foot. 
 
 There are many kinds of coal, but we usually speak of 
 two, hard and soft. The hard coal is " clean," that is, it 
 has little dust in it and gives off little smoke when it burns. 
 
 60 
 
FUELS 
 
 61 
 
 The soft coal is full of dust and its smoke is dense and 
 sooty. 
 
 Hard coal yields about 14,000 B. T. U.'s per pound, when 
 burned; while soft coal yields about 12,000 B. T. U.'s per 
 pound. It is never possible to get all the heat when a fuel 
 
 FIGURE 44. KEROSENE USED AS A FUEL IN THE COOK STOVE. 
 
 is burned, but more is available in some fuels than in others. 
 This is true of coal. Hard coal would give only about 2000 
 B. T. U.'s per pound more than soft coal, if one could get all 
 the heat. But a great deal more heat is lost in the case of 
 soft coal than in the case of hard coal ; so that, in the end, 
 hard coal heats much better than soft coal. 
 
 The following table gives a few of the materials used 
 
62 
 
 SOURCES OF HEAT 
 
 for fuels, and the name or kind of each. Opposite each kind 
 of fuel is the heat value. 
 
 Sources of Heat 
 
 MATERIAL 
 
 KIND 
 
 HEAT VALUE 
 
 Coal 
 
 [Hard 
 
 14000 B.T.U.'s per Ib. 
 
 Wood 
 
 { Soft 
 [Coke 
 /Hard 
 
 12000 B.T.U.'s per Ib. 
 14000 B.T.U.'s per Ib. 
 8400 B T U 's per Ib 
 
 Gas 
 
 \Soft 
 { Natural 
 
 8600 B.T.U.'s per Ib. 
 1200 B.T.U.'s per cu. ft. 
 
 Oils ...... 
 
 [ Artificial 
 [ Kerosene 
 { Naphtha 
 
 600 B.T.U.'s per cu. ft. 
 20000 B.T.U.'s per Ib. 
 9 0000 B T U 's per Ib 
 
 Electricity .... 
 
 [ Crude Oil 
 
 18000 B.T.U.'s per Ib. 
 3411.72 B. T. U.'s per Kw. hr. 
 
 (Electricity is given in this table, though it is not a fuel.) 
 
 69. The Gas Meter. The gas that you use is measured 
 by a gas meter. The gas, flowing through the meter, moves 
 little fans, making the hands move around on the dials. 
 
 1,000,000 100,000 10,000 1,000 
 
 2634 
 
 FIGURE 45. DIALS OF A GAS METER SHOWING A READING OF 
 263,400 Cu. FT. 
 
 These dials indicate how much gas has passed through the 
 meter. The figures above the dials indicate the number of 
 cubic feet that have passed when the hand makes one com- 
 plete revolution. 
 
HEAT FROM FOODS 63 
 
 Figure 45 shows a four-dial meter with a reading of 263,400 
 cu. ft. 
 
 Always begin to read from the right-hand side. 
 
 Your gas bill is made out from these meter readings. 
 The meter man comes round every month and reads the 
 meter. The last month's reading is subtracted from the 
 present month's reading, and the number of thousand (M) 
 cubic feet of gas used during the present month is thus deter- 
 mined. Only integral numbers of thousand cubic feet are 
 counted. Thus, if the meter reads 263,400 cu. ft., the 400 
 is not counted, but the reading is called 263 M. 
 
 The cost of natural gas in Cleveland at present is 30^ 
 per M. while that of artificial gas is 80^ per M. 
 
 Problems 
 
 1. How much hard coal is necessary to melt 150 Ib. of ice when 
 12 per cent of the heat is available? 
 
 2. How much soft coal is necessary to heat 150 Ib. of water from 
 40 F. to 100 F., only 6 per cent of the heat being available? 
 
 3. What will be the cost of the natural gas required to boil 10 Ib. 
 of water away, if 10 per cent of the heat is available? Natural gas 
 costs 30^ per M. 
 
 4. How many B. T. U.'s are given off when a ton of soft coal is 
 burned ? 
 
 5. What is the cost of boiling away 10 Ib. of water, if artificial gas 
 is used at80?f per M? 
 
 6. Draw a 4-dial gas meter showing a reading of 267,300 cu. ft. 
 
 7. What is the month's natural gas bill if the meter read 246,300 cu. 
 ft/last month and 252,600 cu. ft. this month? 
 
 70. Heat from Foods. The energy we use in the body 
 comes from the foods we eat. In other words, our food is 
 fuel. Part of the food is used for building and repairing 
 tissue, but certain kinds are for fuel. 
 
 The United States Government has made charts of the 
 
64 SOURCES OF HEAT 
 
 building value and the heat value of most of our foods. A 
 study of these charts is to be made at this point. 
 
 An average laboring man should have from 3000 to 
 3500 great calories of heat per day. A person not at manual 
 labor should have less it is estimated about 2500 great 
 calories. 
 
 From the table in the Appendix, make up a day's menu 
 so that the person shall get about 2500 calories. Figure the 
 cost of each item and make a total for each meal. Calcu- 
 late the cost for the whole day. 
 
 Review Problems 
 
 1. What is the nature of heat ? 
 
 2. What is meant by the terms hot and cold? 
 
 3. Define temperature. 
 
 4. Change 25 F., - 16 F., 75 F. to the corresponding Centi- 
 grade readings. 
 
 5. Change 10 C., - 8 C., 80 C. to the corresponding Fahrenheit 
 readings. 
 
 6. Define freezing point ; boiling point. 
 
 7. Explain the effect of pressure on the freezing point; on the 
 boiling point. 
 
 8. Name and explain two applications of the effect of pressure 
 on the boiling point. 
 
 9. What are the three heat units used ? Define each. 
 
 10. Discuss heat of fusion. 
 
 11. Discuss the refrigerator as an application of heat of fusion of 
 water. 
 
 12. Discuss heat of vaporization. 
 
 13. Discuss the double boiler as an application of heat of vaporiza- 
 tion of water. 
 
 14. Explain distillation. 
 
 16. What is meant by " iceless refrigeration " ? 
 16. How many calories are necessary to melt 20 kg. of ice without 
 changing its temperature? (One kg. = 1000 grams.) 
 
REVIEW PROBLEMS 65 
 
 17. How many B. T. U.'s are necessary to melt 50 Ib. of ice ? Where 
 does the heat come from if the ice is in a refrigerator ? 
 
 18. If the ice on a lake one mile square is \ foot thick, how many 
 B. T. U.'s are necessary to melt it? (Assume that ice weighs 52 Ib. 
 per cu. ft. and is at Centigrade.) 
 
 19. How many B. T. U.'s are given off when 6 Ib. of steam con- 
 denses in the radiator ? 
 
 20. Explain dew. 
 
 21. Define specific heat. 
 
 22. Name and explain two applications of specific heat. 
 
 23. Explain expansion. 
 
 24. How much will a 40 cm. glass tube expand in length when 
 heated 20 C. ? 
 
 25. How much larger than the rest of the glass will the bottom 
 of a two-inch drinking glass become when the bottom is suddenly 
 thrust into boiling water (212 F.)? (Assume that the original 
 temperature was 80 F.) What will this expansion do to the 
 glass ? 
 
 26. Explain the thermostat. 
 
 27. Why do water pipes burst when they freeze ? 
 
 28. What is the volume coefficient of expansion of a gas? 
 
 29. Explain the meaning of absolute zero. 
 
 30. What application has Charles' Law to the baking of bread and 
 cake? 
 
 31. Explain conduction. 
 
 32. Give three applications of conduction as a method of heat 
 transference. 
 
 33. Explain convection. 
 
 34. Why does the smoke flow out of a chimney? 
 
 35. Explain how the water is heated in the hot-water tank. 
 
 36. How long would the air in a room 20 ft. X 18 ft. X 10 ft. re- 
 main healthful if five persons were in it ? 
 
 37. What are the two types of ventilation ? 
 
 38. Discuss radiation. 
 
 39. Discuss radiators, absorbers, and reflectors. 
 
 40. Name and explain three applications of radiation. 
 
 41. What is a fuel? 
 
 42. How much natural gas is necessary to heat 100 Ib. of water for 
 
66 SOURCES OF HEAT 
 
 a bath, if the water is at 38 F. at the beginning, and 100 F. when 
 heated ? (Assume that 8 per cent of the heat is available.) 
 
 43. How much soft coal is necessary to melt 50 Ib. of ice, if only 
 6 per cent of the heat is available ? 
 
 44. What is the cost per gallon of distilling water, if natural gas 
 is used and 10 per cent of the heat is available ? (Assume that the 
 water has to be raised from 38 F.) 
 
 45. Why should the food one eats have a certain heat value ? 
 
 
CHAPTER V 
 WAVE MOTION 
 
 71. Examples of Wave Motion. Sound and light are 
 the commonest examples of wave motion ; but the example 
 most readily seen is the waves formed on water when some- 
 thing disturbs its surface. If a stone is thrown into still 
 water, a splash occurs at the point where the stone strikes, 
 and waves travel outward in all directions from this point. 
 If a cork, or anything that will float, is placed on the water, 
 it is seen to bob up and down ; but it does not move away 
 from its original position. 
 
 A similar example is the waves produced in a field of 
 grain when the wind blows over it. The individual heads 
 of grain merely rise 
 and fall, but the fX 
 wave travels across 
 the field. 
 
 If a rope or rubber FlGURE 46.-WAVE IN A ROPE. 
 
 hose is held station- 
 ary at one end and the other end is shaken, waves will be 
 sent down the rope. (Figure 46.) The waves travel from 
 one end of the rope to the other, but each particle of the 
 rope, such as P, jumps up and down, but does not move 
 forward. 
 
 Figure 47 shows a spiral spring, attached to a support 
 at the top, having its bottom suddenly jerked downward. 
 
 67 
 
68 
 
 WAVE MOTION 
 
 FIGURE 47. WAVE 
 IN A SPRING. 
 
 A portion of the spring a is stretched, but the rest of the coil 
 b remains the same as before it was jerked. The next in- 
 stant part a pulls down on part b and 
 stretches b, letting a go back to its first 
 position. 
 
 This is a form of wave in which the 
 waves move along the spring, and each 
 particle of the spring jerks backward 
 and forward, parallel with the spring. 
 Waves can be sent along rubber bands 
 just as along the spring mentioned above. 
 Suppose a rubber ball is in the center 
 of the room, fastened by rubber bands 
 to all the walls, the ceiling, and the floor. 
 (Figure 48.) Then suppose the rubber 
 ball contracts suddenly. All the rubber bands next the ball 
 will be stretched, as shown in Figure 49. Each stretched 
 portion will, in turn, stretch the next portion ; and so on, 
 until the effect runs out to the ends of all the rubber bands, 
 just as it did in the spring. 
 Since this effect travels out 
 at the same speed in all the 
 bands, we can think of it as 
 being a wave like the wave 
 on the water. 
 
 72. Origin of Waves. 
 It is seen from all the pre- 
 ceding examples that waves 
 have to be started. This is 
 always true. In the case of 
 
 the water wave, the Stone FIGURE 48. -A RUBBER BALL AT- 
 
 ' TACHED TO THE SlDES OF A ROOM 
 
 Started the disturbance; in BY MEANS OF RUBBER BANDS. 
 
CHARACTERISTICS OF TRANSVERSE WAVES 69 
 
 the field of grain, it was the wind ; in the rope, your hand 
 was the cause. The same thing was true with the spring ; 
 and the contraction of the 
 rubber ball started the wave I 
 in the rubber bands. 
 
 73. Transverse and Longi- FI G URE 49. A stretched PORTION 
 
 .-,, OF A RUBBER BAND NEXT THE 
 
 tudinal Waves. There are BALL> 
 two motions in each case 
 
 mentioned : the motion of the wave, and the motion of the 
 particles of water, rope, spring, rubber, or grain heads. 
 
 The relative direc- 
 
 IE. >TF tions of these two mo- 
 tions determine the 
 
 FIGURE 50. SHOWING DIRECTIONS OF t j * j 
 
 MOTIONS IN A TRANSVERSE WAVE. kmd of wave under 
 
 consideration. Waves 
 
 in which the particles move at right angles to the direction in 
 which the wave moves are called transverse waves. (Figure 
 50.) The long arrow W indicates the direction of the wave, 
 and the arrow P indicates the direction in which the par- 
 ticle moves. 
 
 Waves in which the particles move parallel with the direction 
 in which the wave moves are called longitudinal waves. 
 (Figure 51.) Here the p 
 
 two arrows are parallel, ^^ >w 
 
 and SO show a longi- FIGURE St.- SHOWING DIRECTIONS OF 
 
 MOTIONS IN A LONGITUDINAL WAVE. 
 
 tudinal wave. 
 
 74. Characteristics of Transverse Waves. '- In case of 
 the waves in the water, in the grain, and in the rope, we 
 find that, as the waves follow one another, parts of the 
 material are high and other parts low. The high parts a 
 and c (Figure 52) are called crests, while the low parts b 
 and d are called troughs. 
 
70 WAVE MOTION 
 
 The distance ac from one crest to a corresponding point 
 in the next crest is called a wave length; or it may be from one 
 trough to the corresponding point in the next trough, bd. 
 
 Wave Length 
 
 Wave Length - 
 
 t 1) d 
 
 FIGURE 52. CHARACTERISTICS OF A TRANSVERSE WAVE. 
 
 The distance that each particle moves from the position 
 of rest is called the amplitude, xy. 
 
 When a particle has moved from x to y, to t, to x, it is 
 said to have made one complete vibration. 
 
 The time required to make one complete vibration is 
 called the period; and the number of vibrations the particle 
 makes per second is called the frequency. 
 
 75. Characteristics of Longitudinal Waves. In longi- 
 tudinal waves we have very nearly the same characteristics 
 as in transverse waves. 
 
 Instead of having crests and troughs, we have conden- 
 sations and rarefactions. Figure 53 shows the particles as 
 
 }< Wave Length >\ 
 I 
 i ' 
 
 ! L4LJ 
 
 a \c t> d\ 
 
 [* Wave Length *j 
 
 FIGURE 53. CHARACTERISTICS OF A LONGITUDINAL WAVE. 
 
 they would appear in a rubber band if a wave were traveling 
 in it. 
 The parts a and b where the rubber particles are crowded 
 
HOW LONGITUDINAL WAVES TRAVEL 71 
 
 together, are called condensations. The parts c and d 
 where the particles are stretched apart, are called rarefactions. 
 
 The wave length is the distance from one condensation 
 to the next, or from one rarefaction to the next. 
 
 Amplitude, vibration, period, and frequency mean the 
 same as in transverse waves. 
 
 76. How Transverse Waves Travel. Most transverse 
 waves travel in a substance which has tensile strength, 
 that is, a substance which will resist a pull. The wave 
 moves from one position to another in this way : 
 
 Figure 54 shows a wave in a rope, with some of its parts 
 numbered. As the wave travels along the rope, the particles 
 
 t 8 9 10 11 12 
 
 FIGURE 54. THE START OF A TRANSVERSE WAVE. 
 
 move up and down ; or, as the particles move up and down, 
 the wave travels along the rope. It is the motion of the 
 particles that produces the wave motion. 
 
 In the figure, #1 has been to the top of the swing and 
 has come back to its present position. Since #2 is on the 
 same rope, it is pulled along after $1. Also, #3 is pulled by 
 #2 ; and so on. Thus we see that the different particles are 
 affected in a series, one after the other, and not all at once. 
 
 To state it as briefly as possible : the wave travels by one 
 particle pulling the next one after it. 
 
 77. How Longitudinal Waves Travel. Longitudinal 
 waves may travel in substances that have tensile strength, 
 but they do not depend on the pulling effect to make them 
 travel. Instead, they depend on the crowding effect. 
 
72 WAVE MOTION 
 
 As an example, take the longitudinal wave in a spring. 
 (Figure 55.) The particles of the spring are all crowded 
 together at d and e, and are all spread out at a and c. 
 
 Now, since there is nothing to keep the spring stretched 
 at positions a and c, and compressed at d and e, the crowded 
 portions d and e will expand and tend to compress the parts 
 a arid c. 
 
 If this action should stop when the spring is everywhere 
 stretched alike, the wave would stop ; but it is the same as 
 
 d a e c 
 
 FIGURE 55. How A LONGITUDINAL WAVE TRAVELS. 
 
 when you run fast and then try to stop suddenly. You go 
 farther than you intended. The same is true of the parts 
 of the spring. The crowded portions expand too much, 
 causing an overstretched portion; and the part that was 
 stretched before is compressed. In this way, the crowding 
 effect is passed along, and the wave is said to travel. 
 
 78. Velocity of Waves. Waves travel at different 
 speeds. If the rope is stretched tight, the waves will travel 
 faster than if the rope is loose. 
 
 ~ ~1/-\ l/-v A They would travel more slowly if 
 \y \s V/l the rope were large and heavy. 
 
 On the other hand, the frequency 
 FIGURE 56.- VELOCITY = of the vibration does no t affect the 
 FREQUENCY x WAVE LENGTH. 
 
 speed of the wave, nor does the 
 
 amplitude. If the frequency is high, the waves are short; 
 but if the frequency is low, the waves are long. 
 
 During one vibration the wave travels 1 wave length, 
 L. (Figure 56.) During two vibrations the wave travels 2 
 wave lengths, 2 L ; while during three vibrations it travels 3 
 
VELOCITY OF WAVES 73 
 
 wave lengths, 3 L. From this we see that in N vibrations 
 the wave will travel NL. 
 
 Now, N is the number that usually stands for the fre- 
 quency; so NL is the distance the wave will travel in 1 
 second. 
 
 The distance an object travels in a second is called its 
 velocity; so the velocity of a wave is the frequency times the 
 wave length; or 
 
 Velocity = Frequency X Wave Length 
 or V = NL. 
 
CHAPTER VI 
 SOUND 
 
 79. Definition of Sound. Sound may be defined as a 
 vibration of such a frequency that it may be detected by the ear. 
 
 There are three things necessary for sound : (1) some 
 vibrating object to start the vibration ; (2) some medium 
 to carry the vibration ; (3) something to receive the sound. 
 
 The vibrating object to start the vibration may be a tun- 
 ing fork, piano wire, bell, drum, etc. 
 
 The air is the medium which usually carries the waves 
 from the vibrating body to the ear which receives it. Water 
 will do this very well ; and, in fact, any material body will 
 carry the vibration. A vacuum will not. This can be 
 shown by placing an alarm clock in a jar and then exhausting 
 the air with a pump. The clock will become inaudible, but 
 when the air is let in again it can be heard. 
 
 The thing that usually receives the sound is the ear, 
 but sometimes the vibration is detected by other things. 
 
 80. Nature of Sound. Sound waves travel through the 
 air, but we cannot see the effect, since the air is transparent. 
 Suppose that the air were made so we could see it, and that, 
 just as a sound wave passed through it, an instantaneous 
 photograph were made of the air. How would it look ? 
 
 Figure 57 shows the condition of the air at a certain in- 
 stant when a sound wave is passing through it. At the point, 
 a, where the vibration started, the air is compressed. Around 
 
 74 
 
VELOCITY OF SOUND 75 
 
 this the air is rare, 6; still farther out, it is compressed, c; 
 and it is again rare at d, etc. 
 
 If pressure gauges were placed around in different parts 
 of the room while the sound-wave was passing, some would 
 show high pressures while others showed low pressures. 
 This is because the vibrations crowd the air together at some 
 places and stretch it out at others. These places are in the 
 shape of spheres. The spheres are 
 alternately places of high and low 
 pressures. 
 
 We have described the air at an 
 instant while the wave is traveling 
 through it. The next question is, how 
 do the waves travel ? 
 
 Sound waves are longitudinal, and 
 
 FIGURE 57. A SOUND 
 depend on the crowding enect lor their v/ AVE IN AIR. 
 
 motion. For example, in Figure 57, a, c, 
 etc., are at high pressures ; while b, d, etc., are at low pres- 
 sures; so the air in the high pressures will push outward, 
 crowding the air in the low pressures. This causes the air 
 at the .low pressures to become condensed, and form high 
 pressures. In this way the high pressures travel outward. 
 The low pressures follow in alternate order. 
 
 You will notice that each particle of air moves only back- 
 ward and forward, while the wave always moves forward. 
 
 81. Velocity of Sound. At C. sound travels 1087 
 feet per second. Examples are common which show that 
 sound waves take time to travel. You can always see the 
 steam before you can hear the whistle. Often you can see 
 a carpenter hit a nail and later hear the sound. As in the 
 case of all waves, V =NL. This formula is used in find- 
 ing the velocity of sound. 
 
76 SOUND 
 
 82. Effect of Temperature on Velocity of Sound. You 
 will notice that the temperature C. was mentioned when 
 the velocity of sound was given as 1087 feet per second. 
 This is because a rise or fall in temperature changes the 
 velocity of sound. A rise of 1 C. makes the velocity 2 
 feet per second greater; and a fall of 1 C. makes the velocity 
 2 feet per second less. 
 
 Thus at 20 C. the velocity will be 1087 + (2 X 20) = 1087 + 40 
 = 1127 feet per second. 
 
 Since a rise in temperature causes air to expand, at a 
 higher temperature the air is less dense, and is thus more 
 easily moved. This explains the change in velocity with a 
 change in temperature. 
 
 83. Natural Free Period. Any object such as a pendu- 
 lum, a tuning fork, a swing, a string, etc. will vibrate with a 
 certain period if allowed to swing freely. This period is 
 called its natural free period. 
 
 84. Resonance. In starting to swing some one, the 
 push must always come at a certain time. The push must 
 be in unison with the motion of the swing. This is called 
 resonance. 
 
 Bridges can be set in motion if the even step of those 
 crossing the bridge coincides with the natural free period 
 of the bridge. For this reason, soldiers break step while 
 crossing bridges. 
 
 One tuning fork will be set in vibration by another, if 
 they have the same natural free period. This is true of all 
 musical instruments. 
 
 The principle of resonance can be stated in this manner : 
 Any object free to vibrate will be set in motion by periodic dis- 
 turbances coming in the natural free period of the object. 
 
HOW WE HEAR 77 
 
 85. The Ear. The ear is the instrument with which 
 we receive sound. The receiving is done in accordance 
 with the principle of resonance. Figure 58 shows a section 
 of the ear. The part (a) is that which we can see outside the 
 head, and is called the external ear. From this a tube leads 
 into the middle ear (b) . Over the end of this tube is stretched 
 a membrane (d) called the 
 
 ear-drum. In the middle ear, 
 attached to the ear-drum, is 
 a series of three little bones. 
 The last of these fits up 
 
 against the end of a spiral 
 
 . t n i ji 77 FIGURE 58. DIAGRAM OF THE 
 
 tube called the cochlea or inner EAR 
 
 ear (c). 
 
 The cochlea is a bony tube making two and one half turns 
 like a snail shell. This tube is filled with a liquid; and 
 stretched from one side to the other are about 7000 strings, 
 all of different lengths, and ranging in frequency from about 
 16 to 10,000 vibrations per second. The tube (e) is the 
 eustachian tube, which leads from the middle ear down into 
 the throat. 
 
 86. How We Hear. A sound wave consists of a con- 
 densation and a rarefaction, or a high and a low pressure. 
 The external ear acts as a funnel and directs the sound wave 
 into the tube to the ear-drum. When the high pressure 
 strikes the ear-drum, the membrane is pushed inward, and 
 then when the low pressure comes it is pushed outward. 
 This sets the three bones in motion, and the small 
 stirrup-shaped bone hammers on the opening to the 
 inner ear. This makes the liquid in the shell-like tube 
 vibrate the same as the air outside the ear. One of the 
 7000 strings the one that has the same natural free 
 
78 SOUND 
 
 period will be set to vibrating by the principle of reso- 
 nance. 
 
 Thus far the process is purely mechanica 1 , and would 
 take place whether the person were awake, asleep, or even 
 dead. 
 
 To distinguish between different sounds, or even to become 
 conscious of them, is a psychological process. Each of the 
 7000 strings has a nerve to the brain. Here it affects its 
 own particular brain cell, thus making the person conscious 
 of a sound. After many similar experiences the person is 
 able to recognize a sound and distinguish it from other 
 sounds. 
 
 If sounds of different frequencies come into the ear, the 
 several corresponding strings will vibrate, and the person 
 hears a combination of sounds. 
 
 87. Reenforcement, Interference, and Beats. If two 
 sound waves travel out together and are of different fre- 
 
 FIGURE 59. REENFORCEMENT, INTERFERENCE, AND BEATS ILLUSTRATED. 
 
 quencies, they will reenforce one another at times, and 
 interfere with one another at other times. 
 
 Figure 59 shows two waves of different frequencies start- 
 ing out together. At a they are making condensations and 
 rarefactions at the same time, and thus they increase the 
 effect, or reenforce one another. At b one wave has vibrated 
 faster than the other, and is making a rarefaction while 
 
PITCH 79 
 
 the other is making a condensation. This is an attempt 
 to make both a high pressure and a low pressure at the 
 same place at the same time. The result is neither. One 
 interferes with the other. 
 
 When the two waves reenforce one another, a loud sound 
 is heard, and this is called a beat. A beat occurs every time 
 one vibrating body gains one vibration on the other. 
 
 If the frequencies of the vibrating bodies do not differ 
 by more than ten, the ear is able to distinguish the separate 
 beats ; but if they differ by more than ten, then the beats 
 come so fast that the ear hears the series of beats as a new 
 sound, and not as a series of separate sounds. 
 
 88. Characteristics of Sound. Sounds differ from one 
 another in three different ways. These differences are 
 called the characteristics of sound, and are named intensity, 
 pitch, and quality. 
 
 89. Intensity. The intensity of sound means its loud- 
 ness, and depends upon the amplitude of the vibration. A 
 bell struck very hard with a hammer will give off a loud 
 sound because the sides of the bell are made to swing with a 
 large amplitude. As the amplitude gets smaller, the sound 
 dies out and finally stops. 
 
 90. Pitch. The pitch depends upon the frequency of 
 the vibration. A string vibrating 256 times per second has 
 a different pitch from one vibrating 384 times per second, 
 even if they are struck with the same force. On the other 
 hand, a string may be struck gently or hard, and it will 
 always give off the same pitch. So the pitch is independent 
 of the amplitude. 
 
 A pitch is said to be high or low, according to the frequency 
 of vibration. The greater the frequency, the higher the 
 pitch. 
 
80 SOUND 
 
 91. Quality. The quality of a sound depends upon its 
 overtones. The overtone is the thing which makes it possible 
 to distinguish one person's voice from another's, or to tell 
 the difference between a piano and a violin, etc. 
 
 92. Fundamental and Overtones. When an object, 
 such as a violin string, is giving its lowest tone, it is said to 
 be giving its fundamental. The string vibrates back and 
 
 forth as a whole, just like a rope 
 that is being swung for some one to 
 jump it. We are apt to think this 
 
 FIGURE 60. VIBRATION OF A is the only way a string will vibrate, 
 STRING IN SEGMENTS AND fc h[ { Th j 
 
 ALSO AS A WHOLE. 
 
 will break up into segments which 
 
 vibrate and in that way give off a higher tone. These 
 higher tones are called overtones. 
 
 A string may be giving several overtones and the funda- 
 mental at the same time. It is the presence of the over- 
 tones that changes the quality of the sound. 
 
 Figure 60 shows a string vibrating as a whole and also in 
 segments. 
 
 93. Analysis of Sound Waves. It has been known for 
 many years that sound waves consist of fundamentals and 
 overtones, but it is hard to tell just what overtones are 
 present. In other words, it is hard to analyze a sound wave 
 and tell just what waves it is made of. 
 
 During the latter part of the nineteenth century a scientist 
 named Helmholtz succeeded in analyzing sound waves. 
 He made hundreds of resonators (Figure 61), all of 
 different sizes, ranging from about a half-inch in diameter 
 to several feet in diameter. By testing a certain sound 
 with each of these hundreds of resonators he was able to 
 determine which ones were in tune with that sound. The 
 
LAWS OF VIBRATING STRINGS 81 
 
 ones that had the same free period vibrated; the others 
 did not. 
 
 The most recent and most successful attempt to analyze 
 sound waves was made by Dr. Dayton Miller of Case 
 School of Applied Science, who is still working on the prob- 
 lem. He has made a machine which will transform the 
 sound waves into a vibrating ray of light, so that the wave 
 can be thrown upon a screen and seen with the eye. He also 
 throws this ray on a photographic plate and takes a picture 
 of the wave, making it possible to study the wave at leisure. 
 
 a be 
 
 FIGURE 61. HELMHOLTZ RESONATORS. 
 
 Dr. Miller is now perfecting another machine, which will 
 analyze the wave after it has been taken on a photographic 
 plate. When this is successfully accomplished, he will 
 be able to take any sound wave and tell how many and what 
 overtones are present. 
 
 With Dr. Miller's machine the differences between singing 
 voices are easily seen. Some singers have many harmonious 
 overtones, while others have very few. 
 
 Figures 62, 63, 64, and 65 show samples of waves given 
 by different singers. 
 
 94. Laws of Vibrating Strings. The pitch of a string 
 may be changed in three ways: by changing (1) its length, 
 or (2) its tension, or (3) its diameter. The tighter it is, the 
 
FIGURE 62. PHOTOGRAPH OF SOUND WAVE PRODUCED BY SPEAKING 
 THE VOWEL "A" AS IN "FATHER." 
 
 FIGURE 63. PHOTOGRAPH OF SOUND WAVE PRODUCED BY THE SOPRANO 
 SINGING ALONE IN THE SEXTET FROM " LUCIA." 
 
 u; 
 
 FIGURE 64. PHOTOGRAPH OF SOUND WAVE PRODUCED BY THE SOPRANO 
 AND BARITONE SINGING TOGETHER IN THE SEXTET FROM " LUCIA." 
 
RESONANCE IN CLOSED PIPES 83 
 
 FIGURE 65. PHOTOGRAPH OF SOUND WAVE PRODUCED BY ALL Six 
 SINGING TOGETHER IN THE SEXTET FROM " LUCIA." 
 
 faster it vibrates ; the longer it is or the thicker it is, the 
 slower are its vibrations. 
 
 The laws concerning these three things are stated as 
 follows : 
 
 (1) The diameter and tension remaining the same, the 
 frequency of a string varies inversely as its length. 
 
 (2) The length and tension remaining the same, the fre- 
 quency of a string varies inversely as the diameter. 
 
 (3) The length and diameter remaining the same, the 
 frequency of a string varies directly as the square root of the 
 tension. 
 
 95. Resonance in Closed Pipes. If a tuning fork is 
 struck and then held over a pipe closed at the bottom, the 
 pipe will reenforce the sound of the 
 fork, provided that the tube is of the \ 
 proper length. 
 
 When the fork moves from a to 6 
 (Figure 66), a condensation is made 
 in front of the fork and travels down 
 the tube to the bottom and back to 
 the mouth again, while the fork moves J~ E 66 . _ RESON ANCE 
 down to 6. At this instant the fork IN A CLOSED PIPE. 
 
84 SOUND 
 
 starts back toward a, forming another condensation in 
 front of the fork ; but since a condensation is already com- 
 ing out of the tube at this instant, this forms a double con- 
 densation, making a loud sound. 
 
 In the same way the rarefactions are reenforced. This 
 action will take place only when the tube is of the proper 
 length. The reflected condensation must be just coming 
 out of the tube when the fork is ready to flip back from b to 
 a ; and the reflected rarefaction must be coming out when 
 the fork is ready to flip back from a to b. 
 
 Now, since a condensation travels down and back, or 
 twice the length of the tube, while the fork goes from a to 
 6, or one half vibration, the sound will travel four times the 
 length of the tube during a whole vibration. Therefore the 
 closed pipe is one fourth wave length. 
 
 By this method the velocity of sound may be determined. 
 The wave length is found by multiplying the length of the 
 tube by four. The frequency is al- 
 ways marked on the fork. Then, by 
 formula : 
 
 V = NL. 
 
 96. Resonance in Open Pipes. 
 
 If the pipe is open instead of closed 
 at the bottom (Figure 67), the con- 
 densation will travel down to the end, 
 FIGURE 67 - RESONANCE d m ^ t rarefaction in- 
 
 IN AN OPEN PIPE. t J 
 
 stead of a condensation; so the fork 
 
 must be back at a again before this rarefaction gets to the 
 top. That is, while the sound travels down and back, the 
 fork must make a complete vibration. Therefore the pipe 
 is one half wave length. 
 
CHAPTER VII 
 BASIS FOR MUSIC 
 
 97. Music and Noise. The prime difference between 
 music and noise is that in music the sounds have rhythm 
 while in noise they do not. By rhythm is meant that the 
 sounds come at regular periodic intervals. 
 
 The music of the savages consists almost entirely of beating 
 time, while the music of civilized people goes farther than 
 this, and consists of rhythm and harmony. 
 
 98. Harmony. Two or more tones are said to be in 
 harmony if their combination is pleasant to hear. Har- 
 mony, then, is the combining of musical tones, according to 
 given laws, so that they will be pleasing to the ear. 
 
 One of the laws of harmony is, the ratios of two tones must 
 be in a simple ratio if they are to be in harmony. By " simple 
 ratios " is meant, such ratios as {, f-, f, |,,f, f, f, f, etc. 
 
 The reason why tones having their frequencies in simple 
 ratios are harmonious is a matter of supposition. It is 
 supposed that the mind likes system, and, more than that, 
 simplicity of system. The most simple method in which 
 soldiers can march is in step; the next simplest is every 
 other soldier making two steps to his neighbor's one; the 
 next is three steps to two; and so on. As soon as the ratio 
 gets into large numbers, the mind fails to grasp the system, 
 and the marching soldiers become a mob. 
 
 The same is true of sound. When the ratios are simple, 
 
 85 
 
86 BASIS FOR MUSIC 
 
 the mind grasps the relation and is pleased ; but when the 
 ratios become complex, the mind fails to detect any relation 
 whatever, and a discord results. 
 
 99. Major Triads. When the frequencies of three tones 
 are in the ratio 4:5:6, those three tones are called a triad. 
 In music there are three triads, called major triads. They 
 
 are : 
 
 1. Tonic -C,E,G 
 
 2. Dominant G, B, d z 
 
 3. Subdominant F, A, c 2 
 
 100. Major Scale. The eight notes which form the 
 major triads, when arranged in the proper order, form what 
 is called the major scale. 
 
 CDEFGABc 2 
 
 The frequency of each of the tones in the major scale 
 can be found by the ratios of the major triads. 
 
 C:E:G} 
 
 G: B\(k \ =4:5:6 
 
 The frequency of C can be taken as any number, and then 
 the frequencies of each of the others can be determined from 
 it. 
 
 If C = 256 C = 256 
 
 E _ 5 G _ fi 
 
 C ~ 4 C = 4 
 
 F-* r n 5 r 
 
 *4' -4' 
 
 r /> 
 
 E = '- 256 = 320 G = - - 256 = 384 
 
 By this method the frequencies of all notes can be found. 
 Construct the major scale and calculate all the frequencies. 
 
THE CHROMATIC SCALE 87 
 
 101. The Musical Interval. The ratio of the frequencies 
 of any two tones is called the musical interval between those 
 tones. 
 
 The musical intervals between consecutive tones in the 
 octave, and the intervals between each tone and C are given 
 as follows : 
 
 Letter ...... C D E F G A B c 2 
 
 Frequency .... 256 288 320 341| 384 426| 480 512 
 
 Interval between con- 
 
 secutive tones . . f -V- if I V I If 
 
 Interval between each 
 
 tone and C ... 1 f J | f f - 1 -/ 2 
 
 There are a few musical intervals of more importance 
 than others, and these are given special names. Thus -J- = 
 unison; f = a fifth; J = a fourth; f = a major third; ff = 
 a half step; and f = an octave. 
 
 102. The Chromatic Scale. For certain purposes it is 
 often advisable to start triads on other notes than C, G, and 
 F. This requires other notes than those in the major scale. 
 By starting triads on each of the other notes of the major 
 scale we have : 
 
 E : X v : X 2 
 A Xi : X, 
 
 Figuring out the frequencies of these unknown notes, we 
 find they are in the first triad : 
 
 f =f; Z t -f -288 = 360 
 
 A 2 -?; X*=- A 288=432 
 D 4 4 
 
88 BASIS FOR MUSIC 
 
 Now, 360' falls between F and 0, and 432 falls between A 
 and B; so they are called F -sharp and A-sharp, respectively. 
 Thus the first triad is D, F-sharp, and A-sharp. 
 
 When all these unknown frequencies are calculated, it 
 is found that there are five new notes w r hich fall in betw r een 
 the other notes of the major scale, and a new scale is built 
 up, using the major scale, with the five new notes added 
 in their proper places. This new scale is called the chromatic 
 scale. 
 
 The notes in it are : 
 
 C, C-sharp, D, D-sharp, E, F, F-sharp, G, G-sharp, A, A-sharp, B, c z . 
 
 103. Tempered Scale. The musical intervals between 
 the consecutive notes in the chromatic scale are not all 
 equal. But in the piano and similar instruments the notes 
 are made at equal intervals. This new scale is called the 
 tempered scale. The musical interval between consecutive 
 notes is 
 
 %" = 1.095 
 
 This musical interval is calculated by this method : 
 There are twelve equal intervals in the tempered scale. 
 Suppose we let a- equal the numerical value of this interval. 
 
 Then C-sharp = C x 
 
 D = C-sharp x = C x x 
 D-sharp = D-x = C-x-x-x. 
 
 And so on for the complete scale. 
 
 Thereforec 2 = C x 12 ', 
 but .c 2 = C - 2. 
 
 Therefore x n = 2, 
 
 or x = V2. 
 
 104. Standard Pitch. In order that a piece of music 
 may be played as intended, there must be a standard pitch 
 
MUSICAL INSTRUMENTS 89 
 
 for C. There are several standards, the commonest being 
 the " International Standard Pitch," which makes C = 261. 
 105. Musical Instruments. The student is here asked 
 to report on one musical instrument, covering the following 
 points : 
 
 1 . Description of the instrument. 
 
 2. How the sound is produced. 
 
 3. How the pitch is determined. 
 
 4. What the principal use of the instrument is. 
 
 Review Problems 
 
 1. Give five examples of wave motion. 
 
 2. Distinguish between transverse and longitudinal waves. 
 
 3. What are the characteristics of transverse waves? Define each. 
 
 4. What are the characteristics of longitudinal waves? Define 
 each. 
 
 5. Explain how transverse waves travel. 
 
 6. Explain how longitudinal waves travel. 
 
 7. If a rope be shaken at the rate of 3 vibrations per second, and 
 the waves are 10 feet long, how fast do the waves travel? 
 
 8. Explain the nature of sound. 
 
 9. If 3 seconds after you see the lightning you hear the thunder, 
 how far away was the lightning ? Take the temperature as 18 C. 
 
 10. Why does a vase, or any other small article in the room, often 
 rattle when the piano is played ? 
 
 11. Why is it dangerous for the audience to stamp the feet in a large 
 auditorium ? 
 
 12. Describs the ear. 
 
 13. How do we hear? 
 
 14. Why do heavy explosions, such as the firing of cannon, often 
 cause deafness? 
 
 15. What are beats ? 
 
 16. What is the cause of " dead points " places where it is 
 difficult to hear in an auditorium ? 
 
90 BASIS FOR MUSIC 
 
 17. Name the characteristics of sound. Upon what does each 
 depend ? 
 
 18. What is the difference between a "sweet" and a "harsh" 
 voice ? 
 
 19. If two strings are the same, except that one is 40 cm. long and 
 the other is 60 cm. long, what is the ratio of their frequencies? If 
 the 40-cm. string vibrates 300 times per second, what is the frequency 
 of the other? 
 
 20. Why are some of the strings on a piano large and others small ? 
 
 21. How does a piano tuner tune a piano? Why does this change 
 the pitch ? 
 
 22. Why does a pipe organ have many pipes, all of different lengths? 
 
 23. Explain how to find the velocity of sound. 
 
 24. What is rhythm ? Harmony ? 
 
 25. What is the leason why tones must be in simple ratios to be in 
 harmony ? 
 
 26. Construct a major scale, using C as 96. 
 
 27. Construct a chromatic scale, using E as 409. 
 
 28. What is the tempered scale? 
 
 29. Why is the common musical interval between consecutive notes 
 in the tempered scale 1 .059 ? 
 
 30. Name two other standard pitches besides the International 
 Standard Pitch. (Outside reference.) 
 
 31. What is the use of the sounding board in a piano? 
 
 32. Why does a phonograph give a higher pitch when run fast? 
 
 33. What changes the pitch of a slide trombone ? 
 
 34. What changes the pitch of a cornet? 
 
 35. Why does the piano have the tempered scale? Figure out the 
 frequencies of all notes on the piano, using A as 435. 
 
CHAPTER VIII 
 > 
 
 LIGHT 
 
 106. Nature of Light. Nobody knows the exact nature 
 of light. Many theories have been offered, but none has 
 been accepted as final. But we know a great deal about 
 light, even if we do not know just what it is. In this dis- 
 cussion, we shall take up facts already proved and mention 
 some of the latest theories. 
 
 It is definitely known that light is one of the many forms of 
 energy, and that it has much in common with radiant heat. 
 
 107. Theory of Production of Light. In almost all 
 cases, light is produced by something hot. (Fluorescence 
 and phosphorescence are exceptions.) Our common sources 
 of light are the sun, a fire, a candle, a lamp, or some other 
 very hot body. 
 
 It is thought that the rapid vibration of the molecules of 
 the heated body sets up waves in the ether, and that the 
 ether transmits these waves through space. These waves 
 are of different lengths, depending upon the frequency of 
 the vibration of the molecules. Those waves which are of 
 the right length to affect the eye are called light. 
 
 When a piece of iron becomes hot enough, it gets luminous; 
 in other words, it gives off light. The molecules of the iron 
 vibrate very rapidly, and this vibration sets up waves in 
 the ether, which are transmitted in all directions. These 
 waves we call light. 
 
 91 
 
92 
 
 LIGHT 
 
 108. Propagation of Light Waves. Just how the ether 
 transmits these waves is still a mystery, but it is known that 
 they are transverse, and that they travel in straight lines. 
 
 109. Velocity of Light. It is easy to find examples 
 showing that sound takes time to travel, but all ordinary 
 examples fail to show that the same is true of light, and 
 for many centuries the transmission of light was thought 
 to be instantaneous. 
 
 110. Roemer's Method of Finding Velocity of Light. 
 The first man to prove that the passage of light requires 
 time was Roemer, and he did it by accident. 
 
 FIGURE 68. RELATIVE POSITIONS OF SUN, EARTH, JUPITER, 
 AND MOON OF JUPITER. 
 
 Roemer was an astronomer who lived during the seven- 
 teenth century. About 1676 he was studying the eclipses 
 of one of the moons of Jupiter by Jupiter. He found that 
 the eclipses did not occur at regular intervals, as was ex- 
 pected, but that for six months the time between eclipses be- 
 came shorter and shorter, and then for the next six months 
 it became longer and longer. (Figure 68 shows the relative 
 position of the heavenly bodies under consideration.) 
 
 Every time the moon of Jupiter came into the shadow 
 of Jupiter, there was an eclipse. Roemer knew how long 
 
COMPARATIVE VALUE OF VELOCITY OF LIGHT 93 
 
 it took the moon to make a complete revolution about 
 Jupiter, and so assumed that eclipses ought to come at that 
 interval. He made a schedule something like the follow- 
 ing (assuming that it takes exactly 30 days for the moon to 
 make a revolution) : 
 
 1st eclipse 12 o'clock Jan. 1 
 
 2d eclipse 12 o'clock Jan. 31 
 
 3d eclipse 12 o'clock Mar. 1 
 
 4th eclipse 12 o'clock Mar. 31 
 
 5th eclipse 12 o'clock Apr. 30 
 
 6th eclipse . . ' 12 o'clock May 30 
 
 7th eclipse \ 12 o'clock June 29 
 
 8th eclipse 12 o'clock July 29 
 
 9th eclipse . 12 o'clock Aug. 28 
 
 10th eclipse 12 o'clock Sept. 27 
 
 llth eclipse 12 o'clock Oct. 27 
 
 12th eclipse 12 o'clock Nov. 26 
 
 13th eclipse 12 o'clock Dec. 26 
 
 The earth being at E, at the time of the first eclipse, 
 Roemer found that at each occurrence the eclipses were 
 behind the schedule more and more, and that six months 
 later, when the earth was at E 2 , the eclipse occurred 1000 
 seconds later than the scheduled time (12 o'clock, June 29). 
 Then, for the next six months, the eclipses began to catch 
 up with the schedule, and were exactly on time (12 o'clock 
 Dec. 26) when the earth got back to EI. 
 
 Roemer then reasoned that it took the light 1000 seconds 
 to cross the earth's orbit, a distance of 186,000,000 miles. 
 
 1 SA onn nnn 
 This gave the velocity of light as T ^ = 186.000 miles 
 
 per second. 
 
 111. Comparative Value of Velocity of Light. The 
 velocity of light, 186,000 miles per second, is so great that 
 the mind cannot appreciate it without some comparative 
 
94 LIGHT 
 
 values. It means that a ray of light would travel nearly 
 7J times around the earth in one second. It would take a 
 train, going at 60 miles an hour, over 4 months to travel 
 as far as a ray of light can travel in one second. 
 
 112. Shadows. Since light travels in straight lines and 
 will not go through opaque objects, it is easily shut off by 
 putting one of these objects in its path. When light is 
 shut off from a certain space by an object placed in the 
 path of the light, that space is called a shadow. A shadow 
 is the space from which the light has been cut off. 
 
 A man walking on the sidewalk on a sunny day casts a 
 shadow. Hold your hand in front of a lamp and your hand 
 casts a shadow. The earth shuts off part of the sun's rays 
 and casts a shadow. 
 
 The shadow in each of these cases is the space back of 
 the object. It is not, as we often think, the dark portion 
 
 of the sidewalk or of the 
 wall. Those are only cross 
 sections of the shadows. 
 
 113. Shadow from a Point 
 Source of Light. Figure 
 
 FIGURE 69.- SHADOW FROM A POINT 69 sh WS a shadow Cast b >' 
 SOURCE OF LIGHT. an object in front of light 
 
 coming from a point source. 
 
 The light travels out in all directions from point P, but 
 that which strikes the rectangle abed is shut off, thus making 
 the space S without light, or a shadow. The shadow, then, 
 is a pyramid with the top cut off. 
 
 Had the object been circular, the shadow would have 
 been a cone with the top cut off. 
 
 114. Shadow from a Large Source. Most of our light 
 comes from large sources and not from points. Figure 70 
 
SHADOW FROM A LARGE SOURCE 95 
 
 shows the shadow cast by an object (0) with a large source 
 of light (S). 
 
 It will be seen that the space above be and below ad is 
 lighted by all of S. The space between ac and bd beyond 
 the object gets no light at all, and so is totally dark. This 
 is called the umbra ( U). The space outside the umbra, 
 
 FIGURE 70. SHADOW FROM a LARGE SOURCE OF LIGHT. 
 
 and still inside ad and be, is called the penumbra (P, P). 
 This space is totally dark at ac and bd, but becomes lighter 
 and lighter, as you go outwlard. That is, point y has more 
 light than point x, because more of S is shining on it. 
 
 Shadows play a great part in the arts both of painting 
 and of sculpture. They also enter into the problems of 
 proper illumination, and so will be further discussed under 
 that topic. 
 
CHAPTER IX 
 
 REFLECTION AND MIRRORS 
 
 115. Reflection. If a ray of light strikes a bright sur- 
 face, it will be partially reflected. Reflection is the returning 
 cf a ray of light into the same medium from which it came, 
 when it strikes another medium. 
 
 One of the most common cases of reflection is seen when 
 a ray of light strikes a mirror. Figure 71 shows a ray of 
 
 light striking a mirror and being 
 reflected. 
 
 IR is the incident ray. RR is 
 the reflected ray. MM is the 
 mirror, and OP is the perpen- 
 dicular to the mirror at the point 
 where the ray IR strikes the 
 mirror. 
 
 The angle between the incident 
 ray and the perpendicular to the 
 mirror is called the angle of in- 
 cidence. 
 
 The angle between the reflected ray and the perpendicular to 
 the mirror is called the angle of reflection. 
 
 Light is always reflected so that the angle of reflection equals 
 the angle of incidence. This is called the Law of Reflection. 
 
 116. Pencil of Rays. So far we have spoken of rays of 
 light. Light never goes in single rays, but in bunches of 
 
 96 
 
 M 
 
 FIGURE 71*. SHOWING REFLEC- 
 TION OF A RAY OF LIGHT. 
 
IMAGE IN A PLANE MIRROR 
 
 97 
 
 rays. A small bunch of rays is called a pencil of rays, and 
 
 this is what we have to consider instead of single rays. A 
 
 person gets a pencil of 
 
 rays, or many pencils of 
 
 rays, in his eye, instead 
 
 of just single rays. 
 
 (Figure 72.) 
 
 117. Image in a Plane 
 Mirror. Figure 73 shows 
 the image in a plane mirror, 
 mirror ; and a'b f ', the image. 
 
 FIGURE 72. A PENCIL OF RAYS. 
 
 The object is ab ; MM, the 
 An image is the space occupied 
 by what is apparently the object itself. 
 
 Rays are sent off in all directions from each point of the 
 object. Let us consider the two points a and b, the head 
 and tail of the object. There is just one pencil of rays from 
 
 each of these points 
 which goes out, strikes 
 the mirror at the right 
 angle, and is reflected 
 into the eye. 
 
 The pencil of rays 
 coming from a, after 
 being reflected at c, 
 appears to come from 
 point a' ; and the 
 pencil of. rays coming 
 from 6, after being 
 reflected at d, appears 
 to come from b'. 
 By geometry it is easily proved that the image is as far 
 back of the mirror as the object is in front, and on a line with 
 the object, perpendicular to the mirror. 
 
 M 
 
 FIGURE 73. CONSTRUCTION OF AN IMAGE IN 
 A PLANE MIRROR. 
 
98 REFLECTION AND MIRRORS 
 
 There are two kinds of images, real and virtual '. 
 
 A real image is an image through which the rays of light 
 actually pass. 
 
 A virtual image w an image through which the rays of light 
 apparently pass, but do not. 
 
 It will be seen by these definitions that the image in a 
 plane mirror is virtual. Why? 
 
 118. Concave Mirrors. A concave mirror is a mirror 
 which curves, and has the hollow side towards the object. 
 
 There are several kinds of concave 
 mirrors. The two most common ones 
 
 Ci 
 
 are the spherical mirror (Figure 74) and 
 the parabolical mirror (Figure 75). 
 
 FIGURE 74 -A SPHERI- The sp h e rical mirror is a portion of 
 CAL MIRROR. 
 
 the surface of a sphere, every point of 
 
 which is equidistant from one point (c) called the center of 
 curvature. 
 
 The parabolical mirror is the portion of the surface of a 
 paraboloid and is of the shape shown 
 in Figure 75. The parabolical mirror is 
 much better than the spherical because 
 it gives a perfect image, while the other 
 
 does not. FlGURE 75 -~ A PARA ~ 
 
 BOLICAL MIRROR. 
 119. Meaning of Terms. In Figure 
 
 76 the point c is the center of curvature, and is equidistant 
 from all points in the surface of a spherical mirror. The 
 
 line XO is the prin- 
 cipal axis, and is the 
 
 *- x line passing through 
 
 the center of curvature 
 
 FIGURE 76. -THE PRINCIPAL POINTS OF A and the Center f 
 SPHERICAL MIRROR. the mirror (0). 
 
IMAGE IN A CONCAVE MIRROR 
 
 99 
 
 The focus of a mirror is the point at which the image 
 is located. The point / is the principal focus, and is 
 the point at which all rays parallel to the principal 
 axis are focused. The principal focus is located at one 
 half the distance from c to 0. The focal length is the dis- 
 tance (Of) from the center of the mirror to the principal 
 focus. 
 
 120. Image in a Concave Mirror. Figure 77 shows the 
 construction of an image in a concave mirror. 
 
 First, draw ad from a, the head of the object, parallel 
 to the principal axis. Since this is a ray parallel to the 
 
 FIGURE 77. CONSTRUCTION OF IMAGE IN A CONCAVE MIRROR. 
 
 principal axis, it must be reflected through the principal 
 focus /. This determines line dx. 
 
 Second, draw ag from a through the center of curvature c. 
 This ray is reflected back upon itself, since it strikes the 
 mirror perpendicularly. The point a', where these two 
 reflected rays meet, is the head of the image. 
 
 Third, locate the tail of the image in the same way. This 
 completes the construction of the image. 
 
 This image is seen to be real, inverted, and smaller than the 
 object. The image may be located by this method for any 
 position of the object. The description of the image can 
 then be easily given from the figure. 
 
100 REFLECTION AXD MIRRORS 
 
 121. Convex Mirrors. A convex mirror is a curved 
 mirror which has the hollow side of the curve away from the 
 object. 
 
 The same terms, focus, axis, etc., apply to the convex 
 mirror as to the concave mirror. 
 
 122. Image in a Convex Mirror. The construction of 
 the image in a convex mirror is the same as for the concave 
 mirror. Draw the two lines from the head of the object, 
 
 l b 
 FIGURE 78. CONSTRUCTION OF IMAGE IN A CONVEX MIRROR. 
 
 one (ad, Figure 78) parallel to the principal axis, and the 
 other (ac) through the center of curvature. When reflected, 
 these two rays pass through the principal focus and back upon 
 themselves, respectively. Where they meet (a'} is the 
 image of the head. The image of the tail (b r ) is located in 
 a similar manner. 
 
 In this case the image is virtual, erect, and smaller than the 
 object. 
 
 123. Applications of Mirrors. 1. Plane Mirror. The 
 general use of the plane mirror as a looking glass is too 
 familiar to need discussion. 
 
 2. Concave Mirror. The most general use of the concave 
 mirror is that of a reflector. Since all parallel rays come 
 together at the principal focus, it is seen that the rays from 
 a source of light placed at the principal focus will be sent 
 out as parallel rays. (Figure 79.) 
 
APPLICATIONS OF lf/&jR(pS^; : 101 
 
 The automobile headlight is an example of this. The 
 bulb is so placed that the filament of the lamp is very near 
 the principal focus of the reflector. This sends the rays 
 out in nearly parallel beams. The correct position of the 
 filament is just beyond the principal focus, but close to it. 
 This makes the rays cross and then diverge slightly, so that 
 a large area of the road can be seen. The same use is also 
 made in many different kinds of lamps. 
 
 The concave mirror is used in all telescopes of the re- 
 flector type. The largest telescope of this sort has just 
 been completed by the Warner Swazey Company, of Cleve- 
 land, to be used at the Canadian observatory at Victoria. 
 
 FIGURE 79. A CONCAVE MIRROR USED AS A REFLECTOR. 
 
 The concave mirror for this telescope is 72 inches in diameter, 
 and, like all high-grade concave mirrors, is of the parabolical 
 shape. The telescope will be used to take photographs of 
 distant stars. The mirror is large so that many rays of the 
 star are focused at the image. 
 
 3. Convex Mirror. The convex mirror is often used on 
 automobiles to give the driver a view of vehicles behind 
 him. It is usually placed on the front fender or attached 
 to the side of the windshield. The mirror gives a small 
 but clear image of everything in the rear. 
 
 Large spheres with mirror surfaces are often placed in 
 flower gardens to add to the decorations and to give beau- 
 tiful images of the walks and flowerbeds. 
 
102 \ily', '^tiEFLECTiON AND MIRRORS 
 
 Another use of the convex mirror is that of the " vanity- 
 mirror " carried in ladies' hand bags or pocketbooks. It is 
 much preferred to the plane mirror, for even a small one an 
 inch in diameter will give an image of the whole face. 
 
 6 c 
 
 FIGURE 80. PECULIARLY SHAPED MIRRORS. 
 
 4. Peculiarly Shaped Mirrors. There are many peculiarly 
 shaped mirrors, such as are found in " hilarity halls," etc. 
 Figure 80 shows a few of these. Due to the peculiar shapes, 
 the images are distorted and afford amusement for the 
 patrons. 
 
CHAPTER X 
 
 REFRACTION AND LENSES 
 
 124. Refraction. The term refraction is very often 
 confused with the term reflection, but it must be borne in 
 mind that the two mean entirely different things. 
 
 Refraction is caused by the change in velocity of a ray of 
 light when it passes from one medium to another. This causes 
 a bending of the ray when it strikes at an 
 angle other than 90. 
 
 If a lead pencil be put into a beaker of 
 water (Figure 81), it looks as if the lead 
 pencil were bent at the water line. If you 
 try to touch an object under water very 
 quickly, your hand will pass over the object, 
 
 showing that the object appears higher than 
 
 j. 11 TJ> 11,11 i FIGURE 81. A 
 
 it really is. It you look through a poor grade PENCIL LOOKS 
 
 of window glass at some straight line, such 
 as the side of a tall chimney, the line looks 
 jagged and crooked. (Figure 82.) 
 
 All these illusions are caused by refraction. 
 
 125. Refraction Explained. Figure 83 shows a ray of 
 light (A) passing from air, through a piece of plate glass, 
 back into air. 
 
 The small lines ab represent the wave front of the ray. 
 A ray of light always travels at right angles to its wave 
 front ; so the wave front determines its direction. 
 
 103 
 
 BENT AT THE 
 SURFACE OF 
 THE WATER. 
 
104 
 
 REFRACTION AND LENSES 
 
 The ray travels in a straight line until it strikes the 
 glass. The side a strikes first, and so is retarded, since 
 
 light cannot travel in 
 glass as fast as in air. 
 This allows b to swing 
 ahead, since it is still in 
 air. This continues until 
 both a and b are inside the 
 glass. Then they again 
 go at equal speeds, giving 
 
 FIGURE 82. -A CHIMNEY VIEWED THROUGH the T W a straight path, 
 POOR WINDOW GLASS. but one slightly deviated 
 
 from its original path. 
 
 At the other side of the glass, a comes to the surface 
 first, and so swings ahead of 6, for it now travels in air. 
 It continues to do this until both a and b are again in air. 
 Here they continue again at equal speeds, and the ray 
 again goes in a straight line. 
 
 If the two sides of the glass are parallel, the ray swings 
 back just as much as it deviated in the first place. This 
 makes its path parallel to its path before entering the glass, 
 but not in the same line. 
 
 If the two sides of the glass are not parallel, the ray 
 will not be parallel with its first path, but will deviate ac- 
 cording to the angle of the two surfaces. 
 
 126. Meaning of Terms and Law of Refraction. In 
 refraction., the incident ray is the ray before it strikes the 
 refracting surface (AO for the first surface, and 00' for the 
 second surface, Figure 83). 
 
 The refracted ray is the ray after it strikes the refracting 
 surface (00' for the first surface, and 0' A' for the second 
 surface) . 
 
INDEX OF REFRACTION 
 
 105 
 
 The angle of incidence is the angle between the incident 
 ray and the perpendicular to the surface (angle i for the 
 first surface, and angle i' for the second surface). 
 
 The angle of refraction is the angle between the refracted 
 ray and the perpendicular to the surface (angle r for the 
 first surface, and angle r' for the second surface). 
 
 FIGURE 83. DIAGRAM EXPLAINING REFRACTION OF LIGHT. 
 
 The law of refraction : A ray of light passing from a rare 
 medium into a denser medium always bends toward the per- 
 pendicular, and a ray of light passing from a dense to a rarer 
 medium always bends away from the perpendicular. 
 
 127. Index of Refraction. Different substances refract 
 light in varying degrees. In order to compare and express 
 these amounts of refraction a term called index of refraction 
 is used. 
 
106 REFRACTION AND LENSES 
 
 The index of refraction is equal to the velocity in the rare 
 medium divided by the velocity in the dense medium. 
 
 Index of Refraction = 
 
 Vdense 
 
 There are two kinds of indexes of refraction, relative and 
 absolute. 
 
 The relative index of refraction is the index when the ray 
 passes from one substance to another, and is correct for those 
 two substances only. 
 
 The absolute index of refraction is the index when the ray 
 passes from a vacuum into a substance, and applies to that 
 one substance only. 
 
 The index of refraction is used principally in the manu- 
 facture of lenses. The index determines the amount of 
 curvature that the lens must have. It is the high index of 
 refraction of the diamond that gives it its sparkle. 
 
 128. Applications of Refrac- 
 tion. The applications of re- 
 fraction are used in lenses and 
 prisms. These will be discussed 
 later. 
 
 We have mentioned the effect 
 of looking at a straight line 
 
 through a poor grade of window 
 
 FIGURE 84. -REFRACTION OF LIGHT ] ags Explain this> 
 ABOVE A HOT STOVE. 
 
 It is a common thing to notice 
 
 the wavy effect above a fire or stove (Figure 84). This is 
 not heat waves, as so many think ; but it is due to refrac- 
 tion. The air above the fire is heated and becomes less 
 dense than the surrounding air. Light rays passing through 
 these layers of air of unequal densities are refracted, giving 
 the wavy effect. . 
 
CRITICAL ANGLE 
 
 107 
 
 Our atmosphere acts as a refracting substance to the 
 sun's rays. For this reason we can actually see the sun 
 
 Evening 
 
 FIGURE 85. REFRACTION OF LIGHT BY THE EARTH'S ATMOSPHERE. 
 
 before it is above the horizon in the morning, and also after 
 it has gone below the horizon in the evening. (Figure 85.) 
 
 129. Critical Angle. - 
 Figure 86 shows what 
 takes place when a ray 
 of light passes from a 
 dense medium, such as 
 water, to a rare medium, 
 such as air. 
 
 A ray of light AO passes 
 from the dense medium 
 and goes into the rare 
 medium at 0. According 
 to the law of refraction, 
 
 the ray is bent away from 
 
 ,1 vi r>rtr FIGURE 86. DIAGRAM EXPLAINING 
 
 the perpendicular PP , CRITICAL ANGLE . 
 
 making the angle of re- 
 fraction r larger than the angle of incidence i. (Figure 86.) 
 Now, if the angle of incidence is made larger and larger, 
 the angle of refraction will become larger and larger also 
 
108 
 
 REFRACTION AND LENSES 
 
 and will always be greater than its corresponding angle of 
 incidence. 
 
 If the angle of incidence becomes large enough (i?), the 
 angle of refraction (r 2 ) becomes equal to 90, and the re- 
 fracted ray passes out along the surface of the dense medium 
 (OB'). The angle of incidence is then called the critical 
 angle. 
 
 The critical angle is an angle of incidence which corresponds 
 to an angle of refraction of 90. 
 
 130. Total Reflection. Angle i (Figure 87) is the 
 critical angle, and so the refracted ray OA' passes out 
 
 along the surface of the 
 dense medium, making 
 the angle of refraction (r) 
 equal to 90. 
 
 Now, if the angle of 
 incidence is made still 
 larger, such as i, the 
 angle of refraction be- 
 comes greater than 90. 
 This makes the refracted 
 ray return into the same 
 medium in which it en- 
 tered. But this is reflec- 
 tion instead of refraction, 
 
 and so the ray must obey the law of reflection, making the 
 angle of reflection (r 2 ) equal to the angle of incidence (i^). 
 
 This is called total reflection, because none of the rays 
 can be refracted, but all are reflected. 
 
 Total reflection is reflection against a surface of a rare 
 medium when the angle of incidence is greater than the critical 
 angle. 
 
 FIGURE 87. DIAGRAM EXPLAINING 
 TOTAL REFLECTION. 
 
PRISMATIC WINDOW GLASS 
 
 109 
 
 FIGURE 88. POSITIONS OF PRISMS IN A 
 LIGHTHOUSE REFLECTOR. 
 
 It must be noted that total reflection takes place only 
 when the ray is passing from a dense to a rare medium. 
 
 APPLICATIONS OF TOTAL REFLECTION 
 
 131. The Lighthouse Reflector. The lighthouse re- 
 flector is an application of total reflection. The source of 
 light, a gas flame 
 
 or an electric light 
 
 bulb, is placed at 
 
 the center. (L, 
 
 Figure 88.) Circular 
 
 right-angled prisms 
 
 (Figure 89) are 
 
 placed around the 
 
 light at P, P, P, etc. 
 
 (Figure 88), forming 
 
 an inclosed sphere. Instead of the prisms being far apart, 
 
 as in the figure, they are placed so close together that no 
 
 light gets out between them. 
 
 The light coming from the center 
 strikes one leg of the right-angled 
 prism, enters the glass, and then 
 
 FIGURE 89. A LIGHT- str jk es t h e hypotenuse at an angle 
 
 HOUSE REFLECTOR PRISM. 
 
 greater than the critical angle. Total 
 reflection takes place, and all the light is sent out in a 
 parallel beam. By this means all the light is utilized, top, 
 bottom, and sides. 
 
 132. Prismatic Window Glass. Very often it is im- 
 possible by means of the ordinary windows to get sunlight 
 into rooms shaded by other buildings, especially in large 
 cities where " skyscrapers " are the rule. Prismatic window 
 glass helps to do away with this difficulty. The light com- 
 
110 
 
 REFRACTION AND LENSES 
 
 ing almost straight down (Figure 90) strikes the prismatic 
 
 glass and is totally reflected into the room. 
 
 133. Field Binocu- 
 lars. In the field 
 binoculars, such as 
 are used by officers 
 of the army and 
 navy, the light must 
 pass a distance of 
 several inches after 
 it enters the instru- 
 ment before it reaches 
 the eye. To keep 
 the instrument from 
 becoming too long, 
 the rays of light are 
 reflected back and 
 forth from one end 
 
 to the other by 
 
 FIGURE 90. USE OF PRISMATIC WINDOW . , ' 
 
 GLASS. means of right- 
 
 angled prisms. 
 
 Figure 91 shows a diagram of the path of a light ray in 
 one tube of the binocular. 
 
 134. A Fish's View of the Outside World. It is rather 
 interesting to note just 
 how the outside world 
 looks to the fish below 
 the surface of the 
 water. Figure 92 is a 
 diagram showing how 
 the rays of light come to the fish's eye. 
 
 The sky and all objects above the horizon are seen through 
 
 FIGURE 91. TOTAL REFLECTION USED IN 
 THE BINOCULARS. 
 
LENSES 
 
 111 
 
 a cone whose angle is about 97. Outside of this cone the 
 fish gets rays coming from the bottom and reflected at the 
 surface of the water. This makes the sky look as if it had 
 a fringe of stones or 
 grass, according to 
 whether the bottom 
 is stony or grassy. _ 
 
 135. The Diamond. 
 As mentioned be- 
 fore, the large index 
 
 FIGURE 92. A FISH'S VIEW OF THE 
 of refraction in a dia- OUTSIDE WORLD. 
 
 mond gives it its 
 
 sparkle. As the diamond has a large index of refraction 
 and is cut with many facets, the light is reflected many 
 times within the stone, so that there is scarcely an angle 
 at which you can view it without getting a flash of light. 
 
 LENSES 
 
 136. Lenses. A lens is a transparent body of such a 
 shape that it will focus rays of light. There are two general 
 
 classes of lenses : (a) con- 
 verging, (b) diverging. 
 
 A converging lens is a 
 lens which tends to bring 
 the rays together after they 
 pass through. (Figure 
 93.) 
 
 A diverging lens is a 
 lens which tends to send the rays farther apart after they go 
 through. (Figure 94.) 
 
 Lenses are of different shapes and are given specific 
 names according to these shapes. (Figure 95.) In general, 
 
 FIGURE 93. LIGHT PASSING THROUGH A 
 CONVERGING LENS. 
 
112 
 
 REFRACTION AND LENSES 
 
 lenses that are thicker at the center than at the edges are 
 converging, while those thinner at the center than at the 
 edges are diverging. 
 
 137. Meaning of Terms. The 
 line drawn through the centers of 
 curvature of the two surfaces is 
 called the principal axis. (CC, 
 Figure 96.) 
 
 The optical center (0) is the point 
 on the principal axis, midway be- 
 tween the surfaces of the lens. 
 
 The principal focus (F) is the 
 point at which all rays parallel to 
 the principal axis are focused. 
 
 The focal length (OF) is the distance from the optical 
 center to the principal focus. 
 
 The image is a point, or a series of points, at which the 
 rays coming from an object are focused. The rays coming 
 
 FI.GURE 94. LIGHT PASS- 
 ING THROUGH A DIVERGING 
 
 LENS. 
 
 c d c f 
 
 FIGURE 95. DIFFERENT SHAPED LENSES. 
 
 from one point of the object are focused at one point in the 
 image. 
 
 138. Image through a Converging Lens. There are 
 five possible settings for a converging lens : 
 
 I. The object beyond 2 F. 
 
IMAGE THROUGH A CONVERGING LENS 113 
 
 To construct the image for this position (Figure 97) 
 draw the lens and the principal axis; locate the optical 
 center and the principal focus. (Note: Every lens has its 
 own focal length ; and if this is given, the principal focus 
 can be located by it; but if the focal length is not given, 
 then a focal length 
 must be assumed.) 
 Next, mark off F 
 and 2 F on both sides 
 of the lens, and place 
 
 ' * FIGURE 96. PRINCIPAL POINTS OF A LENS. 
 
 the object beyond 2 F. 
 
 Now, there are an infinite number of rays passing from 
 every point of the object, but two rays are sufficient to 
 locate the image of any one point. Select the two rays, 
 one of which is parallel to the principal axis, and the other 
 which passes through the optical center. 
 
 To locate the head of the image draw these two rays, 
 the one parallel to the axis passes through the principal 
 
 Object 
 
 FIGURE 97. CONSTRUCTION OF IMAGE WHEN OBJECT is BEYOND 2 F. 
 
 focus F (because all parallel rays are focused at this point), 
 and the one through the optical center passes on through 
 the lens in a straight line (it really zigzags just a little at 
 the lens). The point at which these two rays meet is the 
 image of the head. 
 
114 
 
 REFRACTION AND LENSES 
 
 In the same way the tail of the image is located, thus 
 locating the whole image. 
 
 The description of an image gives four things: (1) posi- 
 tion, (2) size, (3) whether it is erect or inverted, (4) whether 
 it is real or virtual. 
 
 When the object is beyond 2 F, the image is (1) between F 
 and 2 F, (2) smaller than the object, (3) inverted, and (4) real. 
 
 II. The object at 2 F. 
 
 To construct the image with the object in this position, 
 proceed exactly as in the former case. (Figure 98.) 
 
 Object 
 
 V 
 
 FIGURE 98. CONSTRUCTION OF IMAGE WHEN OBJECT is AT 2 F. 
 
 The image is (1) at 2 F, (2) of the same size as the object, 
 (3) inverted, and (4) real. 
 
 III. The object between F and 2 F. 
 
 Construct as before. (Figure 99.) The image is (1) be- 
 yond 2 F, (2) larger than the object, (3) inverted, and (4) real. 
 
 Object 
 
 FIGURE 99. CONSTRUCTION OF IMAGE WHEN OBJECT is BETWEEN 
 
 F AND 2 F. 
 
IMAGE THROUGH A CONVERGING LENS 115 
 
 IV. The object at F. 
 
 Construct as before. (Figure 100.) The rays after passing 
 through the lens are parallel, and so never meet. There- 
 fore there is no image. 
 
 FIGURE 100. -CONSTRUCTION OF IMAGE WHEN OBJECT is AT F. 
 
 V. The object between F and the lens. 
 
 The construction is the same as before, except that the 
 rays after passing through the mirror diverge, and so have 
 
 FIGURE 101. CONSTRUCTION OF IMAGE WHEN OBJECT is BETWEEN F 
 AND THE LENS. 
 
 to be produced backward to determine the point where they 
 meet. (Figure 101.) 
 
116 REFRACTION AND LENSES 
 
 The image, then, is (1) on the same side of the lens as the 
 object, (2) larger than the object, (3) erect, and (4) virtual. 
 
 139. Image through a Diverging Lens. There were 
 five distinctive positions for the object in the case of the 
 converging lens, but for the diverging lens there is only one. 
 The image may be constructed in a similar manner to those 
 already studied. 
 
 Draw the two rays from each, the head and tail of the 
 object. (Figure 102.) The two rays parallel to the principal 
 axis diverge at such an angle that, if produced, they pass 
 
 Object 
 
 FIGURE 102. CONSTRUCTION OF IMAGE THROUGH A DIVERGING LENS. 
 
 through the principal focus. These produced rays meet 
 the rays coming from the same points and form an image 
 which is (1) between F and the lens, (2) smaller than the 
 object, (3) erect, and (4) virtual. 
 
 No matter where the object is, the image is formed as 
 described above. If the object is a great distance away, 
 the image approaches F ; and as the object comes closer to 
 the lens, the image also comes closer to the lens, and gets 
 larger. The image reaches the lens and becomes equal to 
 the object when the object reaches the lens. 
 
 APPLICATIONS OF LENSES 
 
 140. The Pinhole Camera. The simplest camera that 
 we have is illustrated by Figure 103. It consists of a light= 
 
THE LENS CAMERA 117 
 
 tight box with a pinhole in the front. A sensitized plate 
 or film may be placed at the back, and a picture can be 
 taken. The principle of the pinhole camera is this : All 
 
 FIGURE 103. THE PINHOLE CAMERA. 
 
 the rays allowed to pass through the pinhole from the same 
 point of the object fall at the same point at the back of the 
 box. A series of these points forms the image. 
 
 141. The Lens Camera. The pinhole camera is not 
 satisfactory, for if the pinhole is very small, the image will 
 be very weak and dim ; and, on the other hand, if the hole 
 
 FIGURE 104. THE LENS CAMERA. 
 
 is made large, then the rays from the same point on the 
 object fall over quite an area of the image, and this makes 
 the image indistinct, or blurred. 
 
 By the use of a converging lens, Fig. 104, the opening may 
 be made large and-, at the same time, the image may be 
 
118 REFRACTION AND LENSES 
 
 kept sharp and distinct. This is an application of the con- 
 verging lens with the object beyond 2 F. 
 
 In order that all the rays coming through the lens from 
 one point of the object be focused at a single point of the 
 
 FIGURE 105. THE 40-lNCH TELESCOPE AT THE YERKES OBSERVATORY, 
 UNIVERSITY OF CHICAGO, WILLIAMS BAY. WISCONSIN. 
 
 This is the largest refracting telescope in existence. The tube is 
 64 ft. long, 52 in. in diameter at the center, and the whole in- 
 strument weighs 75 tons. 
 
THE EYE 119 
 
 image, the lens must be ground with great care. This is 
 why the best cameras are so expensive. 
 
 The plate or film upon which the picture is taken is a 
 piece of glass or other transparent substance covered with a 
 gelatin. This gelatin is of such a composition that when 
 sun light strikes it, it is made insoluble. When a picture is 
 taken, the rays from the light parts of the object affect the 
 plate more than the rays from the dark parts. Then, when 
 the plate is " washed " the unaffected parts dissolve, leaving 
 the insoluble part on the plate. 
 
 The plate is then washed in a " fixing " solution, which 
 makes the remaining gelatin hard to scratch. The plate is 
 now called the negative. It has dark spots where the object 
 is light, and light spots where the object is dark. 
 
 For printing the pictures, either a paper or glass with a 
 sensitive gelatin is used. The " negative " is laid over the 
 sensitive paper or glass and held in the sun for a short time. 
 The sensitive plate is affected just as the negative was 
 when it was made, except that the dark and light spots 
 are reversed, thus reproducing the object as it was seen. 
 
 As all these processes must be done with painstaking 
 care, photography is quite an art. 
 
 142. The Eye. The eye is also an application of the 
 converging lens when the object is placed beyond 2 F. 
 
 The human eye is about an inch in diameter and has 
 three coats. The outer coat is very thick and strong, and 
 is called the sclerotic coat. (Figure 106.) This sclerotic coat 
 covers the entire eyeball, but at the front it is transparent 
 and this portion has the name cornea (C). 
 
 The next coat (D) is dark in color, and is called the choroid 
 coat. At the front, the choroid coat forms a kind of curtain, 
 called the iris (I). The iris is the part that gives color to 
 
120 
 
 REFRACTION AND LENSES 
 
 FIGURE 106. THE EYE. 
 
 the eye. At the back of the eye is a third coat (R) called 
 
 the retina. This is nervous tissue composed of millions 
 
 of small nerve cells. 
 These cells are divided 
 into three classes. In 
 one class are those 
 affected by red light; 
 in another class are 
 those affected by green 
 light; and the third 
 class is composed of 
 those affected by blue 
 light. These different 
 kinds of cells are not 
 
 in separate groups, but are scattered all over the retina, so 
 
 that every point has all three kinds. 
 
 At the front of the eye, fastened into the choroid coat, 
 
 are muscles (m, m). 
 
 These muscles are so attached that they stretch or relax 
 
 a small membrane sack which contains the crystalline lens 
 
 (C. L.) . This crystalline lens is a transparent, jelly-like mass, 
 
 and is a true lens. 
 
 143. How We See. When an object is held before the 
 eye, an image is focused by the crystalline lens upon the 
 retina. The nerve cells are affected according to the color 
 of the light which falls on them. Impulses are sent to the 
 brain, and we become conscious of the image. 
 
 A further study of color will be taken up later, and the 
 subject of the eye should then be reviewed. 
 
 144. Defective Eyes. There are many defects of the 
 eye, but we will mention only three : short-sightedness 
 (myopia), long-sightedness (hypermetropia), and astigmatism. 
 
DEFECTIVE EYES 
 
 121 
 
 FIGURE 107. A SHORT-SIGHTED EYE. 
 
 Short-sightedness is caused by one, or both, of two things. 
 
 The eyeball is too long, or the crystalline lens is too thick. 
 
 When the image falls 
 
 in front of the retina, 
 
 the person has to 
 
 bring the object very 
 
 near the eye to get 
 
 the image to move 
 
 back upon the retina. 
 
 (Figure 107.) 
 To correct this defect, diverging lenses should be used 
 
 for eye-glasses. This makes the image fall upon the retina 
 
 when the object is 
 held at the natural 
 position. (Figure 
 108.) 
 
 Long-sightedness is 
 just the opposite of 
 short-sightedness, 
 and is caused by 
 just the opposite 
 
 things. The eyeball is too short, or the lens is too thin. 
 
 This makes the image fall back of the retina, so that it is 
 
 necessary to hold the 
 
 object far away in 
 
 order to get the 
 
 image to fall on the 
 
 retina. (Figure 
 
 109.) 
 
 Glasses to correct 
 
 this defect should be converging lenses. (Figure 110.) 
 Astigmatism is the most serious of the three defects, and 
 
 FIGURE 108. A SHORT-SIGHTED EYE 
 CORRECTED. 
 
 FIGURE 109. A LONG-SIGHTED EYE. 
 
122 
 
 REFRACTION AND LENSES 
 
 is much the hardest to correct. It may be caused by several 
 things, such as irregularities in the thickness or texture of 
 
 the cornea, or in the 
 crystalline lens. 
 V Figure 111 shows 
 an eye with irregular 
 / thickness of the 
 cornea. The defect 
 must be corrected by 
 having glasses ground 
 to fit this one special 
 Figure 112 shows an at- 
 
 FIGURE 110. A LONG-SIGHTED EYE 
 CORRECTED. 
 
 case, and this requires an expert, 
 tempt to correct astigmatism. 
 145. The Life-size Picture 
 Camera. This camera is 
 just like the ordinary camera 
 except that the box is very 
 long and large and the lens 
 has. a greater focal length. FlGURE 1 1 L ~ AN ^STIGMATIZED EYE. 
 
 This is an application of the second position of the con- 
 verging lens. The object is placed at 2 F in front, and the 
 
 plate is placed at 2 F, 
 back of the lens in the 
 box. (Figure 113.) 
 
 It is used for taking 
 photographs of machinery 
 and parts of machinery, 
 and sometimes of per- 
 
 FlGURE 112. AN ASTIGMATIZED EYE 
 
 CORRECTED. 
 
 sons. 
 
 146. The Projection Lantern. The projection lantern 
 (Figure 114) is an application of the converging lens with 
 the object placed between F and 2 F. 
 
THE MOTION-PICTURE MACHINE 
 
 123 
 
 An arc light is used to illuminate the object (0, Figure 
 114), which is usually a picture on a glass plate called a 
 slide. In order that more of the light from the arc may 
 strike the object, and in order that it may come in parallel 
 
 FIGURE 113. A LIFE-SIZE PICTURE CAMERA. 
 
 rays, condensing lenses (c, c) are placed between the arc 
 and the object. 
 
 Now, the slide or object is placed between F and 2 F be- 
 tween the light and the lens, and the image is thrown on a 
 screen some distance in front, the image appearing very large 
 
 Arc 
 
 FIGURE 114. A PROJECTION LANTERN. 
 
 and inverted. To make the image erect, the slide is placed 
 in the machine upside down. 
 
 147. The Motion-picture Machine. The motion-picture 
 machine is merely a projection lantern with an attachment 
 for changing the slides at the rate of 16 or more per second. 
 
 When images fall on the retina of the eye their effects 
 tend to linger ; that is, after the image has left the retina the 
 
124 
 
 REFRACTION AND LENSES 
 
 FIGURE 115. A 
 DARK LANTERN. 
 
 nerves do not lose the effect immediately, and we continue 
 to see the image for about ^ of a second after it is gone. 
 
 Now, by throwing pictures upon a screen at the rate of 
 16 per second the last picture has not left our mind before 
 the next one has come. This makes the 
 pictures appear to be continuous. 
 
 Thus we see the motion that takes 
 place if pictures are taken at the rate 
 of 16 per second and reproduced at that 
 rate. 
 
 The pictures are taken on a long film 
 and are about f" XI" in size. This film is run off a reel, 
 through the motion-picture machine, on to another reel. 
 
 148. The Dark 
 Lantern. A good 
 example of the con- 
 verging lens with the 
 object at F is the 
 dark lantern. (Figure 
 115.) 
 
 Here the light is 
 placed at the prin- 
 cipal focus, and after 
 passing through the 
 lens it goes in a 
 parallel beam. 
 
 149. The Magnify- 
 ing Glass. Figure 
 116 shows a converg- 
 ing lens used as a 
 
 magnifying glass. Image 
 
 The lens is held at a FIGURE 1 1 6. A POCKET MAGNIFYING GLASS. 
 
DIFFUSED LIGHT 
 
 125 
 
 distance less than F, and a large, erect, virtual image is 
 obtained. 
 
 The magnifying glass is often used as a reading-glass. It 
 is also used by biologists for examining plants and small 
 insects. 
 
 150. Diffused Light. Figure 117 (b) shows a beam of 
 light falling on an irregular surface. Part of the light is 
 absorbed, but the rest 
 is reflected according 
 to the law of reflection, 
 making the angle of 
 reflection equal to the 
 
 angle of incidence. FlGURE 1 17. -EXPLAINING DIFFUSED LIGHT. 
 
 Since the surface is 
 
 irregular, the light is reflected in every direction. These 
 reflected rays are called diffused light. 
 
 FIGURE 118. THE AUTOMOBILE HEAD-LIGHT LENS DIFFUSES 
 THE LIGHT. 
 
 It is by diffused light that we see all bodies which are not 
 incandescent, that is, light giving. An object such as a 
 
126 REFRACTION AND LENSES 
 
 perfect mirror (a, Figure 117), which reflects the light in 
 parallel rays, cannot be seen. This is illustrated by the 
 fact that a person will sometimes walk into a mirror and 
 not know it until he has struck it. One looking into the 
 mirror does not see the mirror, but only the objects re- 
 flected in it. 
 
CHAPTER XI 
 ILLUMINATION AND CANDLE POWER 
 
 151. Intensity of Illumination. One often desires to 
 speak of the amount of light falling on a surface. To ex- 
 press this, the term intensity of illumination is used. 
 
 The intensity of illumination is the light energy per unit 
 area. 
 
 To illustrate this definition, suppose you had a slice of 
 bread and were to spread a serving of butter upon it. The 
 butter would be of a certain thickness. Now, if an equal 
 serving of butter were spread on several slices, its thickness 
 would be much less. This is true of light. 
 
 When a certain amount of light falls on a definite area 
 the intensity of illumination is a certain amount; but if 
 the same light were spread over a larger area, the intensity 
 would be less. 
 
 Every one has noticed that the greater the distance from 
 the source of light, the weaker the light becomes. This is 
 stated in the following law : 
 
 The intensity of illumination is inversely proportional to 
 the square of the distance from the source of light. 
 
 To prove this law, suppose a cardboard (a, Figure 119) 
 is placed before a light (L), the cardboard having a small 
 hole in it. A second cardboard (b) with a square hole, one 
 inch on a side, cut in it is placed one foot from a. A third 
 cardboard (c) is placed two feet from a. 
 
 127 
 
128 
 
 ILLUMINATION AND CANDLE POWER 
 
 Now, the light coming through the square hole in 6 falls 
 on a certain area on c. 
 
 From the figure it will be seen that the side of the illu- 
 minated square on c is twice the side of the square in b. 
 
 a b c 
 
 FIGURE 119. EXPLAINING LAW OF INTENSITY OF ILLUMINATION AS THE 
 DISTANCE VARIES. 
 
 Thus the light falls on an area at c, which is four times as 
 large as on b ; etc. 
 
 Thus the area on which the light falls is directly proportional 
 to the square of the distance from the source. 
 
 Since the intensity of illumination is inversely propor- 
 tional to the area, it is inversely proportional to the square 
 of the distance from the object under consideration to the 
 source of light. 
 
 This law can be applied to reading. If your book is 
 three feet from the lamp the printed pages will be illu- 
 minated four times as strongly as if it were six feet away; 
 nine times as strongly as if it were nine feet away; and 
 10,000 times as strongly as if it were 300 feet away. This 
 shows you why it is so important to get close to the light 
 to get proper illumination. 
 
 152. Candle Power. We have discussed the intensity 
 of illumination of objects lighted by some source other than 
 themselves ; but it is often desired to express the brightness 
 
MEASUREMENT OF CANDLE POWER 
 
 129 
 
 of the source of light itself. The unit used for this is called 
 the candle power. 
 
 One candle power is the light given by a standard candle 
 burning under specified conditions. 
 
 The standard candle is made of sperm oil, weighs ^ of 
 a pound, is usually wrapped in tinfoil, and burns at the 
 rate of 120 grains per hour. 
 
 It will be seen immediately that the unit candle power is, 
 at best, a poor unit, because no matter how much care is 
 taken to get the conditions the same, a candle will never 
 give exactly the same light. It is like using a tape measure 
 made of rubber. Nevertheless, this unit is still used for 
 want of a better one. 
 
 153. Measurement of Candle Power. In measuring the 
 candle power of a source of light, the light is compared to 
 either a standard candle or to 
 another light of which the 
 candle power is known. To 
 make this comparison the 
 photometer is used. 
 
 The photometer is a piece 
 of paper with a grease spot on 
 it. This paper may be either 
 placed in a small black box 
 (Figure 120), or may be put 
 in a standard which holds it 
 in position. 
 
 To compare two lights, the 
 
 photometer is held between them, at such positions that 
 the illuminations on both sides of the paper are the same. 
 (Figure 121.) 
 
 This point can be determined, since the grease spot will 
 
 FIGURE 120. CROSS SECTION OF 
 EUNSEN PHOTOMETER. 
 
130 ILLUMINATION AND CANDLE POWER 
 
 disappear, or look the same shade on both sides, when the 
 correct position is reached. 
 
 By measuring the distance (d x ) of the unknown light ( X} 
 to the photometer, and the distance (d s ) from the known 
 
 S X 
 
 FIGURE 121. COMPARING Two LIGHTS BY USE OF PHOTOMETER. 
 
 standard (S) to the photometer, the candle power of X can 
 be calculated. 
 
 The candle power of a few sources of light are as follows : 
 
 Carbon Lamp ...... about y c. p. per watt 
 
 Tungsten Lamp ...... about 4 c. p. per watt 
 
 Nitrogen Lamp ...... about 1 c. p. per watt 
 
 Mercury Vapor Lamp .... about 1 c. p. per watt 
 
 Arc Light ........ about 1 c. p. per watt 
 
 154. Problems in Illumination. The problem of the 
 proper illumination of different kinds of buildings, streets, 
 etc. is an important one. It is one which cannot be an- 
 swered or solved in this text. Only a few suggestions as 
 to its importance and application can be made. 
 
 In the home, care should be taken to have lights placed 
 in the proper positions. Also, candle power of lamps to be 
 used is largely determined by the decorations of the room. 
 
 For the kitchen, two lamps are usually needed : one above 
 the sink, and one above the stove. Forty-watt tungsten 
 lamps are, as a rule, a good rating. 
 
PROBLEMS IN ILLUMINATION 131 
 
 A bedroom should have at least a 40-watt tungsten. 
 This should be hung above the dresser or dressing table, 
 and not from the center of the ceiling. 
 
 The bathroom should have two lamps, one on each side 
 of the mirror. Twenty-five-watt tungstens are sufficient. 
 
 The lamps in the living rooms, library, etc., cannot be 
 specified, but should be placed so as to be most convenient 
 and at the same time bring out the desired effects of the 
 decorations. 
 
 It is astonishing what different effects may be obtained 
 by different lightings of the same piece of statuary. The 
 same is true of paintings. 
 
CHAPTER XII 
 COLOR 
 
 155. Dispersion. If a ray of white light be passed 
 through a glass prism (Figure 122), it will be refracted and 
 at the same time will be broken up into a band of seven 
 colors, in the order of violet, indigo, blue, green, yellow, orange, 
 and red (vibgyor contains the initials of the colors in the 
 
 FIGURE 122. WHITE LIGHT PASSING THROUGH A PRISM. 
 
 regular order). This breaking up of white light is called 
 dispersion, and the band of seven colors is called the solar 
 spectrum . 
 
 156. Cause of Different Colors. At the beginning of 
 our discussion of light we said that light is a wave motion 
 in the ether. Different wave lengths give differently colored 
 light ; that is, the color of the light depends upon the wave 
 length, just as the high tones in sound have different wave 
 lengths from the low tones. 
 
 132 
 
THE ACHROMATIC LENS 
 
 133 
 
 The violet rays are the shortest waves (about .000033 cm.) 
 which the eye can see, while the red rays are the longest 
 (about .000081 cm.), the other colors falling in between, 
 in the given order. 
 
 When a piece of iron is heated, it first becomes red hot 
 and later white hot. As more heat is applied, the molecules 
 vibrate faster and faster, sending out shorter and shorter 
 wave lengths as well as the longer ones, thus producing all 
 the colors of the spectrum. Just as white light can be 
 broken up into all these colors, so they now combine and 
 make the iron look white. Hence the term white hot. 
 
 This same thing can be noticed in the filament of an 
 electric lamp when it is partially lighted, then fully lighted. 
 
 157. The Achromatic Lens. When a lens is made of 
 one piece of glass, it does not refract all colors equally; in 
 other words, dispersion takes 
 
 place. This makes it impos- 
 sible to get a perfect focus with 
 this kind of lens. 
 
 To correct this defect, lenses 
 are made of crown and flint 
 glass. (Figure 123.) The dis- 
 persive effect of one glass 
 
 counteracts the dispersive effect of the other, but the rays 
 are still refracted, thus producing a perfect focus. This 
 kind of lens is called achromatic without color. These 
 lenses are very expensive and are used only in high-priced 
 cameras, microscopes, and other optical instruments. 
 
 158. Transparent, Translucent, and Opaque Objects. 
 Objects are divided into three classes, according to their 
 ability to transmit light. 
 
 Transparent objects are those which transmit light in 
 
 Whife 
 
 FIGURE 123. AN ACHROMATIC 
 LENS. 
 
134 COLOR 
 
 parallel rays ; and thus objects can be seen in detail through 
 them. 
 
 Translucent objects are those which transmit light, but not 
 in parallel rays, so that objects cannot be seen in detail 
 through them. Light after coming through a translucent 
 object is diffused. 
 
 Opaque objects are those which shut off the light entirely. 
 
 Air, clear plane glass, clear water, etc., are examples of 
 transparent objects. 
 
 Snow, cracked ice, frosted glass, thin paper, etc., are 
 examples of translucent objects. 
 
 Wood, iron, stone, etc., are examples of opaque objects. 
 
 159. Color of Opaque Objects. No object, unless it is 
 self-illuminated, has color. It gets its color from the light 
 that falls on it. 
 
 The light that falls on it is either absorbed or reflected, 
 the object taking on the color of the light that it reflects. 
 Thus a red dress is not red at all, but merely absorbs all 
 colors that fall on it except red, which it reflects, thus giving 
 it the apparent red color. 
 
 This same red dress in a perfectly dark room would be 
 black. It would also be black, or purplish (depending upon 
 the shade of red), if held in the light of a sodium flame, 
 because this light contains only yellow, and so there would 
 be no red to be reflected. 
 
 160. Dyes. A dye is a substance which may be made 
 to stick between the fibers of another object and thus give 
 the object an apparent color by reflecting that colored light. 
 
 Cloth is usually dyed by placing it in a liquid containing 
 certain substances which enter the cloth and stick between 
 the fibers after the dye has dried. If it is a good dye, it is 
 of such a nature that these particles cannot be washed out, 
 
APPLICATION OF COLORED OBJECTS. 135 
 
 causing the cloth to fade. A good dye should also be un- 
 affected by sunlight. 
 
 When a cloth fades, the small particles are either washed 
 out or are so changed chemically that they will not reflect 
 the desired color. 
 
 161. Paints. Paints are different from dyes in that 
 they are colored pigments which are spread over the -surf ace 
 of an object, instead of going in between the fibers. The 
 color of the paint is determined by the colored light which 
 the pigments reflect. 
 
 162. Color of Transparent and Translucent Objects. 
 Transparent and translucent objects get their color from 
 the light which they transmit. A green glass is green be- 
 cause it absorbs all other colors and transmits the green. 
 Objects viewed through green glass appear green because 
 that is the only kind of light that gets through. 
 
 Colored glass is made either by putting the coloring 
 material in the glass when it is manufactured, or else by 
 covering the glass with a film of gelatin containing the 
 coloring-matter. 
 
 163. Application of Colored Objects. From the preced- 
 ing topics it is seen that the color of an object depends upon 
 two things : the kind of light falling on it, and the color which 
 it reflects or transmits. 
 
 The knowledge of this f act is applicable in the selection 
 of dress goods and in the illumination of pictures and other 
 decorations. 
 
 In selecting dress goods, the selection should be made in 
 the same kind of light as that in which the dress is to be 
 worn. For example, if a piece of goods is selected in arti- 
 ficial light, it should be worn in the same kind of artificial 
 light, for it may be of an entirely different color when viewed 
 
136 COLOR 
 
 in daylight. As an exaggerated example, a bright red piece 
 of cloth in daylight would appear dark purple or black in 
 the light of a mercury vapor lamp. This is because there 
 is no red light given off by the mercury lamp, and conse- 
 quently the material has no red to reflect. 
 
 In the same way a blue piece of goods in daylight looks 
 black under a carbon lamp, since the carbon lamp gives off 
 very little blue light. 
 
 The same application can be made in illuminating pic- 
 tures, wall paper, draperies, etc. These decorations will 
 take on an entirely different color when placed under differ- 
 ent colored lights. 
 
 A lamp has recently been put on the market, called the 
 " day-light lamp." It is given this name because the rays 
 sent out by it contain the same colors, and in the same 
 proportion, as are found in sunlight. Most large stores now 
 have these lamps, so that goods selected in this light will 
 have the same color in sunlight. 
 
 164. The Three Primary Colors. It was found that by 
 passing white sunlight through a prism it could be dispersed 
 into seven colors. 
 
 Each of these colors is elementary ; that is, it cannot be 
 broken up into parts or other colors. This would lead us 
 to believe that to get white light we must mix these seven 
 colors, and this is partially true. 
 
 A mixture of these seven colors in the right proportions 
 will give white light, but white light can also be obtained 
 by the mixture of three elementary colors : red, green, and 
 violet. More than that, any color whatsoever can be ob- 
 tained by the correct proportions of these three colors. 
 
 For this reason the three colors red, green, and violet are 
 called the primary colors of light. 
 
MIXING COLORED LIGHTS 137 
 
 165. How We See Color. Referring back to the topic 
 on " The Eye " ( 142), it will be found that the retina, 
 the inner lining of the back of the eye, is composed of 
 countless numbers of nerve-endings or cells, that these 
 cells are divided into three classes, but are all intermingled, 
 so that even the smallest spot on the retina has all three 
 kinds of cells. 
 
 One of these classes of cells is affected by red light, and red 
 only ; another is affected by green light, and green only ; 
 while the third class is affected by violet light, and violet 
 only. 
 
 Now, when an image falls on the retina, these cells are 
 affected by the light that strikes them. Where only red 
 light falls, only those corresponding nerve cells are affected ; 
 the same for green ; and the same for violet. 
 
 If a light such as yellow, which is composed of both red 
 and green, falls on a spot on the retina, both those corre- 
 sponding kinds of cells are affected. 
 
 When these cells are affected, impulses are sent to cor- 
 responding nerve cells in the brain, and we become con- 
 scious of those certain kinds of light falling on their respective 
 positions on the retina. Thus we know the shape of the ob- 
 ject and also its color. 
 
 166. Mixing Colored Lights. It has been noted that 
 lights of different colors may be mixed. When this is 
 done, the result is the combined effects of all he lights each 
 taken separately. This is called the additive method. 
 
 Thus, when the correct proportions of red light and green 
 light are superimposed, the result is the sum of the red and 
 green effects, which gives a yellow. Likewise, any color 
 whatsoever may be produced by adding the proper portions 
 of the three primary colors. 
 
138 
 
 COLOR 
 
 The above statements can be experimentally illustrated 
 by the use of colored disks on a turning table. (Figure 124.) 
 By placing these disks on the spindle, 
 one over the other, in such a manner 
 that a certain portion of each disk is 
 visible, and then by turning the disks 
 at a rapid rate, an apparent mixture of 
 these colors is attained. The mixing 
 
 is done on the same principle as the 
 FIGURE 124. COLORED . . . /e .. 
 
 DlSKS moving-picture ( 147), each color effect 
 
 being superimposed upon the retina of 
 the eye before the other color effects disappear. 
 
 167. Tints and Shades. A tint of a certain color is 
 produced by adding that color to white. In the same way 
 shades of a color are produced by mixing that color with 
 black. 
 
 168. Colored Pigments. Colored pigments are used in 
 paints and dyes, and are small particles of matter of such a 
 nature that they reflect certain colors. 
 
 169. Mixing Pigments. 
 '' Mixing pigments to 
 produce color is called 
 the subtr active method. It 
 is called subtractive be- 
 cause the color that is 
 given out after mixing the 
 pigments is that which is 
 left after the pigments 
 have absorbed their char- 
 acteristic colors. Thus 
 
 Figure 125 illustrates the F , OURE 125 ._ ADDING RED , YELLOW , 
 adding of red and yellow, AND VIOLET LIGHTS. 
 
MIXING PIGMENTS 
 
 139 
 
 White 
 
 Light 
 
 Light Gray 
 
 Light Graj 
 
 Neutral Gray 
 
 -Dark Gra> 
 
 Dark 
 
 Dark Gray 
 
 FIGURE 126. -MIXING Six 
 DIFFERENT COLORED PIG- 
 MENTS. 
 
 yellow and violet, violet and red, and red, yellow, and violet. 
 
 It will be seen that the resulting colors are, respectively, 
 
 orange, green, purple, and black. 
 The three kinds of pigments, 
 
 red, yellow, and violet, are called 
 
 primary, because by adding them 
 
 in the right proportion black is 
 
 obtained. 
 
 Each of the 
 three kinds of 
 pigments absorbs 
 certain colors, 
 giving back only 
 its characteristic 
 
 color. When the three kinds are mixed 
 together, no color is given back, for 
 what one gives back the others absorb. 
 This produces the absence of color, or 
 black. 
 
 Figure 126 is a diagram illustrating the 
 mixing of six kinds of pigments, and the 
 resulting effects. Thus a mixture of red 
 and orange gives a red-orange ; a mix- 
 ture of orange and yellow gives an 
 orange-yellow, etc. 
 
 Opposite colors, such as red and green, 
 orange and blue* yellow and violet, are 
 called complementary colors, because if 
 the one is taken from white the other is 
 the result. For example, if red is taken 
 from white, green is the result, etc. 
 Figure 127 is a diagram showing how 
 
 Black 
 
 FIGURE 127. DIF- 
 FERENT SHADES OF 
 GRAY. 
 
140 COLOR 
 
 to obtain different shades of gray. Half white and half 
 black give what is called neutral gray. Three-fourths black 
 and one-fourth white give a dark gray. Three-fourths white 
 and one-fourth black give a light gray. Greater quantities 
 of black than three-fourths give a dark dark-gray. Greater 
 quantities of white than three-fourths give a light light-gray, 
 etc. Thus any shade from white to black may be obtained 
 by a mixture of the proper proportions. 
 
 170. Limitations of Color Nomenclature. We use the 
 terms red, blue, green, pink, pea-green, sky-blue, etc., very 
 freely, as if they were definite in meaning. The fact of 
 the matter is, they are very indefinite. 
 
 For example, could you tell exactly what color to get if 
 you were sent to buy sky-blue or pea-green silk? The 
 trouble is, our terms are not. definite, but cover a wide 
 range of color. We still use these indefinite terms for 
 want of better substitutes. 
 
 171. Harmony of Color. In music certain tones sound 
 pleasing when given together. The law governing the 
 combining of these tones is called harmony. In the case 
 of colors it is just as true that certain combinations of color 
 are pleasing, while others are not. We speak of this as the 
 harmony of color. 
 
 So far there are few set rules or laws governing these 
 combinations, since they are left to the taste of the in- 
 dividual. What looks well to one individual may be almost 
 shocking to another. 
 
 It is true, however, that the following simple rule can be 
 followed, and that, in general, it will give a pleasing com- 
 bination. All colors harmonize with black and with white. 
 
 172. Half-tone Picture Printing. In half-tone picture 
 printing a negative is obtained from either the object itself 
 
HALF-TONE PICTURE PRINTING 
 
 141 
 
 or from a photograph, in exactly the same manner as in 
 photography. 
 
 Instead of printing on a sensitized paper as in the case 
 of a photograph, the negative is placed over a sensitized 
 plate of copper or other metal, and the picture is printed on 
 this.. 
 
 The copper plate is made sensitive by a covering of gelatin 
 sensitive to light, just as in the case of the paper. 
 
 Before the printing on the metal plate is begun, two glass 
 screens (a and 6, Figure 128) are placed, one over the other, 
 between the negative and 
 the plate. These screens 
 are usually ruled with 
 from 100 to 150 parallel 
 lines to the inch, and, 
 when placed over one 
 another (c), the lines of 
 
 one are perpendicular to the lines of the other; the lines 
 being scratches which shut off light. 
 
 In printing, the light shines through the light part of the 
 negative, turning the sensitive gelatin on the metal plate 
 black, and making it insoluble. The rest of the gelatin 
 is unaffected, and when " washed " dissolves, leaving the 
 black, insoluble part on the plate. The lines of the screens 
 appear as clean lines on the plate. 
 
 This metal plate is then subjected to an acid bath which 
 etches, or eats away the unprotected part of the plate, leav- 
 ing the part covered with gelatin " raised " or level with 
 the original surface. 
 
 After scraping off this gelatin the plate may be inked 
 and used for actual printing of pictures in books, magazines, 
 or newspapers. 
 
 a b c 
 
 FIGURE 128. LIGHT SCREENS. 
 
142 
 
 COLOR 
 
 Since most printing is done from rolls, the impression 
 may be transferred from the metal sheet to the rolls by the 
 electrotype method. ( 280.) 
 
 By referring to Figure 129 it can be seen why the metal 
 plate will produce a picture which is the exact likeness of 
 the object. 
 
 The light part of the negative represents the dark part 
 of the object. The raised part of the metal plate represents 
 the light part of the negative or the dark part of the object, 
 
 Object Negative Plate 
 
 FIGURE 129. DIAGRAM SHOWING OBJECT, NEGATIVE, AND PLATE IN 
 HALF-TONE PICTURE PRINTING. 
 
 the lines of the two screens appearing as depressed parts on 
 the metal plate. 
 
 Now, when the metal plate is inked and a picture is 
 printed with it, the raised portion is the only part that 
 prints, thus reproducing the dark parts of the object in ink. 
 The lines are to keep the ink from " running." They do 
 not show, except upon close examination, in the printed 
 picture. 
 
 173. The Three-color Printing Process. The half-tone 
 picture printing process, discussed in 172, gives a picture 
 in light and shadow only. This process has been enlarged 
 upon, and now pictures in actual colors can be printed by 
 what is called the " three-color process." This process is 
 
THE THREE-COLOR PRINTING PROCESS 143 
 
 used to print the colored cover designs and colored advertise- 
 ments used so much in the better magazines. 
 
 In this process three negatives are taken through three 
 separate light filters. The three filters consist of three plates 
 of glass stained violet, blue-green, and orange, respectively. 
 
 These filters are placed in front of the camera, one at a 
 time, when the three negatives are taken. The negatives 
 are developed and printed on three separate metal plates, 
 as in the half-tone process. 
 
 These plates, or their reproduced rolls, are then inked, 
 the one corresponding to the violet filter with yellow ink, the 
 one corresponding to the blue-green filter with red-orange 
 ink, and the one corresponding to the orange filter with blue 
 ink. Then all three are successively printed on the same 
 sheet of white paper. The result is a picture of the object 
 in actual colors, or at least approximating the actual colors, 
 the degree of accuracy in colors depending on, the trueness 
 of the colors of the filters and inks used. 
 
 The reasons why this process gives the actual colors are 
 as follows : 
 
 In the first place, the negative taken with a violet filter 
 has dark spots only where the violet light strikes, and so the 
 corresponding metal plate has depressed spots representing 
 the violet of the object. 
 
 Likewise, the metal plate corresponding to the blue-green 
 filter has depressed spots representing the blue-green of the 
 object, and the metal plate corresponding to the orange filter 
 has depressed spots representing the orange of the object. 
 
 Now, the three colors, violet, blue-green, and orange, con- 
 tain all the colors of white light, and so the depressions in 
 the three metal plates represent all the actual colors of the 
 object. 
 
144 COLOR 
 
 The plate corresponding to violet in the object, covers all 
 the rest of the white paper with yellow, the complementary 
 pigment of violet. Likewise, the plate corresponding to 
 blue-green in the object covers all the rest of the white paper 
 with red-orange, and the plate corresponding to orange in 
 the object covers all the rest -of the 
 
 Cohr niter /* J . 
 
 white paper with blue. The spots 
 with yellow ink reflect all colors but 
 violet, or, in other words, blue-green 
 and orange. (Figure 130.) Also, 
 the spots with red-orange ink reflect 
 all colors but blue-green, or in other 
 words violet and orange. 
 FIGURE 1 30. - DIAGRAM. Therefore a spot covered by 
 
 yellow and red-orange inks reflects 
 
 only orange. Also a spot covered by yellow and blue inks 
 
 reflects only blue-green, and a spot covered by red-orange and 
 
 blue inks reflects only violet. 
 
 This makes the printed picture reflect the actual colors of 
 
 the object in the correct positions and amounts. 
 
 Review Problems 
 
 1. What is the theory of the nature of light ? 
 
 2. When is a body luminous ? 
 
 3. Why can you see a body which is not luminous? 
 
 4. What is the velocity of light ? 
 
 5. Explain Roemer's method for determining the velocity of light. 
 
 6. Give two comparisons which will show the magnitude of the 
 velocity of light. 
 
 7. Give the law cf reflection. 
 
 8. Does your right hand appear to be the right hand of your image 
 in a plane mirror? 
 
 9. Construct the image in a plane mirror. Describe the image. 
 
REVIEW PROBLEMS 145 
 
 10. Construct the image in a concave mirror, (a) when object is 
 beyond center of curvature, (6) when object is at center of curvature, 
 
 (c) when object is between center of curvature and principal focus, 
 
 (d) when object is at principal focus, (e) when object is between prin- 
 cipal focus and mirror. 
 
 11. Give two uses of the convex mirror. 
 
 12. Give two uses of the concave mirror. 
 
 13. Explain why refraction takes place. 
 
 14. Give five applications of refraction. 
 
 15. Construct the image in the five different settings of the convex 
 lens. 
 
 16. Give an application of each of the five settings of the convex 
 lens. 
 
 17. Explain how a photograph is made. 
 
 18. What is diffused light? 
 
 19. What produces color in a light? 
 
 20. Explain why an opaque object has a certain color. 
 
 21. Explain why a stained glass has a certain color. 
 
 22. Why can you not rely on colors chosen by artificial light ? 
 
 23. What application has color to the decorating and lighting 
 of a home? 
 
 24. Explain why shadows play an important part in the proper 
 illumination of a room. 
 
 25. How are half-tones made ? 
 
 26. What is a tint? What is a shade ? 
 
 27. What is meant by the " additive method " ? 
 
 28. What is meant by the " subtractive method " ? 
 
 29. What is the difference between a dye and a paint ? 
 
 30. What causes a colored piece of goods to " fade "? 
 
CHAPTER XIII 
 MAGNETISM 
 
 174. Properties of Magnetism. We do not know just 
 what magnetism is, but we do know many things about it. 
 For centuries people have known of a peculiar kind of ore 
 called " lodestone," which has the property of attracting 
 iron. The " lodestone " is said to have magnetism, and the 
 best definition we have is : Magnetism is the property some 
 objects have of attracting iron. An object which has mag- 
 netism is said to be a magnet. 
 
 175. Poles of a Magnet. If a magnet be thrust into a 
 box of iron filings, the filings will cling to the ends of the 
 magnet, and will appear to be attracted to one point near 
 each end. This point is called the pole of the magnet, and 
 is located inside the iron some distance from the end. The 
 pole of a magnet is the point at which all the force of attraction 
 is centered. 
 
 A magnet has two poles, one near each end, called north 
 (N) and south (S). It is unfortunate that they were named 
 " north " and " south/' for we are apt to confuse these 
 terms with direction. A magnet may be placed in any 
 position, and yet its poles remain the same, regardless of 
 direction. For example, a magnet may be placed in an 
 east and west position, and yet its poles are called N and S. 
 A magnet may be easily placed so that its N-pole is on the 
 south end (direction) of the magnet. 
 
 146 
 
FIELD OF A MAGNET 147 
 
 176. Law of Attraction and Repulsion. If a magnet is 
 suspended at its middle by a cord, or balanced on a pivot, 
 and another magnet is brought near it, the end of the first 
 magnet is either attracted or repelled by the other magnet. 
 
 If the N-pole of one comes near the S-pole of the other, 
 they are attracted, and if free, will swing together. But if 
 the S-pole of one magnet comes near the S-pole of the other, 
 they are repelled, and if free will swing apart. Thus we 
 have this law : Unlike poles attract and like poles repel. 
 
 177. The Earth a Magnet. The earth itself is a huge 
 magnet, one of its magnetic poles being about 1000 miles 
 from the geographical north pole, while the other magnetic 
 pole is at a similar distance from the geographical south pole. 
 
 A magnet suspended so that it is free to swing in a hori- 
 zontal plane will come to rest in a north and south position. 
 This is due to the magnetic attraction of the earth. The 
 pole that swings towards the north is called " N-pole," 
 while the one that swings towards the south is called " S- 
 pole." At the time the poles were named, people did not 
 know that magnets would ever be used for anything except 
 to tell direction, and the names " N " and " $ " seemed 
 appropriate. 
 
 But now the names are confusing. A N-pole is the pole 
 that points north when the magnet is free to swing, but by 
 the " law of attraction " unlike poles attract ; therefore the 
 magnetic pole near the north geographical pole is really a 
 " S" magnetic pole. Likewise the " N " magnetic pole of 
 the earth is in the south. 
 
 178. Field of a Magnet. We have seen that a magnet 
 will attract iron filings even when they are not touching it. 
 What is it that harnesses the iron filings to the magnet, 
 since we cannot see, or feel, anything between them? 
 
148 
 
 MAGNETISM 
 
 Evidently there is some force in the space about the mag- 
 net. This space is called the " magnetic field," and is said 
 to be filled with " lines of force." 
 
 FIGURE 131. FIELD ABOUT A BAR MAGNET. 
 
 Just what these lines of force are no one is able to explain ; 
 and for want of a better name they are said to be strains in 
 the ether. 
 
 If a piece of paper is placed over a bar magnet and iron 
 filings are sifted on it, the filings will arrange themselves in 
 lines as shown in Figure 131. 
 
 FIGURE 132. ARRANGEMENT OF MOLE- 
 CULES IN A PIECE OF IRON NOT MAG- 
 NETIZED. 
 
 FIGURE 1 33. DIAGRAM 
 OF BALANCED FORCES 
 IN A PIECE OF IRON 
 NOT MAGNETIZED. 
 
THEORY OF MAGNETISM 
 
 149 
 
 179. Properties of Lines of Force. Whatever the lines 
 of force are, they have three known properties : 
 
 FIGURE 134. ARRANGEMENT OF MOLE- 
 CULES IN A MAGNETIZED PIECE OF IRON. 
 
 FIGURE 135. UNBAL- 
 ANCED FORCES IN A MAG- 
 NETIZED PIECE OF IRON. 
 
 1. They have direction and always come out of a N-pole 
 and go in at a S-pole, completing a loop inside the magnet. 
 
 FIGURE 136. How TO MAGNETIZE A PIECE OF IRON. 
 
 2, They have a tendency to contract, like rubber bands, 
 and will contract until they are zero in length. 
 
 3. They repel one another laterally. 
 
 180. Theory of Magnetism. Some substances are said 
 to be magnetic, while others are non-magnetic. Magnetic 
 
 FIGURE 137. FIELD BETWEEN Two UNLIKE POLES. 
 
 substances are substances whose molecules have N- and S- 
 poles, while non-magnetic substances are those whose mole- 
 cules do not have N- and S-poles. 
 
150 
 
 MAGNETISM 
 
 Iron is the most magnetic substance, while cobalt and 
 nickel are only slightly magnetic. Most substances, such as 
 wood, glass, copper, brass, etc., are non-magnetic. 
 
 FIGURE 138. FIELD BETWEEN Two LIKE POLES. 
 
 The fact that iron is magnetic does not necessarily mean 
 that a piece of it is a magnet. It must first be magnetized. 
 
 181. Difference between a Magnetized Piece of Iron and 
 
 One Not Magnetized. - 
 In a piece of iron that is 
 not magnetized the mole- 
 cules have their N-poles 
 and S-poles pointing in 
 various directions (Figure 
 132), and the effect of 
 some molecules neutral- 
 izes the effect of others. 
 It is like several boys 
 pulling in all directions 
 upon a post. (Figure 
 133.) The pull is bal- 
 anced and there is no 
 
 F.GURE 139.-F.ELD ABOUT A HORSE- 
 
 SHOE MAONET. 
 
 " 
 
 But in a piece of iron 
 
HOW TO MAGNETIZE A PIECE OF IRON 151 
 
 which is magnetized, the molecules are all in order; so that 
 all the S-poles point to one end, and all the N-poles to the 
 other. (Figure 134.) 
 
 In this case the effect of each molecule helps the effect of 
 every other, and one end of the bar becomes a N-pole and 
 the other end the S-pole. To 
 illustrate this as before, all the 
 boys pull in the same direction. 
 (Figure 135.) 
 
 FIGURE 140. -FIELD ABOUT A 
 HORSESHOE MAGNET HAVING A 
 BAR OF SOFT IRON IN FRONT 
 OF POLES. 
 
 182. How to Magnet- 
 ize a Piece of Iron. 
 
 To magnetize a piece of 
 iron, place it in a mag- 
 netic field so that the 
 lines of force run through 
 the iron. This lines the 
 
 molecules up as in Figure 136, magnetizing the iron. 
 
 If it is a piece of tempered steel that has been magnetized, 
 
 the molecules will keep their positions, and the steel will hold 
 
 FIGURE 141. FIELD ABOUT A HORSE- 
 SHOE MAGNET HAVING A DISK OF SOFT 
 IRON IN FRONT OF POLES. 
 
152 MAGNETISM 
 
 its magnetism, because the molecules cannot fall back out of 
 line. This is, then, a permanent magnet. 
 
 If the piece of iron is soft and not tempered, the molecules 
 become disarranged as soon as the magnetic field is removed ; 
 and it loses its magnetism. This is a temporary magnet. 
 
 183. Characteristic Fields. The following drawings 
 show the direction of the lines of force in several cases. 
 (Figures 137, 138, 139, 140, 141.) 
 
CHAPTER XIV 
 
 ELECTRICITY 
 
 184. Relation of Electricity to Magnetism. Before 
 studying the subject of electricity we spent some time on 
 magnetism, because magnetism and electricity are very 
 closely related. We shall now find how necessary magnetism 
 is to the production of electricity. 
 
 The question just .what electricity is, has never been satis- 
 factorily answered. The latest theory is that it is some kind 
 of strain in the ether, and that the strain will move along 
 a wire, producing a current of electricity. 
 
 Anything which will transmit elec- 
 tricity from one place to another is 
 called a conductor. 
 
 485. Generation of Electrical Pres- 
 sure. It has been found that if a 
 conductor is moved in a magnetic 
 field so that it cuts the lines of force 
 electrical pressure is produced, or is 
 said to be generated. 
 
 In Figure 142 we have a permanent 
 magnet with the lines of force shown 
 
 coming out of the N-pole. A copper wire, or rod, is held 
 in this magnetic field and moved across the lines of force. 
 This generates electrical pressure in the conductor. 
 
 153 
 
 FIGURE 142. GENERAT- 
 ING ELECTRICAL PRESSURE. 
 
154 ELECTRICITY 
 
 If a complete circuit is made from one end of the bar to 
 the other, a current of electricity will flow. 
 
 The thing that produces the pressure is cutting lines of force 
 with a conductor. This, then, is one of the fundamental 
 principles to learn about electricity. Whenever lines of force 
 are cut by a conductor, electrical pressure is generated. 
 
 186. Nature of Electrical Pressure. But just what is 
 electrical pressure? Since electricity is an invisible some- 
 thing and yet is analogous to the flow of water, we can best 
 get a conception of it by comparing it to the flow of water. 
 
 In the case of water, we say there is a pressure of so many 
 pounds per square inch. Pressure is the thing that makes 
 the water flow when the stop-cock is turned on. The pres- 
 sure is there whether the cock is turned on or not, and when- 
 ever the water has a chance to flow, the pressure forces it to 
 do so. 
 
 Electrical pressure is similar. It is that which makes the 
 electrical current flow. There may be an electrical pressure, 
 and yet no current (if the circuit is not closed) ; but if there 
 is a possibility for the current to flow (as when the circuit 
 is closed) the pressure will make it do so. 
 
 The amount of electrical pressure depends upon the rate of 
 cutting lines of force ; or, we could say, upon the number of 
 lines of force cut per second. 
 
 The direction of the pressure depends upon the direction 
 in which the lines of force are cut. 
 
 187. Electrical Current. The electrical current may be 
 compared to the current of water in a pipe. We say the 
 current is large or small according to the amount of water it 
 will deliver in a certain time. Similarly with electricity, 
 the current is the flow of the electricity, and is measured by the 
 amount of electricity it will deliver per second. 
 
THE SIMPLE GENERATOR 
 
 155 
 
 The size of the current depends upon the pressure forcing 
 it to flow, and upon the resistance offered to it by the con- 
 ductor. 
 
 188. Resistance. If the water pipe in the above case 
 were small, it would be difficult for the water to get through. 
 In other words, the pipe would offer a resistance to the flow 
 of the water current. The same thing takes place in a wire. 
 The resistance is that ivhich tends to hold the current back. 
 
 There are four principal things which affect the resistance 
 of a conductor : (1) size, (2) length, (3) kind of material, 
 (4) temperature. 
 
 The larger the wire, the smaller the resistance. The 
 longer the wire, the greater the resistance. Some kinds of 
 material have more resistance than others. For instance, 
 copper has less resistance than iron. 
 
 Materials which have a low resistance are said to be good 
 conductors. Copper, silver, platinum, and, in fact, nearly 
 all the metals are good conductors. Those materials which 
 have an exceptionally high resistance are called insulators, 
 such as air, wood, glass, mica, rubber, asbestos, etc. 
 
 The temperature affects different materials differently. 
 With some, it increases the 
 resistance ; and with others 
 it decreases it. A carbon 
 lamp has less resistance 
 when hot than when cold, 
 but a tungsten lamp has 
 more resistance when hot. 
 
 189. The Simple Gener- FIOURE ^ _ A ^ GENERATOR 
 ator. Figure 143 shows a 
 
 loop of wire revolving in a magnetic field. The magnetic 
 field is produced by the permanent magnets N and S. The 
 
 , ? r 
 
 
 3 
 3 
 
 ^ 
 
 e 
 
 i 
 
 E 
 
 
 y /fflffitf / //0, 
 
 ^gg^^ 
 
 
 
 WfflW/% 
 
 f/ /W/WM 
 
 
 
 'y/ff/t/Mffff/ 
 
 WfflffMf/: 
 
 
 
 
 W//W/M 
 
 
 
 
 
156 
 
 ELECTRICITY 
 
 lines of force pass from the X-pole across, and into the 
 S-pole. The loop of wire is a conductor ; and when it 
 revolves in this magnetic field, it cuts the lines of force, 
 and electrical pressure is generated. 
 
 190. A. C. Simple Generator. Figure 144 shows a cross 
 section of the simple generator. Since it is a cross section, 
 the ends of the loop of wire, where it is cut off, are dots. In 
 
 FIGURE 144. CROSS SECTION OF SIMPLE A. C. GENERATOR. 
 
 this discussion we shall mention only one side of the loop of 
 wire. 
 
 Suppose we start with the wire at position a and turn it 
 around, or revolve the loop at uniform speed. 
 
 At position a the wire is moving parallel to the lines of 
 force, and so does not cut any. Therefore there is no pres- 
 sure being produced. This can be shown on the curve 
 (Figure 145) at position a. 
 
 Now let the loop revolve until the same wire is at b. 
 Here it is moving perpendicular to the lines of force, and so is 
 cutting them at the greatest rate possible. Therefore there 
 will be the greatest pressure generated, shown by point b 
 on the curve. 
 
 Now, when the loop revolves so that the wire is at posi- 
 tion c, the wire is again moving parallel to the lines of force. 
 Again the pressure is zero, point c on the curve. 
 
A. C. SIMPLE GENERATOR 
 
 157 
 
 As the loop revolves farther, the wire begins to cut the 
 lines of force in the opposite direction ; and so the pressure 
 will be in the other direction, or will be negative. When 
 the wire reaches position d, it is again moving perpendicular 
 to the lines of force, and so is cutting the greatest number 
 
 -\- Pressure 
 o 
 
 Turns 
 1- 
 2- 
 
 3- 
 
 Pressure 
 
 FIGURE 145. CURVE SHOWING PRESSURE AT DIFFERENT PARTS OF THE 
 TURN OF THE ARMATURE IN AN A. C. GENERATOR. 
 
 again; and so the pressure is highest, but in the negative 
 direction, point d on the curve. 
 
 When the loop completes the turn, the wire is at the same 
 point as when it started, so the effect is the same, point 
 c on the curve. 
 
 Reviewing what has just taken place throughout the turn, 
 we find that the pressure started at zero, then gradually 
 increased in the positive direction until the loop had made a 
 quarter turn. Here the pressure was the highest, but imme- 
 diately began to diminish until at the half turn it had died 
 down until it was again zero. At this position the pressure 
 began to increase, but in the opposite direction, and con- 
 tinued to increase until it reached its highest value at the 
 three-quarters turn ; then decreased until it reached zero at 
 the complete turn. 
 
158 
 
 ELECTRICITY 
 
 FIGURE 146. PHOTOGRAPH OF A HAND GENERATOR. 
 
 FIGURE 147. PHOTOGRAPH OF A 300 HORSE POWER D. C. GENERATOR. 
 
SLIP-RINGS 159 
 
 Thus we see that the pressure was first in one direction 
 for half a turn, and then in the opposite direction for half a 
 turn. This is called alternating current pressure, and it 
 makes the current flow first in one direction throughout the 
 circuit, and then stop and flow in the other direction. 
 
 Alternating Current (A. C.) is an electrical current that 
 flows first in one direction and then in the other. 
 
 Direct Current (D. C.) is an electrical current that flows in 
 the same direction all the time. 
 
 191. Slip-rings. From the above discussion we see 
 that whenever a loop of wire revolves in a magnetic field, 
 
 FIGURE 148. - SLIP-RINGS AND WHERE THEY ARE 
 PLACED ON THE ARMATURE OF AN A. C. MACHINE. 
 
 an alternating current is produced in the loop, which is 
 called the armature. If this current is taken off just as it is 
 produced, the current will be alternating, throughout the 
 outside circuit. Current is sometimes taken off by means of 
 slip-rings. Slip-rings are two continuous rings of metal put 
 on the end of the armature, as is shown in Figure 148. 
 
 The ends of the coil are fastened on these rings, one end 
 on one ring and the other end on the other ring. iVletal or 
 carbon "brushes" rest on these rings and pick the current 
 off just as it is made, thus producing an A. C. current in 
 the external circuit. 
 
160 
 
 ELECTRICITY 
 
 192. D. C. Simple Generator. The D. C. simple gen- 
 erator is the same as the. A. C. simple generator, except in 
 the way the current is taken off. In the 
 
 OA. C. generator it is taken off by slip-rings, 
 while in the D. C. generator it is taken off 
 FIGURE 149. A by SL commutator. 
 
 COMMUTATOR Is 193. Commutator. A commutator is the 
 A SLIP-RING r xu i i*. 7-j 
 
 CUT IN PARTS same as one slip-ring, except that it is split. 
 
 It consists of two or more segments, as is 
 shown by Figure 149. 
 
 This is put on the end of the armature instead of the slip- 
 rings. One end of the loop of wire is fastened to one seg- 
 ment, while the other end of the wire is fastened to the 
 other segment. " Brushes " are placed against these seg- 
 ments to take off the current. 
 
 FIGURE 150. A LARGE GENERATOR AT NIAGARA FALLS, DRIVEN BY 
 WATER TURBINE. 
 
COMMUTATOR 161 
 
 Since the current alternates in the loop of wire, first one 
 commutator segment is positive (i.e. the current comes out), 
 and then the other. But the brushes are so set that when 
 the current changes in 
 the loop, the brushes slip 
 from one segment to the 
 other; thus one brush 
 is always positive, and 
 
 the other is always nega- 
 
 TT Vr-i MT FIGURE 1 51. How THE COMMUTATOR 
 
 tivc. Figure ^151 will MAKES A c BECOME a c> 
 
 help to show this change. 
 
 In position a, number 1, commutator-bar is on the right, 
 and is negative, while number 2 bar is on the left, and is 
 positive. This makes the upper brush positive, and the 
 lower brush negative. 
 
 In position b, the coil has turned one-half the way round, 
 putting number 1 on the left and number 2 on the right ; but, 
 in turning, the current is reversed, so that now number 1 is 
 positive and number 2 is negative. This still leaves the upper 
 brush positive and the lower brush negative. 
 
 -|- Pressure 
 
 M I m Turns 3 
 
 FIGURE 152. CURVE SHOWING THE PRESSURE AT DIFFER- 
 ENT PARTS OF THE TURN OF THE ARMATURE IN A D. C. 
 Pressure GENERATOR. 
 
 In position c, the conditions are the same as in a. This 
 shows that the current always comes out of the same brush, 
 or has become D. C. 
 
162 ELECTRICITY 
 
 194. Curve for D. C. Referring back to Figure 145, the 
 curve for the simple generator, we see that the curve changes 
 somewhat when the commutator is put on. It changes to 
 the curve on the preceding page. (Figure 152.) 
 
 The first half-turn is the same, but the second half-turn 
 becomes positive, due to the fact that the brushes slip from 
 
 one bar to the 
 other at the same 
 time the current 
 changes direction. 
 195. A Pulsat- 
 
 FIGURE 153. CROSS SECTION OF A GENERATOR . 
 
 WITH 3 COILS. m g D - c - Made 
 
 Steady. From 
 
 the curve (Figure 152) we see that the current rises and 
 falls with each half-turn of the loop of w r ire. This is what 
 is called a pulsating current. But if, instead of one coil of 
 wire, several coils are put on, as in Figure 153, then the 
 
 -|- Pressure 
 
 <'' '^-S "^ "V "V \S V "V "\/ 
 
 /XxXx>C< A 
 
 Mi 1 IX Turns 2 
 
 FIGURE 154. CURVES SHOWING PRESSURE FROM THREE COILS. 
 The resulting pressure is represented by the tops of the curves. 
 
 current becomes steady. The reason for this is easily seen. 
 There is never an instant when some coil is not cutting 
 the lines of force at right angles, thus constantly keeping 
 the pressure at the highest. (Figure 154.) 
 
CHAPTER XV 
 
 MAGNETIC EFFECT OF AN ELECTRICAL CURRENT 
 
 196. Magnetic Field about a Wire Carrying a Current. 
 We have seen that cutting lines of force by a conductor 
 produces electrical pressure. On the other hand, a current 
 of electricity, like a magnet, has about it a magnetic field. 
 
 If a wire carrying a current of electricity be passed through 
 a cardboard (Figure 155), and iron filings be sifted on the 
 cardboard, the filings 
 will arrange them- 
 selves, in concentric 
 circles, about the 
 wire. This shows that 
 the current has a 
 magnetic field, and 
 that the lines of force 
 are in concentric 
 circles. 
 
 To determine the 
 direction of these 
 
 lines, use this rule : Grasp the wire with the right hand, the 
 thumb in the direction of the current, and the fingers will point 
 out the direction of the lines of force. A magnetic needle set 
 on the cardboard will also show the direction of lines of 
 force. (Figure 156.) 
 
 If a wire carrying a current be held over a magnetic needle, 
 
 163 
 
 FIGURE 155. THE FIELD ABOUT A WIRE 
 CARRYING AN ELECTRIC CURRENT. 
 
164 
 
 MAGNETIC EFFECT OF CURRENT 
 
 the needle will tend to turn at right angles to the wire. (Figure 
 157.) The following rule can be used to tell which directipn 
 
 _ the needle will turn : 
 Extend the fingers of 
 the right hand along the 
 wire with the wire be- 
 tween the palm of the 
 hand and the needle, 
 and the thumb will 
 point the direction the 
 N-pole of the needle 
 will turn. 
 
 197. Currentthrough 
 a Helix. A helix is 
 a coil of wire wound 
 
 round and round in a spiral. It may have a core, or it 
 may not. Let us use a piece of soft iron for a core. Now, 
 
 Current 
 
 FIGURE 156. MAGNETIC NEEDLES SHOW DI- 
 RECTION OF FIELD ABOUT A WIRE CARRYING 
 AN ELECTRIC CURRENT. 
 
 Needle 
 FIGURE 157. MAGNETIC NEEDLE TURNS WITH THE LINES OF FORCE. 
 
 w r hen a current is passed through the helix, it makes the 
 iron a magnet with a north and a south pole. (Figure 158.) 
 
 The coil would become 
 whether the 
 it or not, 
 
 A A 
 
 UU 
 
 ~u b D tr 
 
 a magnet 
 iron were in 
 but the soft iron makes 
 the magnet much 
 stronger. Why ? 
 
 To determine the north pole of an electro-magnet (for that 
 is what the coil is called), use this rule : Grasp the coil with 
 
 FIGURE 1 58. DIAGRAM SHOWING POSI- 
 TIONS OF POLES OF AN ELECTRIC MAGNET. 
 
DOORBELL AND BUZZER 
 
 165 
 
 the right hand with the 
 fingers in the direction of 
 the current, and the thumb 
 will point to the north 
 pole. 
 
 Note that the position 
 of the north pole is de- 
 termined by the direc- 
 tion which the current 
 takes around the coil. 
 The fact that the current 
 goes in at one end or the 
 other has nothing to do 
 with the north pole. 
 
 198. Electro-magnet. 
 - For a definition of an 
 
 electro-magnet we can 
 give this : An electro- 
 magnet is a magnet 
 formed by a current passing around, or near, the magnet. 
 
 APPLICATIONS OF THE ELECTRO-MAGNET 
 
 199. Doorbell and Buzzer. The doorbell is one of the 
 most common applications of the electro-magnet. The cur- 
 rent is started at the battery (B, Figure 160) ; goes through 
 the coils C, C ; then into the vibrator V ; then into the set- 
 screw S; then into the push button P; and, finally, back 
 into the battery, forming a complete circuit. 
 
 When the push button P is held down, the current flows 
 through the circuit, magnetizing the coils C, C. These 
 coils then attract the soft piece of iron on the vibrator, pull- 
 ing it away from contact with -S, and striking the bell with 
 
 FIGURE 159. PHOTOGRAPH OF A 2-ToN 
 LIFTING MAGNET. 
 
166 
 
 MAGNETIC EFFECT OF CURRENT 
 
 Vi 
 
 D 
 
 ;p 
 
 \ \ 
 
 V 
 
 V 
 
 
 c\ 
 
 \ X \ 1 
 
 ^ 
 
 c\ i i \ \ \ i 
 
 
 
 the hammer. As soon as contact is broken, the coils lose 
 their magnetism, and the vibrator flies back in contact with 
 
 S, due to the spring in 
 the vibrator. As long as 
 the button is held down, 
 this operation is repeated 
 again and again, causing 
 a steady ringing of the 
 bell. 
 
 A buzzer is simply a 
 doorbell with the bell left 
 off. The buzzing sound 
 is made by the vibrator. 
 
 200. The Telegraph 
 Sounder. The telegraph 
 sounder consists of two coils of wire (C, C) and a soft iron 
 bar (SI) supported on a pivot (P) in such a manner that a 
 spring (S) holds the end of a bar up 
 against a screw (D). (Figure 162.) 
 
 D 
 
 FIGURE 160. WIRING DIAGRAM CF 
 ELECTRIC DOORBELL. 
 
 FIGURE 161. PHOTO- 
 GRAPH OF ELECTRIC 
 DOORBELL. 
 
 FIGURE 162. WIRING DIAGRAM OF THE 
 TELEGRAPH SOUNDER. 
 
 When a current is sent through the coils C, C by attach- 
 ing a battery at A and B, these coils become magnets and 
 pull the soft iron bar down until it strikes the screw E, 
 
THE TELEGRAPH SYSTEM 
 
 167 
 
 FIGURE 1 63. PHOTOGRAPH OF THE 
 TELEGRAPH SOUNDER. 
 
 making a slight sound. The bar is held in this position as 
 long as the current flows ; but as soon as the current stops, 
 the coils lose their magnetism, and the bar flips back to D, 
 making a loud click. By 
 means of these sounds, 
 the operator is able to 
 read the message. 
 
 201. Telegraph Relay. 
 - The telegraph relay 
 merely uses the electro- 
 magnet to close another 
 electric circuit. 
 
 The main current is 
 sent through coils C, C 
 
 (Figure 164) by connecting the main line to A and B. 
 This magnetizes the coils, and they attract the soft bar of 
 iron SI, pulling it up into contact with screw E. This 
 completes the circuit between C and D, the binding- 
 posts for the local circuit. 
 
 202. TheTele- 
 " ^nsulated graph System. - 
 We have just 
 learned the con- 
 struction of the 
 sounder and re- 
 lay, so now we 
 will see how they 
 are put to use in 
 the telegraph 
 system. 
 
 Figure 167 shows a system through three cities. At 
 Chicago the main wire is grounded ; then a battery (B) is 
 
 37 
 
 : 5J 
 
 1 \ 
 
 lev \ i s 
 
 
 
 
 1 \ 
 
 \c\ \ i| 
 
 V, 
 
 Jl 
 
 FIGURE 1 64. WIRING DIAGRAM OF THE 
 TELEGRAPH RELAY. 
 
168 
 
 MAGNETIC EFFECT OF CURRENT 
 
 put in ; and also a key (K) and a relay (R). Next, the wire 
 runs to Toledo ; and again a key and a relay are connected 
 
 in series with the line. It 
 goes then to Cleveland, 
 where still another key, 
 relay, and battery are put 
 in. Then the wire is 
 grounded. This com- 
 pletes the main circuit. 
 
 Tracing the circuit, we 
 start at the ground at Chicago, go through the battery, 
 relay, and key to the key, and relay at Toledo, then through 
 
 FIGURE 165. PHOTOGRAPH OF THE 
 TELEGRAPH RELAY. 
 
 FIGURE 166. PHOTOGRAPH OF A TELEGRAPH KEY. 
 
 the key, relay, battery, and ground at Cleveland, returning 
 through the ground to Chicago. 
 
 Off each relay is run a local circuit, in which are a battery 
 
 Chicago 
 EB 
 
 FIGURE 167. WIRING DIAGRAM OF A THREE STATION TELEGRAPH SYSTEM. 
 
STREET CAR CIRCUIT-BREAKER 169 
 
 and a sounder. The relay closes the local circuit ; and the 
 battery sends a current through the sounder, making it click. 
 
 Note that the current in 
 the main line never goes 
 through the sounder. 
 
 203. The Electric Clock. 
 - Very often it is desired 
 
 to have several clocks run 
 
 exactly together ; in other 
 
 i , 11 j i FIGURE 168. WIRING DIAGRAM OF 
 
 words, to be controlled by ELECTRIC CLOCK 
 
 a master-clock. This is 
 
 accomplished by the so-called electric clock. (Figure 168.) 
 
 The clock consists of a pair of coils (C, C) so arranged 
 that when an electric current passes through them they 
 turn the soft iron (SI) on the pivot (P), making the pawl 
 (R) slip down a notch on the ratchet wheel. Then, when 
 the current is stopped, the weight (W) turns the bar back, 
 pushing the wheel around one notch. This takes place 
 every minute, thus making the minute hand move one 
 space on the dial. 
 
 For sending the current through the coils an electric cir- 
 cuit is made through the master-clock. The master-clock 
 runs a drum (D, Figure 168) on which is a peg (0). The 
 peg touches the point S every minute, thus making a com- 
 plete circuit through the battery and electric clock. 
 
 204. Street Car Circuit-breaker. As a safety device a 
 so-called circuit-breaker is put on street cars. Its purpose 
 is to break the circuit whenever the current becomes too 
 large. It is constructed as in Figure 169. 
 
 The current from the trolley comes into the point a ; then 
 goes through the coil C ; then to the arm A ; and out of 
 the contact K by point 6. The current makes a magnet of 
 
170 
 
 MAGNETIC EFFECT OF CURRENT 
 
 
 '/ v-. QH j 
 
 c 
 
 5j 
 
 ^c 
 
 US1 
 
 FIGURE 169. WIRING DIAGRAM OF CIRCUIT- 
 BREAKER. 
 
 the coil, its strength depending on the size of the current. 
 If the current becomes sufficiently strong, it lifts the soft 
 
 a iron bar SI, tripping 
 the hook H, allowing 
 the spring S to pull 
 up the arm A, thus 
 breaking the circuit. 
 The motorman must 
 then reach up and pull 
 down the arm again 
 before he can start the 
 car. 
 
 205. The Annunci- 
 ator. The annunci- 
 ator is an instrument 
 used in office buildings, in elevators, etc., etc., for the pur- 
 pose of telling at what 
 place the person calling 
 is located. There may 
 be any number of push- 
 buttons, but the dia- 
 gram (Figure 171) shows 
 an elevator call-system 
 for four floors, or for 
 four push-buttons. 
 
 In the annunciator are 
 four coils (c, c, c, c), five 
 binding-posts (a, b, c, d, 
 and e), and the door- 
 bell (B). 
 
 From the binding- 
 posts a, 6, c, d run wires 
 
 FIGURE 170. PHOTOGRAPH OF A CIRCUIT- 
 BREAKER. 
 
THE AUTOMATIC ARC LAMP 
 
 171 
 
 B 
 
 f 
 
 -UL 
 
 FIGURE 171. WIRING DIAGRAM 
 OF A FOUR-POINT ANNUNCIATOR. 
 
 through coils 1, 2, 3, 4, respectively, these wires all being 
 connected with one wire which runs to the bell and finally 
 to the binding-post e. This con- D 
 
 stitutes the internal connection 
 of the annunciator. 
 
 The external connections are 
 as follows : A battery is attached 
 to the binding-post e, and then a 
 single wire is run up to all of 
 the succeeding push-buttons. 
 Then from each push-button re- 
 turns a wire to its respective 
 binding-post, a, b, c, or d. 
 Whenever a push-button is 
 
 pushed, it completes the circuit, through the corresponding 
 coil and also the bell. Thus the bell is rung, and the needle 
 below the magnet is drawn over, indicating which push- 
 button was operated. 
 
 206. The Automatic Arc Lamp. The automatic arc 
 lamp, which is used principally to light our streets and large 
 factory buildings, is an application of the electro-magnet. 
 
 This principle is used 
 automatically to ad- 
 just the carbons, which 
 are continually burn- 
 ing off. To light the 
 arc, the carbons must 
 first touch ; and then 
 must be drawn just the 
 correct distance apart, 
 
 FIGURE 172. WIRING DIAGRAM OF AN anc * *ept tnere ' ine 
 
 AUTOMATIC ARC LAMP. operation is as follows : 
 
172 MAGNETIC EFFECT OF CURRENT 
 
 The current flows from the line into coil Ci (Figure 172), 
 and then divides. One part goes to the upper carbon, and 
 the other part goes to the coil C 2 . 
 
 When the lamp is not lighted, the upper carbon falls 
 down and touches the lower one; thus when the current 
 first starts, nearly all of it flows through the carbons, instead 
 of through lower coil C 2 , for the resistance of the carbons is 
 much less than that of coil (7 2 . Thus upper coil Ci is mag- 
 netized, but lower coil C 2 is not. This pulls the soft iron 
 bar $7 up, and also the upper carbon which is attached to it. 
 
 As the carbons are separated, the light is formed, and at 
 the same time the resistance of the gap becomes more and 
 more, forcing part of the current to flow through coil C 2 . 
 Whenever this part becomes strong enough to balance the 
 pull of coil Ci, the carbons are held stationary. 
 
 207. Other Applications of the Electro-magnet. Other 
 applications of the electro-magnet are the automatic tele- 
 phone, the electric gas-lighter, and the electric door-latch. 
 
 The automatic telephone takes the place of the operator 
 at the switchboard. The person calling does so by pressing 
 on a dial at his transmitter, thus calling the number he 
 wishes. No telephone operator is necessary to make the 
 connection, as the electro-magnets do it automatically. 
 
 The gas-lighter consists of two electro-magnets, one 
 to turn on the gas and light it, and the other to turn the 
 gas off. It is used where it is desirable to turn the gas off 
 and on from some other place than at the jet. 
 
 The electric door-latch is used principally in apartment 
 houses, and is so arranged that the outer door may be opened 
 by pressing a button in any of the apartments. The pressing 
 of the button closes an electric circuit, causing an electro- 
 magnet to release the latch of the door. 
 
CHAPTER XVI 
 HEATING EFFECT OF AN ELECTRIC CURRENT 
 
 208. Work, Heat, and Electrical Energy. Work is de- 
 fined as a force overcoming a resistance and moving it. 
 Work is energy, and so is heat. There are many cases 
 where work is changed into heat. If you slide down a rope, 
 it burns your hands. Your weight forces you down against 
 the friction of your hand on the rope, thus doing work; 
 and this work is changed to heat. Again, if a piece of iron 
 is hammered, it becomes warm. If you stir cake-dough 
 rapidly for some time, it becomes warmer. The work you 
 do is transformed into heat. 
 
 The same thing is true when a current of electricity is 
 forced through a wire. The pressure is the force ; the cur- 
 rent is the thing forced; and the resistance of the wire is 
 the thing that holds the current back. It is just like your 
 weight forcing your body down the rope against the friction ; 
 and, as in that case, heat is produced. 
 
 Learn this important principle : When an electrical pres- 
 sure forces an electrical current through a resistance, heat is 
 generated. 
 
 209. Electrical Units. Electrical quantities are definite, 
 just like distance, weight, time, etc.; so it is necessary to have 
 units to measure them. 
 
 The following table gives the thing to be measured, the 
 unit of measurement, and the letter used to stand for it : 
 
 173 
 
174 
 
 HEATING EFFECT OF CURRENT 
 
 THING TO BE MEASURED 
 
 UNIT 
 
 LETTER 
 
 
 Volt 
 
 E 
 
 Current 
 
 Ampere 
 
 I 
 
 Resistance 
 
 Ohm 
 
 R 
 
 Power 
 
 fWatt 
 
 W 
 
 Electrical Energy . . . . 
 
 | Kilowatt 
 [ Watt-hour . 
 1 Kilowatt-hour 
 
 Kw 
 
 W-hr. 
 Kw-hr. 
 
 It will be noted that power is a new term, and that it has 
 two units watt and kilowatt. The kilowatt is the larger 
 unit, and is 1000 watts. 
 
 Electrical power is the time rate of delivering electrical energy. 
 
 The electrical power is found by multiplying the pressure 
 by the current ; or 
 
 Watts = Volts X Amperes. 
 W = E I. 
 
 Number of Kilowatts = N b ro f Vdts Dumber of Amperes 
 
 1000 
 
 or Kw = 
 
 E-I 
 
 1000 
 
 The electrical energy is found by multiplying the power 
 by the time, or 
 
 Watt-hours = Watts X Hours. 
 
 W-hr. = WXt. 
 
 Kiloivatt-hours = Kiloivatts X Hours. 
 
 Kw-hr. = KwXt. 
 
 The terms electrical power and electrical energy are often 
 confused. Be sure to get the distinction. 
 
 Electrical power is the rate of delivering energy. It is 
 
OHM'S LAW 175 
 
 the pressure at a certain instant X the current at the same 
 instant. 
 
 On the other hand, electrical energy is a certain amount of 
 energy which is actually delivered. It is not the rate of 
 delivering the energy, but is the energy itself. The power 
 must work for a certain time to give energy. Which do you 
 pay for when you pay your light bill, power or energy? 
 Does it make any difference whether a 40-watt lamp burns 
 
 1 hour or 3 hours? 
 
 Problems 
 
 1. What power is being used when a carbon lamp taking .5 ampere 
 is placed on a 110- volt circuit? 
 
 2. What is the power used when an iron takes 5^ amperes on 110 
 volts? 
 
 3. State, in words, how to find the power in watts and in kilowatts, 
 having given the current and voltage. 
 
 4. Find the cost of running ten 40-watt lamps for 5 hours, if elec- 
 tricity costs 10 cents per Kw-hr. 
 
 6. Figure your monthly light bill, if you run, on an average, 4 lamps 
 of 40 watts each, three hours each day ; an iron taking 5 amperes for 
 
 2 hours, 4 times a month ; and a motor taking 3 amperes for 1 hour, 
 10 times a month. Your lighting circuit is 110 volts, the month has 
 30 days, and the price of electricity is 9 cents per Kw-hr. 
 
 210. Ohm's Law. A great scientist by the name of 
 Ohm worked out this very fundamental law, known as 
 Ohm's Law: 
 
 Voltage = Current X Resistance, or 
 E = I - R. (1) 
 
 Which may also be written : 
 
 I = E (2) 
 
 R 
 
 R = ( 3 ) 
 
176 HEATING EFFECT OF CURRENT 
 
 By these three equations it is possible to find voltage, cur- 
 rent, or resistance, if the other two quantities are given. 
 Always be sure to choose the one which will answer the 
 question to your problem. 
 
 Problems 
 
 1. What current will a lamp take on a 110- volt circuit, if its resist- 
 ance is 220 ohms? 
 
 2. What current would the lamp above take if placed on a 220-volt 
 circuit ? 
 
 3. What current would a lamp take on a 110- volt and a 220-volt 
 circuit, respectively, if its resistance were 44 ohms ? 
 
 4. What voltage is necessary to send 6 amperes through an iron, if 
 its resistance is 15 ohms? 
 
 6. What is the resistance of a stove, if it takes 5.5 amperes on 110 
 volts? 
 
 6. The resistance of the hea ing-element of an iron increases when 
 it gets hot. When does it take more current, hot or cold ? 
 
 7. A carbon lamp takes .5 ampere on a 110- volt circuit, while a 
 tungsten takes .315 ampere on the same circuit. Which one has the 
 higher resistance, and how much ? 
 
 8. A dimmer on a lamp cuts the current down from .315 ampere 
 to .2 ampere. What is the resistance of the dimmer, if the lamp is 
 on a 110- volt circuit? 
 
 APPLICATION OF HEATING EFFECT OF AN ELECTRIC 
 
 CURRENT 
 
 211. The Carbon Incandescent Lamp. The carbon in- 
 candescent lamp was one of the first electric lamps used, and, 
 like all the later lamps, it uses the heating effect of an elec- 
 trical current to produce the light, the principle being to 
 force a large enough Current through a carbon wire to heat 
 it to incandescence. 
 
 The lamp consists of a glass bulb from which the air has 
 been exhausted. (Figure 173.) Inside the bulb is the carbon 
 
THE TUNGSTEN INCANDESCENT LAMP 
 
 177 
 
 wire through which the current must pass. This wire makes 
 connection through the end of the bulb by means of small 
 pieces of platinum wire, platinum being 
 used because its coefficient of linear ex- 
 pansion is nearly that of glass. Other 
 materials would cause the glass to break 
 when it was heated or cooled. 
 
 The glass bulb is sealed with wax into 
 a screw tip, one end of the wire being 
 attached to the side of the tip, while the FlGURE 173. WIR- 
 
 . ING DIAGRAM OF A 
 
 other is attached to a small piece set in CARBON LAMP. 
 
 the middle of the tip. By this means 
 
 the two ends of the wire are insulated from one another. 
 
 Contact is made through the lamp by screwing it into a 
 
 lamp-socket. The screw of the 
 socket is one side of the line, and 
 the middle portion is the other 
 side of the line. 
 
 Carbon lamps can be used on 
 either D. C. or A. C. They are 
 made for almost any voltage 
 (although care must be taken to 
 get the correct voltage for the 
 circuit in question), and take 
 about 3-g- watts per candle power. 
 212. The Tungsten Incandes- 
 cent Lamp. This lamp is con- 
 structed like the carbon lamp, 
 except that the wire filament is 
 
 made of tungsten instead of carbon. Figure 175 shows 
 
 the tungsten lamp. 
 
 The tungsten has almost replaced the carbon lamp, for it 
 
 FIGURE 1 74. PHOTOGRAPH 
 OF A CARBON LAMP. 
 
178 
 
 HEATING EFFECT OF CURRENT 
 
 FIGURE 175. WIR- 
 ING DIAGRAM OF A 
 TUNGSTEN LAMP. 
 
 takes about one- third as much electrical power to light it and 
 costs very little more for the lamp itself. The objection at 
 first to the tungsten lamp was that its 
 filament was so fragile. 
 
 The filaments of the first lamps were 
 made by grinding the tungsten to a 
 powder, making a paste of it and squeez- 
 ing it through holes, and then baking it. 
 These filaments broke with the least jar. 
 Lately manufacturers have learned to 
 draw the tungsten metal into wires for 
 filaments, and these are even more dur- 
 able than the old carbon filaments. 
 
 This lamp can be used the same as the carbon lamp, 
 but it takes only about Ij watts per candle power. 
 
 213. The Gas-filled Lamp. The 
 gas-filled lamp is a tungsten lamp 
 with the bulb filled with a gas, usually 
 argon or nitrogen, instead of having it 
 a vacuum. The filament is put into a 
 more compact coil, so that this lamp 
 is used especially with a reflector. 
 
 The gas-filled lamp can be used in 
 any place that the carbon or tungsten 
 can, and takes about 1 watt per 
 candle power. 
 
 Lamps of 100 watts rating, or over, 
 are usually filled with nitrogen, while 
 lamps of lower ratings are usually 
 filled with argon. 
 
 214. The Mercury Vapor Lamp. This lamp consists of 
 a long glass tube, nearly exhausted of air and containing 
 
 
 FIGURE 1 76. PHOTOGRAPH 
 OF A TUNGSTEN LAMP. 
 
THE ARC LAMP 179 
 
 a small quantity of mercury. In each end platinum wires 
 are sealed, making connections with the electric circuit. 
 (Figure 177.) 
 
 To light the lamp, the tube is brought to a horizontal 
 position, so that the mercury makes contact from one end 
 of the tube to the other. As soon as contact is made, the 
 tube is tilted so as to make the mercury flow to one end. 
 This breaks contact, and at this point the mercury is vapor- 
 ized by the heating effect. This vapor fills the tube, acting 
 as a conductor for the current. The current passing through 
 the vapor heats it to in- 
 candescence, giving off a 
 bluish-green light. Some 
 mercury vapor lamps are 
 lighted by other means 
 
 than tilting, but they all 
 
 FIGURE 177. WIRING DIAGRAM OF A 
 use the same principle for MERCURY VAPOR LAMP. 
 
 producing the light. 
 
 This lamp is used especially in lighting large buildings, 
 such as factories ; for taking photographs ; and for rectify- 
 ing A. C. electricity for storage batteries. 
 
 215. The Arc Lamp. We have already spoken of the 
 arc lamp (Figure 172), but since it is an application of the 
 heating effect of an electrical current, as well as of an electro- 
 magnet, we mention it here. 
 
 The method of lighting is very much the same as in the 
 mercury vapor lamp. To light it, the carbons must touch, 
 allowing the current to flow through them. Then the car- 
 bons must be pulled apart, breaking the electric circuit. 
 
 At the point where the circuit is broken, a high resistance 
 is entered. The current flowing through this high resist- 
 ance produces heat sufficient to vaporize the carbon at that 
 
180 
 
 HEATING EFFECT OF CURRENT 
 
 FIGURE 178. DIA- 
 GRAM OF ELECTRIC 
 FLAT-IRON. 
 
 point. This carbon vapor acts as the conductor, and is 
 
 heated to incandescence, giving off a very bright and power- 
 ful light. The temperature reaches as 
 high as 3500 C. and gives about 1 candle 
 power per watt. 
 
 Arc lamps are used to light streets and 
 large buildings. They are usually placed, 
 100 lamps in a series, on a 5000-volt line, 
 taking from 6 to 9 amperes. They will 
 work either on A. C. or D. C. 
 
 In moving-picture houses the arc lamp is 
 
 used in the picture machine. These arcs usually take from 
 
 50 to 100 amperes, as a very high candle 
 
 power is desired. 
 216. The Electric Flat-iron. The 
 
 electric flat-iron (Figure 178) is very 
 
 much like the ordinary flat-iron, except 
 
 that it has a heating element and an 
 
 attachment to connect it to the lighting FIGURE 179. HEAT- 
 ING ELEMENT IN AN 
 
 system. ELECTRIC FLAT-IRON. 
 
 The heating element is a special kind 
 
 of wire of high resistance wound on an insulator and placed 
 
 inside the iron. 
 Very often ni- 
 chrome wire is 
 wound on a piece 
 of mica (Figure 
 179), and this is 
 then placed be- 
 tween sheets of 
 
 FIGURE 1 80. PHOTOGRAPH OF ELECTRIC mica ' Tne miCa 
 
 FLAT-IRON. acts as an insu- 
 
OTHER APPLICATIONS 
 
 181 
 
 lator. Connection is made through a duplex (double) wire 
 attached to a plug, which can be screwed into an ordinary 
 lamp-socket. 
 
 It is better, however, to have a special socket for the 
 iron, as the current used is often large enough to burn 
 out the connection in an ordinary 
 socket. 
 
 The pressure forcing the current 
 through the heating element pro- 
 duces the heat, and as the current 
 is turned on while using, the iron 
 remains hot. 
 
 If the iron does not get hot 
 enough, it may be fixed by short- 
 circuiting one turn of its heating 
 element, thus letting through more 
 current. If it gets too hot, another 
 turn may be added. Why ? 
 
 217. Other Applications. Along 
 with the flat-iron come many other 
 electrical heating appliances. Some 
 of these are the toaster, curling 
 iron, stove, coffee percolator, and 
 soldering iron. Any, and all, of 
 these can be used on A. C. or D. C., 
 and can be bought for different voltages, although the 
 standard voltage is 110. 
 
 The amount of current taken by these appliances varies 
 with the appliance. A toaster usually requires from 1 to 
 3 amperes; a curling iron from J to 1 ampere; a stove 
 from 3 to 10 amperes; a percolator from 2 to 5 amperes; 
 and a soldering iron from 1 to 2 "amperes. 
 
 FIGURE 181. PARTS OF AN 
 ELECTRIC FLAT-IRON. 
 
 1. Cover and handle. 
 2. Cast iron plate that 
 fits over heating ele- 
 ment. 3. Heating ele- 
 ment. 4. Base on which 
 heating element rests. 
 
182 
 
 HEATING EFFECT OF CURRENT 
 
 FIGURE 182. AN ELECTRIC GRILL. 
 Can be used for several methods of cooking. 
 
 FIGURE 183. ELECTRIC COFFEE FIGURE 184. ELECTRIC COOK STOVE. 
 PERCOLATOR. 
 
OTHER APPLICATIONS 
 
 183 
 
 Electrical heating appliances are coming more and more 
 into common use, principally from the fact that they 
 are very convenient and 
 at the same time are 
 so clean and sanitary. 
 Even the electric cook 
 stove is now quite com- 
 mon. It has become so, 
 largely because it does 
 away with objectionable 
 coal and gas fumes. 
 
 Electric cars are com- 
 monly heated by electric 
 registers, and electric 
 heaters are often used in 
 homes, especially to heat 
 small rooms, like bath- 
 rooms. During weather 
 which is too warm to 
 require a furnace fire, 
 and yet is too cold to 
 keep the house comfort- 
 able without a little heat, electric heaters leave the air 
 purer than those which burn gas or oil. 
 
 In buying any electrical appliance, care should be used 
 to get a good one, as the extra cost at the beginning is soon 
 saved in the saving of electrical energy to run it. 
 
 FIGURE 185. ELECTRIC IRONING MACHINE. 
 HEATED AND RUN BY ELECTRICITY. 
 
CHAPTER XVII 
 
 MOTION-PRODUCING EFFECT OF AN ELECTRIC 
 CURRENT 
 
 218. How Motion is Produced. We saw in the case of 
 a coil of wire revolved in a magnetic field that a current 
 was produced in the coil. The reverse of this is also true. 
 If a coil of wire is put into a magnetic field and a current is 
 
 sent through the coil, 
 it is made to revolve. 
 With the aid of Figure 
 186 we will show why 
 it will revolve, and in 
 which direction the 
 motion will take pla.ce. 
 Let the current go 
 through the coil in the 
 direction ABODE 
 F. Then the coil be- 
 comes a magnet with 
 its north pole (N e ) at the top face of the coil, and its south 
 pole (S e ) at the bottom face of the coil. 
 
 Now, since like poles repel and unlike poles attract, the 
 coil is made to revolve clockwise, or in the direction of the 
 small arrow at E. Thus we see that the coil is made to 
 turn and that the turning effect is due to attraction and re- 
 pulsion of magnetic poles. 
 
 184 
 
 FIGURE 186. How MOTION is PRODUCED 
 BY ELECTRICITY. 
 
THE GALVANOMETER 
 
 185 
 
 APPLICATION OF MOTION- PRODUCING EFFECT OF AN 
 ELECTRIC CURRENT 
 
 219. The Galvanometer. The galvanometer is an in- 
 strument used to detect an electrical current in a conductor. 
 It consists of a coil of wire (C, Figure 187) suspended between 
 the poles (N and S) of a permanent magnet by means of a 
 phospor-bronze ribbon 
 ending in a small spring 
 at the bottom. 
 
 The current to be de- 
 tected is sent through 
 the coil making it an 
 electro-magnet. If the 
 current passes down- 
 ward, as the arrow in- 
 dicates, the north pole 
 of the coil is to the 
 left of the coil. 
 
 The permanent S-pole 
 then attracts it, and 
 the coil is made to turn 
 as the arrows indicate. 
 
 If it were not for the spring, the coil would turn until its 
 north pole would be directly in front of the permanent 
 S-pole, and would then stop. But the spring allows it to 
 turn only so far as the strength of the poles forces it. Since 
 the strength of the poles depends upon the current flowing 
 in the coil, the deflection of the coil indicates not only that 
 there is a current, but its relative strength. 
 
 To make the reading of the deflection easy, a pointer is 
 attached to the coil (or sometimes a mirror is used, so that 
 
 FIGURE 187. WIRING DIAGRAM OF A 
 GALVANOMETER. 
 
186 MOTION-PRODUCING EFFECT OF CURRENT 
 
 FIGURE 188. WIRING DIAGRAM SHOWING WHERE AMMETER AND 
 VOLTMETER ARE PLACED. 
 
 a ray of light may be deflected), showing the amount of 
 deflection. 
 
 220. The Ammeter. The galvanometer detects current 
 flowing, and its relative value, but does not give its amount 
 in amperes. 
 
 FIGURE 1 89. PHOTOGRAPH OF A VOLTMETER WITH THE COVER 
 REMOVED. 
 
THE VOLTMETER 
 
 187 
 
 When the galvanometer has its scale graduated in amperes, 
 it is called an ammeter. Its principle is just the same as the 
 galvanometer, but reads directly in amperes. 
 
 The resistance of the coil in an ammeter is very low, so 
 that it must always be placed in the line (A, Figure 188), 
 and never across the line. 
 
 221. The Voltmeter. The voltmeter is also like the 
 galvanometer, consisting, as it does, of permanent magnets 
 
 FIGURE 190. THE PERMANENT MAGNET, COIL, AND POINTER OF A 
 D. C. VOLTMETER. 
 
 (D. C. meter) and a suspended coil. The scale of the volt- 
 meter is graduated to read directly in volts. 
 
188 MOTION-PRODUCING EFFECT OF CURRENT 
 
 The resistance of the voltmeter is made very high; so it 
 should be placed across, not in, the line (V, Figure 188). 
 
 This resistance is made up of the resistance of the mov- 
 able coil of the instrument. When a high resistance is desired 
 fine wire with a large number of turns is used, but when 
 a low resistance is needed the coil is wound with a coarse 
 wire with few turns. 
 
 It is essential that you know how to connect a voltmeter 
 and an ammeter correctly. Should you put the ammeter 
 
 FIGURE 191. THE MOVABLE COIL AND POINTER OF A VOLTMETER. 
 
 across the line, it will be burned out. Should you place the 
 voltmeter in the line, it will shut off almost all the current. 
 
 222. The Wattmeter. The wattmeter is an instrument 
 made to read the power used in a line, It consists of two 
 
THE WATTMETER 
 
 189 
 
 FIGURE 192. A VOLT-AMMETER WHICH CAN BE USED AS EITHER A 
 VOLTMETER OR AN AMMETER. 
 
 The metal binding posts are ammeter connections, and the rubber 
 ones are voltmeter connections. 
 
 sets of coils. One set takes the place of the permanent 
 magnets in the ammeter, voltmeter, and galvanometer, and the 
 other coil is movable, as in the above instruments. 
 
 Line 
 
 Load 
 
 FIGURE 193. WIRING DIAGRAM OF A WATTMETER. 
 
190 MOTION-PRODUCING EFFECT OF CURRENT 
 
 Since the wattmeter measures power, it must read in 
 watts, or wits X amperes. 
 
 It is so connected (Figure 193) that the current passes 
 through the field coils, measuring the current ; and the 
 movable coil is connected across the line, measuring the volts. 
 The deflection then reads 
 
 Volts X Amperes = Watts. 
 
 223. Meters for A. C. Electricity. The meters here 
 described are for D. C., although the wattmeter will work 
 on either A. C. or D. C. But a special kind of ammeter 
 and voltmeter must be made for A. C. They must have 
 electro-magnets, instead of permanent magnets. 
 
 224. D. C. Motors. We have shown how a loop of wire 
 with a current in it tends to revolve when placed in a mag- 
 netic field. But its tendency is to revolve no farther than 
 to bring the face of the coil which is a N-pole opposite the 
 S-pole of the field magnet, and to remain in this position. 
 
 Now, if the current is reversed in the coil, the face which 
 was a N-pole becomes a S-pole, and vice-versa ; and the coil 
 is made to revolve another half -turn. If the current is 
 again reversed, the coil makes another half-turn ; and so on. 
 Thus the coil is made to turn continuously by reversing the 
 current in the loop every half -turn. 
 
 You will remember that the alternating current generated 
 in the loop of wire of the generator was made direct by 
 means of a commutator. In the same way a direct current 
 is made to reverse in the loop of wire in the motor. Thus 
 by putting a commutator on the loop of wire the coil is 
 made to turn continuously. Do not forget that the turning 
 effect is due to the attraction of magnetic poles. 
 
 The difference between a generator and a motor is this: 
 
THE WATT-HOUR METER 
 
 191 
 
 the generator is supplied with mechanical energy, and trans- 
 forms it into electrical energy; while a motor is supplied 
 with electrical energy, and transforms it back to mechanical 
 energy. A direct current generator may be used also as a 
 motor. 
 
 225. The Watt-hour Meter. The principle of the watt- 
 hour meter is the same as the wattmeter, but instead of the 
 
 movable coil being held 
 
 in position by a spring 
 it is allowed to turn 
 around freely, as a 
 motor. Geared to the 
 movable coil are small 
 hands which pass over 
 dials, just as in the gas- 
 meter. 
 
 With one turn of the 
 coil one watt-hour is 
 registered on the dial; 
 but this is such a small 
 unit that it cannot be 
 detected. One thousand 
 turns make a kilowatt- 
 hour, and this is indi- 
 cated by 1 on the first 
 dial. 
 
 The reading of the watt-hr. meter is the same as the 
 gas-meter (refer to gas-meter, 69). 
 
 At the bottom of the meter is an aluminum disk revolving 
 between permanent magnets. This disk acts as a brake, 
 so that the coil revolves at a speed proportional to the watts 
 used ; it also stops the meter when the current is turned off ; 
 
 FIGURE 194. A DIRECT CURRENT WATT- 
 HOUR METER WITH COVER REMOVED. 
 
192 MOTION-PRODUCING EFFECT OF CURRENT 
 
 FIGURE 195. AN ALTERNATING 
 CURRENT WATT-HOUR METER. 
 
 very small, usually not over 
 
 otherwise the coil would 
 coast and register watt- 
 hours which were never 
 used. 
 
 Watch your meter at 
 home speed up when lights 
 are turned on and slow 
 down when they are turned 
 off. It should stop when 
 all appliances are off; and 
 if it does not, have it re- 
 ported, as you are paying 
 for electricity not used. 
 Be sure that you can read 
 your meter, and then check 
 your light bills. 
 
 226. The Starting-box. - 
 
 The resistance of a motor is 
 
 ohm. If it were attached 
 
 directly to the line, as is shown by Figure 196, the coils of 
 
 the motor would be burned out. The reason for this is 
 
 easily seen. If the voltage is 
 
 110 volts and the resistance is ~liov~ 
 
 J ohm, the current would be 
 
 110 
 
 = 220 amperes, which would 
 
 M 
 
 FIGURE 196. WIRING DIAGRAM 
 OF A MOTOR DIRECTLY ACROSS 
 THE LINE. 
 
 burn out the coils. 
 
 In order to protect the motor 
 
 when starting, a " starting-box " is used. This is made up 
 of coils of resistance wire placed in a convenient box, so 
 that the coils may be cut out of the circuit by merely 
 moving a handle over to the right. (Figure 197.) 
 
C. E. M. F. 
 
 193 
 
 FIGURE 197. WIRING 
 DIAGRAM OF A SIMPLE 
 STARTING-BOX. 
 
 The first coil begins at notch No. I and ends at No. 2. 
 The second coil starts at No. 2 and ends at No. 3, and so on. 
 When the arm is on No. I notch the 
 current must pass through all five coils. 
 As the arm is moved to the right, coils 
 are cut out. 
 
 227. C. E. M. F. It is easy to see 
 why the starting-box keeps the current 
 small, and thus protects the motor while 
 the coils are all in the circuit ; but it is 
 not so easy to see why the current does not get large when 
 the coils are cut out. 
 
 You will remember that we said that whenever lines of 
 force are cut by a conductor an electric pressure is generated. 
 Now, a motor, when running, has loops of wire (the arma- 
 ture) turning in a mag- 
 netic field (field), and 
 thus an electric pressure 
 is generated. This pres- 
 sure is in the opposite 
 direction to the applied 
 pressure or E. M. F., and 
 is hence called counter- 
 E. M. F. or C. E. M. F. 
 A motor, then, when 
 running, generates a 
 C. E. M. F. which 
 opposes the applied 
 E. M. F., thus neutraliz- 
 ing part of it. On account of this, the coils of the starting- 
 box may be cut out, as the C. E. M. F. holds the current 
 down when the motor has gotten up to speed. 
 
 FIGURE 198. PHOTOGRAPH OF A O-POINT 
 STARTING-BOX. 
 
194 MOTION-PRODUCING EFFECT OF CURRENT 
 
 Suppose the motor mentioned above generates 100 volts, 
 C. E. M. F., when running at full speed, 
 110 - 100 10 
 
 then 
 
 = = 20 amperes, the amount of current 
 
 the motor would take when running at full speed. 
 
 FIGURE 199. PHOTOGRAPH OF A 4-poiNT 
 STARTING-BOX. 
 
 228. Series Motor. There are three general classes of 
 D. C. motors : Series, Shunt, and Compound. We shall dis- 
 cuss only the first two. 
 
 IIOV 
 
 r/WWWV ' 
 
 Field s v. 
 I sf \Arma1Lire 
 
 FIGURE 200. WIRING DIAGRAM 
 OF A SERIES MOTOR WITH 
 STARTING-BOX IN THE CIRCUIT. 
 
 FIGURE 201. WIRING DIAGRAM OF 
 A SHUNT MOTOR WITH STARTING- 
 BOX CONNECTIONS. 
 
SHUNT MOTOR 
 
 195 
 
 Figure 200 shows the connection for a series motor with 
 starting-box in the circuit. 
 
 The term series is used because the armature and field 
 are connected in series. The starting-box is put in the line, 
 in series with the arma- 
 ture and field. 
 
 The speed of the 
 series motor is regulated 
 by putting a resistance 
 in series with the motor. 
 To make the motor run 
 fast, cut out resistance ; 
 and to make it run 
 slowly, put in resistance. 
 Why? 
 
 Series motors are used 
 w r here the motor must 
 start under load, as in 
 the case of a street car 
 or an elevator. Why? 
 
 229. Shunt Motor. 
 The term shunt is used 
 because the armature 
 and field are placed in 
 " shunt," or parallel. 
 Figure 201 shows the connections of a shunt motor with 
 starting-box attached. 
 
 The current comes in at the switch, passes to the point 
 on the starting-box marked " Line." From the point 
 marked " A " a wire leads to the armature ; and from the 
 point marked " F " a wire goes to the field. The other 
 ends of the field and armature are connected together, 
 
 FIGURE 202. VACUUM CLEANER DRIVEN 
 BY AN ELECTRIC MOTOR. 
 
196 MOTION-PRODUCING EFFECT OF CURRENT 
 
 FIGURE 203. AN ELECTRIC FAN. 
 
 and then attached to the 
 other side of the line at 
 the switch. 
 
 Inside of the starting- 
 box, a wire goes from 
 the point marked 
 " Line " to the arm. 
 From the last notch goes 
 a wire to the point 
 marked "A" and from 
 the first notch goes a 
 wire to a small coil C, 
 and then to the point 
 " F." 
 
 To start the motor, 
 close the switch; then 
 
 move the arm of the starting-box slowly to the right, 
 
 allowing the motor to 
 
 pick up speed. 
 
 This cuts out the re- 
 sistance in the armature 
 
 circuit, making the arma- 
 ture turn faster; and at 
 
 the same time it puts 
 
 resistance into the field 
 
 circuit, which also makes 
 
 the armature turn faster. 
 
 (Why?) The small coil 
 
 acts as a magnet and 
 
 holds the arm over 
 
 When it is pushed far FlGURE 204 .- A SMALL ELECTRIC MOTOR 
 enough. 
 
 USED TO DRIVE A SEWING MACHINE. 
 
SHUNT MOTOR 
 
 197 
 
 599 
 
 FIGURE 205. AN ELECTRICAL MOTOR DESIGNED TO RUN A 
 WASHING MACHINE. 
 
 The speed is regulated by putting a resistance into the 
 field circuit. Putting in resist- 
 ance makes the motor speed up. 
 Taking out resistance makes it 
 slow down. It may seem unrea- 
 sonable at first that putting in 
 resistance in series with the field 
 of a shunt motor speeds it up, 
 and taking out resistance slows it 
 down. 
 
 The reasons for these charac- 
 teristics are readily understood, 
 however, when it is remembered 
 that the thing that does most to 
 control the current through a 
 motor is the C. E. M. F. which 
 it generates. 
 
 FIGURE 206. AN ELECTRICAL 
 MOTOR ATTACHED TO A WASH- 
 ING MACHINE. 
 
198 MOTION-PRODUCING EFFECT OF CURRENT 
 
 FIGURE 207. A LARGE A. C. POWER MOTOR DISASSEMBLED TO SHOW 
 DIFFERENT PARTS. (SLIP RING TYPE.) 
 
 The armature must turn fast enough to generate a 
 C. E. M. F. almost equal to the applied E. M. F. If the 
 field is weak the armature must burn fast, but if it is strong 
 
 FIGURE 208. ANOTHER LARGE A. C. POWER MOTOR DISASSEMBLED. 
 (SQUIRREL CAGE TYPE.) 
 
SPECIFIC USES OF A. C. AND D. C. MOTORS 199 
 
 then the armature need only turn slowly, to generate this 
 necessary C. E. M. F. 
 
 Therefore, since adding resistance in series with the field 
 makes the field weaker, it causes the motor to speed up, and 
 since taking out resistance in series with the field makes the 
 field stronger, it causes the motor to slow down. 
 
 This motor is used where it can start without load, and 
 can then have the load thrown on gradually, as in the case 
 of motors in a machine-room. 
 
 230. Small Motors. If the motor is small enough, it 
 may be put directly on the line, without a starting-box. In 
 this case the armature is so light in weight that it can start 
 to full speed before the coils have time to burn out. 
 
 231. Specific Uses of A. C. and D. C. Motors in the 
 Home. Motors for either A. C. or D. C. circuits are often 
 used for the following purposes : 
 
 1. Electric fans. 4. Kitchen motors. 
 
 2. Sewing machines. 5. Vacuum cleaners. 
 
 3. Washing machines. 6. Hair driers. 
 
 Name any other uses you know. 
 
CHAPTER XVIII 
 
 INDUCTION 
 
 232. Permanent Magnet in a Coil of Wire. Induction 
 is the producing of an electrical pressure (E. M. F.) by means 
 of a conductor cutting magnetic lines of force. This is not a 
 new idea, but is one which we have been using all through 
 
 the subject of Electricity. 
 We spoke of it when we 
 studied the simple generator. 
 In the simple generator 
 the conductor moved and 
 cut the lines of force, which 
 remained stationary. This 
 action may be reversed, 
 the conductor remaining 
 stationary and the field 
 moving, and the result 
 will be the same. 
 
 Figure 209 shows a per- 
 manent magnet (M) thrust into a coil of wire (C), the ends 
 of the coil being connected through the galvanometer (G). 
 When this is done, the galvanometer will deflect, showing 
 that a current passes through the coil. The lines of force 
 come out of a N-pole and go around and into a S-pole. 
 When the magnet is thrust downward, these lines are cut 
 by the wire in the coil. 
 
 200 
 
 x ^- 
 
 
 
 ' x - 
 
 "N 
 
 f S** 
 
 ^^^ 
 
 \ / / 
 
 
 X y 
 
 '; 
 
 ,. ' 
 
 ' 
 
 M 
 
 r*v 
 
 V 
 
 ''' M 
 
 1 1 
 
 
 --- 
 i 
 
 fi 
 
 , 
 \ 
 i 
 
 1 1 
 
 
 i 
 
 
 I 
 
 
 
 i 
 
 
 1 1 
 
 \\ 
 
 
 i 
 
 R 
 
 | L^- 
 
 \ \ 
 
 
 j 
 
 i, 
 
 j^,.^*-^^** 
 
 V*- 
 
 \ 
 
 rj 
 
 y| 
 
 P 
 
 /' 
 
 I 
 
 "i 
 
 
 
 / 
 
 . 
 
 
 
 
 
 v v a 
 
 
 L \ 
 
 
 r.1-' 
 
 \ 
 
 
 
 
 I ^ 
 
 FIGURE 209. A PERMANENT MAGNET 
 BEING THRUST INTO A COIL OF WIRE. 
 
AN ELECTRO-MAGNET IN A COIL OF WIRE 201 
 
 If the magnet were pulled out, the lines of force would 
 be cut in the opposite direction, and the galvanometer would 
 deflect in the opposite direction, showing that the current is 
 reversed. 
 
 Then, to thrust a N-pole in and pull it out immediately 
 produces an A. C. current in the coil. 
 
 Just the reverse action takes place when a S-pole is thrust in 
 and pulled out, since the lines of force are reversed. That 
 is, to pull a S-pole out is the same as to thrust a N-pole in, 
 and to thrust a S-pole in is the same as to pull a N-pole out. 
 
 233. An Electro-magnet in a Coil of Wire. Figure 209 
 shows a coil of wire with a permanent magnet thrust into it. 
 Figure 210 shows the 
 same coil of wire, but 
 instead of a permanent 
 magnet an electro- 
 magnet has been used. 
 The effect is exactly 
 the same as before. 
 
 Now, if instead of 
 thrusting in and pull- 
 ing out this electro- 
 magnet, the core with 
 the wire around it is 
 placed inside the coil 
 
 of wire, and the key (K) is pressed and released, the same 
 effect is obtained. 
 
 While the key is open, the core is not a magnet; then 
 when it is pressed, the core becomes a magnet, giving the 
 same effect as thrusting a magnet in. Again, when the 
 key is released, the core loses its magnetism, and the result 
 is the same as when the magnet is pulled out. 
 
 : v < 
 
 ^_ 
 
 i %z 
 W 
 
 P 
 
 ^ 
 
 - 
 
 ^ 
 > p 
 
 ^ 
 
 
 1 AT 
 
 
 FIGURE 210. AN ELECTRO-MAGNET IN A 
 COIL OF WIRE. 
 
202 
 
 INDUCTION 
 
 Thus we see that if two coils are placed so that one is 
 inside the other, and a current is made in one, a current 
 is induced in the other. Also, if a current is stopped in 
 one, a current is induced in the other, in the opposite 
 direction. 
 
 The coil in which the current is made or stopped is called 
 the primary, while the coil in which the current is induced 
 is called the secondary. 
 
 234. Mutual and Self-induction. The above case is 
 called mutual induction. It is the producing of a current in 
 one wire by the effect of a current in another. 
 
 FIGURE 211. INDUCTION APPARATUS. 
 
 Self-induction has to do with but one wire. 
 
 It takes time and energy to start an automobile. The 
 tendency of the automobile to hold back, or stay where it 
 is, is called inertia. The tendency for a current not to flow 
 ivhen it is being started, and to keep on flowing when it is being 
 stopped, is called self-induction. 
 
 Self-induction always takes place when a current is 
 
THE INDUCTION COIL 
 
 203 
 
 changed (made larger or smaller) in a circuit. It acts in the 
 opposite direction to the change. 
 
 235. The Induction Coil. The induction coil or " spark- 
 coil," is used to increase the pressure in a D. C. circuit so 
 that a spark will jump across a gap. 
 
 The wiring diagram of an induction coil is shown in Figure 
 212. 
 
 A coil of heavy wire (p) is wound on a soft iron core, with 
 a few turns. Around this is wound a coil of fine wire, with 
 many turns. The coil of 
 heavy wire is called the 
 primary, and is connected 
 in series with a push 
 button (P), a battery (), 
 and a vibrator (F). The 
 fine-wire coil is called the 
 secondary, and ends at 
 opposite sides of a spark 
 gap. A condenser (C) is 
 placed across the gap 
 made by the vibrator. 
 
 A condenser is a storage 
 tank for electricity. It is usually made up of layers of 
 tinfoil insulated from one another by mica or other insulat- 
 ing material, alternate layers being connected together. 
 Positive electricity flows in on one side, and negative on 
 the other. The more leaves or layers, the more it will hold. 
 
 In the primary of the induction coil the action is the same 
 as in the door bell, the vibrator flying backward and for- 
 ward, making and breaking the current. Whenever the 
 current changes in the primary, a current is induced in the 
 secondary by mutual induction. 
 
 FIGURE 212. WIRING DIAGRAM OF 
 THE INDUCTION COIL. 
 
204 INDUCTION 
 
 Since there are several times as many turns in the second- 
 ary as there are in the primary, the voltage of the secondary 
 will be just that many times as great as in the primary. 
 
 To explain : Suppose the primary has 10 turns and the 
 secondary 1000 turns, and that the primary produces a 
 field of a certain strength. Now, for every turn on the 
 primary there are %^, or 100, turns on the secondary. 
 Hence, the secondary cuts 100 times as many lines of force 
 as the primary. Since the voltage depends upon the num- 
 ber of lines cut per second, the voltage in the secondary 
 will be 100 times that in the primary, or 
 
 voltage of secondary turns of secondary 
 voltage of primary turns of primary 
 
 Since there is self-induction wherever a current is started 
 or stopped, the making and breaking of the primary circuit 
 is not accomplished quickly. The condenser is put in over 
 the gap to make this action take place more quickly, thus 
 increasing the voltage of the spark. 
 
 236. Uses of the Induction Coil. The induction coil is 
 used in igniting the gas in gas engines. 
 It is also used for medical purposes. 
 237. The Transformer. The in- 
 duction coil was used on D. C., the 
 vibrator changing the current in the 
 primary. Now if A. C. is used, a 
 vibrator need not be put in, but the 
 primary may be wound about a soft 
 iron without any mechanism to regu- 
 late it. The alternation of the 
 
 FIGURE 213. A Low current takes the place of the make 
 VOLTAGE TRANSFORMER. and break of the induction coil. 
 
THE TRANSFORMER 
 
 205 
 
 Such an arrangement is called & transformer. It consists 
 merely of two coils wound on a soft iron core. One coil is 
 made of fine wire with many turns, while the other is made 
 of heavy wire with few 
 turns. 
 
 As in the induction 
 coil, the voltages of the 
 coils depend upon the 
 ratio of the number of 
 turns. The coil which 
 has the current put into 
 it is called the primary, 
 while the one in which 
 the pressure is induced 
 is called the secondary. 
 
 The commercial trans- 
 former has four coils ; 
 two with fine wire, and 
 two with coarse wire, 
 wound on the same com- 
 mon core of laminated 
 soft iron. The ratio of 
 turns in these coils is 
 10 to 1. That is, for 
 every turn on a coarse- 
 wire coil there are 10 turns on a fine- wire coil. 
 
 By connecting the coils in different combinations different 
 voltages may be obtained. 
 
 With a 110- volt primary line six voltages may be obtained 
 with a commercial transformer three by using the coarse- 
 wire coils as primary, and three by using fine-wire coils as 
 primary. 
 
 FIGURE 214. A HIGH TENSION 
 (VOLTAGE'I TRANSFORMER. 
 
206 
 
 INDUCTION 
 
 ~y I Secondary 
 
 FIGURE 2 1 5. 1 1 VOLTS 
 TRANSFORMED TO 2200 VOLTS. 
 
 238. Coarse-wire Primary. 1. If the primaries are 
 connected in parallel, and the secondaries in series, the volt- 
 age will be ^ X 110 = 2200. 
 (Figure 215.) 
 
 2. If the primaries are con- 
 nected in parallel and the sec- 
 ondaries in parallel, the voltage 
 will be ^ X 1 10 = 1 100. (Figure 
 216.) 
 
 3. If the primaries are connected in series and the 
 secondaries in parallel, the voltage will bo V X 110 = 550. 
 (Figure 217.) 
 
 239. Fine-wire Primary. 1. If the primaries are con- 
 nected in parallel and the second- 
 aries in series, the voltage will be 
 
 AX HO = 22. (Figure 218.) 
 
 2. If the primaries are con- 
 nected in parallel and the second- 
 aries in parallel, the voltage will 
 be A X 110 = 11. (Figure 219.) 
 
 3. If the primaries are connected in series and the 
 secondaries in parallel, the voltage will be ^ X 110 = 5^. 
 (Figure 220.) 
 
 240. Uses and Advantages of the 
 Transformer. First of all, you 
 must remember that transformers 
 can be used only on A. C. 
 
 They are used for stepping the 
 voltage up or down. Your house 
 circuit is not in electrical connec- 
 tion with the power station, but comes from a transformer 
 near the house, where the voltage has been stepped down 
 
 nmar 
 IIOV 
 
 _ 
 
 
 \ , , 
 
 
 Secondanj 
 
 
 ./ ./ JJJ 
 
 
 
 
 
 IIOOV 
 
 
 
 
 C C 
 
 
 
 i I 1 
 
 FIGURE 216. 110 VOLTS 
 TRANSFORMED TO 1100 VOLTS. 
 
 
 ; 
 
 
 550 
 
 
 
 IIOV 
 
 FIGURE 217. 110 VOLTS 
 TRANSFORMED TO 550 
 VOLTS. 
 
USES AND ADVANTAGES OF TRANSFORMER 207 
 
 nabry 
 22V 
 
 1 
 
 i Z~~S 
 
 
 Primary 
 
 
 f i i / ii 
 
 d 
 
 
 
 HOV 
 
 r / 
 
 
 - 1 '! 
 
 FIGURE 218. 110 VOLTS TRANS- 
 FORMED TO 22 VOLTS. 
 
 from 2300 volts to 110 volts. In fact, wherever power 
 is to be delivered some distance it is sent out at high 
 voltage, and then stepped down so that it can be used. 
 
 The transformer has many 
 advantages, but the four prin- 
 cipal ones are these : 
 
 1. It makes it possible to get 
 any voltage you like from any 
 voltage delivered. 
 
 2. It saves cost of wire. Since 
 
 power = E I, if the power is sent out at a large volt- 
 age, the current may be small, and since it is the current 
 
 that heats a wire, the wire may 
 be small w r hen the current is 
 small. 
 
 3. It saves line drop, or fall of 
 voltage. The fall of voltage along 
 a line is the resistance of the line 
 X the current flowing. We saw 
 how the current could be made 
 
 smaller with the transformer, and so line drop is cut down. 
 4. It saves line loss. Line loss is power lost in the line, 
 and is the line drop X current. Since the transformer makes 
 it possible to reduce both the line 
 drop and the current, it makes 
 it possible to reduce the line loss. 
 On account of the advantages 
 just named nearly all transmis- 
 sion lines are of high tension 
 (voltage) . Being of high voltage, 
 they are dangerous, and so are usually put up on strong 
 towers, very well insulated, the wires themselves being bare. 
 
 
 
 
 
 . ( 
 
 
 Primary 
 
 JJJ J \ ) ) 
 
 = 
 
 IIV 
 
 
 - 
 
 
 
 MOV 
 
 
 
 
 
 / 
 
 
 
 
 
 
 FIGURE 219. 110 VOLTS 
 TRANSFORMED TO 1 1 VOLTS. 
 
 condaru 
 
 sv 
 
 
 
 
 p 
 
 i 
 
 Prirvrtf 
 
 - 
 
 
 
 
 10V 
 
 
 
 
 r r 
 
 
 ' -| 
 
 FIGURE 220. 110 VOLTS 
 TRANSFORMED TO 5| VOLTS. 
 
208 
 
 INDUCTION 
 
 241. The Three-phase System. Heretofore we have 
 always considered an electric circuit as having two lines, one 
 line out and one line back. 
 
 The modern system of delivery is 
 what is called the " three-phase " 
 system. It consists of three wires in- 
 stead of two, and carries three times as 
 much power as a two-line system. 
 
 The generator for three-phase current 
 is so arranged that the current goes out 
 on one of the wires and comes back on 
 the other two, or goes out on two and comes back on one. 
 
 For example, at one instant the current is flowing out on 
 line No. 1 (Figure 221), and at the same time is coming back 
 
 Sub-Station 
 Transformer 
 
 ' 2 
 
 FIGURE 221. WIRING 
 DIAGRAM OF A 3- 
 PHASE GENERATOR. 
 
 Generator 
 
 on poles 
 
 FIGURE 222. WIRING DIAGRAM OF A S-PHASE 
 CITY SYSTEM. 
 
 on No. 2 and A T o. 3 ; an instant later it will go out on No. 2, 
 and come back on No. 1 and No. 3, etc. 
 
 This is the system used in Cleveland, Ohio, by the Illu- 
 minating Company. 
 
WIRING DIAGRAM OF HOUSE CIRCUIT 
 
 209 
 
 242. The Wiring Diagram of a City System. Figure 222 
 shows the general wiring diagram of a city using a 3-phase 
 current. The elec- 
 
 n 
 
 3 
 
 tricity is generated 
 at the generator (G) 
 at 11,000 volts, and 
 is sent out to the 
 sub-stations (S) in 
 conduits under 
 ground. Here it 
 runs through trans- 
 formers and is 
 stepped down to 
 2300 volts. This is 
 carried out on poles 
 to the locality in 
 which it is to be used. 
 Here it is stepped down to 110 volts by transformers placed 
 on the poles. This 110-volt line is carried into the houses. 
 243. Wiring Diagram of House Circuit. The current is 
 brought into the house on two insulated wires at 110 volts. 
 
 FIGURE 223. WIRING DIAGRAM OF A HOUSE 
 CIRCUIT. 
 
 B B 
 
 FIGURE 224. WIRING DIAGRAM OF A SIMPLE TELEPHONE CIRCUIT. 
 
210 
 
 INDUCTION 
 
 A city ordinance usually requires that all new wiring must 
 
 enter the house at the basement. Just after it enters the 
 
 ^^^ house it passes through fuses. (Fi, Figure 
 
 off 223.) Then it goes through the service 
 
 ^ f switch (S) to the meter (M) ; then through 
 
 Br^ff another set of fuses (Fz) ; and then to the 
 
 t \ " fixtures in the house ; all the appliances being 
 
 ^^^^^ put in parallel, across the line. 
 
 4feS^ 244. The Telephone. The telephone uses 
 
 FIGURE 225. A 
 PORTABLE TELE- 
 PHONE RECEIVER 
 AND TRANS- 
 
 M1TTER. 
 
 the principle of 
 the transformer. 
 Figure 224 shows 
 a diagram of the 
 simple Bell tele- 
 phone. 
 
 In the trans- 
 mitter is a layer 
 of powdered car- 
 bon (C) between 
 two plates ( P and 
 P). By this ar- 
 rangement an 
 electric circuit is 
 
 established, pass- FIGURE 226 A DESK TELEPHONE SWITCHBOARD 
 ^i i ,1 SUCH AS is USED AS A LOCAL SWITCHBOARD BY 
 
 mg through this A LARGE BUSINESS CONCERN. 
 
 carbon to a bat- 
 tery (B), and through the primary of the transformer (T). 
 The secondary circuit consists of the following parts, all 
 
THE TELEPHONE 211 
 
 being put in series : (a) the secondary coil of the local trans- 
 former, (b) the secondary coil of the transformer at the 
 other station, (c) the coil of wire about the permanent 
 magnet at the local station, (d) the similar coil about the 
 permanent magnet at the other station, and (e) the connect- 
 ing line wires. 
 
 When the speaker talks into the transmitter, the little 
 plate P alternately squeezes and releases the carbon, thus 
 reducing and increasing its resistance. This causes the cur- 
 rent in the primary to fluctuate. This induces an alternat- 
 ing current in the secondary, which in turn strengthens and 
 weakens the permanent horseshoe magnets. As these mag- 
 nets are strengthened and weakened, they first pull, and 
 then release, the steel plate (P 2 ) in the receiver, causing it to 
 flip backward and forward. This plate (P 2 ) then reproduces 
 the sound that enters the transmitter. 
 
CHAPTER XIX 
 CHEMICAL RELATION OF AN ELECTRICAL CURRENT 
 
 245. The Electrolytic Cell. Sometimes liquids instead of 
 solids are used as conductors of electricity. For instance, a 
 salt solution will conduct electricity. When the current 
 passes through a solution like this, a chemical change takes 
 place which is quite different from what happens when a 
 substance like mercury conducts electricity. 
 
 The solution, with the points of contact, is called an 
 electrolytic cell. 
 
 246. Chemical Action in an Electrolytic Cell. When a 
 solution is made, part of its molecules break up into parts 
 or ions, and are said to ionize. Before this can be under- 
 stood a few terms must be learned. 
 
 An atom is the smallest known part of an element which 
 will enter into a chemical change. For example, a copper 
 atom is the smallest known part of the element copper 
 which will enter into a chemical change. We let the 
 symbol Cu stand for it. 
 
 A radical is a group of atoms acting as a single atom in 
 a given chemical change. For example, in CuSO the SO^ 
 is called a radical, and does not break up in a given chem- 
 ical change. 
 
 An ion is an atom or a radical, with an electrical charge. 
 For example, a Cu atom with a charge of electricity is called 
 a copper ion, and is written Cu + . Also, the radical S0 
 
 212 
 
THE ELECTROLYTIC CELL 
 
 213 
 
 with a charge of electricity becomes an ion, and is called a 
 sulphate ion and is written S0 4 ~~. Positive ions carry posi- 
 tive charges, and negative ions carry negative charges. The 
 same kind of atoms or radicals always carry the same kind 
 of charge. 
 
 Thus, when we say a solution ionizes; we mean it breaks 
 up into atoms and radicals carrying electrical charges. 
 
 When an electrical current passes through a solution, the 
 positive ions are made to flow with the current, while the 
 negative ions flow in the other direction. Also, more of 
 the solution ionizes. This is the way a solution conducts the 
 current. 
 
 247. Parts of an Electrolytic Cell. The parts of an 
 electrolytic cell are (1) the solution, which is called the elec- 
 trolyte; (2) the contact, or pole where the current comes in, 
 called the anode; and (3) the contact, or pole where the 
 current goes out, called the cathode. 
 
 248. The Copper Sulphate (CuSO 4 ) Electrolytic Cell. 
 A solution of CuSo^ with a copper anode and any other 
 conductor for a cathode, will 
 
 make an electrolytic cell. 
 (Figure 227.) The action is 
 as follows : 
 
 When the current is turned 
 on, the CuSot ionizes (some of 
 it is already ionized) into Cu + 
 and $0 4 ~. The Cu + passes 
 over to the cathode and gives 
 up its charge, and places the 
 Cu on the cathode. The *S0 4 
 passes over to the anode, unites with an atom of the copper 
 plate, with the aid of the positive charge coming through 
 
 Cu. 
 
 FIGURE 227. A COPPER SUL- 
 PHATE ELECTROLYTIC CELL. 
 
214 
 
 CHEMICAL RELATION OF CURRENT 
 
 the wire, and forms new CuS0 4 . As this action contin- 
 ues, the cathode becomes plated with copper, and the 
 anode is eaten away. 
 
 This action can be expressed by the three following 
 equations : 
 
 Cu + + S0 4 - 
 
 Cu + -+ Cu + ( + 
 
 so,- + Cu + ( + ) 
 
 249. The Sulphuric Acid (H 2 SO 4 ) Electrolytic Cell. - A 
 solution of HzSO* with a cathode and anode of platinum 
 
 will form an electrolytic 
 cell. (Figure 228.) 
 
 The action is as follows : 
 The HzSOi ionizes into 
 ndS04~. The2# 2 + 
 I sl__ ____ H " passes over to the cathode 
 
 and there deposits its 
 charge, the free hydrogen 
 bubbling off as a gas. 
 The S0 4 ~ passes over to 
 the anode, but cannot 
 attack the platinum, so 
 it unites with a molecule 
 of water (H 2 0), with the 
 aid of the positive charge 
 
 ( + ) coming through the wire, and forms a new mole- 
 cule of H 2 SOt, the remaining oxygen bubbling off as a 
 gas. As this action continues, the two plates remain the 
 same, but the solution becomes concentrated, as H-0 is 
 taken off in its two constituent gases. 
 
 This action may be expressed by the three following 
 equations : 
 
 
 FIGURE 228. A SULPHURIC ACID 
 ELECTROLYTIC CELL. 
 
ELECTRO-TYPING 215 
 
 SO,' + H 2 + ( + ) >- H,SO* + 
 
 There are many different electrolytic cells but the action 
 in all is similar to that in the two just studied. 
 
 250. Electro-plating. The electrolytic cell is used in 
 plating. A solution containing a salt of the metal to be 
 plated on the object is used as an electrolyte. The object to 
 be plated is used as a cathode, and the anode is of the same 
 material as the metal to be plated on the object. The 
 action is exactly the same as in the case studied under 
 the CuSOt electrolytic cell. 
 
 Many precautions are required to make plating success- 
 ful. The solution must be of just the right strength, the 
 object to be plated must be perfectly clean, and the rate 
 of plating, or the size of the plating current, must be just 
 right. 
 
 It is by this process that nearly all modern plating is done. 
 Name some things that are silver-plated. Some that are 
 nickel-plated, some that are gold-plated. 
 
 251. Electro-typing. Electro-typing is another of the 
 useful things done by means of the electrolytic cell. All 
 the cuts in books, magazines, and newspapers as well as the 
 reading matter of most of our books are made by electro- 
 typing. (The reading matter of most newspapers is not 
 electro-typed.) 
 
 If the thing to be electrotyped is a page of printed matter, 
 the type is first set up. Then an impression is made in wax. 
 This impression is next sprinkled with graphite to make it 
 a smooth conducting surface. Then this form is used as the 
 cathode in a plating cell. Copper about the thickness of 
 
216 CHEMICAL RELATION OF CURRENT 
 
 paper is plated on the graphite surface. This is then backed 
 with type-metal to make it strong, and the wax is melted 
 off. This plate can then be used as often as desired, and is 
 easily stored away. The type used at the beginning can 
 be used over and over again. 
 
CHAPTER XX 
 BATTERIES 
 
 252. The Simple Voltaic Cell. We have learned that 
 an electrical pressure is generated whenever lines of force 
 are cut by a conductor. Here are three other known ways 
 by which an electrical pressure may be produced : 
 
 1. By chemical action. 
 
 2. By certain kinds of friction. 
 
 3. By heating two metals in contact. 
 
 If a glass jar has a solution of common salt put into it, 
 and a zinc strip and copper strip be put into the solution 
 and joined together by a conductor, an electrical current 
 will flow. The jar of salt water with its copper and zinc 
 strips is called a voltaic cell, for it generates an electrical pres- 
 sure. The pressure is set up by the chemical action which 
 takes place in the cell. 
 
 Care should be taken not to confuse the terms " voltaic 
 cell " and " electrolytic cell." The latter is merely a con- 
 ductor of electricity, while the former produces an electrical 
 pressure. 
 
 253. The H 2 SO 4 Voltaic Cell. There are several kinds 
 of voltaic cells. We just learned that salt water with 
 copper and zinc strips for " electrodes " forms a voltaic cell. 
 So, also, does dilute H^SO* with copper and zinc electrodes. 
 
 Let us note the chemical action that takes place in the 
 H 2 SO, voltaic cell. (Figure 229.) 
 
 217 
 
218 
 
 BATTERIES 
 
 As soon as the circuit is closed, the ionized HiSOi sepa- 
 rates, the H 2 going to the Cu electrode and giving up its 
 
 charge, the 2 H being given 
 off as a gas. The S0 4 goes 
 to the Zn plate, receives the 
 positive charge coming around 
 the wire, and unites with the 
 Zn to form ZnSO (zinc sul- 
 phate). 
 
 This action may be shown by 
 the three following equations : 
 
 Hf >- 2 H + ( + ) 
 SOt- + Zn + (+) > ZnSO* 
 
 FIGURE 229. A SULPHURIC ACID , 
 
 VOLTAIC CELL. Ihus we see that an elec- 
 
 trical current is sent through 
 
 the wire, that the HzSO^ is used up, that ZnSO* is made in 
 its place, and that the Zn strip is eaten up. 
 
 254. Polarization. It was seen above that hydrogen gas 
 is given off at the copper plate. In all cells where this is 
 done there is a tendency for these hydrogen bubbles to stick 
 to the plate, and thus insulate it. This is called polarization. 
 
 255. Open-circuit Cells. Cells which polarize cannot 
 be run for long periods, because the positive plate becomes 
 insulated by the hydrogen. Therefore these cells are called 
 " open-circuit cells," because the circuit on which they are 
 placed must remain open most of the time and can be closed 
 for only short periods. 
 
 Name some uses of open-circuit cells. 
 
 256. The Wet Salammoniac Cell. An open-circuit cell 
 may be made by placing a handful of ammonium chloride 
 
THE ADDWATER CELL 
 
 219 
 
 Zn 
 NH.CI 
 
 MnO, 
 
 FIGURE 230.- CROSS 
 
 SECTION OF A SIMPLE 
 
 DRY CELL. 
 
 in a quart jar filled with water, using a strip of 
 carbon for a positive electrode and a zinc strip for a nega- 
 tive electrode. This cell is often used 
 for doorbells. 
 
 257. The Dry Cell. The dry cell 
 has the same chemical action as the wet 
 NH 4 Cl cell, but it is constructed differ- 
 ently, so that it may be handled much 
 easier. 
 
 Figure 230 shows a cross section of 
 this cell. The outside, or case, is zinc, 
 and acts as the negative electrode. The 
 center portion (C) is a stick of carbon, which is the positive 
 electrode. Packed in around this carbon stick is a paste of 
 NH 4 Cl and manganese dioxide (MriOz). The NH^Cl is the 
 active portion, and the manganese dioxide is put in to 
 
 retard polarization. This is an 
 open-circuit cell. 
 
 The top shaded portion is tar, 
 or wax, used to seal the cell so 
 that the moisture will not dry out. 
 This cell gives about 1.4 volts, 
 and, when new, will give as high 
 as 30 amperes on short circuit. 
 Name some uses of the dry cell. 
 258. The Addwater Cell. - 
 The Addwater cell is an open- 
 circuit cell, the construction of 
 which is kept secret by the manu- 
 facturers. Its advantage over the 
 ordinary dry cell is the fact it will last longer, as it has a 
 well to be filled with water, thus keeping it from drying out. 
 
 COP 
 
 Knur/ Hut 
 Acorn Head Post 
 
 Carbon E/ec/rode 
 
 - Pu/pboord Bottom 
 
 FIGURE 231. CROSS SECTION 
 OF A COMMERCIAL DRY CELL, 
 AS IT is NOW MANUFACTURED. 
 
220 
 
 BATTERIES 
 
 259. Closed-circuit Cells. In the case of some voltaic 
 cells there is no hydrogen given off in the form of a gas, and 
 so these cells do not polarize. Keeping the circuit closed 
 for a long period does not harm them, 
 and they are called " closed-circuit 
 cells." 
 
 Name some uses for closed-circuit 
 cells. 
 
 260. The Gravity Cell. The gravity 
 cell consists of two solutions placed in a 
 glass jar with copper and zinc electrodes. 
 These two solutions are concentrated 
 CuSO* and dilute ZnSO* (5-1 ) . The CuSO* 
 is placed in the bottom, and the ZnSO^ on 
 top. They keep these relative positions 
 on account of their difference in density, 
 hence the name " gravity cell." 
 
 The copper plate is placed in the CuSO*, 
 and the zinc plate, or " crowfoot," is hung 
 in the ZtiSO*. The circuit must be kept 
 closed, or the two liquids will diffuse, thus 
 These cells are used on telegraph lines. 
 
 261. The Daniell Cell. The Daniell cell is similar to the 
 gravity cell, except that the ZnSO* is placed in a clay porous 
 cup so that the cell may be handled without danger of mix- 
 ing the solutions. The action is exactly the same as in the 
 gravity cell. 
 
 262. Secondary or Storage-cells. The voltaic cells we 
 have been studying are capable of giving an electrical pres- 
 sure as soon as they are set up, and are therefore called 
 primary cells. It has been found that cells may be made 
 which will not at first give an electrical pressure, but which 
 
 FIGURE 232. THE 
 ADDWATER CELL, 
 WHICH is A SPECIAL 
 KIND OF DRY 
 CELL. 
 
 spoiling the cell. 
 
THE LEAD WET STORAGE-CELL 
 
 221 
 
 will do so if " charged." These cells are called " second- 
 ary cells " or " storage-cells" 
 
 263. The Lead Wet Storage-cell. A storage-cell may 
 be made by using two lead plates for electrodes and dilute 
 H<iS0 4 for an electrolyte. 
 (Figure 233.) 
 
 When first set up, this 
 cell will not give a pres- 
 sure, but if a D. C. current 
 is allowed to flow through 
 it for a time it is said to 
 become " charged," and 
 will then give an electrical 
 pressure. 
 
 The charging current 
 causes a chemical action 
 
 to take place within the cell, thus storing up chemical 
 energy. No electricity is stored in the cell. Then, when 
 the cell is used to give 
 pressure, the current 
 flows in the opposite di- 
 rection, at the expense of 
 the chemical energy stored 
 
 FIGURE 233. A DIAGRAM OF A WET 
 LEAD STORAGE BATTERY. 
 
 Source of 
 Pressure 
 
 FIGURE 234. --WIRING DIA- 
 GRAM OF A STORAGE BATTERY 
 CHARGING CIRCUIT. 
 
 FIGURE 235. A COMMERCIAL LEAD 
 STORAGE BATTERY. 
 
 in it. When this energy is exhausted, the cell must be 
 recharged. 
 
222 BATTERIES 
 
 To charge the cell, a D. C. must be used, and the + pole 
 of the charging circuit must be connected to the + pole of 
 the cell. (Figure 234.) 
 
 If A. C. is used, it must first be rectified, that is, changed 
 into D. C. by a motor generator, a rotary converter, or a 
 mercury vapor lamp. 
 
 The lead storage-cell is easily injured, so a few precau- 
 tions may be appropriately named : 
 
 1. D. C. current must be used for charging. 
 
 2. Do not overcharge. 
 
 3. Do not short circuit. 
 
 4. Do not charge too fast. 
 
 5. Do not let it remain uncharged. 
 
 6. Keep it filled with pure water. 
 
 The lead storage battery is used for many things. Some 
 of these uses are : 
 
 1. To run electric motor cars. 
 
 2. To start motors and to light cars. 
 
 3. To light houses in the country. 
 
 4. For plating. 
 
 The lead storage-cell gives about 2 volts per cell, regardless 
 of the size of the cell. 
 
 264. The Dry Lead Storage-cell. There has just re- 
 cently been put on the market a dry lead storage-cell 
 (Figure 236), but as yet, its success 
 has not been shown. It may, or may 
 not, be good. Its principle is ex- 
 actly the same as the wet lead cell, 
 but instead of the acid being in a 
 free state, it is absorbed by a com- 
 
 FIGURE 236. DIAGRAM OF , ,, < S ,, n 
 
 A DRY LEAD STORAGE P ound > thus forming a dry cell. 
 BATTERY. The electrodes are lead plates wound 
 
THE EDISON STORAGE-CELL 
 
 223 
 
 in concentric spirals, thus giving a large active area. The ab- 
 sorbing compound is pressed in between the plates with such 
 force that the active material on the plates cannot come out. 
 
 If this cell proves to be good, it will be a great step in 
 storage battery construction, for free acid is a dangerous 
 thing to handle. 
 
 265. The Edi- 
 son Storage-cell. 
 -Thomas A. Edi- 
 son has had an 
 
 QSlTIVC POLE 
 
 FIGURE 238. THE 
 POSITIVE AND NEGA- 
 TIVE PLATES OF AN 
 EDISON CELL. 
 
 FIGURE 237. DISSECTED VIEW OF. AN EDISON 
 STORAGE BATTERY CELL. 
 
 altogether different storage-cell on the market for some 
 time. This cell has potassium hydroxide (KOH} for an 
 electrolyte, and patented nickel and steel electrodes. The 
 container is a pressed-steel box, so that it is almost in- 
 destructible. The Edison cell does not need the care 
 that a lead cell does, and can be subjected to much more 
 
224 BATTERIES 
 
 rough handling, without injury. A short circuit does not 
 permanently harm it, if it is immediately recharged. 
 
 FIGURE 239. A WOODEN TRAY CONTAINING 
 5 EDISON CELLS. 
 
 The voltage of the Edison storage-cell is lower than that 
 of the lead cell, it being about 1.5 volts; and its efficiency 
 runs lower than the lead cells. 
 
 STATIC ELECTRICITY 
 
 266. Static Electricity. Till now we have been study- 
 ing about dynamic or current electricity. But there is 
 another kind called static electricity. 
 
 There are many applications of this form of electricity, 
 such as lightning, wireless telegraphy, and medical uses. 
 When we scuff across a thick rug in a cold room and then 
 touch a metal door-knob or gas-fixture, we get a slight shock 
 due to static electricity. 
 
 Although the applications of static electricity are spectac- 
 ular and interesting, it has not the widespread practical 
 
STATIC ELECTRICITY 
 
 225 
 
 FIGURE 240. AN ACTUAL PHOTOGRAPH OF A STROKE OF LIGHTNING 
 TAKEN ON THE SHORE OF LAKE MICHIGAN. 
 
 value of current electricity. For this reason a complete 
 treatment of it is not embodied in this book. 
 
 Review Problems 
 
 1. Discuss the field about a magnet. 
 
 2. Distinguish between a magnetized piece of iron and one which 
 is not magnetized. 
 
 3. Why is magnetism studied before electricity ? 
 
 4. How may an electrical pressure be generated? What deter- 
 mines its amount and its direction? 
 
 5. Discuss pressure, current, and resistance. 
 
 6. Distinguish between A. C. and D. C. 
 
 7. How is an A. C. made D. C. ? 
 
 8. Describe the space about a wire carrying a current. 
 
 9. What determines the poles of an electro-magnet? 
 
 10. Name ten applications of the electro-magnet. 
 
 11. How does electricity produce heat? 
 
226 BATTERIES 
 
 12. Name five electrical quantities to be measured, the unit used for 
 each, and the letter used to denote each. 
 
 13. If a door bell has 180 ohms resistance, what current will it take 
 if 6 volts are applied to it ? 
 
 14. What is the cost of running a motor for 2 hours, if it takes 3 
 amperes on 1 10 volts, the cost of electricity being 9^ per Kw.-hr. ? 
 
 15. How long would a starting-battery last if it contained 600 watt- 
 hours and gave a pressure of 6 volts at a 300-ampere discharging rate? 
 
 16. Compare the cost of running four 25-watt lamps to that of three 
 40-watt lamps. 
 
 17. How much would you save on your electricity bill if you had an 
 attachment like the " dim-a-lite," which would throw in an additional 
 100 ohms to the 340 ohms if the lamp were to burn 8 hours on a 110- 
 volt circuit, and cost 9^ per Kw.-hr.? 
 
 18. In problem 17 would the lamp be as bright with the extra 100 
 ohms in the circuit? 
 
 19. What heats an electrical flat-iron? 
 
 20. How does electricity produce motion? 
 
 21. Explain how the ammeter measures current. 
 
 22. Show where a voltmeter and an ammeter should go in a circuit. 
 
 23. What is the difference between A. C. and D. C. meters? 
 
 24. Discuss the essential parts of a watt-hour meter. 
 26. What is C. E. M. R? 
 
 26. Tell briefly the difference between a series and a shunt motor. 
 
 27. What is induction ? 
 
 28. Discuss mutual- and self-induction. 
 
 29. How could you get 6 volts from a 120-volt A. C. line? 
 
 30. If the two coils of a transformer have their turns in the ratio of 
 3 and 24, what voltages could you get from a 110-volt A. C. line? 
 
 31. What is the advantage of the 3-phase system? 
 
 32. Discuss the wiring diagram of a house. 
 
 33. What is the difference between an electrolytic cell and a voltaic 
 cell? 
 
 34. Explain how silverware is plated. 
 
 35. Why is a dry-cell called an " open-circuit cell "? 
 
 36. Give some applications of static electricity. 
 
CHAPTER XXI 
 MECHANICS OF SOLIDS 
 
 267. Units of Measurement. The things with which 
 physics deals are very definite, and so require definite units 
 to measure them. For example, the houses we live in are of 
 definite sizes, the food we eat has a certain weight, and you 
 go to class for a definite length of time. All these quantities 
 are definite, and in order to express them we must have 
 definite units. 
 
 This is not a new thing, for we have been using units all 
 through this course, but it is advisable to study them for 
 their own sake. 
 
 268. The English System. There are two great sys- 
 tems of measurement the English and the Metric. There 
 is no necessity for two systems, but we have them, and 
 people will continue to use both for many years to come. 
 
 There are other things to be measured, but the three 
 principal ones are space, mass (incorrectly called weight), and 
 time. 
 
 Under space, come length, area, and volume. The English 
 unit of length is the foot. Other units are derived from 
 this ; the yard = 3 ft. ; the inch = yV ft. ; the mile = 5280 ft. 
 
 The unit foot is made definite by the fact that the original 
 is kept in London. Copies of it are made and used as 
 standards of measurement. Our standard is kept at 
 Washington. 
 
 227 
 
228 
 
 MECHANICS OF SOLIDS 
 
 FIGURE 241. A 
 CUBIC FOOT. 
 
 The units of area and volume are derived from the units 
 of length. Thus the square foot is an area which is one foot 
 on a side ; the cubic foot is a cube which 
 is one foot on each edge. (Figure 241.) 
 
 Other units, such as square yard, cubic 
 yard, square inch, cubic inch, etc., have 
 similar meanings. 
 
 The unit of mass is the pound (lb.), 
 and it denotes a certain amount of 
 matter determined by a standard kept 
 in the same way as the standard foot. Other units are 
 derived from it, such as the ounce (oz.) = -j^ lb. ; the ton 
 (T.) = 2000 lb. ; etc. 
 
 The unit of time is 
 the second (sec.) ; it is 
 based on the time it 
 takes the earth to 
 
 make one rotation on its axis. The second is ^eJoT f that 
 time. The other units derived from it are the minute 
 (min.) = 60 sec. ; the hour (hr.) = 60 min. ; the day = 24 
 hr. ; the year = 365 \ days. 
 
 FIGURE 242. THE STANDARD METER. 
 
 FIGURE 243. UNITED STATES NATIONAL PROTOTYPE METER BAR, 
 Bureau of Standards, Washington, D. C. 
 
THE TWO SYSTEMS COMPARED 229 
 
 269. The Metric System. The same quantities can be 
 measured in the metric system, but the units are different. 
 The unit of length is the meter (m.) ; and it is defined as the 
 distance between two scratches made on a platinum bar 
 kept at Paris. (Figure 242.) 
 
 Table of Lengths 
 
 10 millimeters (mm.) = 1 centimeter (cm.) 
 100 cm. = 1 meter (m.) 
 
 1000 m. =1 kilometer (km.) 
 
 The metric unit of mass is the gram (gm.), and it is YoVo 
 part of a piece of brass kept in Paris along with the standard 
 meter. This piece of brass was so made that it has the 
 same mass as 1000 c.c. of pure water at 4 C. That makes 
 the gram equal to the mass of 1 c.c. of pure water at 4 C. 
 
 Other units are given in the table. 
 
 Table of Masses 
 
 1000 milligrams (mg.) = 1 gram (gm.) 
 1000 gm. = 1 kilogram (kg.) 
 
 The metric unit of time is the second. It is identical with 
 that of the English unit. 
 
 270. The Two Systems Compared. Just a glance at 
 the two systems is sufficient to show that the metric is much 
 the simpler. 
 
 All the derived units in the metric system are multiples 
 of ten. For example, 10 mm. = 1 cm., 100 cm. = 1 m., 
 1000 m. = 1 km., etc. This makes it easy to remember and, 
 at the same time, easy to change from one unit to another. 
 All that is necessary is to move the decimal point either to 
 the right or left. For example : 
 
230 MECHANICS OF SOLIDS 
 
 1.273 m. = 127.3 cm. 
 467.8 cm. = 4.678 m. 
 3.642 kg. = 3642 gm. 
 
 In the English system this is not true. There is no 
 regularity whatever. This makes it hard to change from 
 one unit to another. For example : 
 
 15 ft. = 15 X 12 = 180 in. 
 231 in. = ^ = 19J ft. 
 
 3 Ib. = 3 X 16 =48 oz. 
 90 oz. = f jf = 5| Ib. 
 
 271. Relation between the Two Systems. So long as 
 there are two systems in use, we shall at times be obliged 
 to change readings in one to readings in the other. For this 
 reason we need a table of equivalents. The fact that the 
 two systems are entirely independent makes these equiva- 
 lents irregular and burdensome. 
 
 Table of Equivalents 
 ENGLISH METRIC 
 
 in 2.54 cm. 
 
 Ib. . 453.6 gm. 
 
 sec. 1 sec. 
 
 sq. in. . . . . . , 6.452 sq. cm. 
 
 cu. in. . . . .... 16.39 c.c. 
 
 liquid qt - .945 liter (liquid unit) 
 
 Using this table we can change from any reading in one 
 system to the corresponding readings in the other system. 
 
 272. Force. Besides space, mass, and time there are 
 many other physical quantities which have to be measured. 
 One of these is force. 
 
 Force is a push, or a pull, on an object, that tends to make 
 the object move. The force may, or may not, make the object 
 move, but it always tends to do so. For example, you can 
 
UNITS OF WORK 231 
 
 pull on a chair and make it slide on the floor. Again, you 
 can pull or push on the corner of a house, and it will not 
 move, but there is a tendency to move, and if the push or 
 pull were large enough, it would move. These are examples 
 of force. 
 
 273. Units of Force. Force is measured in both the 
 English and metric systems. 
 
 The unit most used in the English system is the pound. 
 You will notice that this is the same name as that given to 
 the unit of mass, but the idea is different. 
 
 A pound mass is a certain amount of matter. A pound 
 force is the pull of the earth on a pound mass at sea level. 
 
 The unit most used in the metric system is the gram. 
 Again, this is the same name as that given to the unit of 
 mass, and, as in the English system, it represents the pull of 
 the earth on a gram mass at sea level. 
 
 274. Work. When a force produces motion, it is said 
 to do work. Work is a definite physical quantity and can 
 be measured. When you pull on a chair, and it slides on the 
 floor, you do work; but if you do not pull hard enough to 
 make it slide or move, there is no work done. 
 
 Work is the result of a force acting against a resistance and 
 moving it. The amount of work is measured by the force 
 multiplied by the distance the force moves. 
 
 Work = Force X Distance. 
 
 It will be seen that if the object is not moved, no work 
 will be done ; or, if the body be moving without any force 
 applied, no work is done. 
 
 275. Units of Work. The unit of work in the English 
 system is. the foot-pound, and in the metric system it is the 
 gram -centimeter. 
 
232 MECHANICS OF SOLIDS 
 
 A foot-pound is the work done when a pound force acts 
 through a distance of one foot. 
 
 If you were to pull a chair on the floor a distance of 3 
 ft. and it took a force of 5 lb., the work done would be 
 
 3 X 5 = 15 ft. lb. 
 
 To find the work done, multiply the force by the distance 
 it moves. 
 
CHAPTER XXII 
 
 MACHINES 
 
 276. Machines. A machine is a mechanical apparatus 
 which either transforms or transfers energy. There are six 
 simple machines. They are lever, wheel and axle, inclined 
 plane, pulley, screw, and wedge. 
 
 All other machines are composed of a combination of one 
 or more of these six. For example, a sewing machine has a 
 combination of the lever, pulley, and screw. Even the most 
 complicated machine, such as the modern printing-press, is 
 made of groups of the six simple machines. 
 
 277. The Lever. The lever consists of a rigid bar (B) 
 Figure 244, a weight (W), a force (F), and a pivot (P). W 
 represents the force 
 
 overcome, which is 
 often the weight of 
 an object being lifted ; 
 F represents the force 
 applied; while P is the 
 point about which the 
 bar turns. 
 
 The distance (a) from the force to the pivot is called 
 the force-arm. The distance (b) from the weight to the pivot 
 is called the weight-arm. The product of the force and 
 the force-arm is the force moment (F a), and the product of 
 the weight and weight-arm is the weight moment (W b). 
 
 233 
 
 FIGURE 244. THE LEVER; 
 
234 
 
 MACHINES 
 
 The law of the lever is that the force moment equals the 
 weight moment, or F a = W b. 
 
 278. Classes of Levers. Levers are divided into three 
 classes, according to the relative positions of the force, the 
 
 weight, and the pivot. 
 The first class has 
 the weight and the 
 force on the ends and 
 the pivot in the 
 middle. (Figure 245.) 
 The second class 
 has the force and 
 
 on 
 
 the 
 
 FIGURE 246. SECOND CLASS LEVER. 
 
 FIGURE 245. FIRST CLASS LEVER. 
 
 the pivot 
 ends and the weight in the middle. (Figure 246.) 
 
 The third class has the weight and the pivot on the ends 
 and the force in the 
 middle. (Figure 247.) 
 279. Mechanical 
 Advantage. In dis- 
 cussing a machine, 
 the term mechanical 
 advantage is used. 
 Every machine has a mechanical advantage, and this is 
 found by dividing the weight by the force, or by finding an 
 
 equal ratio. Thus it 
 has a definite mean- 
 ing, and is defined as 
 
 W 
 
 the fraction 
 r 
 
 In the case of the 
 Therefore to find the 
 
 FIGURE 247. THIRD CLASS LEVER. 
 
 lever - 
 
 b 
 
 W 
 F 
 
 ~; (Figure 244.) 
 
APPLICATIONS OF THE LEVER 
 
 235 
 
 mechanical advantage of a lever, divide the force-arm by 
 the weight-arm, or 
 
 I IT L / j 4 Force-arm 
 
 Mechanical advantage = rrr . 
 Weight-arm 
 
 280. Efficiency. Another term used in discussing a 
 machine is efficiency. This term also has a definite meaning, 
 
 j j .c i ,v j , work-out 
 
 and is denned as the traction - 
 
 work-in 
 
 No machine will do work of its own accord. Work must 
 first be put into it, and then it will do work, giving a cer- 
 tain amount out. The 
 work-in is the work put 
 into the machine. The 
 work-out is the work 
 that the machine gives 
 out when operated. 
 
 A machine never gives 
 out as much work as is 
 put into it, because some 
 
 FIGURE 248. BALL BEARINGS REDUCE 
 FRICTION AND INCREASE THE EFFICIENCY. 
 
 of the work is always 
 
 lost in the machine, 
 
 overcoming friction. Therefore the efficiency of a machine 
 
 is always less than 100 per cent. 
 
 In the case of a lever there is usually very little friction 
 and so the efficiency is usually from 95 per cent to 99.9 per 
 cent. 
 
 281. Applications of the Lever. There are many appli- 
 cations of the lever, but one that needs especial mention is 
 the balance used for weighing objects. (Figure 249.) 
 
 The balance consists of a beam (B) supported on a knife- 
 edge (K). At each end of the beam is hung a scale pan (S). 
 These are also supported on knife-edges. A pointer (P) 
 
236 
 
 MACHINES 
 
 is attached to the beam 
 to show when a balance 
 of the weights is ob- 
 tained. 
 
 To make a weighing, 
 the object to be weighed 
 is placed in the left-hand 
 pan and is the W of the 
 lever. Standard weights 
 are placed in the right- 
 hand pan, so that a 
 balance is obtained. 
 
 The best method to 
 get a balance is to start 
 with the largest weight. 
 If it is too small, add 
 the next one, and so on. 
 If it is too large, take it 
 off and use the next smallest. Repeat this operation until 
 a balance is obtained, that is, until the pointer will swing 
 the same distance on one side as 
 on the other. 
 
 The balance is a lever of the first 
 class. Other examples are shown in 
 Figures 250, 251, 252. 
 
 Figures 253, 254, 255 show applica- 
 tions of the second class lever. 
 
 Figures 256, 257, 258 show applica- 
 tions of the third class lever. 
 
 Make a simple drawing and 
 classify the levers in the following 
 examples. 
 
 FIGURE 249. THE WEIGHING BALANCE 
 is A LEVER. 
 
 FIGURE 250. THE CAN 
 OPENER USED AS A FIRST 
 CLASS LEVER. 
 
APPLICATIONS OF THE LEVER 
 
 237 
 
 FIGURE 251. THE TACK 
 PULLER USED AS A 
 FIRST CLASS LEVER. 
 
 FIGURE 253. A CAN 
 OPENER USED AS A 
 SECOND CLASS LEVER. 
 
 FIGURE 252. SCISSORS ILLUSTRATE 
 A FIRST CLASS LEVER. 
 
 FIGURE 254. A POTATO RICER 
 USED AS A SECOND CLASS LEVER. 
 
 FIGURE 255. A NUT CRACKER is 
 A SECOND CLASS LEVER. 
 
 FIGURE 256. GRASS CUTTERS OR 
 SHEEP SHEARS ILLUSTRATE THIRD 
 CLASS LEVER. 
 
 FIGURE 257. THE SUGAR TONGS 
 is A THIRD CLASS LEVER. 
 
238 
 
 MACHINES 
 
 1. Wire pliers 
 
 2. Pitcher pump 
 
 3. Lemon squeezer 
 
 4. Spoon 
 
 5. Knife 
 
 6. Fork 
 
 7. Claw hammer pulling a nail 
 
 8. Oar of rowboat 
 
 9. Paddle of canoe 
 
 10. The human arm 
 
 1 1 . Wheelbarrow 
 
 12. See-saw 
 
 13. Spring-board 
 
 14. Shovel 
 
 Name five other applica- 
 tions of the lever, and 
 classify them. 
 
 282. Wheel and Axle. - 
 The wheel and axle is an- 
 other simple machine very 
 similar in action to the lever. 
 
 It consists of a wheel and 
 an axle rigidly fastened to- 
 gether. (Figure 259.) The 
 force (F) acts on a rope 
 wound around the wheel, 
 
 FIGURE 258. -A BROOM USED AS A 
 THIRD CLASS LEVER. 
 
 FIGURE 259. THE 
 WHEEL AND AXLE. 
 
INCLINED PLANE 
 
 239 
 
 and the weight (IV) is hung on a rope wound in the opposite 
 direction on the axle. 
 
 When the force moves down, the weight moves up. The 
 action is the same as in the lever. 
 The radius (R) of the wheel acts as 
 
 the force-arm, and the radius (r) of / &// \\F 
 
 the axle acts as the weight-arm. 
 
 The mechanical advantage of the 
 
 wheel and axle is or, as in the 
 r 
 
 R 
 
 lever, 
 r 
 
 The efficiency of this machine is 
 
 less than that of the lever, ranging FIGURE 260. ANOTHER 
 FORM OF THE WHEEL AND 
 
 from 60 per cent to 99 per cent. AxLE 
 The efficiency depends upon the 
 
 bearings of the machine and upon the flexibility of the cord. 
 Sometimes a crank is used instead of the wheel. (Figure 
 260.) This does not change the action. 
 
 283. Applications of Wheel and Axle. The windlass 
 used in removing dirt from wells or manholes in the street is 
 an application of the wheel and axle. (Figure 261.) 
 
 Another application of the wheel and axle is the device 
 
 used for raising awnings. 
 (Figure 262.) 
 
 Name and draw two other 
 applications of the wheel and 
 axle. 
 
 284. Inclined Plane. The 
 inclined plane consists of a 
 
 FIGURE 261. -THE WINDLASS Is A P lane set at . an an S le to the 
 
 WHEEL AND AXLE. horizon. (Figure 263.) The 
 
240 
 
 MACHINES 
 
 weight (W) always acts downward, and the force (F) acts 
 along the plane. The vertical distance (h) is called the 
 
 height of the plane, while the 
 distance along the plane (L) 
 is called the length of the plane. 
 The force (F) must move 
 the length of the plane (L) 
 in order to raise the weight 
 (W) the height (h). 
 The mechanical advantage 
 
 of the inclined 
 
 i W 
 plane is - 
 
 r 
 
 or 
 
 It will be seen from 
 
 this that the more nearly the 
 FIGURE 262. -A WHEEL AND AXLE lane comeg to the horizontal, 
 Is OFTEN USED TO LIFT AWNINGS. , ... 
 
 the greater will be the me- 
 chanical advantage. Then, in order to lift a large weight, 
 use a long plane. 
 
 285. Applications of Inclined Plane. There are many 
 applications of the inclined plane. Figure 264 shows an in- 
 clined plane used for loading a piano into a truck. A heavy 
 plank is used for the plane and the height of the truck is the 
 height of the plane. 
 By this means one or 
 two men can push 
 the piano into the 
 truck. 
 
 Another applica- 
 tion of the inclined plane is the rolling stairway. (Figure 
 265.) This is often used in large department stores instead 
 of elevators. A person wishing to go from one floor to 
 
 FIGURE 263. THE INCLINED PLANE. 
 
PULLEY 
 
 241 
 
 another steps on the moving 
 stairway and is carried up, or 
 down, according to the direction 
 in which the stairway moves. 
 Usually there are two of .these side 
 by side, one going up, and the 
 other down. 
 
 Graded roads are excellent ex- 
 amples of inclined planes. 
 
 FIGURE 264. AN INCLINED 
 PLANE USED TO LOAD A 
 PIANO INTO A TRUCK. 
 
 (Figure 
 
 286. Pulley. There are two types of pulleys. 
 266 and Figure 267.) 
 
 Figure 266 shows two pulleys belted together. The 
 one which supplies the power is called the driver, and 
 
 the other the driven. 
 
 FIGURE 265. A MOVING STAIRWAY Is AN 
 INCLINED PLANE. 
 
 The larger the driven pulley is, 
 the greater the mechanical ad- 
 vantage. 
 
 FIGURE 266. Two PULLEYS BELTED 
 TOGETHER. 
 
 a 
 
 w 
 
 FIGURE 267. AN- 
 OTHER TYPE OF 
 PULLEY. 
 
242 
 
 MACHINES 
 
 Tne mechanical advantage = 
 
 radius of driven R 
 
 radius of driver r 
 Figure 267 shows the other type of pulley, often called a 
 block. A block consists of one or more pulleys or sheaves 
 
 fastened side by side, or 
 one above the other, so 
 that they are free to turn. 
 Two blocks are used 
 to lift a weight. One 
 block is made fast, and 
 the weight is attached 
 to the other one. A 
 rope or chain is threaded 
 through the blocks, as 
 shown in the figure. 
 
 The mechanical ad- 
 vantage is equal to the 
 number of strands sup- 
 porting the weight. 
 
 From the figure it will 
 be seen that if the weight 
 be lifted 1 foot, there are 
 six strands to be short- 
 ened 1 foot. This allows 
 the force (F) to move 6 
 feet while the weight 
 moves 1 foot. Thus the 
 mechanical advantage is 
 six. 
 
 287. Applications of the Pulley. A familiar example of 
 the first type of pulley is the sewing machine. (Figure 269.) 
 Here the large wheel is the driver, and the small wheel is the 
 
 FIGURE 268. A LABORATORY SET OF 
 PULLEYS. 
 
APPLICATIONS OF THE PULLEY 
 
 243 
 
 driven. This arrangement makes it harder to turn, but a 
 greater speed can be obtained. 
 
 The revolutions per minute (R. P. M.) of two pulleys belted 
 together are inversely as their diameters. This means that the 
 large pulley runs slowly while the small 
 one runs fast. 
 
 Problem: If a driver is 2 ft. in diameter, 
 and makes 500 R. P. M., what is the speed of 
 the driven, which is f ft. in diameter ? 
 
 The second type of pulley is often I 
 used in lifting safes or other heavy FIGURE 269. THE 
 
 i . /-rr ^i-rn \ A 1 PULLEY AS USED IN 
 
 Objects. (Figure 270.) A gin pole IS THE SEWING MACH.NE. 
 
 placed in the window above, and the 
 
 upper block is fastened to this. By pulling on the free end 
 
 of the rope the safe is raised to the open window. From 
 
 here it is swung inside. 
 
 Elevators are usually lifted up 
 
 and let down by means of this 
 
 type of pulley. 
 
 FIGURE 270. A SET OF PUL- 
 LEYS USED TO LIFT HEAVY 
 OBJECTS TO THE UPPER 
 STORIES OF HIGH BUILDINGS. 
 
 FIGURE 271. A JACK SCREW. 
 
244 
 
 MACHINES 
 
 FIGURE 272. A WEDGE. 
 
 288. Screw and Wedge. The screw and the wedge are 
 both very much the same as the inclined plane. As is shown 
 by Figure 271 , the screw is merely a spiral inclined plane which 
 
 is made to move 
 under the weight, 
 thus forcing the 
 weight to move. 
 
 Likewise Figure 
 272 shows that the 
 wedge is a double 
 inclined plane, made 
 to move under the 
 weight, causing the 
 latter to move. 
 
 The pitch of a screw is the number of threads per inch, and 
 the distance from one thread to the next is called the lead 
 (L). The mechanical advantage is the circumference of the 
 circle that the force moves divided by the lead, or 
 
 Mechanical advantage = 
 
 j 
 
 The mechanical advantage of the wedge is the length of the 
 wedge (L) divided by the thickness of the wedge (h), or 
 
 Mechanical advantage = 
 
 h 
 
 The efficiency of the screw and the wedge is small, because 
 there is always much friction. 
 
 289. Application of the Screw and Wedge. The use of 
 the screw is common, and many illustrations could be named. 
 A few are the piano stool (Figure 273), the ordinary wood 
 screw (Figure 274), and the bolt and nut (Figure 275). 
 
 The wedge is not in such common use, but many examples 
 
POWER 
 
 245 
 
 can be found. Figure 276 
 shows a hatchet used as a 
 wedge to split kindling. 
 
 290. Power. Power is 
 the time rate of doing work. 
 It is very often confused 
 with the term work; but it 
 is different, for it involves 
 the idea of time, while work 
 does not. 
 
 A boy could carry a thou- 
 sand bricks up a ladder 10 ft. 
 high as well as a man, but it 
 would take him longer. 
 
 The amount of work done 
 by the boy and man would 
 
 be the same, but the rate at which the man would do the 
 work would be greater ; so we say he has the more power. 
 
 The units of power are the foot-pound per second, and the 
 gram-centimeter per second. These units are so small that 
 larger units are commonly used. The horsepower is the one 
 most common in this country. A horsepower is the power 
 that will do 33000 foot-pounds of work per minute. 
 
 To find the horsepower delivered in any case, find the 
 work in foot-pounds done per minute, 
 and divide by 33000 ; thus : 
 
 FIGURE 273. THE PIANO STOOL Is 
 AN APPLICATION OF THE SCREW. 
 
 FIGURE 274. THE 
 WOOD SCREW. 
 
 FIGURE 275. THE BOLT AND NUT Is 
 AN APPLICATION OF THE SCREW. 
 
246 
 
 MACHINES 
 
 If a girl weighs 120 pounds and climbs the stairs from one floor to the 
 next, a distance of 15 ft., in 30 seconds, she does 120 X 15 = 1800 ft.- 
 Ib. in .5 min. (30 sec.) or 
 
 1800 
 .5 
 
 3600 ft.-lb. per min. 
 
 33000 
 
 6. 
 55 
 
 ho ower 
 
 291. Power Delivered by Pulleys. It is often desirable 
 to know the power necessary to run certain appliances in the 
 
 home, such, for example, 
 as the sewing-machine, 
 the vacuum cleaner, the 
 washing-machine, food 
 chopper, bread mixer, 
 etc. Most of these are 
 either run by pulleys 
 driven by belts or by 
 
 gears, SO the method for 
 findm ^ horsep ower is 
 the same. 
 
 Let us compute the horsepower for a sewing machine as an 
 example. 
 
 Suppose the small 3-in. wheel of the sewing machine must make 
 500 R. P. M., and that the belt has an effective pull of 2 Ib. What is 
 the horsepower necessary to run it ? 
 
 Method : 
 
 3 inches = = .25 ft. 
 
 .25 X 3.1416 = .7854 ft., cir. of wheel 
 .7854 X 500 = 392.7 ft., distance the belt moves in 1 min. 
 392.7 X 2 = 785.4 ft.-lb. per min. 
 
 785.4 
 
 FIGURE 276. THE HATCHET USED IN 
 SPLITTING KINDLING Is AN APPLICATION 
 OF THE WEDGE. 
 
 33000 
 
 .0238, horsepower required. 
 
PROBLEMS 247 
 
 What horsepower is necessary to run a food chopper that requires a 
 force of 10 Ib. on the end of a 1-ft. crank making 60 R. P. M. ? 
 Method : 
 
 2 ft. = diameter of circle 
 2 X 3.1416 = 6.2832 ft., cir. of circle 
 6.2832 X 60 = 376.992 ft., distance force moves in 1 min. 
 376.992 X 10 = 3769.92 ft.-lb. per min. 
 3769.92 
 
 33000 
 
 .114, horsepower required. 
 
 Problems 
 
 1. The pulley on a washing-machine is 10" in diameter and makes 
 100 R. P. M. The belt has an effective pull of 25 Ib. What horse- 
 power is required ? 
 
 2. The pulley on a kitchen power-table is 6" in diameter and makes 
 600 R. P. M. ; the effective pull on the belt is 10 Ib. What horsepower 
 is required ? 
 
 3. If a motor of 80 per cent efficiency runs the pulley in Prob. 1, 
 how many watts does it require? (746 watts = 1 horsepower.) 
 
 4. If a motor of 85 per cent efficiency runs the pulley in Prob. 2, 
 how many watts does it require ? 
 
 5. When you turn an ice-cream freezer handle 1 ft. long, 50 R. P. M., 
 and it requires a force of 8 Ib., what horsepower are you producing? 
 
CHAPTER XXIII 
 DYNAMICS 
 
 292. Motion. Motion is a change of position with refer- 
 ence to some other object. 
 
 If you were to look at a book lying near the center of a 
 table and were then to close your eyes, and if, while they 
 were closed, some one were to change the book to the edge 
 of the table, could you tell that it had been moved, when 
 you opened your eyes ? You say " Yes " ; for it has changed 
 its position with reference to the table. 
 
 Now, if you were to try the experiment again, and the 
 person changed the table and let the book remain in the 
 center of the table, could you tell whether the book had been 
 moved? Some would say " Yes," and some "No." Both 
 are right and both are wrong, depending on what is taken as 
 a point of reference. Explain. 
 
 293. Newton's Three Laws of Motion. It always takes 
 force to produce, or to change, motion. A chair cannot be 
 moved unless some force is applied. Also, anything in mo- 
 tion requires a force to stop it or make it change its direction. 
 
 Newton learned this fact and put it into three laws : 
 
 1. Every body continues in a state of rest, or of uniform 
 motion in a straight line, unless acted upon by some external 
 force. 
 
 2. Every motion is proportional to the acting force, and 
 takes place in the direction in which the force acts. 
 
 248 
 
APPLICATION OF NEWTON'S LAWS 249 
 
 3. To every force there is an equal force in the opposite direc- 
 tion. 
 
 294. Meaning and Application of Newton's Laws. The 
 first law means that if a body is at rest, it has a tendency to 
 remain at rest. This is shown when you undertake to move 
 a table or some other heavy object, even though it be on cas- 
 ters. On the other hand, a body in motion tends to keep on 
 going in a straight line. This is illustrated by the skidding 
 of an automobile, either around corners or when the brakes 
 are set quickly. 
 
 The tendency which a body has to remain at rest, when at 
 rest, or to continue in motion, when in motion, is called 
 inertia. It is the inertia of your body which throws you 
 over in a street car when it turns a corner, or which jerks 
 you backward or forward when the car starts or stops 
 suddenly. 
 
 The second law means that the resulting motion is doubled 
 if the force is doubled, or multiplied by 3 if the force is multi- 
 plied by 3, etc. It also means that the object tends to move 
 in the direction in which the force acts. 
 
 To illustrate : If you throw a ball with a certain force, 
 it will have a certain quantity of motion ; but, if it is thrown 
 with twice the force, it will go twice as fast ; also it will go in 
 the direction in which it is thrown, if no other force acts 
 upon it. 
 
 The third law means that there is always a force, called the 
 reaction, which acts in the opposite direction to any given 
 force. 
 
 To illustrate this, consider your own weight. This force is 
 downward, but the floor pushes upward with the same force ; 
 otherwise you would go through the floor. You cannot take 
 hold of your shoe-tops and lift yourself, for every pound that 
 
250 
 
 DYNAMICS 
 
 FIGURE 277. A CLOTHES-LINE POST 
 WITH BALANCED FORCES. 
 
 you lift is counteracted by a pound in excess of your weight 
 which is pushed downward by your feet. 
 
 295. The Parallelogram of 
 Forces. When two forces act 
 -J? upon a body, the body cannot 
 move in both directions, but 
 moves in the direction of the 
 resultant of those two forces. 
 For example, a clothes-line 
 post, as in Figure 277, cannot 
 move in both the directions 
 AB and AC, but tends to move along the resultant AR, 
 which is somewhere between AB and AC. 
 
 To find the resultant of two forces such as those men- 
 tioned above we use what is called the parallelogram of forces. 
 First, lay off to scale lines representing the forces in both 
 amount and direction. 
 (Figure 279.) 
 
 For example, if the force 
 AB were 50 pounds, and the 
 force AC were 30 pounds, let 
 5 inches represent the 50 
 pounds and 3 inches repre- 
 sent the 30 pounds. Upon 
 these two sides construct a 
 parallelogram. The diagonal, 
 which is 5.83 inches, repre- 
 sents the resultant of 5.83 X 
 10 = 58.3 pounds. 
 
 In this way the result- 
 ant of any two forces 
 may be found. If the 
 original forces are laid 
 
 FIGURE 278. A LABORATORY EXPERIMENT 
 SHOWING BALANCED FORCES. 
 
APPLICATIONS OF PARALLELOGRAM OF FORCES 251 
 
 off to a certain scale, then the length of every line in the 
 figure represents the amount of force in that line. 
 
 296. Applications of Parallelogram of Forces. The 
 parallelogram of forces can be used to determine the tension 
 in the wires in picture-hanging. 
 
 R 
 
 20 
 
 A 50*= 5" B 
 
 Scale 1*=10* 
 
 FIGURE 279. THE PARALLELOGRAM 
 OF FORCES. 
 
 FIGURE 280, THE 
 PARALLELOGRAM OF 
 FORCES APPLIED TO 
 PICTURE HANGING. 
 
 Figure 280 shows a picture hanging from a hook in one of the usual 
 ways. The distance between the supporting screws in the picture is 
 20 in. The distance from the hook to the line of screws is 25 in. Find 
 the tension in each wire, if the picture weighs 10 pounds. 
 
 Method: 
 
 If the picture were supported from two hooks (A and B}, the wires 
 would each be 25 in. long and would support - 1 ? - = 5 pounds. 
 
 Since each line in the figure represents the amount of force in that 
 line, then 
 
 25 in. = 5 Ib. 
 1 in. = & of 5= ilb. 
 
 The actual wire CD = ^1 (AC) 2 + (AD) 2 = 
 
 -25 2 
 /. the tension in CD 
 
 -625 = 
 26.9 Xi = 5.38 Ib. 
 
 = 26.9+ 
 
 Problems 
 
 1. Find the tension in the wire of a picture hung from a hook which 
 is 12 in. above the line of the screws in the picture, if the two screws 
 are 18 in. apart and the picture weighs 8 Ib. 
 
252 DYNAMICS 
 
 2. What is the tension in a guy-wire for a clothes-line post, if the 
 post is 6 ft. high and the guy- wire is set 4 ft. from the base of the post, 
 the clothes-line having a tension of 75 Ib. ? 
 
 297. Velocity and Acceleration. Any body in motion 
 has a definite speed or velocity two terms meaning the same 
 thing. 
 
 Velocity is the time rate of motion. This means that the 
 number of units of distance passed over per unit of time is 
 velocity. 
 
 To say that the velocity of a train is 30 miles per hour 
 
 (sometimes written 30 '- } means it would travel 30 miles 
 hr. J 
 
 in one hour, if it ran at that rate of speed. Other units of 
 velocity are 
 
 ft. cm. km. 
 
 J , -, , etc. 
 
 sec. sec. hr. 
 
 If the speed of an object is the same continuously, it is 
 said to have uniform velocity. But if the velocity changes 
 it is said to be accelerated. 
 
 Acceleration is the change in velocity per unit time. For 
 example, if a body starts from rest and is going at the rate 
 
 of 5 - at the end of the first second ; 10 J! ^- L at the end of 
 sec. sec. 
 
 the second second ; 15 at the end of the third second, 
 sec. 
 
 etc., the motion is said to have an acceleration of 5 ft. per sec- 
 ond, per second, meaning that it has gained 5 - 1 ^ of velocity 
 
 sec. 
 
 every second. 
 
 Acceleration is either positive or negative, according as the 
 change in velocity is an increase or a decrease. 
 
UNIFORMLY ACCELERATED MOTION 253 
 
 The pull of gravity gives all bodies an acceleration down- 
 ward of 32.2 ft. per second, per second, or 980 cm. per second, 
 per second. This is called the acceleration due to gravity, 
 and is represented by the letter g. 
 
 298. Uniformly Accelerated Motion. When a body is 
 uniformly accelerated, it is very often desirable to find : 
 
 (1) The velocity (v) in terms of the acceleration (a) and 
 the time (t) which the body has traveled 
 
 v = at] 
 
 (2) The distance (S) which the body has traveled in terms 
 of the acceleration (a) and the time (t) which the body has 
 
 traveled 
 
 S = i at 2 ; 
 
 (3) The distance (d) which the body has traveled in any 
 particular second in terms of the acceleration (a) and the 
 second (t) in question 
 
 d = J a (2 t - 1) ; 
 
 (4) The velocity 0) in terms of the acceleration (a) and the 
 distance passed over (S) - 
 
 v z = 2 aS. 
 The following problems illustrate the use of these formulae : 
 
 Problem (1) : What is the velocity of an automobile at the end of 
 5 seconds, if it has an acceleration of 2 ft. per second, per second ? 
 Method : 
 
 v = at 
 
 .-. v = 2.5 = 10^- (ans.) 
 sec. 
 
 Problem (2) : How far will a train travel in 10 seconds, if it has an 
 acceleration of \ ft. per second, per second ? 
 
 Method : 
 
 S = \ off- 
 /. S = * . J. 10 2 = \ J - 100 = 25 ft. (ans.) 
 
254 DYNAMICS 
 
 Problem (3) : How far will a train travel during the 8th sscond after 
 starting, if it has an acceleration of ^ ft. per second, per second ? 
 Method: 
 
 d = \ a (2 t - 1) 
 
 /. d = }. }(2. 8-1) =1-1(16-1) 
 = 1-i. 15 =3f//. (ans.) 
 
 Problem (4) : What is the velocity of an automobile after it has gone 
 25 ft., if it has an acceleration of 2 ft. per second, per second? 
 Method : 
 
 v* = 2 aS 
 /. v* = 2 . 2 25 = 100 
 
 v = VlOO = 10 -^- (ans.) 
 
 All the examples above were given in the English system. 
 The same formulae and methods of solution are used in the 
 metric system. Instead of feet use centimeters. 
 
 Since the pull of the earth gives all bodies a uniform accel- 
 eration, these same formulae apply to freely falling bodies. 
 
 For falling bodies the above formulae may be written and 
 used in the special forms : 
 
 v = gt. 
 S = \ cjt\ 
 d = g(2t- 1). 
 v* = 2 gS. 
 
 299. Momentum. The quantity of motion which a body 
 possesses is called momentum. It is measured by multiplying 
 the mass of a body by its velocity. Thus an automobile 
 
 7777 
 
 weighing 2500 Ib. and going 20 ~^ has 2500 X 20 = 50,000 
 
 hr. 
 
 Ib.-miles per hour of momentum. 
 
 Likewise, a baseball weighing 5 oz. and going 100 ft. per 
 sec. has a momentum of ^ 100 = 31 J Ib.-ft. per sec. 
 
 There is no definite unit for momentum, so terms such as 
 
FORCE TO OVERCOME INERTIA 255 
 
 lb.-mi. per hr., Ib.-ft. per sec., etc., have to be used. In com- 
 paring momenta, care must be taken that they are ex- 
 pressed in the same units. 
 
 300. Force to Overcome Inertia. By Newton's first law 
 of motion every body tends to remain at rest or to continue in 
 a straight line at a uniform speed unless some force acts 
 upon it ; hence a force setting a body in motion (or stopping 
 its motion) must overcome this inertia, together with the 
 other forces acting upon the body, such as friction, weight, 
 etc. 
 
 The force to overcome inertia is proportional to both the 
 mass of the body and the acceleration given it. Thus : 
 
 F = Ma (1) 
 
 F = ^- (2) 
 
 9 
 
 If the mass is given in grams and the acceleration in centi- 
 meters per second, per second, equation (1) gives the force in 
 dynes. If the weight is given in pounds or grams and the accel- 
 eration in feet per second, per second, or centimeters per second, 
 per second, equation (2) gives the force in pounds or grams re- 
 spectively. 
 
 Thus a girl weighing 1 10 Ib. and standing in an elevator going down 
 with an acceleration of 2 ft. per second, per second, will apparently 
 weigh 103.2 b. 
 
 p= Wa 
 
 32.2 
 
 .'. she weighs 6.8 Ib. less than 110 = 103.2 Ib., her apparent weight. 
 
 If the elevator were going up with an acceleration of 2 ft. per sec- 
 ond, per second, she would weigh 6.8 Ib. more, or 110 + 6.8 = 116.8 Ib., 
 her apparent weight. 
 
256 
 
 DYNAMICS 
 
 The force required to overcome the inertia of any body can 
 be found in a similar manner. 
 
 301. Force to Overcome Friction. Excepting the mo- 
 tions of the heavenly bodies, all motions are opposed 
 by a certain amount of friction, so that the force 
 changing the motion of a body must overcome the fric- 
 tion besides overcoming inertia and other forces, such as 
 weight, etc. 
 
 In calculating the force necessary to produce motion of a 
 body, each part must be calculated separately and the results 
 added. 
 
 302. Centrifugal Force. Any body moving in the cir- 
 cumference of a circle (Figure 281) tends to fly away from 
 
 the center. This is due to Newton's 
 first law of motion. Explain. 
 
 The force tending to throw the body 
 away from the center is called the 
 centrifugal force. 
 
 A pail of water may be swung in a 
 vertical plane without spilling the 
 water on account of the centrifugal 
 force. Centrifugal force causes ve- 
 hicles to skid around corners. The 
 cream separator uses centrifugal force to separate the cream 
 from the milk. This can be done because cream is lighter 
 than plain milk. 
 
 303. Energy of Motion. All bodies in motion have 
 energy due to that motion. An automobile moving 
 60 mi. per hour will do more damage, if it smashes into 
 a building, than if it were running 10 mi. per hour. A 
 hammer swung with the arm will drive a nail farther than 
 if the hammer were just laid on the nail. These are all 
 
 FIGURE 281. CENTRIF- 
 UGAL FORCE. 
 
GRAVITATION 257 
 
 illustrations of energy of motion, usually called Kinetic 
 Energy (KE). 
 
 KE->. 
 
 The above formula will give the kinetic energy in foot- 
 pounds if W is expressed in pounds; v = feet per second; 
 and g = 32.2. 
 
 304. Gravitation. Every bit of matter in the universe 
 exerts a pull on every other bit of matter. This pull is called 
 gravitation. 
 
 The earth, being a very large bit of matter, exerts a pull 
 on all objects on or near it. This pull is called the weight 
 of the object. 
 
 Newton formulated three laws, called Newton's three laws 
 of gravitation. They are : 
 
 1 . The weight of an object at any given place is directly pro- 
 portional to its mass. 
 
 2. The weight of an object above the surface of the earth is 
 inversely proportional to the square of the distance from the 
 center of the body to the 
 
 center of the earth. 
 
 3. The weight of a body 
 below the surface of the 
 earth is directly propor- 
 tional to the distance be- 
 
 FIGURE 282. ILLUSTRATING THE SECOND 
 
 tween the center of the body LAW OF GRAVITATION. 
 
 and the center of the earth. 
 
 The first law needs no explanation. The second law can 
 be made more clear by the use of Figure 282. 
 
 It will be seen that the farther the body is away from the 
 earth, the fewer are the lines of gravitation which pass 
 
258 
 
 DYNAMICS 
 
 through it. This is why the pull gets less as the distance 
 gets greater. 
 
 Figure 283 illustrates the third law. A body inside the 
 earth has part of the earth (A ) pulling to the right, while the 
 
 other part (B) pulls to the 
 left. Thus we see that the 
 resulting force becomes 
 smaller as the distance be- 
 tween the center of the body 
 and the center of the earth 
 becomes smaller. 
 
 305. Pendulum. A pen- 
 dulum is a body supported 
 from a pivot and free to 
 swing because of its weight. 
 (Figure 284.) L represents 
 the length of the pendulum ; 
 
 a, the amplitude of the swing; g, the acceleration due to 
 
 gravity ; t, the time of the pendulum the time it takes the 
 
 pendulum to move from one side of the swing to the other. 
 
 There are four laws governing the time of a pendulum : 
 
 1. The time is independent 
 of the mass. 
 
 2. The time is independent 
 of the amplitude. 
 
 3. The time is directly pro- 
 portional to the square root of 
 the length. 
 
 4. The time is inversely proportional to the square root cf 
 the acceleration due to gravity. 
 
 The pendulum is used to regulate clocks, etc. To make 
 a clock run faster, shorten the pendulum. 
 
 FIGURE 283. ILLUSTRATING THE 
 THIRD LAW OF GRAVITATION. 
 
 FIGURE 284. THE PENDULUM. 
 
CHAPTER XXIV 
 MECHANICS OF FLUIDS 
 
 306. The Three States of Matter. All matter exists in 
 one or more of three states solid, liquid, or gas. Some 
 substances are found in all three states. Water is the most 
 common of these. Other substances existing in the three 
 states are iron, copper, lead, mercury, etc. 
 
 The apparent difference between the three states of matter 
 is as follows : 
 
 1. A solid has a definite shape and volume. 
 
 2. A liquid has a definite volume, but takes the shape of 
 the containing vessel. 
 
 3. A gas has neither a definite shape nor volume, but 
 takes the shape of the containing vessel and fills it com- 
 pletely. 
 
 The theoretical difference between the three states of 
 matter depends upon the molecular construction of the 
 substance in these different states. 
 
 In a solid, the molecules are close together and are held 
 firmly together by a force called cohesion. This force is 
 sufficient to keep the molecules from changing their relative 
 positions, but it allows them to vibrate. 
 
 In a liquid, the molecules are farther apart, and the force 
 of cohesion is not so great. The molecules can slide over 
 one another, but still the force is great enough to keep them 
 from separating. 
 
 259 
 
260 
 
 MECHANICS OF FLUIDS 
 
 In a gas, the molecules are far apart, the force of cohesion 
 is too small to count, and the molecules fly about with perfect 
 
 freedom, bumping against one 
 another and the sides of the con- 
 taining vessel. 
 
 307. Gases and Liquids through 
 Pipes. The fact that gases and 
 liquids have no definite shape 
 makes it possible to deliver them 
 through pipes. 
 
 Consider the two pipes (a) and 
 (b) (Figure 285) filled with water 
 coal, respectively, and then a force put 
 In the first case, the water molecules 
 
 FIGURE 285. LIQUIDS AND 
 SOLIDS IN PIPES. 
 
 and chunks of 
 on both of them, 
 would slide over one another at the bend 
 of the pipe, and so would flow around the 
 bend ; but, in the second case, the chunks 
 of coal would not slip past one another, 
 but would push against the end of the 
 pipe and would clog the pipe. A gas 
 would act in the same way as the water. 
 
 Thus we see why it is possible to de- 
 liver gas and water through pipes, but 
 why we have to haul our coal, wood, and 
 all other solids. 
 
 308. Pressure. Figure 287 shows a 
 cylinder with water in it, and a piston 
 (K) being forced against the water with 
 a force of 100 Ib. 
 
 It will be seen that the water will 
 push on the end of the cylinder with a 
 force of 100 Ib. If the end of the cylinder 
 
 FIGURE 286. PRES- 
 SURE is USED IN 
 THE FIRE EXTIN- 
 GUISHER 
 
THE HYDRAULIC ELEVATOR 
 
 261 
 
 has an area of 25 sq. in., this 100 Ib. will be distributed over 
 the total 25 sq. in. Thus each square inch will receive 
 
 (P, Figure 287.) 
 
 = 4 Ib. 
 
 The force on the one square 
 inch is called the pressure. 
 
 Pressure is the force per unit 
 area. It is found by dividing the 
 force by the area of the surface. 
 
 FIGURE 287. MEANING OF 
 THE TERM "PRESSURE." 
 
 Force applies to the total area, while pressure applies 
 only to unit area. 
 
 309. Pascal's Law. In Figure 287 the water would 
 press not only on the end of the cylinder, but also on 
 the sides ; that is, every square inch of surface would also 
 
 have a force of 4 Ib. ; 
 or, as we say, the pres- 
 sure would be 4 Ib. per 
 square inch. 
 
 Pascal stated these 
 facts in the form of a 
 law : The pressure on a 
 confined liquid is trans- 
 mitted undiminished in 
 all directions, and acts at 
 right angles to all sur- 
 faces. 
 
 310. The Hydraulic 
 Elevator. The hydrau- 
 lic elevator (Figure 288) 
 
 FIGURE 288. THE HYDRAULIC ELEVATOR, uses the principle ex- 
 
262 
 
 MECHANICS OF FLUIDS 
 
 pressed by Pascal's Law. A large piston (P) on the bottom 
 of the elevator fits into a cylinder in the ground. A pipe 
 ( K) runs down the side of the cylinder and enters it at the 
 bottom. 
 
 To go up, the stopcock (S) is turned so that water enters 
 the pipe (K) from the water-main (a). The water flows 
 down the pipe ( K) and into the cylinder, pushing up on the 
 piston (P). Since the pressure in the water-main is about 
 60 Ib. per square inch, there is also a pressure of 60 Ib. per 
 square inch exerted on the bottom of the piston. 
 
 If this piston contains 100 sq. in., the elevator will be 
 pushed up with a force of 60 X 100 = 6000 Ib. 
 
 To come down, the stopcock is turned so that no more 
 water can get into the pipe, but the pipe is opened to the 
 outlet or sewer. The weight of the ele- 
 vator pushes the water out, and the 
 elevator comes down slowly. 
 
 311. Breaking Jugs or Fruit Jars. 
 Jugs and fruit jars are very often broken 
 by filling them with a liquid and then 
 forcing in the stopper or pressing on the 
 lid. The force is applied to a small area, 
 and this produces a large pressure. This 
 pressure being transmitted to the total 
 area of the sides and bottom is sufficient 
 to break the jar. 
 
 312. A Liquid in an Open Vessel. 
 FIGURE 289. PRES- When a liquid is in an open vessel, the 
 
 SURE IN AN OPEN . . 
 
 VESSEL. pressure acts in all directions, just as in 
 
 the closed vessel, but the amount of 
 pressure depends on the weight of the liquid above, and 
 not on an outside force. 
 
 
A LIQUID IN AN OPEN VESSEL 
 
 263 
 
 Figure 289 shows water in a rectangular tank 2 ft. square and 6 ft. 
 
 deep. It is seen that the total weight of the water rests on the bottom. 
 
 Since water weighs 62^ Ib. per cubic foot, the force on the bottom is 
 
 2 x 2 X 6 = 24 cu. ft. 
 24 X 62| = 1500 Ib. 
 
 Since the 1500 Ib. is on 4 sq. ft., 
 
 1500 
 
 Pressure = = 375 Ib. per square foot, 
 
 4 
 
 375' 
 
 or Pressure = - - = 2.6 Ib. per square inch. / 
 144 
 
 It has been proven that the pressure on the bottom of a 
 vessel has nothing to do with the shape of the vessel, but 
 depends solely upon the depth of 
 the liquid and the area of the 
 base. 
 
 Problem: Find the pressure on the 
 bottom of the irregular vessel rilled 
 with water. (Figure 290.) 
 
 Assume a column of water 6 ft. 
 high standing on a base one foot 
 square. 
 
 Then its 
 weight = 1 X 1 X 6 X 62| = 375 Ib. 
 
 Thus the pressure is 375 Ib. per 
 square foot, regardless of the shape of 
 the vessel. 
 
 = 2.6 Ib. per square inch. 
 144 
 
 Rule : To find the pressure in pounds per square foot of a 
 liquid in an open vessel, multiply the height (h) in feet, by the 
 weight of the liquid per cubic foot (D). 
 
 FIGURE 290. PRESSURE IN AN 
 
 IRREGULAR 
 VESSEL. 
 
 SHAPED OPEN 
 
 P = h-D. 
 
264 MECHANICS OF FLUIDS 
 
 If the pressure is wanted in pounds per square inch, 
 divide by 144. 
 
 h-D 
 
 P = 
 
 144 
 
 Problem : What is the pressure in pounds per square inch 20 ft. 
 below the surface of water ? 
 
 144 
 
 P = = 8.68 pounds per square inch. 
 
 144 
 
 Problem: What is the pressure 3 ft. under mercury, if it is 13.6 
 times as heavy as water ? 
 
 P =*J?. 
 144* 
 
 D 3 X 62.5 X 13.6 
 
 P = - - = 17.7 pounds per square inch. 
 
 J.TX 
 
 Problems 
 
 1. The water in a tank stands 18 ft. above a faucet. What is the 
 pressure at the faucet? 
 
 2. How high does the water rise in the spout of a teakettle? 
 
 3. Could a large tank of water, on a level with the second story, and 
 a hose, be used to fight fire on the third story ? Why ? - 
 
 4. What is the pressure on a deep-sea diver when he goes down 
 180 ft., if sea water is 1.1 times as heavy as fresh water? 
 
 5. What is the pressure at a faucet on the third floor, if the pressure 
 in the water-main in the basement 45 ft. below is 60 Ib. per square inch ? 
 
 313. Air-Pressure. Air, like water, has weight, but not 
 so great as water. The atmosphere is estimated to reach 
 from 300 to 400 miles above the surface of the earth ; and 
 all this great weight of air above is resting on the lower 
 layers, producing a pressure just as the weight of the water 
 above produces a pressure on the water beneath. 
 
THE SIMPLE BAROMETER 
 
 265 
 
 At sea-level the air-pressure is normally 14.7 Ib. per square 
 inch. Places above sea-level have less pressure, because 
 there are fewer layers of air resting on them. The upper 
 layers are not so heavy, since they are less compressed, 
 consequently the pressure falls rapidly 
 as you rise above sea-level. 
 
 The air-pressure is measured by an 
 instrument called the barometer. 
 
 314. The Simple Barometer. A 
 simple barometer may be constructed in 
 this way : Take a glass tube about 32 in. 
 long, closed at one end, and fill it with 
 mercury. Then invert it in a cup of 
 mercury, being careful not to let in any 
 air. (Figure 291.) 
 
 The mercury will fall away from the top 
 of the tube, and stand at 30 in., more or 
 less, according to the air-pressure. The 
 space above the mercury in the tube is 
 almost a vacuum, since there is nothing 
 in it except a little mercury vapor. 
 
 The pressure of the mercury in the tube 
 is exactly balanced by the pressure of 
 the air on the surface of the mercury in the cup. This 
 pressure can be expressed in inches of mercury, centimeters 
 of mercury, pounds per square inch, or grams per square 
 centimeter. 
 
 If the pressure is wanted in inches of mercury, or centimeters 
 of mercury, it is read directly from the column of mercury ; 
 but if it is wanted in pounds per square inch, or grams per 
 square centimeter, it has to be calculated as one calculates 
 the pressure in a liquid. 
 
 FIGURE 29 1. THE 
 SIMPLE BAROMETER. 
 
266 
 
 MECHANICS OF FLUIDS 
 
 FIGURE 292. THE WEIGHT OF THE AIR MAKES IT POSSIBLE TO FLY. 
 
 Example : What is the pressure in pounds per square inch, when the 
 barometer reads 28 in. ? 
 
 h X D 
 
 144 
 
 h = ft. 
 12 
 
 D = 62.5 X 13.6 = 850 Ib. per cubic foot. 
 . p = HX850 = 2|^850 = 13 , b per square inch 
 
 144 
 
 12 X 144 
 
 315. The Commercial Barometer. The commercial ba- 
 rometer, which is used for accurate readings of the air- 
 pressure, is a modified form of the simple barometer. 
 
 Figure 293 is a diagram of this instrument. The glass 
 tube is inclosed in a brass tube having part of it cut away 
 so that the glass tube can be seen at the upper end. The 
 
THE COMMERCIAL BAROMETER 
 
 267 
 
 mercury cup has a rubber or leather bottom, so that it can 
 be raised or lowered by a set-screw (a). 
 
 A small movable scale (I 7 ), called a vernier, is operated 
 by a set-screw (b), and slides at the side of a scale (S) marked 
 off in inches and tenths 
 of inches. 
 
 To make a reading : 
 First, adjust the mer- 
 cury in the cup with 
 V the set-screw (a) so that 
 the top of the mercury 
 just touches the point 
 of the ivory plug (P). 
 This point is the zero 
 of the scale (S). 
 
 Second, slide the ver- 
 nier (V) by means of 
 screw (6) so that the 
 bottom of the vernier 
 is just at the top of 
 the mercury in the 
 tube. 
 
 Third, read the scale 
 (S) and the vernier (F). 
 Figure 295 shows an 
 FIGURE 293. DIAGRAM enlarged drawing of the 
 
 FIGURE 294. 
 PHOTOGRAPH OF 
 
 % ' A BAROMETER. 
 
 nier (V). 
 
 First, note where the zero of the vernier (V) comes on the 
 scale (S). In the figure it- is past 28.3, and not quite to 28 A ; 
 then the scale reading is the smaller of these, or 28.3. 
 
 Second, note where a mark on the vernier (V) coincides 
 
268 
 
 MECHANICS OF FLUIDS 
 
 with a mark on the scale (S). In the 
 figure it is 5 on the vernier. (It 
 makes no difference which one on the 
 scale.) This determines the next 
 figure to be annexed to the scale 
 reading, which makes the completed 
 reading. Thus the reading in Figure 
 295 is 28.3 with 5 annexed, or 28.35". 
 316. Weather Maps. Weather 
 conditions are usually accompanied 
 by certain air-pressure and tempera- 
 ture changes. Knowing this fact, the 
 government has a branch of the De- 
 partment of Agriculture called the 
 United States Weather Bureau, part of whose duties it is 
 to make weather maps and from them send out weather 
 forecasts. 
 
 FIGURE 295. ENLARGED 
 DRAWING OF THE VER- 
 NIER OF A BAROMETER. 
 
 FIGURE 296. A TYPICAL WEATHER MAP. 
 
THE LIFT-PUMP 
 
 269 
 
 The Weather Bureau has stations established all over 
 the United States, and every 24 hours these stations report 
 to the head office at Washington, D. C., on the weather 
 conditions. Some of the things reported are barometer 
 reading (reduced to normal conditions), temperature, clear, 
 cloudy, rain, or snow, direction and velocity of wind. These 
 reports are then summarized and reported back to all the 
 stations. Each station then draws up a weather map and 
 forecasts the local weather for the next 48 hours. 
 
 A weather map (Figure 296) is made by drawing heavy 
 lines, called isobars, through all stations of equal pressure ; 
 dotted lines, called isotherms, through all stations of equal 
 temperature ; an arrow at 
 each station, indicating 
 the direction of the wind ; 
 and small circles marked 
 to show whether it is 
 clear, partly cloudy, cloudy, 
 rain, or snow, respectively. 
 The cloudy areas are 
 shaded, the low pressure 
 areas are marked " LOW," 
 and the high pressure areas 
 are marked " HIGH." 
 
 For a further study of 
 the weather map read 
 some good physical geog- 
 raphy. 
 
 317. The Lift-Pump. - 
 Figure 296 is a diagram of 
 the lift-pump, which is an 
 application of air-pressure. FIGURE 297. THE LIFT-PUMP. 
 
270 
 
 MECHANICS OF FLUIDS 
 
 The piston (P) works air-tight in the cylinder of the 
 pump. When the piston is drawn up, the valve (B) closes, 
 and a partial vacuum is left behind the piston. The air- 
 pressure, acting on the surface of the water (C, C) in the 
 well, forces the water up to fill this partial vacuum. 
 
 On the down stroke of the piston, valve (A) closes and 
 (B) opens. After several strokes, the water reaches up 
 into the pump. The operation is continued, and the water 
 flows through the valves, instead of air. When the water 
 gets high enough, it runs out of the spout. 
 
 Sometimes the pump will not start, but has to be 
 " primed." This is because the valves or piston will not 
 hold air, so water has to be put in to make them air-tight. 
 
 This kind of pump can be used only to pump water from 
 shallow wells and cisterns, since the air-pressure will raise 
 
 water only 34 ft. 
 under ideal condi- 
 tions ; and only 
 about 28 ft., practi- 
 cally. 
 
 318. The Force- 
 Pump. The force- 
 pumps used to drive 
 water into mains, 
 pressure tanks, and 
 fire hose are much 
 like the lift-pump, 
 only instead of allow- 
 ing the water to flow out of the spout of its own accord, it 
 is confined in the top of the pump and forced out. (Figure 
 298.) 
 
 An air-chamber (C) is attached to the pump, so that the 
 
 FIGURE 298. THE FORCE-PUMP. 
 
OTHER APPLICATIONS OF AIR-PRESSURE 271 
 
 d 
 
 air, when compressed, acts as a spring to keep the pump 
 from bursting and to keep the water flowing between strokes. 
 
 319. The Siphon. Figure 299 represents a siphon, 
 which consists of a tube with its ends in water, at different 
 levels. If the tube is completely filled with liquid, the 
 liquid will run through the tube from the higher level to 
 the lower. 
 
 The air-pressure on the surface of the water (c) tends to 
 lift the water 34 ft. in the tube. Also the same air-pressure 
 
 at (d) tends to lift the x ^ 
 
 water 34 ft. on the 
 other side of the tube. 
 But the water presses 
 downward on the two 
 sides with a pressure 
 of a ft. and b ft., re- 
 spectively. This 
 leaves a pressure of 
 34 - a and 34 - 6, 
 respectively. Since b 
 is greater than a, the 
 greater pressure is to- 
 wards (6), and the water runs in that direction. The 
 greater the difference in (a) and (b), the faster the liquid 
 will flow. 
 
 The siphon is used for getting acids out of carboys, cider 
 out of barrels, water out of tanks, etc. 
 
 320. Other Applications of Air-Pressure. Drawing soda 
 water through a straw could not be done if it were not for 
 air-pressure. The air is drawn out of the straw, leaving a 
 partial vacuum, and the air-pressure forces the soda water 
 up to take the place of the air. 
 
 FIGURE 299. THE SIPHON. 
 
272 MECHANICS OF FLUIDS 
 
 Ordinary breathing depends upon air-pressure. The 
 muscles of the chest act and make the cavity in which the 
 lungs are located larger. This reduces the pressure in the 
 lungs, and the air is forced in to equalize the pressure. 
 
 Fruit-jar lids are often hard to get off on account of the 
 pressure of the air. When the jar is sealed, the liquid 
 and air in the jar are hot. On cooling, they both contract, 
 thus reducing the pressure inside the jar. The outside 
 air-pressure then holds the lid on very tight. Corks 
 drawn into bottles in the same way are often hard to get 
 out. 
 
 Air-pressure enables the house-fly to stick to the ceiling. 
 His feet have tiny pads on them, and when he sets them 
 down all the air is squeezed out from under them, and then 
 the pressure of the air makes them stick to the wall or ceil- 
 ing. A fly will fall off the side of a bell jar and will crawl 
 around on the bottom, if he is put inside and the air is 
 pumped out. 
 
 " Suction soles " on gymnasium shoes are similar to the 
 foot-pads of the fly. The soles have holes, or depressions, 
 on the bottoms, and when the weight of the wearer comes 
 down on them, the air is squeezed out, and then the air- 
 pressure outside tends to make them " stick." " Suction 
 tread " tires work on exactly the same principle. 
 
 321. Boyle's Law. All gases can be compressed by 
 putting pressure on them. That is, more and more gas may 
 be forced into the same space, or a certain amount of gas 
 may be forced into a smaller space. In either case the 
 pressure in the gas is increased. 
 
 On the other hand, a gas will expand if allowed space to 
 do it in. In this case the pressure is decreased. 
 
 Boyle stated these facts in a law, called Boyle's Law. 
 
SURFACE TENSION 273 
 
 The volume of a gas at a constant temperature varies in- 
 versely as the pressure exerted upon it. 
 
 This means that if the pressure is doubled, the volume is 
 halved; or if the pressure is halved, the volume is doubled, 
 etc. 
 
 The law applies to natural or artificial gas used as a fuel. 
 The higher the pressure, the more gas there is squeezed into 
 a cubic foot ; and, since gas is usually sold by the cubic foot, 
 the pressure affects the cost of the gas. 
 
 This change in cost due to change in pressure is not as 
 great as some people think. An illustration will show how 
 much the effect is. 
 
 Suppose the normal pressure is 6 oz. per square inch. 
 (This is the average pressure maintained for natural gas.) 
 This means 6 oz. per square inch above atmospheric pres- 
 sure. Since atmospheric pressure is about 14.5 lb., or 232 
 oz., per square inch, this makes the actual pressure in the 
 gas main 232 + 6 = 238 oz. per square inch. 
 
 Now, if the gas pressure should fall 50 per cent, or to 3 oz. 
 above atmospheric pressure, the actual pressure in the main 
 would be 238 3 = 235 oz. per square inch. 
 
 235 
 Thus there will be - - as much gas in a cubic foot as 
 
 there was at the normal pressure of 6 oz. per square inch. 
 
 The inflation of tires with air under pressure is also an 
 application of Boyle's Law. 
 
 322. Surface Tension. All liquids act as if they have a 
 " skin " or " membrane " stretched over their surfaces. A 
 needle may be laid on the surface of water (Figure 300), if 
 care is taken. The surface of the water is curved under the 
 needle just as if there were a cover over the water. This 
 apparent " skin " or membrane is called surface tension. 
 
274 
 
 MECHANICS OF FLUIDS 
 
 The fact is, there is no membrane on the liquid. The 
 molecules at the surface are exactly the same as inside the 
 
 liquid. Surface tension is ex- 
 plained as follows : 
 
 Consider a molecule of water 
 
 FIGURE 300. A NEEDLE 
 LYING ON WATER, 
 
 FIGURE 301. SURFACE 
 TENSION EXPLAINED. 
 
 (M, Figure 301) at the surface of the water, 
 in quadrants (a) and (d) 
 attracts the molecule (m) 
 and tends to pull it down- 
 ward. As there is no 
 water in (b) and (c), 
 but only air, which 
 attracts the molecule (m) 
 but slightly, the result- 
 ing effect is for the mole- 
 cule (m) to be pulled 
 toward the center of the 
 water, and every other 
 molecule on the surface 
 pulled toward the 
 
 The water 
 
 is 
 
 center in the same way. 
 
 FIGURE 302. WATER IN CONTACT WITH 
 GLASS. 
 
CAPILLARITY 
 
 275 
 
 This gives the effect of a stretched covering over the surface 
 of the liquid. 
 
 323. Capillarity. Capillarity is an application of sur- 
 face tension. Figure 302 shows water in contact with glass. 
 The water against the glass 
 is curved up ; because 
 glass has a greater attrac- 
 tion for water than water 
 has for water; therefore 
 the glass in quadrant (c) 
 pulls the molecule of water 
 (m) more than the water 
 in quadrant (a). Also the 
 glass in (6) pulls (m) more 
 than does the air in (d). 
 This makes the surface of 
 the water curve as shown 
 
 in the figure. 
 
 FIGURE 303. MERCURY IN CONTACT 
 WITH GLASS. 
 
 Figure 303 shows mer- 
 
 cury in contact with glass. The mercury against the glass 
 is curved down. Mercury attracts mercury more than 
 glass attracts mercury, therefore the mercury in quadrant 
 (a) pulls the molecule of mercury (m) 
 more than the glass in quadrant (c). 
 Also the glass in (6) attracts (m) more 
 than the air in (d). Thus the sur- 
 face curves downward as shown in the 
 figure. 
 
 When a tube is put into a vessel of 
 water, the water creeps up the tube, as 
 
 A GLASS TUBE. put into a vessel of mercury, the mercury 
 
276 
 
 MECHANICS OF FLUIDS 
 
 creeps down the tube. (Figure 305.) This is called 
 
 capillarity. 
 
 The steps in this process are as follows : 
 
 When the tube is placed 
 in the water (Figure 304), 
 the surface of the water 
 curves up the glass; but 
 since the surface tension on 
 the water acts like a rubber 
 covering, the surface straight- , 
 ens out; and then curves 
 again. This alternation is 
 kept up until the weight of 
 water in the tube is so great 
 that the surface tension is 
 not able to lift it and 
 straighten out the surface. 
 
 In the case of mercury and 
 glass the mercury is pressed 
 down (Figure 305), the pro- 
 cess being the same as for 
 water, except that the sur- 
 face curves in the opposite 
 direction. 
 
 324. Other Applications of 
 Surface Tension. Rain- 
 drops become spherical on 
 
 account of surface tension. The elastic surface tends to pull 
 
 all molecules towards the center, thus producing a sphere. 
 Drops of water on a greased surface become spherical for 
 
 the same reason. Similarly, drops of mercury on a table or 
 
 your hand become spherical. 
 
 \ 
 
 FIGURE 305. How MERCURY CREEPS 
 DOWN A GLASS TUBE. 
 
ARCHIMEDES' PRINCIPLE 
 
 277 
 
 Soap-bubbles are thin films of soapy water with a double 
 surface tension one on the inside, and one on the outside. 
 Sometimes you can see the water run down between the 
 two surfaces. 
 
 The fact that the white of an egg has a high surface tension 
 makes it possible to " beat " it into a white fluffy mass. 
 This fluffy mass is made up of thousands of tiny bubbles 
 which depend on surface tension for their existence. 
 
 Oil is sometimes poured on stormy 
 seas to stop the breaking of the 
 waves and thus save the ship. The 
 three surface tensions act as a 
 blanket over the water. Explain 
 why there are three surface tensions. 
 
 325. Archimedes' Principle. - 
 Archimedes formulated the follow- 
 ing principle : 
 
 A body immersed in a fluid loses in 
 weight an amount equal to the weight 
 of the fluid displaced. 
 
 This principle can be demon- 
 strated as follows : Suppose a cube 
 1 ft. on an edge be immersed in water so that the top of 
 the cube is 5 ft. below the surface. (Figure 306.) Then 
 the bottom of the cube is 6 ft. below the surface. The force 
 downward on the top of the cube equals 
 
 FIGURE 306.- ARCHIMEDES' 
 PRINCIPLE VERIFIED. 
 
 h- D-A 
 
 5 X 62^ X 1 = 
 
 Ib. 
 
 The force upward on the bottom of the cube equals 
 
 F = h'D-A 
 F = 6 X 62 X 1 
 
 375 Ib. 
 
278 MECHANICS OF FLUIDS 
 
 This leaves a force upward of 375 - 31 2i Ib. = 62^ Ib. 
 But 62^ Ib. is the weight of a cubic foot of water, which is 
 also the volume of the cube. 
 
 The illustration above assumed that the body was com- 
 pletely submerged. If the weight of the body is less than 
 the weight of an equal volume of liquid, then the body will 
 sink to a depth where it displaces a weight of liquid equal 
 to the weight of the body. 
 
 For example, if a body of one cubic foot weighs 40 Ib., it 
 
 will sink in water until it displaces 40 Ib. of water, or 
 
 cu. ft. 
 
 Thus a body heavier than a liquid sinks, and one lighter 
 than a liquid floats. 
 
 326. Applications of Archimedes' Principle. A stone 
 submerged in water is much easier to lift than one out of 
 water. 
 
 A person in water weighs very little. This makes swim- 
 ming possible. Why does the swimmer keep as much of 
 his body under water as possible ? 
 
 An egg will sink in fresh water but will float in salt water. 
 Explain. 
 
 Grapefruit and oranges may be tested for juiciness by 
 dropping them into water. If they are juicy and heavy, they 
 will float very low in the water, but if dry and light, they will 
 float high. 
 
 A ship sinks in water until the weight of the water dis- 
 placed equals the weight of the ship and its cargo. That is 
 the reason why an empty freighter rides high and a loaded 
 one rides low in the water. 
 
 327. Density and Specific Gravity. The term density 
 means the mass per unit volume. A cubic foot of water 
 
PROBLEMS 279 
 
 contains G2| lb., and a cubic centimeter contains 1 gram. 
 Therefore the density of water is 62| lb. per cubic foot, or 1 
 gram per cubic centimeter. 
 
 Specific gravity is the ratio of the mass of a body to the mass 
 of an equal volume of water. 
 
 P . - . mass of body 
 
 specific gravity = 
 
 mass of equal vol. of water' 
 
 Specific gravity is a comparison of the density of a body 
 to the density of water. 
 
 Since the density of water in the metric system is nu- 
 merically 1 (1 gram per cubic centimeter), the specific grav- 
 ity and the density of a body in that system are numerically 
 equal. 
 
 By the use of the table on the next page the weight of any 
 certain volume of a substance can be found, or the volume 
 of any certain weight can be found. 
 
 Example : What is the weight of 25 cu. ft. of copper ? 
 From the table : 1 cu. ft. copper = 550.6 lb. 
 
 25 cu. ft. = 25 X 550.6 = 13765 lb. 
 
 Example : What is the volume of 1000 lb. of cast iron ? 
 From the table : 1 cu. ft. cast iron = 449 lb. 
 
 2.23 cu. ft. 
 
 449 
 
 Problems 
 
 1. What is the weight of a cedar chest that is made of 2 cu. ft. of 
 lumber ? 
 
 2. If a gold chain weighs 30 grams, how many cubic centimeters of 
 gold does it contain ? 
 
 3. Why is cork used in life preservers ? 
 
 4. A gallon contains 231 cu. in., and a cu. ft. contains 1728 cu. in. 
 What is the weight of a gallon of water ? 
 
280 
 
 MECHANICS OF FLUIDS 
 
 TABLE OF DENSITIES AND SPECIFIC GRAVITIES OF SOME 
 SUBSTANCES 
 
 
 j 
 
 DENSITY 
 
 . 
 
 SUBSTANCE 
 
 Lb. per 
 Cu. Ft. 
 
 Gms. per c.c. 
 
 SPECIFIC GRAVITY 
 
 Ash (dry) . . . . 
 
 43.7 
 
 .70 
 
 .70 
 
 Ash (green) 
 
 52.8 
 
 .84 
 
 84 
 
 Acetic Acid 
 Alcohol ...... 
 
 66.4 
 50.0 
 
 1.062 
 .80 
 
 1.062 
 .80 
 
 Aluminum 
 
 165.6 
 
 2.65 
 
 265 
 
 Beech ........ 
 Cedar 
 
 53.2 
 35.0 
 
 .69 to .852 
 .561 
 
 .69 to .852 
 .561 
 
 Cork . 
 
 15.0 
 
 24 
 
 24 
 
 Copper (cast) .... 
 Copper (sheet) .... 
 Brass 
 
 550.6 
 555.0 
 527 5 
 
 8.81 
 8.88 
 8 38 to 8 44 
 
 8.81 
 8.88 
 8 38 to 8.44 
 
 Gold 
 
 12188 
 
 19 50 
 
 19 50 
 
 Hydrochloric Acid . 
 Iron (wrought) .... 
 Iron (cast) 
 
 75.2 
 480-0 
 449 
 
 1.22 
 7.68 
 720 
 
 1.22 
 7.68 
 720 
 
 Lead ,' . 
 Maple ... . 
 
 709.6 
 460 
 
 11.36 
 75 
 
 11.36 
 75 
 
 Mercury 
 
 8500 
 
 136 
 
 136 
 
 Milk ....... 
 
 64 5 
 
 1.032 
 
 1.032 
 
 Nitric Acid 
 
 763 
 
 1 22 
 
 1.22 
 
 Oak 
 
 53.1 
 
 .85 
 
 .85 
 
 Pine 
 Platinum 
 
 28.8 
 13488 
 
 .46 
 21 5 
 
 .46 
 21.5 
 
 Sea Water . , 
 
 644 
 
 1 03 
 
 1 03 
 
 Silver . 
 
 656 3 
 
 10 5 
 
 105 
 
 
 31 2 
 
 5 
 
 .5 
 
 Steel 
 
 5900 
 
 784 
 
 7.84 
 
 Sulphuric Acid .... 
 Tin (cast) 
 
 115.1 
 4558 
 
 1.84 
 729 
 
 1.84 
 7.29 
 
 Walnut .... 
 
 41 6 
 
 67 
 
 .67 
 
 Water . 
 
 62 5 
 
 1 00 
 
 1.00 
 
 Zinc " 
 
 431 3 
 
 69 
 
 69 
 
 
 
 
 
METHODS OF FINDING SPECIFIC GRAVITY 281 
 
 5. If there were 12 cubes of gold, 1 in., 2 in., 3 in., etc., on an edge 
 respectively, and you were told you could have whichever one you 
 could lift at the first trial, which one would you try? Why? 
 
 6. If a bucket containing water is placed on the platform of a set of 
 scales and is found to weigh 40 lb., what weight will the scales show if 
 a cast iron cube 3 in. on an edge is supported just under the surface of 
 the water by a string, care being taken that the cube does not touch 
 the bucket? 
 
 7. How could you find the cubical contents of an egg? 
 
 8. From the table determine the order of the heaviest substances 
 named. 
 
 328. Methods of Finding Specific Gravity. (1) If it is 
 possible to weigh a body and also to determine its volume, 
 the density can be found by dividing the weight by the volume. 
 If the body can be weighed and the dimensions taken, then 
 the weight divided by the volume gives the density. This 
 density divided by the density of water gives its specific 
 gravity. 
 
 Example : What is the specific gravity of a piece of metal if it weighs 
 40 lb., and is 2" X 4" X 12"? 
 Solution : 
 
 2 X 4 X 12 = 96 cu. in. 
 96 1 
 
 40 
 
 7 = 40 X 18 = 720 lb. per cubic foot. 
 
 fl 
 
 Density of water = 62.5 lb. per cubic foot. 
 
 720 
 /. - 11.5 = sp.gr. 
 
 (2) The hydrometer (Figure 307) is an instrument used 
 to determine the specific gravity of liquids. It is a tube, 
 weighted at the bottom, that has a scale marked on the side. 
 The depth to which it sinks gives the specific gravity reading. 
 
282 
 
 MECHANICS OF FLUIDS 
 
 An hydrometer, made to read the specific gravities of liquids 
 lighter than water, has the zero of the scale at the bottom, but 
 
 one for liquids heavier than water 
 has the zero at the top. Why? 
 (3) Another method for find- 
 ing the specific gravity of a 
 body, and the one generally 
 used if the body is irregular in 
 shape, is to weigh the body in 
 air, and then in water. The 
 difference represents the weight 
 of the water displaced. Why? 
 Then the weight in air divided 
 by the loss in weight equals 
 specific gravity. 
 
 Example : What is the specific 
 gravity of a body which weighs 19 
 grams in air and 12 grams in water? 
 
 Solution : 
 
 FIGURE 307. THE HYDROM- 
 ETER. 
 
 19 - 12 = 7 grams, wt. of water 
 displaced. 
 
 1Q 
 
 ^=2.71= sp.gr. 
 
 (4) Other cases : (a) If the body is lighter than water, a 
 sinker must be used ; but the principle is similar. 
 
 (6) If the object is soluble in water, it can be weighed in a 
 liquid in which it is not soluble, but whose specific gravity 
 is known. 
 
 (c) If it is a liquid whose specific gravity is to be found, a 
 sinkter, first weighed in air, then in water, and then in the 
 liquid, will give the data necessary for finding the specific 
 gravity. 
 
REVIEW PROBLEMS 283 
 
 Explain, with an example, how to find the specific gravity in (a), 
 (6), and (c). 
 
 Problems 
 
 1. What is the density and specific gravity of a piece of butter 
 which is 2|" X 2" X 4" and weighs 1 Ib. ? 
 
 2. What is the specific gravity of an egg, if it weighs 1 oz. in air and 
 .1 oz. in water? 
 
 3. What is the specific gravity of a grapefruit, if the following data 
 are taken ? Weight of grapefruit in air, with a sinker attached, but in 
 water, equals 1.5 Ib. ; weight of sinker alone in water equals .3 Ib. ; 
 weight of grapefruit in water with sinker attached and in water equals 
 .lib. 
 
 4. What is *the specific gravity of a crystal of a substance, if it 
 weighs .24 gram in air, and .05 gram in a liquid whose specific gravity 
 is 1.5? 
 
 5. What is the specific gravity of a liquid, if a sinker weighs 12 grams 
 in air, 5 grams in the liquid, and 4 grams in water? 
 
 Review Problems 
 
 1. Define force, work, mechanical advantage, and efficiency. 
 
 2. Classify and describe levers. 
 
 3. If a force of 15 Ib. is exerted on the handles of a nutcracker 6 
 inches from the pivot when the nut is placed 1| inches from the pivot, 
 what is the pressure on the nut? 
 
 4. The crank on an awning lifter is 15 inches long, and the radius 
 of the axle on which the rope is wound is 1 inch. What force on the 
 crank is necessary to lift the awning if it pulls down on the rope with 
 a weight of 50 Ib. ? 
 
 5. If a piano weighs 600 Ib. and is rolled up a plank 16 ft. long 
 into a truck 4 ft. high, what force is necessary, ignoring friction ? 
 
 6. How fast will the blades of an egg-beater run, if the handle is 
 fastened to a wheel with 50 cogs, which in turn drives a wheel, with 
 8 cogs, directly connected to the blades, the handle being turned 96 
 R. P. M.? 
 
 7. What horsepower is exerted when a 120-lb. girl climbs a stairs 
 15 ft. high in \ min. ? 
 
 8. Define motion. 
 
284 MECHANICS OF FLUIDS 
 
 9. What are Newton's three laws of motion? 
 
 10. Explain the use of the parallelogram of force. 
 
 11. How far will a train travel in 10 seconds if it has an accelera- 
 tion of | ft. per second, per second, and starts from rest? 
 
 12. How long will it take a stone to fall 100 ft. ? 
 
 13. How far will an automobile coast if it has a velocity of 36 ft. 
 per second and slows down at the rate of 2 ft. per second, per second ? 
 
 14. What is the apparent weight of a girl going up in an elevator 
 which is increasing its speed at the rate of 3 ft. per second, per second, 
 if her actual weight is 110 Ib. ? 
 
 15. Give two uses of the pendulum. 
 
 16. Explain why gases and liquids can be delivered through pipes 
 while solids cannot. 
 
 17. How does force on a surface differ from pressure on a surface? 
 
 18. What is the pressure in pounds per square inch at the bottom 
 of a tank of water 8 ft. deep ? 
 
 19. If the water main pressure is 60 Ib. per square inch, how high 
 will the water rise in a pipe? 
 
 20. Why do high buildings have ext a pumping systems of their 
 own? 
 
 21. If you were to supply water to a house, from an open tank, where 
 would you locate the tank ? 
 
 22. Give five applications of air-pressure. 
 
 23. Explain capillarity. 
 
 24. State Archimedes' principle. 
 
 25. What is meant when we say the specific gravity of brass is 8.3 ? 
 
 26. Why will an egg sink in fresh water and float in salt water ? 
 
 27. How could you test a grapefruit for juiceness in a simple manner ? 
 
 28. What is the specific gravity of an egg, if it weighs 1.1 oz. in 
 air, and .08 oz. in water ? 
 
APPENDIX 
 
 I. Freezing and Boiling Points of Some Common Substances 
 Under Normal Atmospheric Pressure 
 
 SUBSTANCE 
 
 FREEZING POINT 
 
 BOILING POINT 
 
 Oxygen 
 
 Centigrade 
 
 - 235 
 
 Centigrade 
 - 18 9 
 
 Ammonia 
 Ether 
 
 - 75 
 - 113 
 
 - 39 
 35 
 
 Methylic Alcohol 
 
 - 112 
 
 66 
 
 Distilled Water 
 Acetic Acid 
 
 
 - 17 
 
 100 
 117 
 
 Turpentine 
 
 - 27 
 
 157 
 
 Fat, Oil, etc 
 Mercury 
 
 - 33 
 
 - 388 
 
 210 
 357 
 
 
 
 
 II. Vapor Tension of Water 
 
 Temperatures Given in Centigrade Degrees, and Vapor Tension in 
 Centimeters of Mercury 
 
 TEMPERATURES 
 
 VAPOR TENSIONS 
 
 TEMPERATURES 
 
 VAPOR TENSIONS 
 
 - 10 
 
 .22 
 
 3 
 
 .57 
 
 - 9 
 
 .23 
 
 4 
 
 .61 
 
 - 8 
 
 .25 
 
 5 
 
 .65 
 
 - 7 
 
 .27 
 
 6 
 
 .70 
 
 - 6 
 
 .29 
 
 7 
 
 .75 
 
 - 5 
 
 .32 
 
 8 
 
 .80 
 
 - 4 
 
 .34 
 
 9 
 
 .86 
 
 - 3 
 
 .37 
 
 10 
 
 .92 
 
 - 2 
 
 .39 
 
 11 
 
 .98 
 
 1 
 
 .42 
 
 12 
 
 1.05 
 
 
 
 .46 
 
 13 
 
 1.12 
 
 1 
 
 .49 
 
 14 
 
 1.19 
 
 2 
 
 .53 
 
 15 
 
 1.27 
 
 285 
 
286 
 
 HOUSEHOLD PHYSICS 
 II. Vapor Tension of Water Continued 
 
 TEMPERATURES 
 
 VAPOR TENSIONS 
 
 TEMPERATURES 
 
 VAPOR TENSIONS 
 
 16 
 
 1.35 
 
 30 
 
 3.15 
 
 17 
 
 1.44 
 
 31 
 
 3.34 
 
 18 
 
 1.54 
 
 32 
 
 3.54 
 
 19 
 
 1.63 
 
 33 
 
 3.74 
 
 20 
 
 1.74 
 
 34 
 
 3.96 
 
 21 
 
 1.85 
 
 35 
 
 4.18 
 
 22 
 
 1.97 
 
 36 
 
 4.42 
 
 23 
 
 2.09 
 
 37 
 
 4.67 
 
 24 
 
 2.22 
 
 38 
 
 4.93 
 
 25 
 
 2.35 
 
 39 
 
 5.20 
 
 26 
 
 2.51 
 
 40 
 
 5.49 
 
 27 
 
 2.65 
 
 41 
 
 5.79 
 
 28 
 
 2.81 
 
 45 
 
 7.14 
 
 29 
 
 2.98 
 
 100 
 
 76.00 
 
 III. Table of Specific Heats of Some of Our Most Common Substances 
 SUBSTANCE SPECIFIC HEAT 
 
 Aluminum 22 
 
 Brass 094 
 
 Copper .095 
 
 Iron 1138 
 
 Mercury 038 
 
 Lead .031 
 
 Ice 5 
 
 Air (at constant pressure) 2375 
 
 Hydrogen (at constant pressure) ...... 3.4 
 
 Steam (at constant pressure) 48 
 
 IV. Table of Coefficients of Linear Expansion 
 
 SUBSTANCES COEFFICIENT 
 
 Aluminum 0000222 
 
 Brass 0000187 
 
 Copper 000017 
 
 Glass .0000083 
 
 SUBSTANCES COEFFICIENT 
 
 Iron . . .0000112 
 
 Platinum . .0000088 
 
 Steel . . .000013 (tempered) 
 
 Steel . . .000011 (untempered) 
 
 If the range in temperature is given in Fahrenheit degrees, then the 
 above coefficients must be multiplied by -jj-. 
 
APPENDIX 
 V. Sources of Heat 
 
 287 
 
 MATERIAL 
 
 KIND 
 
 HEAT VALUE 
 
 Coal 
 
 Wood 
 Gas 
 
 [Hard 
 | Soft 
 I Coke 
 Hard 
 Soft 
 Natural 
 
 14000 B. T. U.'s per Ib. 
 12000 B. T. U.'s per Ib. 
 14000 B. T. U.'s per Ib. 
 8400 B. T. U.'s per Ib. 
 8600 B. T. U.'s per Ib. 
 1200 B. T. U.'s per cu. ft. 
 
 Oils 
 Electricity . 
 
 Artificial 
 ( Kerosene 
 { Naphtha 
 [ Crude Oil 
 
 600 B. T. U.'s per cu. ft. 
 20000 B. T. U.'s per Ib. 
 20000 B. T. U.'s per Ib. 
 18000 B. T. U.'s per Ib 
 3411.72 B. T. U.'s per kw. hr. 
 
 (Electricity is given in this table, though it is not a fuel.) 
 VI. Heat Value of Foods 
 
 FOOD (edible portion) 
 
 APPROXIMATE MEASURE OP 
 1 00-G HEAT-CALORY 
 PORTION 
 
 WEIGHT IN 
 OUNCES OP 
 100-GREAT- 
 CALORY 
 PORTION 
 
 Almonds 
 
 15 average 
 
 0.5 
 
 Apples 
 
 2 medium .... 
 
 6.5 
 
 Apricots fresh 
 
 2 large 
 
 6 1 
 
 Asparagus, cooked .... 
 Bacon, smoked (uncooked) . 
 Bananas 
 
 2 servings 
 1 thin slice, small . . . 
 1 large 
 
 7.5 
 0.6 
 3.6 
 
 Beans, baked, canned . . . 
 strin^ canned 
 
 1 small serving (^ cupful) 
 5 servings 
 
 2.8 
 17.2 
 
 lima canned 
 
 1 large side-dish . 
 
 46 
 
 Beef corned 
 
 
 1 2 
 
 dried salted and smoked 
 
 4 large slices 
 
 20 
 
 porterhouse steak 
 
 1 small 
 
 1.3 
 
 ribs lean 
 
 1 average serving . 
 
 1.9 
 
 ribs fat 
 
 
 09 
 
 round, free from visible fat 
 rump lean 
 
 1 generous serving . . . 
 
 3.1 
 
 1 7 
 
 rump fat; 
 
 
 0.9 
 
 sirloin steak 
 
 1 average serving . 
 
 1.4 
 
288 
 
 HOUSEHOLD PHYSICS 
 
 VI. Heat Value of Foods Continued 
 
 FOOD (edible portion) 
 
 APPROXIMATE MEASURE op 
 IOO-GREAT-CALORY 
 PORTION 
 
 WEIGHT IN 
 OUNCES OF 
 IOO-GREAT- 
 CALORY 
 PORTION 
 
 Beets cooked 
 
 3 servings . * 
 
 89 
 
 Brazil nuts .... 
 
 3 average size ...... 
 
 0.5 
 
 Bread, graham . 
 
 1 thick slice . .... 
 
 1.3 
 
 toasted 
 
 2 medium slices (baker's) 
 
 1 2 
 
 white homemade 
 
 1 medium slice 
 
 1.3 
 
 average 
 
 1 thick slice ...... 
 
 1.3 
 
 whole-wheat 
 
 1 thick slice . . 
 
 1.4 
 
 Buckwheat flour 
 
 T cupful . 
 
 1.0 
 
 Butter 
 
 1 tablespoonful (ordinarv pat) 
 
 0.5 
 
 Buttermilk .... 
 Cabbage 
 
 lj cupfuls (1^ glasses) . . . 
 2 servings . . 
 
 9.9 
 11 2 
 
 Calf's foot jellv . . . 
 
 
 4.1 
 
 Carrots, fresh . . . 
 
 2 medium 
 
 7.8 
 
 Cauliflower . . . 
 
 
 11.6 
 
 
 
 19.1 
 
 Celery soup, canned 
 
 2 servings ...... 
 
 6.6 
 
 Cheese, American pale 
 
 1^ cubic inches . 
 
 0.8 
 
 American red . . . 
 
 1| cubic inches 
 
 0.8 
 
 Cheddar .... 
 
 1^ cubic inches 
 
 0.8 
 
 Cottaee . 
 
 4 cubic inches (^ cupful) . . 
 
 3.2 
 
 Neufchatel. ... 
 Roquefort . . . . 
 
 l cubic inches (j cupful) . . 
 1^ cubic inches ...... 
 
 1.1 
 1.0 
 
 Swiss 
 
 1^ cubic inches . . . - 
 
 0.8 
 
 Chicken broilers 
 
 1 large serving 
 
 3.3 
 
 Chocolate . . . . 
 
 1 generous half souare 
 
 0.6 
 
 Cocoa 
 
 2i tablespoonfuls 
 
 1 
 
 Cod salt 
 
 
 3.4 
 
 Corn green 
 
 1 side-dish 
 
 3.6 
 
 Corn meal .... 
 
 
 1.0 
 
 Crackers, graham . 
 
 3 crackers 
 
 0.9 
 
 soda 
 
 3 crackers 
 
 0.9 
 
 water . 
 
 3 crackers . . 
 
 0.9 
 
 Cranberries, cooked 
 
 
 7.5 
 
 Cream 
 
 x cupful . 
 
 1.8 
 
 Cucumbers .... 
 
 2 laree 
 
 20.3 
 
 Dates dried 
 
 4 medium .... 
 
 1.0 
 
 Doughnuts . 
 
 ^ doughnut 
 
 0.8 
 
 
 
 
APPENDIX 
 
 289 
 
 VI. Heat Value of Foods Continued 
 
 FOOD (edible portion) 
 
 
 
 APPROXIMATE MEASURE op 
 IOO-GREAT-CALORY 
 PORTION 
 
 WEIGHT IN 
 OUNCES OF 
 IOO-GREAT- 
 CALORY 
 PORTION 
 
 Eggs uncooked 
 
 I^T medium or 2 small . 
 
 2.4 
 
 Farina 
 
 
 1 
 
 Figs, dried 
 Flour rye 
 
 1 large 
 j cupful 
 
 1.1 
 1.0 
 
 wheat entire 
 
 -i cupful 
 
 1 
 
 wheat, graham 
 wheat, average high, medium 
 
 ^ cupful 
 cupful 
 
 1.0 
 
 1.0 
 
 Gelatin . . 
 
 4 tablespoonfuls 
 
 1 
 
 Grapes 
 
 1 large bunch 
 
 3 7 
 
 Haddock 
 
 
 4.9 
 
 Halibut steaks . 
 
 1 average serving 
 
 29 
 
 Ham fresh lean 
 
 
 1 5 
 
 fresh, medium 
 
 1 average serving . 
 
 1.1 
 
 smoked lean 
 
 
 1.3 
 
 Herring whole 
 
 
 25 
 
 Hominy, uncooked .... 
 
 i cupful 
 
 1.0 
 
 Lamb, chops, broiled . . . 
 leg roast 
 
 1 small chop .... 
 1 average serving 
 
 1.0 
 
 1 8 
 
 Lard, refined 
 Lemons .... 
 
 1 tablespoonful (scant) 
 3 medium 
 
 0.4 
 8.0 
 
 Lettuce 
 
 50 large leaves 
 
 20.4 
 
 Liver, veal, uncooked 
 Macaroni, uncooked . 
 Macaroons .... . . 
 
 2 small servings . 
 cupful (4 sticks) . 
 
 2 
 
 2.9 
 1.0 
 0.8 
 
 Mackerel, uncooked .... 
 salt . 
 
 1 large serving . . . 
 
 2.5 
 1.2 
 
 Marmalade, orange .... 
 Milk, condensed, sweetened 
 skimmed 
 
 1 tablespoonful . 
 l~rV cupful 8 .... 
 1^ cupfuls 
 
 1.0 
 1.1 
 9.6 
 
 whole 
 Molasses cane . 
 
 f cupful (half glass) . 
 I cupful 
 
 5.1 
 
 1.2 
 
 
 ^ large serving . 
 
 8.9 
 
 Mutton, leg 
 
 1 average serving . 
 
 1.8 
 
 Oatmeal, uncooked .... 
 
 i cupful 
 
 0.9 
 
 Olives green 
 
 7 to 10 . . 
 
 1.2 
 
 Onions fresh 
 
 2 medium 
 
 7.3 
 
 Oranges 
 
 1 very large .... 
 
 6.9 
 
290 HOUSEHOLD PHYSICS 
 
 VI. Heat Value of Foods Continued 
 
 FOOD (edible portion) 
 
 APPROXIMATE MEASURE OP 
 IOO-GREAT-CALORY 
 PORTION 
 
 WEIGHT IN 
 OUNCES OF 
 IOO-GREAT- 
 CALORY 
 PORTION 
 
 Oysters, canned .... 
 
 5 oysters 
 
 49 
 
 Parsnips 
 
 1 large 
 
 54 
 
 Pea soup, canned . 
 Peaches, canned . . . 
 fresh 
 
 1 large serving 
 1 large 
 4 medium 
 
 3.5 
 7.5 
 S ^ 
 
 Peanuts 
 
 10 to 12 (double kernels) 
 
 06 
 
 Peas, dried, uncooked . . 
 canned 
 
 2 tablespoonfuls .... 
 2 servings 
 
 1.6 
 6 3 
 
 green 
 Pies, apple 
 
 1 generous serving 
 \ piece . 
 
 3.5 
 1 3 
 
 custard 
 
 ? piece . 
 
 20 
 
 lemon 
 
 -3- piece . 
 
 1 4 
 
 mince 
 
 A piece . 
 
 1 2 
 
 squash 
 
 J- piece . 
 
 20 
 
 Pineapples, fresh 
 
 5 slices 
 
 82 
 
 canned 
 
 1 small serving 
 
 2 3 
 
 Pork, chops, medium 
 fat, salt 
 
 1 very small serving . . . 
 
 1.1 
 05 
 
 Potatoes, white, uncooked 
 sweet, uncooked . 
 
 1 medium 
 \ medium .... 
 
 4.2 
 2.9 
 
 Prunes, dried .... 
 
 3 large 
 
 1 2 
 
 Raisins 
 Rhubarb, uncooked . . 
 Rice, uncooked .... 
 Salmon, whole .... 
 
 \ cupful (packed solid) . . 
 3| cupfuls (scant) .... 
 2 tablespoonfuls .... 
 1 small serving . 
 
 1.0 
 15.3 
 1.0 
 
 1 7 
 
 Shad, w r hole 
 Shredded wheat .... 
 
 1 average serving .... 
 1 biscuit 
 
 2.2 
 1.0 
 
 Spinach, fresh .... 
 Succotash, canned . . . 
 Sugar 
 
 3 ordinary servings (cooked) 
 1 average serving . . 
 3 lumps 5 tea spoonfuls 
 
 14.7 
 3.6 
 
 Tomatoes, fresh .... 
 canned 
 
 granulated, 6^ teaspoon- 
 fuls powdered 
 4 average servings .... 
 1-2- cupfuls .... 
 
 0.9 
 15.5 
 156 
 
 Turkev 
 
 1 serving . 
 
 1 2 
 
 Turnips 
 
 2 large servings (2 turnips) 
 
 90 
 
 Veal, cutlet 
 
 
 2.3 
 
APPENDIX 
 
 291 
 
 VI. Heat Value of Foods Continued 
 
 FOOD (edible portion) 
 
 APPROXIMATE MEASURE OF 
 IOO-GREAT-CALORY 
 PORTION 
 
 WEIGHT IN 
 OUNCES OF 
 IOO-GREAT- 
 CALORY 
 PORTION 
 
 fore Quarter 
 
 ] thick slice 
 
 2.3 
 
 hind Quarter 
 
 
 23 
 
 Vegetable soup canned 
 
 
 259 
 
 \Valnuts California 
 
 
 0.5 
 
 Wheat, cracked 
 Whitefish 
 
 4 nuts 
 
 1.0 
 2.4 
 
 Zwiebach 
 
 
 0.8 
 
 
 
 
 VII. Tables of Measurements 
 English Lineal Measure 
 12 inches = 1 foot 
 3 feet = 1 yard 
 5| yards = 1 rod 
 320 rods = 1 mile 
 
 Lineal Chain Measure 
 7.92 inches = 1 link 
 100 links = 1 chain 
 80 chains = 1 mile 
 
 Rope and Cable Measure 
 6 feet = 1 fathom 
 120 fathoms = 1 cable's length 
 
 Cloth Measure 
 2.25 inches = 1 nail 
 
 4 nails = 1 quarter 
 
 5 quarters = 1 ell 
 
 Metric Lineal 
 10 millimeters = 
 10 centimeters = 
 10 decimeters = 
 10 meters 
 10 dekameters = 
 10 hektameters = 
 10 kilometers = 
 
 Measure 
 1 centimeter 
 1 decimeter 
 1 meter 
 1 dekameter 
 1 hektameter 
 1 kilometer 
 1 myriameter 
 
292 HOUSEHOLD PHYSICS 
 
 Equivalent values in English and Metric Lineal Measure 
 
 1 inch = 2.54 centimeters 
 
 1 foot = 30.48 centimeters 
 
 1 yard = 91.44 centimeters 
 
 1 rod = 502.92 centimeters 
 
 1 mile = 160,934.72 centimeters 
 
 1 centimeter = .394 inch 
 
 English Surface Measure 
 
 144 square inches = 1 square foot 
 9 square feet = 1 square yard 
 30| square yards = 1 square rod 
 160 square rods = 1 acre 
 640 acres = 1 square mile 
 
 Architect's Measure 
 1 square = 100 square feet 
 
 Metric Surface Measure 
 
 100 square millimeters = 1 square centimeter 
 100 square centimeters = 1 square decimeter 
 100 square decimeters = 1 square meter 
 100 square meters = 1 square dekameter 
 100 square dekameters = 1 square hektameter 
 100 square hektameters = 1 square kilometer 
 100 square kilometers = 1 square myriameter 
 
 Equivalent values in English and Metric Measure 
 
 1 square inch = 6.45 square centimeters 
 
 1 square foot = 929.03 square centimeters 
 
 1 square yard = 8361.29 square centimeters 
 
 1 square rod = 252,929.04 square centimeters 
 
 1 square centimeter = .155 square inch 
 
 English Measure Volume 
 
 1728 cubic inches = 1 cubic foot 
 27 cubic feet = 1 cubic yard 
 
 A standard gallon contains 231 cubic inches, and a standard struck 
 bushel contains 2150.42 cubic inches. 
 
APPENDIX 293 
 
 English Liquid Measure 
 
 4 gills = 1 pint 
 2 pints = 1 quart 
 4 quarts = 1 gallon 
 
 English Dry Measure 
 
 2 pints = 1 quart 
 4 quarts = 1 gallon 
 2 gallons = 1 peck 
 4 pecks = 1 bushel 
 
 English Fluid Measure 
 
 8 drams = 1 ounce 
 16 ounces = 1 pint 
 2 pints = 1 quart 
 4 quarts = 1 gallon 
 
 Metric Measure of Volume 
 
 1000 cubic millimeters = 1 cubic centimeter 
 
 1000 cubic centimeters = 1 cubic decimeter 
 
 1000 cubic decimeters = 1 cubic meter 
 
 1000 cubic meters = 1 cubic dekameter 
 
 1000 cubic dekameters = 1 cubic hektameter 
 
 1000 cubic hektameters = 1 cubic kilometer 
 
 1000 cubic kilometers = 1 cubic myriameter 
 
 Metric Liquid and Dry Measure 
 
 10 milliliters = 1 centiliter 
 
 10 centiliters = 1 deciliter 
 
 10 deciliters = 1 liter 
 
 10 liters = 1 dekaliter 
 
 10 dekaliters = 1 hektaliter 
 
 10 hektaliters = 1 kiloliter 
 
 10 kiloliters = 1 myrialiter 
 The liter contains 1 cubic decimeter or 1000 cubic centimeters. 
 
 Equivalent values in English and Metric Volume Measure 
 
 1 cubic centimeter = .061 cubic inch 
 
 1 cubic meter = 1.308 cubic yards 
 
 1 liter = .908 dry quart = 1.057 liquid quarts 
 
294 
 
 HOUSEHOLD PHYSICS 
 
 English Measures of Weight 
 
 16 ounces = 1 pound 
 2000 pounds = 1 ton 
 
 Metric Measures of Weight 
 
 10 milligrams = 1 centigram 
 
 10 centigrams = 1 decigram 
 
 10 decigrams = 1 gram 
 
 10 grams = 1 dekagram 
 
 10 dekagrams = 1 hektogram 
 
 10 hektograms = 1 kilogram 
 
 10 kilograms = 1 myriagram 
 
 Equivalent values in English and Metric Measures of Weight 
 
 453.6 grams = 1 pound 
 VIII. Vibrations of Musical Sounds 
 
 Letter C 
 
 Frequency 256 
 
 Interval between con- 
 secutive tones . . 
 Interval between each 
 tone and C . 1 
 
 D 
 
 288 
 
 E 
 
 320 
 
 10 i < 
 
 9^ 
 
 F 
 
 3411 
 
 G 
 
 384 
 
 A 
 
 I 
 
 B 
 
 480 
 
 IX. Candle-Power of a Few Sources of Light 
 
 Carbon Lamp about f c. p. per watt 
 
 Tungsten Lamp ...... about ^ c. p. per watt 
 
 Nitrogen Lamp about 1 c. p. per watt 
 
 Mercury Vapor Lamp .... about 1 c. p. per watt 
 Arc Light about 1 c. p. per watt 
 
 X. Terms and Abbreviations in Electricity 
 
 512 
 
 THING TO BE MEASURED 
 
 UNIT 
 
 LETTER 
 
 Pressure 
 
 Volt 
 
 E 
 
 Current 
 
 Ampere 
 
 I 
 
 
 Ohm 
 
 R 
 
 
 Watt 
 
 W 
 
 Electrical Energy 
 
 Kilowatt 
 Watt-hour 
 Kilowatt-hour 
 
 Kw 
 
 W-hr. 
 Kw-hr. 
 
APPENDIX 
 
 295 
 
 XI. Table of Densities and Specific Gravities of Some Substances 
 
 SUBSTANCE 
 
 DENSITY 
 
 SPECIFIC 
 GRAVITY 
 
 Lbs. Per 
 Cu. Ft. 
 
 Gms. Per c. c. 
 
 Ash (dry) 
 
 43.7 
 
 52.8 
 66.4 
 50.0 
 165.6 
 53.2 
 35.0 
 15.0 
 550.6 
 555.0 
 527.5 
 1218.8 
 75.2 
 480.0 
 449.0 
 709.6 
 46.0 
 850.0 
 64.5 
 76.3 
 53.1 
 28.8 
 1348.8 
 64.4 
 656.3 
 31.2 
 590.0 
 115.1 
 455.8 
 41.6 
 62.5 
 431.3 
 
 .70 
 .84 
 1.062 
 .80 
 2.65 
 .69 to .852 
 .561 
 .24 
 8.81 
 8.88 
 8.38 to 8.44 
 19.50 
 1.22 
 7.68 
 7.20 
 11.36 
 .75 
 13.6 
 1.032 
 1.22 
 .85 
 .46 
 21.5 
 1.03 
 10.5 
 .5 
 7.84 
 1.84 
 7.29 
 .67 
 1.00 
 6.9 
 
 .70 
 .84 
 1.062 
 .80 
 2.65 
 .69 to .852 
 .561 
 .24 
 8.81 
 8.88 
 8.38 to 8.44 
 19.50 
 1.22 
 7.68 
 7.20 
 11.36 
 .75 
 13.6 
 1.032 
 1.22 
 .85 
 .46 
 21.5 
 1.03 
 10.5 
 .5 
 7.84 
 1.84 
 7.29 
 .67 
 1.00 
 6.9 
 
 Ash (green) 
 Acetic Acid 
 Alcohol 
 Aluminum 
 Beech 
 Cedar 
 Cork 
 
 Copper (cast) 
 Copper (sheet) 
 Brass 
 
 Gold 
 Hydrochloric Acid .... 
 Iron (wrought) 
 Iron (cast) 
 
 Lead 
 Maple 
 
 Mercurv 
 
 Milk 
 Nitric Acid 
 
 Oak 
 
 Pine 
 
 Platinum .... 
 
 Sea Water . 
 
 Silver . . 
 
 Spruce 
 
 Steel 
 Sulphuric Acid . . . 
 
 Tin (cast) . 
 
 Walnut 
 
 Water 
 
 Zinc 
 
 
INDEX 
 
 Numbers refer to pages. 
 
 Absolute zero 38 
 
 Absorbers 58 
 
 Acceleration ... . 252, 255 
 Additive method in color . . 137 
 Air necessary for a person . . 56 
 
 Air pressure . 264 
 
 Air pressure, other applications 
 
 of 271 
 
 Alcohol used in thermometers . 4 
 Alternating current rectified . 222 
 
 Ammeter 186 
 
 Ammonia used in ice plant . 21, 22 
 
 Amperes 174 
 
 Amplitude . . . 70, 71, 79, 258 
 
 Angle of incidence 96 
 
 Angle of reflection ..... 96 
 
 Annunciator 170 
 
 Anode 213 
 
 Arc lamp, automatic . . 171, 179 
 
 candle power of 130 
 
 Archimedes' principle . . . 277 
 
 applications of 278 
 
 Area 227 
 
 Armature of generator . . . 159 
 
 Artificial ice 8 
 
 plant 21, 22 
 
 rinks 22, 23 
 
 Astigmatism 121 
 
 Atmosphere as a refracting sub- 
 stance 107 
 
 Atmospheric pressure . . . 8, 9 
 
 Atom 212 
 
 Attraction, law of magnetic . . 147 
 Axis . . . . 100 
 
 Balance wheel of a watch . . 33 
 
 Barometer .... 265, 266, 267 
 
 Batteries 217 
 
 Beats 78 
 
 Bell, door . . 165 
 
 Binoculars, field 110 
 
 Blue 132 
 
 Boiling point . . . . 3, 4, 5, 7, 8, 9 
 
 Boyle's law 272 
 
 British Thermal Unit (B. T. U.), 
 
 definition of 10 
 
 Brushes of generator .... 159 
 
 Buzzer 165 
 
 Calory, definition 10 
 
 Calory, great, definition of . . 10 
 
 Camera lens 117 
 
 Camera, life-sized picture . . 122 
 Camera, pinhole . . . 116, 117 
 Candle power .... 127, 128 
 
 Candle power, measurement of 129 
 
 Capillarity 275 
 
 Carbon lamp, candle power of 130 
 
 color of 136 
 
 Cathode 213 
 
 Center of curvature of mirror . 98 
 
 Centigrade thermometer . . 4 
 
 construction of 4 
 
 Centrifugal force 256 
 
 Charles' law 39 
 
 applications of 39 
 
 applied to baking ... 39, 40 
 
 applied to clay modeling . . 40 
 
 other applications .... 40 
 
 Chemical energy 221 
 
 Choroid coat of eye . . . . 119 
 
 Chromatic scale 87 
 
 Circuit breaker 169 
 
 City system wiring diagram . 200 
 
 Closed pipes, resonance in . . 83 
 
 Clothes . 46 
 
 Clouds 25 
 
 Coal, as a fuel 60, 61 
 
 Cochlea 77 
 
 Coefficient of cubical expansion 35 
 
 linear expansion 29 
 
 linear expansion, table of . . 30 
 
 volume expansion .... 35 
 
 Cohesion 11, 16, 259 
 
 297 
 
298 
 
 INDEX 
 
 Numbers refer to pages. 
 
 Cold body, differs from hot body 2 
 
 definition of 3 
 
 Color 132 
 
 niters 143 
 
 harmony of . 140 
 
 how we see 137 
 
 nomenclature 140 
 
 of opaque objects .... 134 
 of transparent and translu- 
 cent objects 135 
 
 screens 141 
 
 Colored objects, application of 135 
 
 Colors, cause of 132 
 
 elementary 136 
 
 Commutator of generator . . 160 
 
 Concave mirror 98, 100 
 
 Condensation . . . . 70, 71, 84 
 
 Condenser 203 
 
 Conduction 41, 58 
 
 Conductor, electrical . . 153, 155 
 
 Conductors 41 
 
 Convection 41, 47, 58 
 
 Convection currents 48, 50, 51, 52, 53 
 Convex mirror . . . . 100, 101 
 
 Cornea of the eye 119 
 
 Counter-electromotive force . 193 
 
 Crest 69 
 
 Critical angle 107 
 
 Crystalline lens 120 
 
 Current of electricity .... 153 
 
 Dark lantern 124 
 
 Daylight lamp 136 
 
 Decorations, selection of, ac- 
 cording to color 136 
 
 Degree, unit used on thermom- 
 eter 5 
 
 Density 278, 279, 280 
 
 Dew 25 
 
 Diamond Ill 
 
 Diffused light ...... 125 
 
 Discord 86 
 
 Disks, colored 138 
 
 Dispersion 132 
 
 Distillation 19 
 
 fractional 20 
 
 Domestic science, relation of, to 
 
 physics 2 
 
 Dominant 86 
 
 Double boiler 18 
 
 Drafts in chimney 48 
 
 Dress goods, selection of, ac- 
 cording to colors .... 136 
 
 Driven pulley 241 
 
 Driver pulley 241 
 
 Dry cell 219 
 
 Dyes .134 
 
 Dynamics 248 
 
 Ear 
 
 drum . . . . 
 
 external . . . 
 
 how we hear . 
 
 inner . . . . 
 
 middle . . . . 
 Edison storage cell 
 Efficiency . . . . 
 Electric clock 
 
 77 
 
 77 
 
 77 
 
 77 
 
 77 
 
 77 
 
 223 
 
 235 
 
 169 
 
 curling iron 181 
 
 door latch 172 
 
 flat iron 180 
 
 gas lighter 172 
 
 grill 182 
 
 mangle 183 
 
 percolator 181 
 
 soldering iron 181 
 
 stove 181 
 
 toaster 181 
 
 Electrical current 154 
 
 application of heating effect of 176 
 
 chemical relation of ... 212 
 
 D C pulsating, made steady 162 
 
 heating effect of 173 
 
 induced 202 
 
 magnetic effect of . . . . 163 
 
 magnetic field about a 163 
 
 motion producing effect of . 184 
 
 through a helix 164 
 
 Electrical energy 174 
 
 generator, simple . . . . 155 
 
 generator, simple AC. . . 156 
 
 generator, simple DC. . . 160 
 
 power 174 
 
 Electrical pressure .... 200 
 
 alternating current . . . . 159 
 
 amount of 154 
 
 curve of, in A C generator . 157 
 
 curve of, in D C generator . 161 
 
 direct current 159 
 
 direction of .... 154 
 
INDEX 
 
 Numbers refer to pages. 
 
 299 
 
 generation of 153 
 
 nature of 154 
 
 of a voltaic cell 215 
 
 stepped up 204 
 
 Electrical units 173 
 
 Electricity 153 
 
 analogous to water . 154, 155 
 
 relation to magnetism . . . 153 
 
 static 224 
 
 Electrodes 217 
 
 Electrolyte 213 
 
 Electrolytic cell 212 
 
 chemical action in . . . . 212 
 
 copper sulphate 213 
 
 parts of 213 
 
 sulphuric acid 214 
 
 Electro-magnet . . . . 164, 165 
 
 applications of 165 
 
 in a coil of wire 201 
 
 other applications of ... 172 
 
 poles of 164, 165 
 
 Electro-plating 215 
 
 Electro-typing 215 
 
 Energy 91 
 
 definition of 1 
 
 kinetic 257 
 
 of motion 256 
 
 English system compared to 
 
 metric 229, 230 
 
 of measurement 227 
 
 Ether 57 
 
 vibrations in 3 
 
 waves in 91 
 
 Eustachian tube 77 
 
 Evaporization 23 
 
 Expansion 29 
 
 effect on balance wheel of a 
 
 watch 33 
 
 effect on glass ware ... 34 
 
 effect on pendulum of a clock 32 
 
 effect on water pipes ... 37 
 
 of gases 38 
 
 other effects of 34 
 
 peculiar effects on water 35, 36 
 
 tank 52 
 
 Eye 119, 137 
 
 defective 120 
 
 Fahrenheit thermometer 
 Fifth . . . . , 
 
 4, 5 
 
 87 
 
 Fireless cooker 43 
 
 First class lever 234 
 
 Flowing of gases and liquids 260 
 
 Fluorescence 91 
 
 Focal length 99, 112 
 
 Focus 100, 111 
 
 principal 99 
 
 Fog 25 
 
 Foods, heating value ... 63, 64 
 Foot-pound ...... 231, 232 
 
 Force 230, 233 
 
 arm 233 
 
 centrifugal 256 
 
 moment 233 
 
 parallelogram of 250 
 
 to overcome friction . . . 256 
 to overcome inertia . . . 255 
 
 units of 231 
 
 Forced systems of ventilation 55, 56 
 
 Fourth 87 
 
 Freezing, effect on water pipes 37 
 
 point 1, 4, 5, 8, 9 
 
 point, definition of .... 6 
 
 points, table of 7 
 
 Frequency . . 70, 71, 73, 79, 86 
 
 Friction 256 
 
 Fuels 68 
 
 Fundamental 80 
 
 Fusion, heat of, definition of . 11 
 
 Galvanometer 185 
 
 Gas, artificial 60 
 
 Gases 259 
 
 Gases and liquids through pipes 260 
 
 Gas meter 62 
 
 Gas, natural 60 
 
 Gelatin, extraction of .... 9 
 
 Gram-centimeter 231 
 
 Gravitation 257 
 
 Newton's three laws of . . 257 
 
 Gravity cell ....... 220 
 
 Green 132 
 
 Hail 25 
 
 Half-step 87 
 
 Half-tone picture printing . . 140 
 
 Harmony 85 
 
 laws of 86 
 
 of color 14 
 
300 
 
 INDEX 
 
 Numbers refer to pages. 
 
 Heat, absorption of .... 2 
 
 and heat measurement . . 1 
 
 capacity 26 
 
 changed from one form to 
 
 another 2 
 
 definition of 2 
 
 insensible 57 
 
 kinds of 2 
 
 molecular 2 
 
 nature of 1 
 
 necessary for one person . . 64 
 
 of fusion 11 
 
 of fusion, effect on climate 14, 15 
 of fusion, protection by . . 14 
 of vaporization .... 15, 16 
 of vaporization, effect on cli- 
 mate 21 
 
 of vaporization, other effects 
 
 of 21 
 
 quantity of 10 
 
 radiant .... 
 
 sensible . . . 
 
 sources of ... 
 
 transference of . 
 
 travels .... 
 
 units .... 
 
 units compared . 
 
 value of foods 
 
 value of fuels . . 
 Helmholtz resonators 
 Horse-power . . 
 Hot air heating . . 
 
 . . 2 
 . . 57 
 
 . . 60 
 . . 41 
 . . 2 
 . ; 10 
 . .10 
 . 63, 64 
 60, 61, 62 
 . 80, 81 
 . . 245 
 50 
 
 Hot bodies, definition of ... 3 
 Hot body, how different from a 
 
 cold body 2 
 
 Hot water bottle 28 
 
 Hot water heating system 51 , 52, 53 
 
 Hot water tank 50, 51 
 
 House circuit, wiring diagram of 209 
 Hydraulic elevator . . . . ' 261 
 
 Hygrometer 24 
 
 Hypermetropia 120 
 
 Ice cream freezer . . 
 Ice cream, making of . 
 Iceless refrigerators . 
 Ices, freezer of ... 
 Ices, making of ... 
 Illumination . 
 
 13 
 
 6 
 
 22 
 
 13 
 
 6 
 
 127 
 
 problems of ...... 130 
 
 Image 97, 112 
 
 how to find in a plane mirror 97 
 
 in a concave mirror ... 99 
 
 in a convex mirror . . . . 100 
 
 real 98 
 
 through a converging lens 112, 
 
 113, 114, 115 
 
 through a diverging lens . . 11.6 
 
 virtual 98 
 
 Incandescent lamp, carbon . . 176 
 
 gas filled 178 
 
 mercury vapor 178 
 
 tungsten 177 
 
 Incidence, angle of .... 105 
 
 Incident ray 96, 104 
 
 Inclined plane . 233, 239, 240, 241 
 
 height of 240 
 
 length of ....... 240 
 
 Index of refraction, absolute . 106 
 
 relative 106 
 
 Indigo ......... 132 
 
 Induction 200 
 
 coil . . ., . . . ./ . . 203 
 
 coil, uses of 204 
 
 mutual 202 
 
 self . . .. . ,.. .... 202 
 
 Inertia 202, 249, 255 
 
 Insensible heat 2 
 
 Insulation 44 
 
 Insulators ....... 41 
 
 Insulators, electrical .... 155 
 
 Intensity of illumination . . . 127 
 
 Intensity of sound 79 
 
 Interference . 78 
 
 Interval, musical ....... 87 
 
 Ion 212 
 
 lonization 212 
 
 Iris of the eye 119 
 
 Isobars 269 
 
 Isotherms 269 
 
 Kilowatt-hours 174 
 
 Kilowatts 174 
 
 Kinetic energy 257 
 
 Kitchen range 49 
 
 Lantern slide 123 
 
 Lead of a screw 244 
 
 Length 227 
 
INDEX 
 
 Numbers refer to pages. 
 
 301 
 
 Lens, achromatic 133 
 
 condensing 123 
 
 converging Ill 
 
 diverging Ill 
 
 Lenses Ill 
 
 Lever . . . 233, 234, 235, 236, 237 
 
 classes of 234 
 
 Light 01 
 
 velocity of . . . . .92, 93, 94 
 
 Lighthouse reflector .... 109 
 
 Light, nature of 91 
 
 theory of production of . . 91 
 
 waves, propagation of 92 
 
 Line drop 207 
 
 Lines of force 148, 163 
 
 properties of 149 
 
 Liquids 259 
 
 Lodestone 146 
 
 Long-sightedness 120 
 
 Loudness 79 
 
 Luminous bodies 91 
 
 Machines ....... 233 
 
 Magnet, electro 164 
 
 field of 147 
 
 permanent 152 
 
 poles of 146 
 
 tempo rary 152 
 
 Magnetic fields, characteristic 152 
 
 needle 163 
 
 poles of the earth .... 147 
 
 substances 150 
 
 Magnetism 146 
 
 of earth 147 
 
 theory of 149 
 
 Magnetized piece of iron com- 
 pared to one not magnetized 150 
 
 Magnetizing iron 151 
 
 Magnifying glass 124 
 
 Major scale 86 
 
 triad 86 
 
 Mass 227, 255 
 
 Matter, composition of ... 2 
 
 definition of 1 
 
 Mechanical advantage . . . 234 
 
 Mechanics of fluids .... 259 
 
 of solids 227 
 
 Melting point 1 
 
 Mercury in contact with glass 275 
 
 in thermometers . 4, 8 
 
 vapor lamp, candle power of 130 
 
 vapor lamp, color of ... 136 
 
 Meters for A C 190 
 
 Metric system of measurement 229 
 Miller, Dr. Dayton, of Case 
 
 School of Applied Science 81 
 
 Mirror 96 
 
 concave 98 
 
 convex . 100 
 
 parabolical 98 
 
 peculiarly shaped . . . . 102 
 
 spherical 98 
 
 Mist 25 
 
 Mixing colored lights . . . 137 
 Molecular construction of mat- 
 ter 2 
 
 heat 2 
 
 Molecules 7, 11 
 
 Moment 233 
 
 Moments, law of 234 
 
 Momentum . 254 
 
 Motion . . . 230, 231, 248, 252 
 
 energy of 250 
 
 formulae for uniformly ac- 
 celerated 253 
 
 Newton's three laws of 248, 249 
 
 picture machine 123 
 
 Motor and generator compared 191 
 
 Motor, compound 194 
 
 DC 190 
 
 series 194 
 
 shunt ...... 194, 195 
 
 small 197 
 
 use of, in home 199 
 
 Music, basis for 85 
 
 Musical instruments .... 89 
 
 interval 87 
 
 Myopia 120 
 
 Natural system of ventilation 55 
 
 Negative plate 119 
 
 Newton's three laws of gravita- 
 tion 257 
 
 motion 248, 249 
 
 Nitrogen lamp, candle-power of 130 
 
 Noise 85 
 
 Non-conducting materials . 41, 42 
 
 magnetic substances . . . 150 
 
 Octave 87 
 
 Ohm 174 
 
302 
 
 INDEX 
 
 Numbers refer to pages. 
 
 Ohm's law 175 
 
 Opaque objects . . . .58, 94, 133 
 
 Open pipes, resonance in . . 84 
 
 Optical center 112 
 
 Orange 132 
 
 Overtones 80 
 
 Paints 135 
 
 Parabolical mirror 98 
 
 Parallelogram of forces . . . 250 
 
 Pascal's law 261 
 
 Pencil of rays 96, 97 
 
 Pendulum 258 
 
 laws of 258 
 
 of a clock, compensating . . 32 
 
 Penumbra 95 
 
 Period 70, 71 
 
 natural free 76 
 
 Phosphorescence 91 
 
 Photograph, how made . . 118, 119 
 
 Photometer 129 
 
 Physics, definition of .... 1 
 relation of, to domestic 
 
 science 2 
 
 Pigments 135 
 
 colored 138 
 
 mixing colored 138 
 
 Pitch 79 
 
 international standard . . 89 
 
 of screw 244 
 
 standard 88 
 
 Pivot 233 
 
 Plane mirror 97, 100 
 
 Plaster 45 
 
 lath 45 
 
 Polarization 218 
 
 Power 245 
 
 delivered by pulleys . . . 246 
 
 Pressure, application of water 262 
 
 applied 7 
 
 effect of, on boiling point . 8, 9 
 
 effect of, on freezing point . 8 
 how to calculate, in an open 
 
 vessel 262 
 
 kettle . 9 
 
 water 260 
 
 Primary cells 220 
 
 coil 202 
 
 Principal axis 112 
 
 of mirror 98 
 
 Principal focus 99, 112 
 
 Prismatic window glass . . . 109 
 
 Prisms, refracting 109 
 
 Projecting lantern 122 
 
 Pulley .... 233, 241, 242, 243 
 
 Pump, force . 270 
 
 lift 269 
 
 Quality of Sound . . . . 79, 80 
 
 Radiant heat 2, 91 
 
 Radiation 41, 57 
 
 Radiators 58 
 
 Radical 212 
 
 Rain ......... 25 
 
 Range, kitchen 49 
 
 Rarefaction 70, 71, 84 
 
 Red 132 
 
 Reflected ray 97 
 
 Reflection .... 96, 103, 104 
 
 Reflection, law of 96 
 
 total . .... ... 108 
 
 Reflectors 58 
 
 Refracted ray 104 
 
 Refraction 103 
 
 angle of 105 
 
 index of 105 
 
 law of 104 
 
 Refrigerator 44 
 
 tested 13 
 
 uses of 12 
 
 Repulsion, law of, magnetic . 147 
 
 Resistance 155 
 
 what determines amount of . 155 
 
 Resonance 76 
 
 in closed pipes 83 
 
 in open pipes 84 
 
 principle of 76, 78 
 
 Resonators 80, 81 
 
 Retina, eye 120, 137 
 
 Roemer's method of finding 
 
 velocity of light .... 92 
 
 Salt on ice, effect of ... 13, 14 
 
 Saturation point 23 
 
 Scale, chromatic 87 
 
 major 86 
 
 tempered 88 
 
 Sclerotic coat of eye . . . . 119 
 Screw 233, 244 
 
INDEX 
 
 Numbers refer to pages. 
 
 303 
 
 Second class lever 234 
 
 Secondary cell 221 
 
 Secondary coil 202 
 
 See, how we 120 
 
 Sensible heat 2 
 
 Shades 138 
 
 Shadows 94, 95 
 
 Sheathing 45 
 
 Short-sightedness 120 
 
 Siphon 271 
 
 Slip-rings 159 
 
 Snow 25 
 
 Solids 259 
 
 Sound 74 
 
 characteristics of .... 79 
 effect of temperature on ve- 
 locity of 76 
 
 intensity of 79 
 
 interference of 78 
 
 nature of 74 
 
 quality of 79 
 
 reinforcement of ... 78, 83 
 things necessary for ... 74 
 
 velocity of 75, 84 
 
 waves, analysis of . . . 80, 81 
 waves, how they travel . . 75 
 waves, photographs of . 82, 83 
 
 Space 227 
 
 Specific gravity 278, 279, 280, 281, 282 
 method of calculating . . 281, 282 
 
 Specific heat 27 
 
 effect on climate .... 28 
 
 Spectrum, solar 132 
 
 Speed 252 
 
 Spherical mirror 98 
 
 Standard candle 129 
 
 pitch 88 
 
 Starting box 192 
 
 need of 193 
 
 Static electricity 224 
 
 Steam cooker 18 
 
 heating 16, 17, 18 
 
 Storage cell 220 
 
 charging of 222 
 
 dry, lead 222 
 
 Edison 223 
 
 lead, wet 221 
 
 uses of 222 
 
 Studding 45 
 
 Sub dominant 86 
 
 Subtractive method .... 138 
 
 Sugar, manufacture of ... 9 
 
 Surface tension . . . ' . . . 273 
 
 applications of 273, 274, 275, 276 
 
 Telegraph relay 167 
 
 sounder 166 
 
 system 167 
 
 Telephone . . 210 
 
 switch hoard, automatic . . 172 
 Temperature at which water is 
 
 densest 36 
 
 and quantity of heat compared 10 
 
 definition of 3 
 
 Tempered scale 88 
 
 Thermometer, alcohol used in 4 
 
 centigrade 4 
 
 changing from centigrade 
 reading to Fahrenheit read- 
 ing 5, 6 
 
 expansion involved in ... 4 
 
 Fahrenheit 4 
 
 fixed points of 4, 5 
 
 kinds of 4 
 
 mercury in 4 
 
 relation of centigrade and 
 
 Fahrenheit scales ... 5 
 
 uses of 4 
 
 Thermos bottle 44, 58 
 
 Thermostat 30, 31 
 
 Third class lever 234 
 
 Third, major 87 
 
 Three color printing process . 142 
 
 Three phase system .... 208 
 
 Three primary colors .... 136 
 
 Three states of matter . . . 259 
 
 Time 227 
 
 Tints 138 
 
 Tonic 86 
 
 Transformer . . ^204, 205, 206, 207 
 
 advantages and uses of . 206, 207 
 
 Translucent object .... 133 
 
 Transparent object .... 133 
 
 Triad, major 86 
 
 Trough 69 
 
 Tungsten lamp, candle-power of 130 
 
 Umbra 95 
 
 Unison 87 
 
 Units of measurement . . . 227 
 
 velocity 252 
 
304 
 
 INDEX 
 
 Numbers refer, to pages. 
 
 Vacuum 44 
 
 pan 9 
 
 Vapor tension 7 
 
 Velocity ........ 252 
 
 Ventilation 54, 55, 56 
 
 Vernier 267 
 
 Vibrating strings, laws of . . 81 
 
 Vibration, complete .... 70 
 
 Violet 132 
 
 Volt 174 
 
 Voltaic cell 217 
 
 addwater. 219 
 
 closed circuit 220 
 
 Daniell 220 
 
 dry 219 
 
 gravity 220 
 
 open circuit 218 
 
 secondary or storage . . . 220 
 
 sulphuric acid 217 
 
 wet salammoniac .... 218 
 
 Voltmeter 187 
 
 Volume 227 
 
 expansion 35 
 
 Walls of houses 45 
 
 Water in contact with glass . 274 
 Water vapor in the air 23, 24, 25, 26 
 
 Water, when densest .... 36 
 
 Watt 174 
 
 Watt-hour 174 
 
 Watt-hour meter 191 
 
 Wattmeter 188 
 
 Wave, characteristics of longi- 
 tudinal 70, 71 
 
 transverse 69, 70 
 
 Wave length . . 70, 71, 73, 132 
 longitudinal . . . 69, 71, 72, 75 
 motion ....... 67 
 
 motion, examples of ... 67 
 
 origin of 68 
 
 transverse . . 69, 70, 71, 92 
 
 velocity of 72, 73 
 
 Weather-board 45 
 
 Weather maps 268 
 
 Wedge .233,244 
 
 Weighing balance . . . 235, 236 
 
 Weight 233 
 
 Weight-arm 233 
 
 moment 233 
 
 Wheel and axle . . 233, 238, 239 
 Work . . . f . . '. . 173, 231 
 
 Work-in . ... 235 
 
 Work-out . . . :.. . . . . 235 
 
 Work, units of 231 
 
 Yellow . 132 
 
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 UNIVERSITY OF CALIFORNIA LIBRARY