HY51C3 BY S H AW S53 ,^xTH"vwm. <-?<^y Jtkaca, N.y Cornell Xllnivetstt^ Xibrar^ OF THE mew l^ork State College of Hflrtc ulture ^.f2..i<&o. .aajA.|.i.^ ^^ S^ jjt-^ g>^ tf>^ g^ ^^ -SS^-«g-^ 1 If ! < > 4 i \ c a Pig. B7. the centre of gravity of the wood C is. THE PENDULUM. 69 Start both pendulums at the same time, and in the same direction. Do they make the same number of vibra- tions in a minute ? Does a change in the amount of weight at the end of a pendulum affect the time of vibra- tion ? Let G vibrate through a small arc, and count the number of vibrations it makes in a minute. Then count the number of vibrations it makes through a little longer arc. Then through as long an arc as will allow the pendu- lum to vibrate steadily. Make the same trials with pendu- lum D. Does it make any difference in the time, whether the arc is long or short ? Experiment 52. — Take a string five feet long, and tie to the end of it an iron nut or a stone. Pass the string through the cross-piece used in the last experiment, and draw it up until it forms a pendulum equal in length to C or D. According to the facts already observed, all these pendulums will vibrate in nearly equal times. Pull up the string until you have made pendulum E half as long as pendulum C or D. Set it vibrating, and count the number of vibrations, or oscillations, in a minute. Has changing the length of the pendulum changed the time of vibration? How does the number of vibrations compare with the number of vibrations when it was twice as long? Make it one-fourth as long as C or D, and compare the times of the vibrations. If one pendulum is one foot long, and another four feet, their times of vibration will be compared as follows : 70 GRAVITATION. Length of Length of Time of Vibra- Time of Vibra- First. Second. tion of First. tion of Second. That is, it will take a pendulum four feet long twice the time to vibrate that it will a pendulum one foot long ; a pendulum nine feet long, three halves of the time it will take a pendulum four feet long. The time of vibration of one pendulum is to the time of vibration of a second pendulum as the square- root of the length of the first is to the square-root of the length of the second. Length of a Second's Pendulum at New Tork. — The length of a pendulum that vibrates just sixty times per minute, or once each second, is 39.1 inches. This is at New York ; for as gravity causes the motion of a pendulum, it follows that if the attraction of gravity changes, the vibrations of the pendulum will vary at different points on the earth's sur- face. If two pendulums of equal length be vibrating, one near the pole and the other at the equator, the one near the pole will vibrate the more rapidly. Questions and Problems. 1. What is meant by the amplitude of a pendulum's vibration ? 3. Make a drawing on the black-board, and explain with its aid how the pendulum of a clock is kept vibrating. 3. What is the length of a pendulum vibrating seconds at New York? 4. How long must a pendulum be at New York to vibrate once in two seconds ? 5. A clock loses time. What is the fault with its pendulum ? 6. Two pendulums are respectively sixteen and thirty-six inches long. How do their times of vibration compare ? kewton's thibd law of motion. 71 NEWTON'S THIRD LAW OF MOTION. Experiment 53.— Take two ivory or glass balls, and suspend them* by cords so that they may hang in contact, as shown in Fig. 58. Draw the ball A aside, and let it swing against ball JB. N'otice that when A strikes against Ji, A is instantly stopped, and ball J3 takes up the motion of A, and is moved to the point D. The first ball acts upon the sec- T • • • . . ]y " — " 111 OnQ, imparting its motion to it, BiiiiiiiimmmiiniuuiiimmuimiimmmuiiiuiiiliimiumiuiiimiAir^ and the second ball reacts upon the first, depriving it of its motion. How does the momentum of ball A, when it strikes ball Ji, compare with the momentum ball Ji has the instant it begins to move toward the point D ? Third Law. — To every action there is an equal and opposite reaction.^ * These balls may be suspended by gluing a narrow strip of leather to each ball, letting the middle part of the strip project up from the ball, thus forming a loop to which the string can be fastened. f When a shot is fired from a gun, if the gun be free to move, there is considerable recoil, the shot moving forward and the gun backwards. If the gun be fixed to the ground, the shot is apparently the only thing which moves. If the shot were held fast, and the gun were free to move, the gun would move backwards. In this case we see, then, that to the action which impels the shot forward, there is a contrary reaction which impels the gun backwards. When a horse is loosely harnessed to a car, it may sometimes be observed that an inexperienced animal starts forward quickly ; but suddenly the traces tighten, the car is jolted forward, and the horse is jolted backwards. If a locomotive with a heavy train be suddenly started, it will be 72 GRAVITATION. ENERGY. Kinetic and Potential Energ-y. — We have seen (p. 59) that whenever motion is effected in over- coming resistance, work has been done. If, then, a pendulum he raised from a state of rest at S to A (Fig. 59), motion has been eifected in overcoming the resist- ance of gravitj-, and a definite amount of work has been done. If held at A, the pendulum has no motion. It has a tendency, how- ever, to move, and is only kept from motion by the restrain- ing power of the hand. Remove the hand ; the pendulum^ falls to its original position S, but does not stop there. It still has a tendency to move, and lawless some external force is applied, it will rise to C. In the position C, also, it possesses the same tendency to move. It may be said, then, that the pendulum, when at A or C, has an ability to do work, due to its iMsHlon ■ but at B it has an ability to do work, due to its motion. The ability to do tvorh (s called Energy. The energy due to the motion of a hody is called Kinetic Energy. seen that its wheels may uselessly turn round ; it has given a sudden pull to the carriages, and their reaction upon it is equivalent to a backward pull given to a moving engine. When a stone is thi-own upwards from the earth, the earth is thrown back by recoil, and moves downwards to a very small extent as long as the stone continues to ascend ; when the stone is at its highest point the earth is at its lowest, and as the stone falls, the earth ascends to meet it. This is, of course, not the result of direct observation, but is deduced by way of inference from Newton's third law of motion, which ENERGY. 73 The energy due to the position of a body is called Potential Energy.* At A the pendulum has no motion, but it has energy, as it has the ability to do work. Which kind of energy, then, has it ? Which kind at G ? At B it has motion, but if stopped all ability to do work is destroyed. Which kind of energy, then, has it at B ? At E it has motion, but if stopped it still has the ability io move when released. What can you say of its energy at this point ? The potential energy of the pendulum at A is greater than at E ; for if it be let fall from A, it will do more work, i.e., move farther than if let fall from E. But at E there is a corresponding increase of motion, i.e., of kinetic energy. The kinetic energy, then, of the swinging pendulum increases from A, where it is nothing, to B, where it is is confirmed by all phenomena, terrestrial and astronomical, by which it can be put to the test. — Principles of Physics. Daniell. * All the forms of energy may exist in one or other of two condi- tions; the one an active, motive condition, the other a, passive condition. . In order to prevent misconception, the specific terms hinetic a,ui poten- tial are respectively applied to these two conditions of energy, the former being derived from a Greek word meaning motive, and the latter from a Latin word meaning ^ossi'JZe. Gravitation offers the most simple illustration of these two condi- tions. A stone falling from a height ; a stream of water running, that is, falling slowly from a height ; a hammer-head descending — all obviously possess active, actual, or kinetic energy, for they are capable of overcoming resistance in virtue of their mass in motion. On the other hand, a stone lodged on the face of a mountain, a clock- weight wound up, a head of water pent up in a cistern, though all actually at rest, possess, in virtue of the very position of their masses relatively to the earth, a potential energy which we may at any moment convert into actual. The one is as real as the other, and the one represents, and has been called into existence at the expense of a 74 GEAVITATION. greatest ; while the potential energy decreases correspond- ingly from A, where it is greatest, to £, where it is nothing. What is the case when the pendulum swings from JB to C? The sum of the kinetic and the potential energy at any point in the arc is equal to their sum at any other point. What is lost in kinetic energy is gained in potential energy, and the reverse. A stone is thrown perpendicularly into the air. Which kind of energy has it the instant it leaves the hand ? Which, at its highest point ? Which, just before it strikes the ground ? What part is kinetic energy, and what poten- tial, when it has passed over one-third of the entire distance in its descent ? The hinetic energif of a body in motion is reckoned in foot-pounds, and is equal to one-half the protluct of the weight by the square of the velociti/ per second, divided by thirty-two ; or, expressed in formula : K. E. 33 Conservation of Energy. — Sound, heat, and light are forms of energy, and are mutually convertible. Instances of this convertibility are : The energy of heat of the burning coal in the fire-box of the locomotive is con- verted into the energy of motion of the revolving drive- definife amount of the other. We cannot have actual motive energy under gravity from a body lying on the ground, or from a clock-weight run down ; it must have been raised to a height first, and have pos- sessed the possible or potential energy. And when we do have it, it is merely the restoration of the actual energy which has been expended in raising the mass to a height. Thus the energy of a clock-weight shown in moving the machinery for the space of a week Is merely the doling out of the potential energy imparted to it in a few seconds in winding up. — I>r. Neil Amot. SUMMAEY. 75 wheel. In the production of the electric light, the energy of heat drives the engine ; this energy of motion is im- parted to the dynamo, and changed by it into electrical energy ; this is in turn changed in the lamps to the energy of light. The principle that the sum total of energy in all forms in the universe is always equal to a fixed quantity is called the principle of Conservation of Energy. SUMMARY. Gravitation is the attractive force which acts between masses. Law of Gravitation. — Every particle of matter in the universe attracts every other particle. This attraction is directly as the mass and inversely as the square of the distance through which it acts. Gravity is the attraction existing between the earth and the bodies on it. Laws of Gravity. — I. The earth attracts all bodies towards its centre. II. The weight of a body above the surface of the earth decreases as the square of the distance from the centre of the earth increases. III. A body on the surface of the earth will weigh more as it approaches the poles, and less as it approaches the equator. The Centre of Gravity of a body is the middle point of that body with reference to weight. A body is in Stable Equilibrium when any disturbance of the body affecting the centre of gravity raises it. A body is in Unstable Equilibrium when any disturbance of the body affecting the centre of gravity lowers it. A body is in Indifferent Equilibrium when every change in its position neither raises nor lowers its centre of gravity. The Line of Direction of a body is a line from the centre of gravity of that body towards the centre of the earth. Motion is change of position. Force is that cause which moves or stops a body or tends to alter its state of rest or motion. Weight is the force exerted by a body, due to the effect of gravitation Work is the overcoming of resistance of any kind. 76 GRAVITATION. A Foot-pound is the amount of work expended in raising one pound one foot high against the force of gravity. Momentum is the quantity of motion a body has. Velocity is the rate of speed at which a body moves. Law of Falling Bodies. — Bodies fall to the earth with uniformly accelerated motion. Newton's Laws of Motion. — I. A body in motion moves uniformly in a straight line, unless acted upon by some outside force. Inertia is the inability of a body to change its own state of rest or motion. II. A force produces the same effect upon a moving body or a body at rest, whether it acts alone or with other forces. The Resultant of two forces equals the diagonal of a parallelogram, whose sides represent in length the respective forces, and whose angle is equal to the angle between the forces. A Pendulum is a body suspended so that it can swing freely to and fro. Laws of the Pendulum — I The time of vibration is independent of the material of the pendulum, and of the length of the arc through which it vibrates. II. The time of vibration varies directly as the square root of the length of the pendulum. III. The time of vibration diminishes as the pendulum moves from the equator towards the poles. Third Law of Motion. — To every action there is an equal and opposite reaction. Energy is the ability to do work. Kinetic energy is the ability to do work, due to the motion of a body. Potential Energy is the ability to do work due to the position of a body. Formula for computing kinetic energy : K.B. = iWV» 33 The principle of Conservation of Energy is that the sum total of energy in all forms in the universe is always equal to a fixed quantity. QTJESTIONS AND PROBLEMS. 77 Questions and Problems. 1. If the earth's mass were twice its present mass, with no increase in volume, what would be the effect upon one's weight ? 2. The distance between the centres of attraction of two bodies is doubled. What is the effect upon the attraction they exert towards each other ? 3. A body on the surface of the earth four thousand miles from the centre weighs four thousand pounds. If it were placed four thousand miles above the surface, what would it weigh ? 4. Suppose two bodies, ten miles apart, weighing respectively one hundred and fifty and four hundred and fifty pounds, were free to move towards each other as the result of their mutual attraction, what distance would each pass over ? 5. A body having a uniform motion passes over one hundred and fifty feet in three seconds. What is its velocity per second ? 6. A round body rolls more easily than a square body. Why ? 7. Draw a diagram on the blackboard, and explain why a ball rolls down hill. 8. How do the times of vibration of two pendulums, respectively sixteen inches and thirty-six inches in length, compare ? 9. It takes four seconds for a stone to fall from the top of a tower to the ground. How high is the tower ? 10. A cannon ball weighing twelve pounds is moving at the rate of eight hundred feet per second ; another cannon ball weighing fifteen pounds is moving at the rate of five hundred feet per second. Which has the greater momentum ? 11. Why does a steamboat continue to move after the paddle-wheels have stopped ? 12. A rifle ball shot vertically upward returns to the earth in eigh teen seconds. How far did it ascend ? 13. Explain why two plumb lines are not exactly parallel. 14. A stone is let fall from a steeple. It strikes the ground in three seconds. What is its velocity per second when it strikes ? The stone weighed two and a half pounds ; what was its momentum when it struck ? 15. How far must the earth be removed from the sun that the attrac- tion of the two bodies may become one-ninth of what it now is ? 16. Where is the centre of gravity of a ring of uniform size and density ? 17. Make a "witch'' (Pig. 60) by taking a piece of eider pith ^, FlO. 60. 78 GRAVITATION. and gluing it to a piece of lead B (half a large shot). Explain why, ^ when it is laid on the table, it suddenly takes _ an upright position. 18. Explain why a pendulum vibrates. How long would a pendulum encountering no friction vibrate in a perfect vacuum ? 19. How far will a body fall in the ninth second ? How far in nine seconds ? 30. A body weighs five hundred pounds on the surface of the earth. What would be the weight of the same body were it sunk fifteen hundred miles below the surface ? 21. A ball is shot upward with a velocity of three hundred and fifty- two feet. How many seconds will it continue to rise ? How high will it rise ? What time will elapse before it reaches the ground ? 33. Define energy. 23. How is the kinetic energy of a body found ? 34. Could a bird move itself were it suspended in space, the space, except that occupied by its body, being perfectly empty ? 25. A man weighing one hundred and fifty pounds ascends by the stairs to the top of a lighthouse, a distance of one hundred and forty feet. How much work has he done ? 26. Find the energy of a body weighing eighty-two pounds, and moving at the rate of twenty-six feet per second. 37. What is the energy of a locomotive and tender weighing eighty tons, and moving at the rate of a mile a minute ? 28. Suppose two ferryboats, each weighing a thousand tons, to collide when the first is moving at the rate of seven hundred and twenty and the second at the rate of eight hundred and fifteen feet per second. What amount of energy would be expended to crush the boats ? 29. What must be the time of the rotation of the earth in order that at the equator the centrifugal tendency of bodies may just overcome the attraction of gravity ? CHAPTER IV. Fig. 61. HYDROSTATICS. Experiment 54. — Tie a piece of thin rubber tissue * (see Appendix, § 3) over tlie mouth of a small bottle (Fig. 61). Load the bottle with a piece of lead so that it will sink. Nearly fill a glass jar with water. Place the bottle on the bottom of the jar in the water (Fig. 62). Notice the rub- ber covering over the mouth of the bottle. Stand the bottle up. Notice again the thin rubber over the mouth of the bottle. Turn the bottle upside down. Place the bottle in several other positions. Can you find any position in which the water does not press JTiG. 62. the rubber inward ? f- a Experiment 55. — Cut out of an ordinary business-card a circular piece, a trifle larger than the large end of a lamp- chimney. Wet the end of the lamp-chimney. Put this disk over the wet end, and press the chimney down into a glass jar filled with water. What holds the disk in posi- tion ? Now pour water very carefully down into the chim- ney. To what height does the water rise when the disk drops ? Repeat the experiment, plunging the chimney to * The rubber from a bpokep tov b^JJopu is very good for this purpose. 80 HTDKOSTATICS. a greater and a lesser depth. Push the chimney obliquely down into the water. Experiment 56. — Tie a piece of rubber tissue over the end of a lamp-chimney which has been shortened (see Appendix, § 2), and to the other end of the chimney fasten a bent glass tube, as in Fig. 63. Fill the chimney with water colored with red ink, putting in enough water to rise one-third the distance to the top of the small tube. Mark on the small tube the height to which the colored water as- cends. Notice that the weight of the water bulges the rubber outward. Press the rubber slightly with the fin- ger. What is the effect upon the water in the glass tube ? Immerse the chimney in a pail of water just below the surface. Does the pressure of the water cause the liquid in the tube to rise ? Put the chimnej'' a little farther under ; what do you notice as to the rising of the water in the tube ? Move it down to the bottom. Where do you find the pressure of the water in the vessel the greater, near the surface or at the bottom ? Fig. &i. Water exerts pressure in every direction, and the pressure increases tvith the depth. Experiment 57. — Fill a thin glass bottle with water nearly to the top, and place a tight-fitting cork so that it just rests on the water in the neck of the bottle. DIRECTION OF PRESSURE. 81 as in Fig. 64. Put the bottle upon the floor, press heavily upon the cork with a stick, and notice what happens. Do not hold the bottle while pressing. Water is only slightly compressible. Experimeut 58. — Into the small end of a lamp- chimney put a rubber cork having two holes in it. Through these holes pass short pieces of glass ^'°- **• tubing, and over the outer ends of these fit rubber tubing, letting the rubber tubing end in pieces of glass tubing, as shown ^ in Fig. 65. Fasten the apparatus by clamps. Now pour water into the lamp-chintney. Compare the height to which the water rises in the glass tubes with the height in the chimney. Water in pipes will rise as high as its source. I'm. 65. Experiment 59. — Put into the larger end of a lamp- chimney a 'So. 10 rubber cork (see Appendix, § .3), with a glass tube inserted, as in Fig. 66. Connect this with the tin apparatus a (see Appendix, § 3), by means of a piece of rubber tubing. Over each opening in the tin tie a piece of rubber tissue ; suspend a lamp-chimney, as in Fig. 66, and fill the whole apparatus with water by pouring it into the small end of the chimney. Then, with the chimney brimful of water, tie a piece of rubber tissue over the 82 HYDROSTATICS. small end. Press in the rubber, and notice carefully the effect on each of the rubber coverings of the tin apparatus. If the pressure is exerted by the fin- ger at A, and the effect of the press- ure is noticed at each of the coverings of the tin apparatus, what is it that transmits, or carries, the pressure from the point ^-1 to each of the rubber coverings on the tin ? Wlmt is the direction of the pressure ex- ^a \X: erted at £ ? What is the direction at C ? What is the direction at D ? What, at JS ? Is the pressure, then, transmitted 1)V the water in all directions? Jftitcr frail. •oil its presstife equally in all directions. Exi>eriuieut 60. — Take out the rubber cork at ^, and allow all the water to flow out of the ajDparatus. Replace the cork again at JEJ. Xow press in the rubber at ^1. Did the rubber at -B, C, and J) again show the effect of the pressure of the finger ? In Experiment 59, we found that water transmits pressure in all directions. Wliat is the substance in the tubes now that transmits the pressure from the point A to the dif- ferent points £, C, and D ? DIRECTION OF PRESSURE. 83 We learn, then, that air, as well as liquids, will transmit pressure ; but it will be found that as air is very com- pressible it is not so useful for this purpose as water. You discovered in Exp. 57 that you could not compress water to any noticeable extent. Experiment 61. — Take three tubes an inch in diameter (parts of lamp-chimneys of the same size will an- swer), insert rubber stop- pers in the bottom, and connect these tubes by bent glass tubing, as^. shown in Fig. 67. Secure the apparatus in an up- right position by any con- venient means. Pour water into one of the tubes. It will stand at the same level in all. Why ? When the tubes are about half full, mark the height to which the water ascends in each. Into one of the holes of a rubber stopper that can be easily moved up and down in one of the tubes fasten a stick (see A, Fig. 67) ; close the other hole of the stopper. Now wind a strip of cotton cloth around the stopper, so that it will fit water-tight the tube £. Press the stopper down on the water until you have lowered the surface of the water in tube JB one inch, and measure the height to wliich the water has risen in each of the tubes C and Z>. Fig. 67. 84 HYDROSTATICS. If we pressed down on the surface of the water in tube B with a force of twelve pounds, how many pounds press- ure would the water in each of the tubes C and D exert upward ? Why ? Suppose that we connect the tube -Z? of Fig. 67 with one tube E, Fig. 68, as large as both C and D of Fig. 67. How much upward pressure will the surface of -E" exert if there is a downward pressure on the surface of H of five pounds ? If the stopper in S is pushed down two inches, how high would a water-tight stopper in E be raised ? The Hydraulic Press. — Hydraulic presses and jacks work upon the principles shown in Experiments 59 and 61. In Fig. 69 a hydraulic press is shown. B is the cylin- der and C the piston, to the upper part of which is attached the platform. D is the pipe running from the force-pumps FF ; W is the pipe connecting the force- pumps with the water supply. The force-pumps are worked by the eccentrics EE attached to the driving shaft and pulley P. The pulley is run by a belt. One revolu- tion of the pulley gives one stroke of the pump. »S' is a safety-valve, and It is used to let the water out of the cylinder when it has been filled with water. This press is built to exert a power of 6,000 pounds to the square inch. HYDRAULIC PBESS. 85 Why is it necessary to let the water out of the cylinder 7 How often must the water be let out of the cylinder ? Fio. 69. Sometimes called " Bramah's Press." — The hydraulic press is sometimes called JBramah's Press, because Bra- mah, by the invention of a water-tight collar, overcame 86 HYDROSTATICS. Hif n the leaking of water around the piston. This collar (Fig. 70) is made of leather and iits closely around the piston. Fig. 71 shows a sec- tion of the collar. A, B, (7, i» is a groove cut in the cylinder, in which is placed the col- lar. "When pressure is exerted the water passes up the side of the pis- ton and forces the lips of the leather collar in the directions F, P. The greater the pressure the more difR- ^"*- ''i- cult it is for the water to pass heyond the collar. The Hydraulic Jack. — Hydraulic jacks are used for lift. ing. Fig. 72 is a picture of a hydraulic jack. G is the cylinder, P the pis- ton, F the force-pump, worked by the handle H. E X p e r i lu ent 62. — Procure a pickle or olive bottle having its mouth on the inside the same size as the large end of a lamp- chimney {A and _B, Fig. 73). Procure also a glass tube C the same size as the large end of the chimney (Fig. 73), and about eight inches long. Remove the bottom of the bottle (see Appendix, § 2), Fig. 78. PRESSURE OF A COLUMN OP WATER. 87 and grind the nozzle smooth with emery. Cut a circular piece of wood out of a piece of cigar-box, a little larger than the mouth of the bottle, and stretch smoothly over this a piece of rubber tissue. Drive a small wire staple or screw- eye into the middle point of the smooth side of the disk, and tie a string to the staple. With the straight piece of glass tubing set up a piece of apparatus, as shown in Pig. 73. Put weights on the scale-pan to hold the disk against the bot- tom of the tube with a little force. In addition, place on the scale-pan some definite weight, say one ounce. Now pour water carefully into the glass until the pressure on the disk causes the water to run out. Measure the height of the water above the di Remove the glass tub( bottle. Pour in water Measure the height of t the experiment, putting 1 bottle. Does the quantil upon the bottom. Upon 88 HYDROSTATICS. To Calculate the Pressure of Water. — A cubic foot of water weighs one tiiousand ounces, or sixty-two and a half pounds. Wliat is the pressure upon the bottom of a rect- angular tank three by four feet, in which the water is two and a half feet deep ? What is the pressure upon the bottom of a tank five feet square, with seventy-eight inches of water in it ? The 2>)'essiire upon fJie side of a tank, or reservoir, is equal to file weight of a column of water, whose base is the surface jjressed upon, and whose height is the depth of the water to the middle point of that sur- face. How many times the weight of tlje water is the entire pressure upon a cubical tank filled with water ? What is the pressure upon a dam thirty feet long, with the water twenty-six feet deep? SPECIFIC GRAVITY. Experiment 63. — With the balance get the exact weight of a stone about the size of an egg by tying a thread to the stone. Then place a tumbler of water so that the stone hangs in it from the beam of the balance, as in Fig. 74. Weigh the stone while thus immersed. Is there any dif- ference in the weight ? Ascertain carefully these Aveights and their difference. Set a tumbler in a saucer, and pour water into the tumbler until it just flows over the edge. Carefully sponge up out of the saucer the water that flows over. Lower slowly into the water in the SPECIFIC GRAVITY. 89 tumbler the stone just weighed, and then take the stone out again. Lift the tumbler from the saucer, weigh the water which has flowed over the tumbler into the saucer, and compare its weight with the difference in weight of the stone in water and out of water. It is, of course, very plain that the stone when placed in the tumbler displaces an amount of water equal to its own bulk. If this amount of water was exactly what the stone lost in weight when placed in watev, it is also plain that the water buoyed up the stone with a force equal to the weight of the water displaced by the stone. If the displaced water weighed half as much as the stone weighed, it is evident that the stone weighed twice as much as the water ; hence, we say the weight of this stone when compared with its own bulk of water is two, meaning that its weight is , twice that of water. If the water dis- placed by the stone was one-fourth its weight, the weight of the stone when compared with its own bulk of water would be four. This comparing the weight of a solid with the weight of an equal bulk of water is called finding the Speeific Gravity of that solid. Find the specific gravity of a piece of iron. Of a piece of lead. Of a piece of glass. Of a piece of copper or brass. Experiment 64. — Place a tumbler in a saucer, and fill it with water until it overflows a little. Carefully sponge up all the water in the saucer. Take a piece of wood containing about eight cubic inches and weigh it carefully in the air. Now with an awl or a pen push the wood 90 HTDROSTATICS. down carefully into the tumbler of water (Fig. 75). Weigh accurately the water that overflows. How does the bulk of water in the saucer compare with the bulk of the wood ? The weight of the wood is what fractional part of the weight of the water? Or, to put the ques- tion in other words, What is the specific gravity of the wood ? Pig. 75. ■Experiment 65. — Balance an ounce bottle on the scales. Fill the bottle even full of water, and weigh the water care- fully. Pour out the water, dry the bottle, and fill with alcohol or oil. Find the weight of the alcohol or oil. What part of the weight of the water is the weight of the alcohol or oil ? Or, what is the specific gravity of the alcohol or oil ? It requires delicate apparatus to obtain the specific gravity of gases, hence experiments of that nature are omitted. The weight of a stibstance compared with the iveight of an equal bulk of some other substance taken as a standard is called its specific gravity. For solids and liquids, distilled water is the standard; for gases, hydrogen is the standard. The specific gravity of the standard, whether water or hydrogen, is 1. CARTESIAK DIVEB. 91 In the following table will be found the specific gravity of several Important substances (at 33° F., compared with water at 39.3° P.). Solids. SUBSTA2TCE. SPECIFIC GRAVITY. Platinum (rolled) 33.069 Gold (cast) 19.358 Lead " 11.352 Silver " 10.474 Copper " 8.788 Brass 8.383 Iron (cast) 7.207 Tin " 7.291 Zinc " 6.861 Marble 2.837 Cork 240 Liquids. Substance. Specific Geatitt. Mercury 13.598 Sulphuric acid 1.841 Milk 1.033 Sea water 1.036 Olive oil 915 Alcohol (absolute) 793 Ether 713 CARTESIAN DIVER. Experiment 66. — Procure a small bottle two inches long and about half an inch in diameter. Fit a small cork into the mouth, and cut the cork off even so that it does not project beyond the bottle. Then remove the cork, and cut a small notch lengthwise of the cork so that by shaking the bottle slowly, after it has been filled with water, a small drop of water can be forced out. Fill the bottle with colored water and insert the notched cork. Procure another bottle about six inches long and fill with water to within three-fourths of an inch of the top. Drop 92 HYDROSTATICS. 1 '■_ JElDl fi ' ■ - o .' ■ i| !" Szl^. l] :: 1 the small bottle filled with water into the larger one. If it sinks, shake out with a quick movement of the hand a drop of water. Test whether the bottle still sinks. If so, shake out another drop and try again. Repeat this till the small bottle with bottom up will just float on the top of the water (Fig. 76). Xow tie tightly a piece of rubber tissue over the mouth of the large bottle. Press upon the rubber and the small bottle will move down- ward. Lessen the pressure on the rub- ber and the small bottle rises. Watch the effect of the pressure upon the water in the small bottle. Why does the small bottle sink when you press the rubber on the larger bottle ? Pig. 76. Cartesian Imp. — A little figure in the form of an imp, made so it would just float under the ordinary pressure of the atmosphere, was first used when this principle was dis- covered. This figure was made to descend and ascend through the water at will by the operator, who pressed upon some portion of the vessel containing the water, and what caused the movements of the imp, or diver, was for some time a mystery. It was supposed to be a piece of witchcraft, and the little object was called a Cartesian Imp. BARKER'S MILL. Experiment 67. — Close one end of a quarter-inch glass tube ten inches long by melting it in the flame of an alcohol lamp. Pass the tube through the centre of a cork, barker's mill. 93 and place the cork firmly in the smaller end of an argand lamp-chimney. Then fit into this cork four short tubes, bent as shown in Fig. 77. Take a knitting-needle, or a piece of a steel umbrella-frame, sharpen one end, and fasten the other end in a block of wood. Set the chimney and tube down over the piece of wire, so that the closed end of the tube revolves freely on the sharp point of the wire (Fig. 77). Pour water into the chimney, and as it flows out of the end of the smaller tubes notice what oc- curs. Can you explain why the piece of apparatus , turns ? Do not say that it is caused by the water pushing out against the air, for the " mill " would revolve without air. Let the outer end of any one of the bent tubes be illustrated by Fig. 78. Compare the amount of pressure at Q with the amount at JP, due to the head of water in the chimney. ^ This principle was discovered by a man named Barker, and the apparatus has ever since been called Barker's Mill. The principle is frequently illustrated in revolving fountains sometimes seen on lawns, and in revolving sprinklers often used for wetting lawns. Fig. 77. -^0. Fi6. 78. 94 HYDROSTATICS. SUMMARY. Hydrostatics treats of liquids. Liquids exert pressure in every direction. This pressure Increases with the depth. Water in comnnunicating pipes will rise as high as its source. Witer transmita-pressure equally In all directions. The pressure upon the side of a vessel containing a liquid is the product of the surface pressed by the depth of water to the middle point of the surface. Specific Gravity Is the weight of a substance compared with the weight of an equal bulk of some other substance taken as standard. For solids and liquids, distilled water is the standard; for gases, hydrogen is the standard. Questions and Problems. 1. In what directions do liquids at rest exert pressure ? 2. State the three conditions upon which the pressure of a liquid upon the bottom of a vessel depends. 3. How much water does a piece of floating wood displace ? 4. What is meant by the specific gravity of a substance ? 5. Give the weight of a cubic foot of water. 6. Put the palm of the hand over the top of a lamp-chimney, and push the chimney down into water. Why does the water rush up the tube when the hand is removed ? 7. Explain why a liquid stands at the same height in the spout as in the body of a tea-pot. 8. If you try to lift out of water a large stone that is on the bottom, it moves easily till it emerges from the water, when it suddenly becomes very heavy. Explain this. 9. Why is it difficult to keep one's feet on the bottom when water is chin-deep ? 10. Explain the principle of the hydraulic press. 11. Iron is heavier than water. Why, then, does an iron ship float ? 12. A tank iilled with water is two feet long, one foot deep, and eighteen inches wide. It has a pipe three feet long, the area of whose cro.5S section is two square inches, leading down from the bottom. What is the pressure upon the end of the pipe when closed ? 13. A solid weighs fifteen pounds in air and ten pounds in water. What does an equal volume of water weigh ? What is the specific gravity of the solid ? QUESTIONS AND PROBLEMS. 95 14. Would a piece of iron float in melted copper ? 15. Refer to the specific gravity table, and tell what solids will float on mercury. 16. Does a person float more easily in salt water or in fresh water ? 17. A piece of ore weighed forty grains in air, but lost thirty grains when weighed in water. "What was its specific gravity ? 18. Suppose the specific gravity of petroleum is .88, and that a quart of water weighs forty ounces ; find how many gallons of petro- leum will weigh thirty-eight and a half pounds. 19. A vessel draws more water in a river than in the ocean. Why ? 20. The double gate of a canal is twenty-five feet wide and ten feet high. When the water is ten feet deep in the canal, what is the pressure on the gate ? CHAPTER V. PNEUMATICS, Experiment 68. — Take a small empty bottle. Is it really empty ? If it is not empty, what does it contain ? Tie over the top a piece of thin rubber tissue. Place the bottle in a large pickle bottle, as in Fig. V9. Put a rubber stopper in the pickle bottle with a tightly fitting glass tube in one hole. Stop the other hole of the cork. To the glass tube attach a piece of thick rubber tubing. Put the tube to the mouth, and draw out the air. What effect is observed in the small bottle standing on the bottom of the pickle bottle ? Is it the air that exerts pressure ? If so, why did we not notice the pressure against the rubber before drawing the air out of the larger bottle ? Experiment 69. — Take two bottles, as in Fig. 80. Fit the tube tightly in the cork, and put fife ffl A it in the bottle A. Let the '*■ ■ tube be bent so as to extend nearly to the bottom of both bottles. Be- Fig. so. fore putting in the cork with the tube, fill bottle A about PRESSURE EXERTED BY AIR. 97 half full of water and jjut just enough in B to cover the end of the tuhe. Place these under the receiver of an air-pump or in the pickle bottle used in Exp. 68, and ex- haust the air, observing closely what happens to the water in A. Air exerts an expansive force when the outer press- ure is removed. Experiment 70. — Tie a piece of rubber tissue over the large end of the lamp-chimney. What substance is there on each side of the rubber ? Put a rubber stopper in the other end of the chimney, stop one of the holes, and into the other fit tightly a short piece of glass, to which is attached a piece of rubber tubing (Fig. 81). Draw the air out of the chimney through the tubing, noticing whether the air outside presses the thin rubber down into the chimney. In what direction is the pressure ? Air exerts a pressure downward. Fm. 81. Experiment 71, known as Torricelli's Experiment. Take a piece of quarter-inch glass tubing thirty-three inches long. Tie tightly over one end a piece of mem 7 98 PNEUMATICS. brane such as comes tied over the stoppers of perfumery bottles, having previously soaked it in water to make it pliable. Put the tube aside till the membrane dries. Then fill the tube Pljjf with mercury,* put the fin- ger over the open end, and carefully invert it into a small cup containing a lit- tle mercury (Fig. 82). Ses that no air passes up through the mercury into the top of the tube. No- tice that the mercury flows out until it is at a height of about thirty inches. With a pin prick a hole through the membrane, and notice what takes place. Why did not the mercury flow com- pletely out of the tube before the membrane was pricked ? Pig. 82. Explanation. — In this experiment it is evident that some force equal to the weight of the column of mercury is hold- ing the mercury up in the tube. As air exerts pressure downward, and as air is the only substance in contact with the mercury, it is quite plain that the air presses the mer- cury up the tube, and that as soon as the air is allowed to * Twist a piece of paper into the shape of a hollow cone, and use this as a funnel to fill the tube. THE BAROMETER. 99 enter the top of the tube the mercury falls, because the air pressure is the same at both ends of the tube. A tube of mercury thirty inches high and one inch square weighs nearly* fifteen pounds, therefore, a column of air one inch square and the height of the atmosphere above the earth at the level of the sea must Vfeigh fifteen pounds to balance the column of mercury. At the level of the sea, air exerts pressure on a square inch of surface equal to a force of about fifteen pounds. This pressure we shall sometimes call one atmosphere. If some liquid lighter than mercury were used to per- form Exp. 71, would the tube have to be longer or shorter than for mercury ? Mercury is 13.6 times as heavy as water. How long then must a tube be to perform Exp. Tl with water. If the mercury stands at twenty-eight inches, how long must a tube be to perform Exp. 71 with water ? The Barometer. — A tube of mercury as used in Torri- celli's experiment illustrates a common barometer, for if you should allow the tube to remain with the top closed, you would notice from day to day that the height of the mercury varied. Some days the height of the column of mercury would be less than thirty inches, some days more than thirty inches. This shows that the pressure of air is not always the same. When the pressure of air becomes light the column of mercury falls. This fact is explained as follows : The earth is surrounded by a great ocean of air, concern- ing the depth of which authorities differ. Some scientists say that the air extends fifty miles above the surface, while * 14.7 pounds. 100 PNEUMATICS. others say it extends as high as five hundred miles. Whatever its height may be on an average, it is not always the same height in one place or the same height at all places. The barometer shows exactly all changes of atmospheric pressure. The height of the barom- eter does not tell the kind of weather, but any sudden rise or fall indicates a change of weather. The atmosphere becomes less dense as we ascend from the surface of the earth. If a barometer is carried up a mountain the. mercury is found constantly to fall as the elevation in- creases. As a rough rule the barometer falls an inch in ascending nine hundred feet. Fig. 84 is a sketch of a siphon barometer. » // Tlie space above the mepciirij in the tube of a barometer in free frotn air, and is called a Vacuum. THE COMMON PUMP. Fig. 84. Experiment 73. — Take the piece of glass tubing used in performing Torricelli's experiment, place the end in a tumbler of water, and, by applying the mouth to the upper end, draw out the air. Why does the water rise in the tube ? If you draw out some of the water, why does more water rise in the tube ? If the tube were thirty-six feet long, and you could exhaust all the air from the tube, how high would the water rise when the barometer stands at thirty inches? See p. 99. Experiment 73,— Fit up the following piece of appa- Bureau Nature Study, '"ORWELL UNTVEFSITT. ^ llkai^iLm N I / THE COMMON PUMP. 101 ratus : Take a lamp-chimney and place in the lower end a rubber stopper, closing one hole A of the stopper (Fig. 85). Through a small piece of soft leather thrust a wire nail, and drop this into the hole in the rubber stopper to act as a valve (see D in the figure). Next select a rubber stopper with two holes, that will slide easily up and down the inside of the chimney. Wind a strip of cloth about this stopper H, so that it will work water-tight in the chim- ney. Into one hole of the stopper j)ut a stick for a handle, and into the other hole ^put a wire nail thrust through a piece of leather, to act as a valve. A hole may be bored through the chimney at F (see Appendix, § 2), and a piece of rubber tubing inserted for a spout. Now place the lower end of the chimney in water, with the plunger -S pushed down as far as convenient. Raise the plunger nearly to the top of the chimney. What do you push out when you raise the plunger? What comes up through the valve D ? What causes it to rush up ? Now lower the plunger. Why does the water not run back through D ? Raise and lower the plunger several times, and explain fully how the pump acts. Fig. 85. Fia. Test. — Explain how the common pump 102 PNEUMATICS. (Fig. 86) works. A is the valve in the lower part of the barrel, or cylinder ; Ji is the valve in the piston, or plunger. The pipe G leads to water. THE FORCE-PUMP. Experiment 74. — Take the piece of apparatus you fitted up for Exp. 73, remove the valve E, and stop up the hole in the plunger. Take out the piece of glass used to close the hole A, and insert in the hole a piece of bent glass tubing, like G, Fig. 87. With rubber corks and glass tubing complete the piece of ap- ^ paratus, as shown in Fig. 87, being care- ful to have the valve O work easily. Support the piece of apparatus in any convenient way, and insert the tube II in water. Raise and lower the plunger. Explain how this force- pump works. Remove the air-cham- ber and the valve 0, and work the pump. Why does a continuous stream of water flow from the pump when the air-chamber is attached ? Fia. 87. THE FOBOE-PUMP. 103 Test. — Fig. 88 is an illustration of a force-pump, fre- quently seen. The plunger, instead of being solid as in some kinds of force- pumps, has a valve A. The cylinder, however, is air - tight, as the collar C, through which the plunger-rod moves, is packed to prevent the water from passing out around the rod. D is the valve in the base of the air-chamber ; J^ the delivery pipe. ^ is a faucet, used to draw water, but closed when water is to be forced up F/ B is the valve at the base of the cylinder. Ex- plain how the pump works. Pia. 88. Experiment 75. — Draw out a glass tube to a fine point over an alcohol flame (see Appendix, § 2), and push the end through a bored cork and into a bottle, as in Fig. 89. Attach a short piece of rubber tubing to the outer end of the tube. By means of tlie mouth draw all the air from the bottle' possi- ble, pressing the rubber tubing so as to prevent the air from going back into the bottle. Place the end of the rubber tubing under water and remove the pressure. A little Fio. 89. 104 PNEUMATICS. fountain is thus produced. Explain why the fountain flows. THE SIPHON. Experiment 76. — Take a small glass tube about eight inches long, and bend it in the shape shown in Fig. 88 (see Appendix, § 2), so that one end may be longer than the other. Fill the tube with water, holding the finger tightly over the short end. Insert the short end in a tumbler of water, letting the long end hang over the tumbk'i-, as in Fig. 91. Notice what takes place. Again fill the tube with water and place the finger over the end as before. This time place the longer end in the tumbler. Which way does the ■«' a t e r flow now ? ^Vhy is it that the water flows up the short end of the tube and down the long end ? Fio. 90. Fig. 91. Explanation. — The siphon is filled with water, and the shorter arm is immersed, as shown in Fig. 92 ; or, having placed the shorter arm in the water, the air is exhausted by applying the mouth at the orifice D. In the latter way, a vacuum is produced in the siphon, and the water in A will rise and fill the tube. The water will flow as long as the shorter arm dips below the surface of the water in A, or until the water stands at the same level in the two vessels A and D. The con- tinuous flow is caused by the difference in pressure at THE AIK-PUMP. 105 A and D. At A there is a pressure of one atmosphere minus the weight of a column of water equivalent in height to .4 ^. At Z> there ^. C is a pressure of one atmosphere minus the weight of a col- umn of water equiv- alent in height to CD. Since a column of water C D weighs more than a column of water A B, and the upward pressures at A and D are equal, each being one atmos- phere, there is less pressure at D than at A, hence the flow. The greater the difference between the levels of the liquid in the two vessels, the more rapid will the flow of water be ? Why ? Does the size of the tubing of which the siphon is made make any difference in the flow of the liquid? Why ? Is there any limit to the height of the shorter arm? Why? When the liquid is at the same level in the two vessels, why does the flow stop ? If we exhaust the air from the siphon by placing the mouth at the orifice B, what causes the water in the vessel A to rise and fill the siphon ? THE AIR-PUMP. The air-pump (Fig. 93) is a machine used to exhaust air from vessels. It consists of a piston B (Fig. 94) working air-tight in a cylinder. In the piston is a valve opening upward. The cylinder is connected by a tube F, with a ground glass plate P (see larger figure) covered by a bell 106 PNEUMATICS. jar A, called the receiver. The receiver has a ground edge 80 that when it stands on the plate it will be air-tight. Fig. D is a, screw to admit air under the receiver when necessary. As the piston Ji is lowered to the bottom of the cylinder, the valve opens in the piston, and the valve C closes (Fig. 94). When the piston is raised to the top of the cylinder, the valve in the piston closes, the air in the cylinder above JB is pushed out, thus tending to form a vacuum in the cylinder under the piston. But air by its expansive force will spread out through all the space in a vessel confining it, hence the air that was confined in the receiver expands through the tube i^, lifts the valve C, and fills both the receiver and the cylinder. If the capacity of the receiver is as large as the capacity of the cylinder, there is but half the quantitj'^ of air in the receiver that was there before the piston was raised. When the pis- ton is lowered again, the valve C closes, and as the air in the Fig. 94. THE AIE-PUMP. 107 cylinder is prevented by C from rushing back into the re- ceiver, it escapes through the valve in the piston B, so long as the piston is moving down. The air can never be entirely exhausted from the receiver, because only a part of the air is taken out at each stroke of the piston. A perfect vacuum cannot, therefore, be produced by an air-pump. Experiment 77. — Take a large flask having an air-tight stopper to which a piece of rubber tubing is attached, and a clamp to press the tubing together. Weigh these accurately. Pump the air out of the flask, and close up the tubing with the clamp so that air cannot rush back into the flask. Weigh again. Is there any difference in the two weights ? Suppose you should force air into the flask and stop the flask tight. Would the flask weigh more or less than it did before forcing air into it ? Air has weight. Experiment 78. — With two glass tubes, one A, ten inches long, and the other B, fif- teen inches long, and a piece of rubber tubing C, forty-four inches in length, fit up an appa- ratus, as in Fig. 95. Secure the rubber tubing to the glass by twisting copper wire tightly Fig. 95. about the tubing. Fasten the shorter glass tube A to the window casing or any upright standard. Fit a cork in the end of tube A, and measure downward four inches from the bottom of the cork. Calling this point zero, graduate the standard or window casing in inches in 108 PNEUMATICS. either direction. Now remove the cork and hold the apparatus, as in Fig. 95. Pour in mercury until it stands at zero, and is on a level in both tubes. Replace the cork in the shorter tube, and to secure the cork drive a nail over the top of it into the standard. How many atmospheres of pressure are there on the mercury in each tube ? Now raise the tube _B, as in Fig. 96, until the s four inches of air in the tube A have been com- pressed into the space of two inches. What is the height of the mercury in the g longer tube ? How many inches higher is it than the mercury in the shorter tube ? How does this dif- ference compare with the reading of the barometer ? * How many atmospheres of pressure are now exerted on the air confined in the shorter tube ? Lower the tube B, as in Fig. 97, until the air in the tube A occupies double the space that it did at first, or eight inches. How much higher does the mercury stand in tube A than in tube J3 ? What is the ratio between this dif- ference and the reading of the barometer? If, then, the pressure of the air on the surface of the mercury in the m Fib. 96. Pio. 97. * Very few schools possess a barometer. Some member of the school, however, can ascertain the reading of a barometer on the morning that Exp. 78 is to be performed. SUMMARY. 109 open tube B sustains a weight of mercury in the closed tube A equal to one-half an atmosphere, how many atmos- pheres of pressure are exerted on the air in the tube A ? Raise the tube B until the air in the shorter tube is compressed one-fourth, and note the difference between the heights of the mercury in the two tubes. How many atmospheres of pressure are exerted on the air in tube A ? Lower the tube B as before, until the air expands one- fourth. How many atmospheres of pressure are exerted on the air in tube A ? Comparing the amount of contraction and expansion of the air in the closed tube with the number of atmospheres of pressure upon it, can you state the law of the compressi- bility of gases ? The volume of a gas is inversely proportiotial to the pressure exerted upon it. This law is called Boyle's Law, sometimes MarioU^s Law, from the names of the men who discovered it about the same time. The law is true for all gases within cer- tain limits, but under great pressure there are variations from it. SUMMARY. Pneumatics treats of gases. Air expands when the outer pressure is removed. Air exerts pressure downward. Air exerts a pressure of about fifteen pounds to the square inch at the sea level. The barometer is an instrument for measuring atmospheric pressure. The valves of a common pump should not be more than thirty feet apart. In the force-pump advantage is taken of the elasticity of the air to pro- duce a steady stream of water. 110 PNEUMATICS. The siphon acts upon the principle of atmospheric pressure, and the difference in weight between the water in the long arm and that in the short arm. Only a partial vacuum can be formed with an air-pump. The Torri- cellian vacuum approaches nearest to a perfect vacuum. Air has weight. The volume of a gas is Inversely proportional to the pressure exerted upon it. Questions and Problems. 1. Upon what principle does the common elder pop-gun work ? 2. How can you show that air exerts a pressure downwards ? 3. Two siphons have their bore i inch in diameter, and their short arms each 1 foot long ; but the long arm of one is 3 feet in length, and the long arm of the other 18 inches. Will one empty a vessel any quicker than the other ? Give reasons for your answer. 4. If you blow strongly through the glass tube into the Bottle (Fig. 98), the water will flow out when the mouth is removed. Explain why. 5. What is a valve ? 6. When the mercury in a barometer stands at 30 inches, the atmospheric pressure is 14.7 pounds to the , square inch. What is the atmospheric pressure when the barometer stands at 27 inches ? 7. A well 20 feet deep is under the floor of a house. Can water be pumped out of the well with' a pump set on the ground beside the house, 25 feet from the well ? 8. A barometer which stood at 29 inches at the foot of a mountain is carried to the top, where it stands at 22 inches. How high is the moun- tain, and what is the atmospheric pressure at the top? 9. Explain why, when a glass is filled with water and a piece of paper placed over the top, the glass may be inverted (Pig. 99), and the water remain in the glass. 10. What is a vacuum ? 11. Explain how water may be raised with a, pump out of a well 40 feet deep. 13, The blood enters the right auricle of the heart, as shown in the Fie. 99. QUESTIONS AND PROBLEMS. Ill diagram, at A, passes through the opening B into the right ventricle, is thence forced through C, through the lungs, and into the left auricle at D, whence it passes into the left ventricle through U, being finally forced into general circulation through J'. There is a valve at each openiag. Can you show the action of the valves in each auricle, and in each ventricle, when it contracts ? 13. When the barometer stands at 28 inches, how high above the sur- face of the water can the lower valve of a pump be placed, disregarding friction ? 14. Can you explain why, if you exhaust the air from the chimney (Pig. 81), and turn it in any direc- tion, the rubber tissue over the end of the chimney will be pressed in ? 15. Explain why, when a door is suddenly closed, a remote door to a connecting room often closes quickly. 16. State Boyle's or Mariotte's Law. 17. A quantity of gas occupying a cubic foot of space at a press- ure of one atmosphere is compressed by a piston forced down with an additional pressure of one atmosphere. What volume will the gas occupy ? CHAPTEE VI. HEAT. HEAT AND TEMPERATURE. Experiment 79. — Touch the lower part of a stove in which a fire is burning ; how does it feel ? Touch a stove- lifter after it has been on the stove awhile ; how does it feel ? Touch a piece of iron in a distant part of the room ; does it feel cold or hot ? The lower part of the stove feels warm, the top feels hot, and the iron in a distant part of the room is cold. The agent tvhtch 2it'0(luces the sensations of warmth, hotness, and coldness is called Heat. Experiment 80. — Put on the stove a vessel containing snow or ice, and heat it. When first put on the stove it is ice-cold : after the ice or snow has melted, put the hand in the water and it is still found to be cold ; heat it still further and we say it is lukewarm ; apply more heat and it becomes warm ; still more will make it hot, and then it becomes boiling hot. "We use the terms boiling hot, hot, warm, lukewarm, cold, and ice-cold to express the state or condition the water is in with respect to the heat that affects the hand. These names are the names of temperature ; and in these experi- ments we have used the hand to measure the temperature. TEMPERATURE. 113 Experiment 81. — Place a piece of hot iron upon a piece of cold iron ; after a while the hot iron loses heat, and we say its temperature falls. What do you notice about the temperature of the piece of cold iron ? What is Temperature J — When a substance is growing colder we say its temperature is falling, and when it is growing warmer we say its temperature is rising. The state or condition of a body with reference to its heat is called Temperature. Temperature is not heat itself any more than the level of water in a pond is the water itself ; and as water falls from a higher to a lower level, so heat passes from bodies at a given temperature to bodies at a lower temperature. Experiment 83. — Take three vessels, the first contain- ing water as hot as the hand can bear ; the second con- taining lukewarm water, and the third very cold water ; plunge the right hand into the first vessel and the left hand into the third, holding them there a short time. Then plunge both hands into the second vessel. How does the water in the second vessel feel to each hand ? To which hand does the water feel warm, the hand that was in the first vessel, or the one that was in the third vessel ? How can it feel warm to one hand and cold to the other, when we know that both hands are in the same vessel ? Bodily Sensation not to be relied on to determine Tem- perature. — Exp. 82 proves that we cannot depend upon the hand or bodily sensation to determine temperature. This fact is also noticeable in a room containing several people. Some will feel cold, while others will feel warm. 8 114 HEAT. Instruments are made by which temperature can be accurately measured. Such instruments are called ther- mometers. GENERAL EFFECT OF HEAT ON SOLIDS, LIQUIDS, AND GASES. Experiment 83. — Take a piece of iron about twelve inches long, fasten one end to a block of wood with nails, as shown in Fig. 100. Rest the other end upon a fine knitting- needle, placed upon a hard block of wood which should be very smooth. If the block is not smooth, place a smooth piece of metal under the needle. Fasten a broom splint to the end of the needle with sealing-wax and let it hang down over the end of block B, Fig. 100. Tack a semi- circular scale A on the end of the block. Heat the iron bar by means of an alcohol lamp or some other hot flame, by moving the flame from one end toward the other along the bar. Does the splint move to the right or to the left? Pig. 100. "What effect, then, does the heat have upon the iron bar? Repeat the experiment, using brass instead of the iron, also copper and glass, and note the efl^ect upon each material that is used. Expansion in length is called linear expansion. Experiment 84. — Take a piece of round iron three- fourths of an inch in diameter, and file a hole in a piece of GENEEAL EFFECT OF HEAT. 115 sheet iron or tin, so that the piece of iron will pass through the hole closely. Heat the piece of round iron and attempt to put it in the hole through which it passed before being heated. You will notice that the heat has expanded the iron so much that it will not pass through the opening. In Exp. 84 the round iron grew larger in area than the area of the hole. This showed what is called superficial expansion. When in addition to this superficial expansion we take into account that the bar also expanded in length, there results, as a total, what is called cubical expansion. Experiment 85. — Take a two- ounce flask or thin bottle and fill it with clear water that has been boiled, and allow it to cool. Color the water with red ink. Pass a tube twelve inches long through one of the holes of a rubber cork, close the other hole, and insert the cork and tube in the flask (Fig. 101). Be sure that the flask is so full of water that when the cork is pressed in, the water will rise in the tube a short distance above the cork. Mark on the tube the height to which the water rises, by tying a fine thread around the tube. Immerse the flask in hot water. Notice how the water in the tube moves. Give reasons for the two movements j,^^ j^j which the column of water makes. After a few minutes remove the flask. Notice the effect upon the water in the tube. Experiment in the same way with alcohol. Do not, how- ever, color the alcohol, and be sure there are no flames 116 HEAT. near. If possible, try the experiment with mercury, effect has heat upon liquids ? What Experiment 86. — Take a large flask, having a capacity of one quart. Bend a tube, as shown in Fig. 102, and put in the bend a small quantity of mercury. Hold the bulb of the flask between the two hands. Watch the mercurj^. What does the experiment teach ? The Thermometer. — You noticed in Exp. 85 that water rose in the tube when heated, and when the heat was withdrawn the water slowly fell again ; this was true also of the mercury and the alcohol. The fact that liquids expand by means of heat, and contract when heat is taken from them, suggested the method of making the ther- mometer. Alcohol is more suitable than either water or mercury, because of its greater expansion when heated ; but alco- hol and water both boil and become vapor at a much lower temperature than mer- cury, and mercury is therefore used in making most ther- mometers. Then, too, if the tube is left open, as in Exp. 85, the liquid would evaporate, and the pressure of the atmosphere would also affect the rising of the liquid. Hence the tube must be closed, and there must be no air above the liquid, because the elasticity of the confined air would affect the rising of the liquid upon being heated. Then there is so much liquid in the flask that it takes a long time for it to feel the effect of the heat. By using a small vessel and a tube ^vith a very fine bore the best results can be obtained. Fig. 102. KINDS OF THERMOMETEES. 117 The standard merourial thermometer of commerce is m"ade as follows : It consists of a glass bulb and a stem with a minute hair-like bore. Through the opening in the end of the tube, the bulb and a portion of the stem are filled with mercury, which is then boiled to expel all the air and moisture. While the mercury is boiling, and has thus reached its greatest expansion, the top of the tube is twisted after being heated by means of a blow-pipe. It is necessary to twist the tube when melted to close the opening ; for if it were simply drawn out to a fine thread the opening would still exist. After the open end is closed the mercury is allowed to cool, and as it settles in the tube it leaves a vacuum above it. The thermometer is now ready for a scale. The thermometer is plunged into melting ice or snow, and the point to which the mercury then settles is mai'ked the freezing point. It has been proven that the mercury will always fall to this same point when subjected to the same temperature. The next point to be fixed is the point at which water boils ; but here great care m^ust be taken, for water contain- ing different minerals or other impurities does not always boil at the same temperature. Then, too, the pressure of the atmosphere affects the point or temperature at which water will boil. In order, therefore, to get a standard, pure water is boiled, the barometer standing at a height of thirty inches. The thermometer having been taken from the .melting ice is confined in the steam rising from the boiling water, and the height to which the mercury rises is called the hoiling point. Different Kinds of Thermometers. — All thermometers are made thus far alike, the point at which the water boils being called the hoiling point on the thermometer, and the point at which the mercury stopped while the bulb was plunged in melting ice is called the freezing point. In the thermometers most in use the distance between these two 118 HEAT. points is divided into one hundred and eighty divisions, and thirty-two more divisions of the same length are con- tinued below the freezing point, the thirty-second being marked 0, or zero. The boiling point on this thermometer is one hundred and eighty divisions, or degrees as they are called, above the freezing point, or two kundred and twelve degrees above the zero point. A thermometer graduated thus is called the Fahrenheit thermometer. There are two other scales in use to mark temperature be- tween the freezing and boiling points (see Fig. 103). Study this figure so tliat you can draw it from memory. Notice that the mercury is at the same height in each tube. Both of these thermometers make the freezing point zero, but one makes one hundred divisions between the freezing and the boiling points, while the other makes but eighty divisions between the two points. The one having eighty degrees be- tween these two points is called Reaumur's ; the one having Fall. Cent K^o- One hundred degrees "1212° Boiling Point. Freezing Point, 6 |o°_ is called a Centigrade thermometer. If these three thermometers, then, with their dif- ferent scales attached, were plunged into a freezing mixture, i n two of them mercury would stand at zero, while in the other it would stand thirty-two degrees above zero ; for yon will remember that in the Fahrenheit thermometer the zero point was placed thirty-two degrees below the freezing point. The Fahrenheit thermometer is used mostly in the United States, England, and Holland ; O Fig. 103. <^ LIMITS OF THERMOMETERS. 119 the Reaumur is used in Germany, and the Centigrade princi- pally in France. As the parts of the Centigrade scale can be conveniently written as decimals, it is employed almost universally for scientitio purposes. Questions and Problems. 1. The difference between two temperatures is 72° Centigrade. What is the difference in Fahrenheit degrees ? 3. A Fahrenheit thermometer reads 60° ; what would be the reading on a Centigrade tliermometer ? 3. Change the following readings Fahrenheit to Centigrade read- ings : 50°, 12°, - 40°*, 200°, 18°. 4. Change 80°, 15°, 5°, — 18° Centigrade to Fahrenheit readings. 5. Change 120° F. to C. reading ; 80° C. to F. ; 64° F. to C. ; - 18° C. to F. ; 150° F. to C. ; - 32° F. to C. ; 100° F. to C. The Selection of a Thermometer. — To select a thermom- eter, take the one that marks the average temperature of several thermometers which differ only three or four de- grees from each other. In a good thermometer the bore must be of uniform size and the tube free from air. Limits of Temperature a Mercurial Thermometer will Measure. — Thermometers made with mercury can be used for all ordinary purposes, but when mercury reaches — 39.2 degrees F., that is 39.2 degrees below zero, it solidifies. It is therefore of no use in measuring very low temperatures. To measure temperature in very cold climates, the ther- mometer tube is filled with alcohol containing some color- ing matter. Pure alcohol never freezes in the greatest cold that has been found. When heat is applied to the mercurial thermometers the mercury will rise until it reaches 660 degrees F., then it boils and passes oif in vapor. When it is needful to meas- ure temperatures greater than 660 degrees F., other in- * The minus sign denotes that the temperature is below zero. 120 HEAT. struments are used, based upon the expansion of metals, or the alteration, by change of temperature, of some property of the metals, as electrical conductivity. An Air Thermometer. — Take two flasks, A and Ji, Fig. 104. Fit into each a rubber stopper, and connect the two flasks with a piece of glass tubing eighteen inches long. The stopper in flask H should have a second hole opening out to the air. Put water colored with red ink into flask J5, and let the glass tube dip below the water, as shown in Fig. 104. Wai-m the flask ^i so as to exjjel a little air ; the colored water rises in the tube. The apparatus forms a simple Air Thermometer, the movements of the colored water in the tube showing the effect of a change of temperature upon the confined air in flask A. Place the air thermometer and a mercurial one in a cool place, note the point to which the water rises 'WW) ii the tube and the temperature marked by the mer- Fio 104 curial thermometer. Put both thermometers in a warm place. Note the point where the water stands in the tube and the tem- perature indicated by the mercurial thermometer. Make a scale on paper dividing the distance between the highest and lowest points in the tube into as many divisions as there arc degrees between the highest and lowest tempera- ture marked by the mercurial thermometer. Which is the more sensitive to slight changes of tempera- ture, a mercurial thermometer or an " air thermometer "2 CONVECTION. Experiment 87. — Take a small beaker half full of water and sprinkle into it some bran or fine sawdust, then CONVECTION. 121 liiiii!iiiiiiiiiiiiiiniiiiiiniii Fig. 105. 1,,/ place it over a flame and boil the water. Describe the movement of the bran or sawdust. In this experiment the water is heated by circulation. That is, the portion of the water that the heat reaches becomes heated and rises through the colder water to the surface, while the cooler portions sink and take their places over the flame, until all portions of the water reach the boiling point. The process of heating by circula- tion of molecules is called Convec- tion. Experiment 88. — Fit up a piece of apparatus as shown in Fig. 105. A is an inverted bottle with the bottom out off (see Appendix, § 2). B is an eight-ounce flask. The tube G con- nects the lower part of A with the lower part of £ ; the tube J) con- nects the upper part of A with the upper part of £. Fill the whole ap- paratus with cold water, taking great care that all the bubbles of air are out of flask _S. Color the water in A and apply heat to the flask. Note the re- sult. What does the experiment teach ? Gases are heated by convection in the same manner that liquids are. 122 HEAT. CONDUCTION. Experiment 89. — Heat a piece of iron at one end. Touch the head of a match to the iron and see how far from the source of heat the match can be ignited. Place an iron rod and a copper rod of equal size in the flame of a lamp, and note the point farthest from the flame on each rod that a match Can he lighted by simply holding it on the metal. The heat passes along the rod, being communi- cated from one molecule to another. The flaw of lieat through an unequally heated body, from molecule to molecule, is called Conduction. Experiment 90. — Place a fine piece of muslin on a sheet of lead and drop a burning coal upon it. Note what occurs, and tell what you think is the reason. Place another piece of muslin upon a block of wood and drop a burning coal upon it. Note what occurs, and ex-, plain why. Is lead a good conductor of heat .' Is wood a good conductor of heat ? Place a few pieces of lead in a paper pill-box. Heat gradually. The lead melts. Place a strip of a business card in the molten lead. It is charred. Why was not the box charred in melting the lead ? Experiment 91, — Take a test-tube, place in the lower end a piece of ice, around which a strip of sheet lead is wound to sink the ice, and nearly fill the tube with water. Place the tube so that the water in the upper portion of the tube is directly over the flame of the lamp. Notice CONDUCTION. 123 that the water in the upper part of the tube boils, while the ice remains in the lower part. Or put water in a test- tube and hold in the flame, as shown in Fig. 107. Is the heat applied to the upper portion of the tube conducted by Pig. 106. Fig. 107. the water to the lower portion ? Is water a conductor of heat ? Other liquids would give the same result that the water does in this experiment. Are liquids good or poor conductors of heat ? In the following list the metals are given in order of the conducting power, the best conductor being placed first. 1. Silver. 4. Zinc. 7. Lead. 2. Copper. 5. Tin. 8. Platinum 3. Gold. 6. Iron. 9. Bismuth. Experiment 92. — Hold a piece of wire gauze over a flame (Fig. 108), and then lower it slowly to the bottom of the flame. Does the flame pass through the gauze ? If you have a gas burner, place the gauze half an inch above the 124 HEAT. jet (Fig. 109), turn on the gas, and apply a match above the gauze. Does the gas burn below the gauze ? Pig. 108. Fig. 109. Experiment 93. — Wrap a thin piece of writing paper once around a smooth, round piece of copper, brass, or iron, about an inch in diameter, and hold it in the flame. The paper will not be burnt, because the metal inside con- ducts the heat away so rapidly that the paper does not become hot enough to burn. Can you now explain what was observed in Exp. 92 ? The Safety Lamp. — Sir Humphry Davy discovered the fact you ob- served in Exp. 92, and took advan- tage of it in making his safety-lamp. The safety-lamp consists of wire gauze enclosing a lamp (Fig. 110), and is used by miners when combustible gases are likely to be encountered in mines. EVAPORATION. Experiment 94. — Half fill a tumbler with water, and place over it a clean piece of glass — a piece of broken Fig. 110. EBULLITION. 125 window-pane will answer. Let it remain over night, and in the morning examine the glass which covered the tum- bler. What do you see on the piece of glass ? How did it get there ? If the piece of glass had not been placed over the tum- bler, what would have become of the water which now adheres to the glass ? If the tumbler were allowed to stand uncovered, what would become in time of the water in it ? Set a pail of water out of doors and let it remain a few days. What becomes of this water ? What do you mean by the statement, " It dries up " ? The quiet formation of vapor at the surface of a liquid is called Evaporation. Evaporation is one of the most useful processes in the distribution of moisture. EBULLITION. Experiment 96. — Fill a beaker half full of water, and place it over a Bunsen burner or the flame of an alcohol lamp. Watch the water until large bubbles of vapor form rapidly at the bottom and rise to the surface. Put a thermometer in the water and note the tempera- ture. Continue the boiling of the water for some time, and notice whether there is any change of temperature pro- 126 HEAT. duced when the water is made to boil more rapidly than it boiled at the beginning. Do you think you can make the water hotter than two hundred and twelve degrees ? * The rapid formation ofbuhbles of vapor at thebottom of a liquid is called Ebullition, or Boiling. The boiling point remains constant during ebulli- tion. Fig. m. Experiment 96. — With a flask, a rubber stopper, a * Water that boils at two hundred and twelve degrees must be pure, and it must be boiled in the open air. EBULLITION. 127 piece of glass tubing, and a thermometer, fit up a piece of apparatus, as shown in Pig. 111. Boil the water in the flask with the bottle a third full of water. At what temperature does the water boil ? When the steam escapes from the end of the tube, fill the Fig. 112. bottle nearly full of water, and again notice the tempera- ture at which the water boils in the flask. What caused the temperature to rise ? Why did it take a lygher temperature to keep the water boiling when t]l« bottle was nearly full than it did when there was only a small amount of water in it ? How does pressure affect the boiling point ? An increase of pressure raises the temperature at which a liquid boils. 128 HEAT. Experiment 97. — With parts of tlie same apparatus, shown in Fig. Ill, fit up a piece like that shown in Fig. 112. Be sure that the thermometer and glass tube fit tightly in the rubber cork. Fill the bottle D half full of water. Boil the water in the flask and remove the lamp. Note the temperature at which the water boils. When a short time has passed, apply the end of the tube A to the air-pump, and exhaust the air from the bottle. The pressure of the air is thus removed from the surface of the water in the bottle, and this lessens the pressure upon the water in the flask. Or close the rubber tube at B, detach the flask at C, and turn it upside down. Pour cold water over the flask to hasten the condensation of the steam. What effect does the removal of pressure from its sur- face have upon the water? What was the temperature shown by the thermometer when the water was boiling, and the space above it was filled with steam ? What was the temperature at which it boiled after the tube had been closed, and the pressure removed by the partial condensation of the steam ? What effect does removing pressure have upon the boil- ing point of a liquid ? A decrease of pressure lowers the temperature at which a liquid boils. Have you watched the process of boiling closely enough to tell whether there is any difference between bubbles of air and bubbles of vapor ? DISTILLATION. 129 Can you tell to what the clicking sound is due which is heard while the water is boiling ? CONDITIONS AFFECTING EVAPORATION. Bxperimeut 98. — Heat a quart of water nearly to the boiling point, and pour it into a pan about three inches deep. Notice that vapor rises from the water. Do bubbles of steam or vapor come from the bottom of the pan as in ebullition. From what part of the liquid does evaporation take place ? Pour into a pan three inches deep a quart of cold water. Compare the evaporation from the cold water with that from the hot water by holding a piece of clean, cold glass over each. From which does evaporation take place the more rapidly ? From what you have learned in the foregoing experi- ments, answer these questions : What effect has temperature on evaporation ? What effect has extent of surface on evaporation ? What effect has the removal of the pressure of the atmosphere on evaporation ? DISTILLATION. Experiment 99. — Fit up such a piece of apparatus as is shown in Fig. 113. See that the lamp-chimney is full of water. Keep the vessel A constantly filled with cold water, and let the water run from the rubber tube S into a pail. Place in the flask a mixture of salt and water, and boil it for at least a quarter of an hour. Taste the water that has collected in the tumbler. 9 130 HEAT. Why does the water in the lamp-chimney become warm? The process of changing a liquid into vapor by means of heat, and then condensing the vapor by cold, is Dis- tillation. The Use of Distillation. — Distillation is frequently used in the arts to separate liquids from solids. Water may be iihMiiiiiiMiiiiiiiiiiiiiiiiUDiiiiiiiiiiiiiiiniiiinuuiiiuiiiiitiiiiiiiiMiiiiiiiiitiiiiiiiiiniiiiiiiiiMt'ii,. Flu. 113. purified by this means, as water in evaporating is freed from its impurities. LATENT HEAT. Experiment lOO. — Take two beakers or tumblers of the same size. Fill one beaker two-thirds full of small pieces of ice, and put into the other an equal weight of ice-water. Put a thermometer in each, and note the temperature. Then place both beakers in a pan of boiling water that you have just removed from the flame. Stir constantly the ice in one of the beakers. Let the beakers stand in the hot water till the ice has SPECIFIC HEAT. 131 all melted, and then quickly note the temperature of each. ' As both beakers are of the same size, and have stobd in the hot water the same length of time, each must have received the same amount of heat. In which one does the temperature remain the same ? As both were at the same temperature when placed in the water, and both have received the same amount of heat, what has' become of the heat which was absorbed by the beaker in which the temperature did not rise ? In what state was the matter in this beaker before it was put, into the hot water ? In what state was the matter after it had been subjected to heat, though the temperature was not raised ? The heat which changes a body from a solid to a liquid, or from a liquid to a gas, without change of temperature, is called Latent Heat. The heat which is expended in raising the temperature of a body is called Sensible Heat. SPECIFIC HEAT. Experiment lOl. — Put a pound of mercury into a ves- sel, and a pound of water into another vessel of the same size and the same material. Place each over the same source of heat. When the water has risen five degrees, note the temperature to which the mei'cury has risen. If the experiment is carefully made, the temperature of the water will be to that of the mercury as one to thirty. Ascertain how long the water must be kept over the source of heat to raise its temperature as high as the tern- 132 HEAT. perature of the mercury rose while that of the water was rising five degrees. You will find that it takes about thirty times as long to raise water to a given temperature as it does mercury. Next, take half a pound of sheet lead, roll it up so as to form a hollow cylinder, and suspend it by a string in boil- ing water for five minutes. At the end of that time the lead has the same temperature as the boiling water. Now quickly immerse the lead in a half pound of water at thirty- two degrees F. Find the temperature of the water when it ceases to rise. The lead falls in temperature much faster than the water rises in temperature. Heat the half pound of water to boiling, remove it from the flame, and immerse the roll of sheet lead in it. When the lead has remained in the water five minutes, remove it and ascertain as accurately as possible its temperature. Note the temperature of the water when the lead is re- moved. We see that equal weights of different substances receiv- ing the same amount of heat have their temperatures raised unequally. This is expressed in other words by saying that all substances do not have the same specific heat. The amount of heat required to warm a given iveigM of any substance one degree co)npared with the amount of heat required to warm au equal weight of water one degree is called the Specific Heat of that substance. For comparison, a standard of specific heat is necessary ; water is taken as the standard, its specific heat being 1. ARTIFICIAL COLD. 133 Specific Heat of a Pew Substances. Substance, Specific Heat. Hydrogen 3.4090 Water 1.0000 Ice 4890 Air 2375 Iron 1188 Copper 0952 Silver 0570 Tin 0563 Mercury 0333 Lead 0314 ARTIFICIAL COLD. Experiment 102. — Tie some cotton wool upon the bulb of a mercurial thermometer and wet the wool with ether. Why does the temperature fall ? Tie a piece of cotton cloth over the bulb of the air ther- mometer. Wet the cloth with alcohol. Exjilain the result. Experiment 103. — Cut out a round piece of sheet copi- per an inch and a half in diameter. Form it into a little capsule or basin by hammering it upon some concave sur- face. Pour a few drops of water upon the table, set the little basin in the water, and partly fill the basin with car- bon disulphide. With a pair of bellows blow over the sur- face of the carbon disulphide till it is evaporated. Ice is formed under the capsule. Repeat the experiment, but instead of placing the little basin upon the wooden table, place it upon a piece of sheet copper. Why are you unable to form ice as in the first part of the experiment ? Experiment 104. — Crush a quantity of ice. Take a bottle holding four or five ounces ; fill it to the top with 134 HEAT. water. Push into the nozzle a rubber cork with a ther- mometer through one hole and a piece of glass tubing about twenty inches long through the other. The water will rise five or six inches in the tube. See that there is no air in the bottle. Put this piece of apparatus in a wooden pail or a small box and pack around it snow or ice. Do not cover the neck of the bottle with ice. A little salt may be mixed with the ice. Tie a fine thread around the tube to show the height of the water in the tube. Follow the movement of the water in the tube by tying another piece of thread about the tube and moving it down as the water falls. When the water has fallen to the lowest point, let the thread remain, and note the temperature marked by the thermometer. As the water is cooled by the fi-eezing mixture, does it occupy less or more space ? in other words, does it become rarer or denser ? At what temperature does it reach its greatest density ? When the water began to freeze, did it occupy more or less space than it did when put into the freezing mixture ? Wafer reaches its maximum density at 39.2 de- grees F. Water freezes at 32 degrees F., and expands* when freezi)ig. RADIATION. Experiment 105. — Heat an iron or a brick very hot. Hold one hand a little distance from the brick, and with * Like water, east iron, bismuth, and antimony expand when solidi- fying, and can therefore be used for casting, as they will take up the impressions of the moulds in which they are allowed to solidify. Gold, silver, and copper contract, and hence coins must be stamped with a die. RADIATION. 135 a small piece of cardboard, fan the air between the brick and the hand held in front of it. Your hand feels the heat, but the heat is not communicated to your hand by the air. Go near a hot stove and remove the lid. The heat seems to strike out into your face at once. The heat which is thus thrown across space is called radiated heat. Move your hand from one position to another around the hot brick ; over it ; under it ; to the right ; to the left ; in every direction. Is the heat radiated in every direction ? Hold the hand at various distances from the brick. Does the distance from the heated body affect the amount of heat which the hand receives ? The heat received from a hot body at a distance of two feet is one-quarter of what it is at a distance of one foot ; at a distance of three feet it is one-ninth ; at a distance of four feet it is one-sixteenth ; at a distance of five feet it is one twenty-fifth, etc. Beat is radiated in every direction, and decreases in intensity as the square of the distance increases. Experiment 106. — Procure two tin fruit-cans — one which has just been opened, and the other very rusty. Polish the tin of the new one as bright as possible. Heat two quarts of water to boiling, and pour it into the two quart fruit-cans. Hold the hand at equal distances from each. Which radiates the greater amount of heat? 136 HEAT. Hough surfaces radiate well; smooth surfaces are poor radiators. In which can will the water retain its heat the longer — the bright can or the rusty one ? Why ? Which stove will " throw out " the more heat, one that is highly polished or one that is rusty ? ABSORPTION. Experiment 107. — At short but equal distances from a heated brick or a two-quart pail of boiling water, set up a piece of bright tin and a piece of sheet iron or tin painted black. Let them remain for a short time. Which feels warmer, the bright, smooth substance, or the dark, rough substance ? Which do you conclude is the better absorb- ent of heat, a bright, smooth body, or a dark, rough one ? REFLECTION. Experiment 108. — Heat a brick or a large piece of iron Fig. 114. Fery hot. Set it upon some body so that it will not burn THE STEAM-ENGINE. 137 the table. Place in front of the brick a pasteboard screen, as in Fig. 114. Upon the opposite side of the screen sus- pend a thermometer. Hold a bright piece of tin above the screen at different angles until a position is found in which the other hand placed over the thermometer feels the heat. Remove the hand from over the thermometer, and keep the tin in the same position. See whether the temperature marked by the thermometer rises. The heat was not radiated in an unbroken straight line from the brick to the thermometer, but was reflected to the thermometer by the bright tin surface. The rays of heat which pass from the hot body to the reflector are called incident rays ; those which pass from the reflector to the hand or thermometer are called reflected rays. The Steam-Engine. — The steam-engine is a machine in which the expansive force of steam is the motive power. The expansive force or tension of steam formed when the temperature of water is two hundred and twelve degrees F. is equal to one atmosphere, or fifteen pounds, to the square inch. When the pressure upon boiling water is increased, its boiling point is raised and the expansive force of steam is increased. As the expansive force of steam is due to heat, the steam engine is a machine by means of which heat is transformed into work. The essential parts of a steam-engine are the boiler, in which steam is generated, and the cylinder, in which the expansive force of the steam acting alternately on a piston moves it to and fro. Experiment 109. — Cut oif a piece of lamp-chimney [A, Fig. 115) three and a half inches long (see Appendix, § 2). 138 HEAT. Into each end of it fit a cork, with a quarter-inch hole near the edge. In the centre of one cork bore a second hole M, about one-eighth of an inch in diameter, so as to allow the stick J3 to move easily back and forth. On the end of the stick J3 fasten a disk of cardboard C, that will move easily back and forth in the glass cylinder. Then take a block of wood J), two and a half inches long, seven-eighths of an inch wide, and five-eighths of an inch thick, and chisel out smoothly a hole in this block, one and three- fourths of an inch long, five-eighths of an inch wide, and half an inch deep. Fig. 115. In one side of the little box thus made bore three quarter- inch holes, X, iVJ and 0, one-fourth of an inch from each other and the same distance from the inside ends of the box. In the middle of the opposite side of the box bore one such hole K. Cut out a piece of cork (^, Figs. 115 and 116) one and THE STEAM-ENGIKE. 139 one-fourth inches long, a scant half mch thick, and three- eighths of an inch high. In the bottom of this small piece of cork cut out a. hole, as indicated by the y \ a dotted lines (Fig. 116). Put this piece of cork \^^^^^V in the box, as shown at E, Fig. 115, and pass through the ends of the box and close to the top of the cork a small iron wire F. Fasten the iron wire to the cork with shellac. Over the front of the box glue a piece of glass, making it air-tight. Next bend two pieces of quarter-inch glass tubing G and H, as in Fig. 115, and insert them into the end holes L and in the lower side of the box, and into the holes in the corks, making them air-tight with shellac or sealing- wax. The straight parts of the tube G should be a little longer than those of the tube H, so that the apparatus may be easily put together. Fit into the upper hole ^in the box a straight piece of glass tubing, and attach to this a piece of rubber tubing. We have now a piece of apparatus to show how steam is made to pass into and out of the cylinder of an engine, driving the piston back and forth. A represents the cylin- der, C the piston, IB the piston-rod, D the steam-chest, E the slide-valve, -K'the pipe leading to the boiler, and the hole iV^in the steam-chest represents the escape-pipe. Blow into the rubber tube attached to K, moving the wire F in and out, and notice the motion of the piston. The disk C must move easily and the rod B fit easily in the hole M, in order that the breath may move the piston. Why 18 the hole iV necessary ? Of what use is the hole cut in the lower side of the slide-valve Ef 140 HEAT. In the real engine a fly-wheel attached to ^ by a crank gives the machinery a steady motion. The rod F is made to move out and in at the proper times by an eccentric attached to the shaft of the fly-wheel. Pig. 117. Fig. 117 represents a cross-section of the cylinder and steam-chest of an engine. In the figure the slide is in such a position that the piston is at rest. Make a drawing similar to Fig. 117 to represent the position of the slide-valve when the piston-rod is moving out. Make another drawing to represent the position of the slide-valve when the piston-rod is moving in. The corresponding parts of Fig. 115 and Fig. 117 are lettered alike. SOME SOURCES OF HEAT. Experiment 110. — Place upon a large piece of iron a piece of copper or lead or iron, and hammer it. Notice that its temperature is raised. In this instance the rise in temperature is due to percussion. SUMMARY. 141 Experiment 111. — Rub vigorously together two pieces of wood ; or bore a hole in a hard piece of wood with a brace and bit. Notice that the temperature of the bit and also that the temperature of the surfaces of the two pieces of wood is raised. In this case the rise in temperature is due to friction. Experiment 112. — With a pair of hand-bellows blow upon the bulb of a thermometer for a few minutes. No- tice that the temperature of the mercury rises. Air on being heated expands (Exp. 110), and when it is compressed, as it is with the bellows, it gives out heat. The rise of temperature observed in performing Ex- periments 110, 111, and 112 resulted from the conversion of mechanical energy into heat. SUMMARY. Heat is that agent which produces the sensations of warnnth or cold- ness. Temperature is the state or condition of a body with reference to its heat. The thermonneter Is an instrument for measuring temperature. Heating bodies causes them to expand ; cooling bodies causes them to contract. (For exceptions to this, see p. 134.) The freezing point is the temperature at which pure water freezes — 32° F.,0° C, 0° R. The boiling point is the temperature at which pure water boils when the barometer stands at thirty Inches— 212° P., 100° C, 80° R. Convection is the process of heating by the circulation of molecules. Conduction Is that process of heating In which the heat is diffused through a body from molecule to molecule. Fluids (liquids and gases) are heated by convection. Solids are heated by conduction. 142 HEAT. Combustible matter will not burn until heated to the proper tempera- ture. Evaporation is the quiet formation of vapor at the surface of a liquid. Ebullition, or boiling, is the rapid formation of vapor within the body of a liquid. The boiling point of a liquid is raised in temperature (1) by an increase of pressure, (2) by the presence of foreign matter in solution. Evaporation is affected by the amount of heat, by the extent of surface, and by the pressure of the atmosphere. Distillation is the process of changing a liquid into vapor by means of heat and then condensing the vapor by cold. Latent heat is the amount of heat which is expended in changing the condition of a body without changing its temperature. Sensible heat is the heat expended in raising the temperature of a body. The specific heat of a substance is the amount of heat required to warm it one degree compared with the amount required to warm an equal weight of water one degree. Artificial cold may be produced by evaporation. Water reaches its maximum density at 39.2° F Radiation is the passage of heat from the source of heat through space. Heat is radiated in every direction, and decreases in intensity as the square of the distance increases. Rough surfaces are good radiators. Smooth surfaces are poor radi- ators. Rays of heat are reflected by smooth surfaces. Mechanical energy can be transformed into heat. Questions and Problems. 1. What is heat ? 2. What is cold ? 8. Has ice any heat ? 4. What is temperature ? 5. Distinguish between heat and temperature. 6. What is a thermometer ? 7. Distinguish between a thermometer and a barometer. QUESTIONS AND PROBLEMS. 143 8. Change — 40° C. to Fahrenheit reading, and — 40° F. to Centi- grade reading. 9. — 36° F. equals what reading on the Centigrade thermometer ? 10. 20° F. equals what reading on the Centigrade thermometer ? 11. 30° C. equals what reading Fahrenheit ? 13. Why should the bore of a thermometer be uniform through- out ? 13. Can you pass heat from a colder to a hotter body ? 14. If we heat an object, how will it aflEect its size ? 15. Ca» you mention an exception to the answer to the foregoing question ? 16. Why is the temperature of the body not a safe standard by which to determine the amount of heat an object contains ? 17. In Bxp. 83, page 113, why did the same vessel of water feel both hot and cold ? 18. What kind of a thermometer would you employ to measure high temperatures ? Why ? 19. To measure low temperatures ? Why ? 20. To measure extremely low temperatures ? Why ? 31. Why do coach-wheels become set when the axles are not suf- ficiently greased ? 33. The spaces between the ends of the rails of a railroad are greater in winter than in summer. Why ? 33. Why do blacksmiths heat wagon-tires before putting them on wheels ? 24. What are the fixed points on a mercurial thermometer, and how are they determined ? 35. Why do glasses crack when suddenly heated ? 36. Explain why vessels and pipes in which water is confined burst when the water is frozen. 27. Show that water is a bad conductor. 38. Why does a soap-bubble ascend when first blown ? 29. Why does it descend after a time ? 30. Explain the process of ventilation. 31. What causes the fire to burn better on a cold day than on a warm day ? 33. What do you mean when you say that the fire draws well, or has a good draught ? 33. Why does vapor ascend ? 34. Why dg chjnineys sometimes smoke when the fire is first lighted ? 144 HEAT. 35. Why is it that on a cold morniDg iron feels colder than wood, both having been exposed to the same degree of cold ? 36. Do good conductors feel warmer or colder to the touch than poor conductors of the same temperature ? JVhy ? 37. Why is ice preserved from melting by being wrapped in flannel ? 38. Does the carpet feel warmer than the marble hearth ? Why ? 39. If they were both warmer tlian the body, what sensation would they produce ? 40. "Why do cotton or linen sheets feel cool, while woollen blankets feel warm ? 41. Why does the mercury in the thermometer, though sealed up, indicate external temperature ? 43. Explain why the glass covering of conservatories, or hot-beds, renders the confined air warmer than the atmosphere outside. 43. Why are double windows often put up in the winter ? 44. Why does snow keep the earth warm ? 45. Why cannot the warmth of the earth escape through the snow ? 46. Why do ponds become dried up in dry, hot weather ? 47. Why are bright tin pans not so good to bake bread in as dark ones ? 48. Mention three circumstances which will afEect the rapidity of evaporation. 49. Will water boil at a lower temperature on the top of a mountain, or at the sea level ? Why ? 50. Mention three circumstances which will affect the temperature at which a liquid will boil. 51. What is boiling ? 52. What causes the clicking sound heard when water boils ? 53. Why does boiling water bubble ? 54. Upon what fact or principle of physics does distillation depend ? 55. Can latent heat be changed into sensible heat ? 56. Why does fanning one's self produce a sensation of coldness ? 57. When a pitcher of ice-water is brought into a warm room, why is it soon covered with moisture ? 58. Why is salt mixed with ice in freezing ice cream ? 59. If a tumbler is held near the mouth of a boiling kettle, why is it soon covered with moisture ? 60. Why is the window-pane covered with mmsture if we breathe against it ? 61. What is meant by the maximum density of water ? At what temperature does water reach its maximum density ? QUESTIONS AND PROBLEMS. 145 63. Why does ice float ? 63. Why does water when frozen occupv more space than when it is in the liquid form ? 64. How may heat be diffused ? 65. How is the earth's surface heated ? 66. How is the atmosphere heated ? 67. Is a good absorbent a good reflector ? 68. Is a good conductor a good radiator ? 69. We learned in Exp. 95 that the boiling point is constant during ebullition. After the water becomes 212°, what becomes of the heat that does not raise the temperature of the water ? 70. Distinguish between boiling and evaporation. 71. The temperature of a closed room is 85°. Water is sprinkled on the floor, and the temperature of the room falls several degrees. Explain why. 72. A pond of water at sundown has a, temperature of 45°. The weather grows very cold during the night, and before morning the pond is frozen over. Was there any circulation of the water produced by the cold ? When did the circulation stop ? 10 CHAPTEE VII. SOUND. Experiment 113. — Stretch a cord tightly, as shpwn in W«|?H A Fig. 118. Fig. 118. Now pull it aside suddenly. A sound is pro- duced. a Place a small glass 1/ bead in a bell-jar, hold Flo. 119. the jar in one hand (Fig. 119), ^ ^ e c and strike the edge with the knuckle or a piece of wood. A sound comes from the bell-jar, and the bead is heard tapping the glass. The edge of the bell- jar is in motion. Strike a tuning-fork, and hold a piece of paper so that it will touch the side of the fork near the end of the prong. A quick succession of slight taps is heard. Grasp at the middle a piece of quarter-inch glass tubing about thirty inches long. "With a piece of damp cloth ^ Fie. 130. SONOROUS BODIES. 147 grasp the tubing near the hand holding it, and draw the cloth quickly up and away from the tubing (Fig. 121). A sound is heard. The movements, back and forth, of the cord, of the bell- jar, of the tuning-fork, and of the glass tubing are called vibrations. Sound is produced by the rapid vibrations of a body. A body which produces a sound is called a Sonorous body. A Pig. 121. of its length. Terms. — A complete vibration is the movement from A to C, and back again to A (Figs. 118, 120, and 122). The distance from A to G, or from C to ^, is the amplitude of the vibration. The vibrations of the cord, of the bell-jar, of the tuning-fork are from side to side, or at right angles to the length of the body, and are called transverse vibrations. The vibration of the glass tubing is in the direction "^' Such vibrations are called longitudinal. Experiment 114. — Pull the cord aside lightly, then strongly. In what respect do the vibrations differ ? Hold the glass tubing at the middle, and set the tube vibrating by drawing the damp cloth along the tube. Begin to draw the cloth at one-third of the distance towards the end, and set the tube vibrating. In what respect do the vibrations differ ? 148 SOUND. &t SOUND NOT PROPAGATED IN A VACUUM. Experiment 115. — Fit a flask with a rubber cork, hav- ing two holes. Pass a piece of wood through one hole, and attach to the end of the wood a toy-bell or a small sleigh-bell (Fig. 123). Stop the other hole of the cork, insert it in the flask, shake, and notice the sound. Remove the cork, put a little water into the flask, and replace the cork, leaving one hole open. Dry the outside of the flask. Heat the flask slow- ly at first, and then hold it steadily in "'^ " ■ the flame until the water boils briskly and the steam issues rapidly. Stop the hole in the cork, and remove the flask instantly from the flame. As the flask cools, a vacuum is formed in it. Shake the flask, and compare the sound heard with that when the flask was full of air. Sound is not propagated in a vacuum. Experiment 116. — Dry the flask, and fill it with illu- minating gas by holding the flask over a jet. Replace the rubber cork and shake the bell. VELOCITY OF SOUND. 149 How does the sound compare with that made when the flask was filled with air ? Fill the flask with carbonic acid gas if you have any means for generating it. Replace the cork and shake the bell. How does the sound compare with that made when the flask was filled with air ? The loudness of sound depends upon the density of the gas in tvhich the sound originates, not upon the density of the gas or air ivhich surrounds the person hearing the sound. Velocity of Sound in Air. — Every student must have made observations like the following : The flash of a gun is seen and then the report is heard. Steam is seen issuing from a whistle and afterward the sound is heard. The axe of a woodcutter falls, and as he is raising it again the sound of the blow is heard. The velocity of sound in air is, therefore, less than the velocity of light. Sound travels through air at the temperature of freezing f 1,090 feet in a second. At ordinary temper- ature (sixty degrees F.) the velocity of sound is 1,120 feet in a second. Velocity of Sound in Water and in Solids. — In water sound travels about four times as fast as in air, or 4,708 feet per second. Let one student place his ear against a telegraph pole, while another strikes the next post with a stone. Two sounds will be heard ; the first con- ducted by the wood and wire, the second conducted by the air. Was the first or the second sound the more distinct ? 150 SOUND. Experiment 117. — Obtain a strip of wood eight or ten feet long and about half an inch thick ; place a watch upon the end of this strip, and see whether its ticking can be heard at the other end. Or place a watch on one end of a table or bench, and see whether its ticking can be heard when the ear is placed at the other end. Does the air or a solid convey sound with the greater intensity ? Sounds travel quicker in solids than in air or water. Waves. — If we watch a flag when the wind is blowing, we see waves running along the flag from the part near the staff to the end. Shake a breadth of carpet, and waves pass from the edge in the hands to the outer edge. Neither the flag nor the carpet moves forward, though the waves do. The form of the wave, therefore, travels from one end to the other, moving parts of the flag or carpet up and down, but not lengthwise. In water waves, also, the form only moves forward, while the particles of water simply rise and fall, or have a mo- tion trans- versely to the direction of the wave. The top of the wave is called the crest C, the hollow is called the trough T (Fig. 124). A 10 av e leiKjth is the distance from crest to crest, C to C, or from trough to trough, Tto 7'(Fig. 125). SOUND WAVES. 151 Experiment 118. — Procure a piece of brass or steel wire wound in the form of a spiral. Fasten both ends firmly to two fixed pieces of board. Then press four or five turns of the spiral next to one board closely together. Remove the hand quickly, and the spiral vibrates in the direction of its length, or longitudinally. If you notice closely, you will see that there is a movement of the crowded part of the spiral forward and then back again. This is followed by the stretched part. The spiral does not move as a whole ; it is only the movement lengthwise and back of the compressed part, followed by the stretched part. We may regard the compressed part as a condensation, and the stretched part as a rarefaction. It is the elasticity of the spiral that allows the conden- sation and the rarefaction to move from end to end. Were the wire not elastic, the condensation would not pass along the spiral. When a tuning-fork is set vibrating, the prong moves out and compresses the particles of air next to it A (Fig. 126). The particles of air are elastic, and when crowd- ed together tend to ex- pa n d. In expanding they crowd together the particles of air just be- yond, and in this way the compression, or con- densation, is propagated. When the prong moves back, it causes the par- ticles of air to separate, producing a rarefaction. A. FlQ. 136. This rarefaction is propagated 152 souxo. in the same way. The jparticles of air are vibrating longi- tudinally, or in the direction the wave is moving. A sound wave is composed of a condensation A, and a rarefaction £ (Fig. 126). A wave length consists of a condensation and a rarefaction. When a fire-cracker is lighted and tossed into the air to explode, the report is heard by those on the ground, by those across the street, by those who may be near the upper-story windows, and by those up and down the street. The explosion produced but one sound wave, and as the sound was heard in every direction from the spot where the explosion occurred, the sound wave must have moved out in the form of a constantly enlarging sphere. When a sonorous body is set vibrating, as, for instance, a bell, there is a series of sound waves travelling out from the bell in constantly enlarging spheres. Sound travels in all directions from a sounding body. REFLECTION OF SOUND. Experiment 119. — Take a large concave reflector, such as is used behind wall lamps, and support it as shown in Fig. 127. In the focus of the reflector hang a watch. Place another reflector four or five feet from the first one, EEFLEOTION OF SOUND. 163 and hold in the focus of the second reflector a small funnel with a rubber tube attached. Place the extremity of the tube in the ear, and the ticking of the watch can be distinctly heard. Remove the reflectors, and compare the sound heard with that heard before the removal of the reflectors. The focus of a reflector can be found approximately by letting the sunlight fall upon it, and measuring the distance to the reflector from the point in front of it, where a piece of dark paper will be set on fire. The point where the paper burns is the focus. When sound waves are reflected, they follow the same law that waves of light do. (See p. 180.) In speaking-tubes the sound waves are reflected from the inside of the tube, and cannot spread. The waves are therefore transmitted to long distances with very little loss in the intensity of the sound. Echoes. — Echoes are caused by the reflection of sound from some surface, as the walls of a building, the sides of a mountain, the trees and foliage of a wood. The time that elapses between producing a sound and hearing its echo is the time it takes for the sound to go from the sounding body to the reflecting surface and back to the ear. It is difficult to distinguish more than ten syllables a second— that is, there must be one-tenth of a second be- tween hearing one syllable and hearing the next. Keeping this fact in mind, and also the fact that at ordinary temperatures sound travels about eleven hundred feet per second, we can easily determine how far away a reflecting surface must be in order to hear a distinct echo. 154 SOUND. Experiment 120.- For sounds to be one-tenth of a second apart, the sound waves must follow each other at a distance of a hundred and ten feet (1,100 f eet -=- 10 = 110 feet). But as the sound wave travels to the reflecting surface and back again, the reflect- ing surface cannot be nearer than fifty -five feet (110 feet•-^ 2 = 55 feet). INTENSITY OF SOUND. -Construct a piece of apparatus like that shown in Fig. 128 (see Ap- pendix, § 4). It is a crude form of an instrument called the siren. Insert a short piece of quar- ter-inch glass tubing in a piece of rubber tubing. Hold the end of the glass tubing oppo- site the outer row of holes in the siren. Turn the siren stead- ily and blow through the rubber tubing. A tone is produced. As each hole in the siren comes opposite the end of the tubing a puff of breath passes through. These puffs of breath produce on the opposite side of the siren a series of condensations. Each of these is followed by a rare- faction, owing to the elasticity of the particles of air. Although the siren itself does not vibrate^ it produces sound waves the same as a vibrating body. INTENSITY OF SOUND. 165 Experiment 121. — Turn the pulley-wheel of the siren steadily, so that the cardboard disk shall make, as nearly as possible, the same number of revolutions in each second, and blow lightly through the glass tube at any row of holes. Notice the sound. . Now, while the disk is revolv- ing at the same rate, blow as hard as possible.* Notice the sound. How does the first sound differ from the second ? When you blew hard through the disk,- a greater con- densation and rarefaction was produced than when you blew lightly. The amplitude, therefore, of the vibrating particles was greater when blowing hard than when blow- ing lightly. Loudness or intensity of sound depends upon the amplitude of the vibrations. Experiment 122. — Stretch a string or wire between two supports. Pull the string lightly. Notice the sound. Pull the string with considerable force. Notice the sound. To what was the difference between the two sounds due ? The farther the ear is from a body which is giving forth sound, the fainter is the sound. If a person who has been standing a short distance from a sounding body moves twice as far away, the sound is only one-fourth as loud. If he moves three times the dis- tance, the sound is only one-ninth as loud. If four times the distance, it is one-sixteenth as loud. Suppose a person to stand within two feet of a sounding * Blow-pipe bellows attached to the tubing will give a light and a heavy stream of air more evenly. 156 SOUND. body, and then to move twelve feet away from the body ; how does the loudness at twelve feet compare with that at a distance of two feet ? The intensity of sound decreases inversely as the square of the distance from the sonorous body.* MUSICAL SOUNDS. Pitch. — In the study of sound thus far no distinction has been made between sounds that are musical and sounds that are mere noise. If we strike a ruler upon the desk, the sound is a noise. The ear hears but one sound wave. If we strike the desk several times in succession, a number of sound waves strike the ear, but there is an interval between, and the ear dis- tinguishes the sounds as so many separate noises. "When, liowever, the sound waves follow one another with perfect regularity, and so rapidly that the ear cannot distinguish the intervals, but one continuous sound pleasing to the ear is produced, we have a musical sound or tone. Experiment 123. — Turn the pulley- wheel of the siren and blow through the outer row of holes. Blow steadily and turn the wheel faster and faster. You notice that the sound becomes higher the faster you turn. Remembering that every puff of air that passes through a hole in the disk produces beyond the disk a condensation, which is followed by a rarefaction, you will see that the faster the wheel turns the greater is the number of sound waves or vibra- tions produced. * The intensity of sound also depends upon the density of the me- dium in which the sound originates. See p. 149. MUSICAL SOUNDS. 167 The pitch of a sound depends upon the number of vibrations in a given time. The greater the number of vibrations the higher the tone. Experiment 124. — Stretch a wire or catgut string tightly upon a sonometer* (Fig. 129) (see Appendix, § 4). Near the bridge A place another bridge, and move it toward i|i!iggr:::?!iiii;aiiiiiiis!;s"::::::"aM^^^^^^^^ PiQ. 189. £, stopping every two or three inches of distance and sounding the string. Is the pitch of each note the same ? How does the num- ber of vibrations the shortened string makes compare with the number it made before being shortened by the movable bridge ? Experiment 126. — Take away the movable bridge, and attach a tin pail, C, to the string (Fig. 129). Put into the pail a weight of three or four pounds. Sound the string, noticing the tone. Put twice as much weight * Fair results can be obtained by stretching the wire tightly length- wise of a table or board, with the bridges arranged as stated above. 158 SOUND. into the pail, and again sound the string. Put in three tunes as much weight as at first and sound the string. "What effect does increasing the tension have upon the pitch ? How does the number of vibrations made by the string when last sounded compare with the number it made when it had less tension upon it ? Experiment 136. — Stretch upon the sonometer two strings of the same material and of such size that the diameter of one is about twice that of the other. Fasten to the end of each string the same amount of weight, so that the tension of each will be the same. Sound first one string and then the other. Which gives forth the higher note ? Which string, then, makes the greater number of vibrations, the one having the larger or the one with the smaller diameter ? The pitch of a vibrating string depends upon the length, the tension, and the size of the string. Experiment 127. — Pull a string on the sonometer. Produce a sound of the same pitch upon a piano or an organ, and also upon a flute or other wind instrument. The three sounds differ in what is termed quality. Upon what does the loudness of sounds depend ? Upon what does pitch depend ? Upon what does the quality of sounds depend ? THE MUSICAL SCALE. Experiment 128. — Turn the siren steadily, and with the tube blow through the inner row of holes. Then blow THE MUSICAL SCALE. 159 successively through the second, third, and fourth row. Repeat this several times to gain a correct idea of the notes produced. Calling the first note Do, we recognize the other notes as Mi, Sol, Do, of the musical scale. Turn the siren more rapidly. The pitch of the four notes is raised, but relative to each other, they are Do, Mi, Sol, Do. Count the number of holes in each row. The numbers stand : 48 60 73 96 or in the ratio of 4 5 6 8 If we turn the siren so that it revolves two and three- quarter times a second, we have for Do : Do 132 vibrations per second. Mi 165 Sol 198 Do 364 The musical scale consists of seven notes : Names, Do Ke Mi Fa Sol La Si Do Letters, C DEP GABC No. of Vibrations, 133 148i 165 176 198 230 247i 364 Ratios, 1 Iff? § ¥ 3 The note having two hundred and sixty-four vibrations Is called the "middle C," and the interval, or difference in pitch, between that and the G below, is called an octave. The middle C is the first note of a second octave. Write out the number of vibrations each note of this second octave has, and place the number under its proper letter. 160 SOUND. The interval from C to D, D to E, etc., is called a second. " " " C to E, D to P, " " a third. " CtoG, AtoE, " " & fifth- Between all consecutive notes, except E and F, and B and C, another note is inserted. This note is named from the note below, as C sharp (CJ), D sharp (DJ) ; or from the note above, as BJ!at (Bb), AJlat (Aj?). Experiment 129. — Fasten the bell-jar in any way sim- ilar to that shown in Fig. 1-30. Set the edge vibrating by drawing a violin bow across it in one spot, or by tapping the edge with the finger. Hold near the vibrating edge a bead, a spherical piece of cork, or a bit of sealing- wax suspended by a thread. As- certain by trials the four points where the bead is thrown off farthest by the vibi-ating edge. Hold the bead midway between these points, and it will be thrown off very little, if any. The parts which have the least motion in a vibrating body are called the nodes. The circle in Figs. 131 and 132 represents the edge of the bell-jar, and the points JV, JST, JV, JV, the nodes. The segments A, A, £, B, show the parts of the bell-jar that move in and out during vibration. Fig. 130. Fio. 132. CHLADHri'S FIGURES. 161 Fill the bell-jar partly full of water, bow the edge, and notice the agitation of the water at the nodes and the middle points of the segments. CHLADNl'S FIGURES. Experiment 130. — Cut out a piece of window-glass Pio. 383. seven inches square. Find the middle point, and mark this with ink, or paste a small piece of paper there. Place the middle point over the hole of a spool. Sprinkle fine sand or emery evenly on the glass, and hold the glass firmly upon the spool with the ball of the thumb (Fig. 133). Let another student touch the edge at iV(Fig. 134), while 11 162 SOUND. you set the glass into vibration by drawing a violin-bow across the edge at V. The sand collects on the nodes. iV Fig. 135. Fig. 136. Repeat the experiment, touching at iV^and bowing at V, as shown in Figs. 135 and 136. Make drawings to show where the sand collected when the plate was bowed. See whether you can produce any different figures by touching and bowing at other points than those indicated in Figs. 134, 135, and 136. The figures formed by sand on vibrating plates are called Chladni's Figures, after their discoverer, a famous musician. Experiment 131. — Sprinkle lycopodium * upon the plate, touch and bow, as in the previous experiment. The lycopodium does not collect on the nodes, but on the parts of the segments where there is tlie greatest vibration. Can you explain this ? OVERTONES, OR HARMONICS. Experiment 132. — First Part. Stretch tightly a piece of cord or wire eight to ten feet long. Pull the wire aside and let it vibrate. Notice the tone. * Do not bring lycopodium near a flame. OVERTONES, OB HARMONICS. 163 The wire can be seen moving from side to side, as shown in Fig. 137. The vibrating part A C^, or.4Z»^, is^*^ called a ventral seg- ""£ merit. ^i"- 137. The lowest tone that can be obtained from a strinrf without changing its length or tension is called the Fundamental Tone of the string. Experiment 133. — Second Part. Pull the string near the end, or, what is better, draw a violin bow across it, and hold a feather or camel's-hair pencil lightly against the middle point of the string. Remove the feather or pencil just after the bow is withdrawn. Repeat this a few times, and determine whether the string vibrates in one ventral segment or in more than one. Make a drawing to show the segments you observe, and write the word riocle where you have observed a node. Again touch the string one-third of the distance from the end, and bow near the end. How many ventral segments do you notice ? Make a drawing to show these segments, and write the word node wherever you have observed one. Wis. 138. Fig. 138 represents a string A B vibrating in four ven- tral segments. 164 SOUND. Experiment 134. — Cut out three or four rings* of light paper, as shown in Fig. 139, and slip these on the wire. Place these rings at equal dis- tances apart, damp the wire with the pencil or feather in the middle, and bow strongly. To what places do the rinffs move ? Tig. 139. ^ ° Again place the rings at equal distances, damp the wire one-third of the distance from the end, and bow strongly. To what places do the rings move ? Damp the wire at one-fourth the distance from the end. Make a drawing to show what you observe with refer- ence to segments and nodes. Experiment 135. — Sound the wire again. Notice the fundamental tone. Damp the wire at its middle point ; sound it. On placing the ear near the wire the octave is heard. Experienced ears can detect the octave while the fundamental tone is sounding. The octave heard with the fundamental tone is called the first overtone, or harmonic. Damp the wire one-third of the distance from the end, and sound it. The tone heard with the fundamental is the second overtone, the twelfth higher than the fundamental. The higher tones heard when a sotmding body vibrates in two, three, or more ventral segments are called Overtones, or Harmonics. The combination of * Instead of rings, several riders, or little strips of paper one inch long and one-qaarter of an inch wide, doubled thus /\, may be put on. Find by trial the places where the riders will not be thrown off. REINFORCEMENT OF SOUND. 165 overtones with the fundamental tone gives the quality of sound in ani/ instrument. REINFORCEMENT OF SOUND. Experiiiicnt 136. — Stop the large end of a lamp-chim- ney, and pour in water till it rises above the narrow part of the chimney. Set a tuning-fork vibrating, hold it over the top of the chimney (Fig. 140), and pour in water. Stop pouring the water the moment the sound is louder. Meas- ure the distance between the fork and the water. This strengthening of the sound is called reinforcement or reso- J/M nance. Empty the chimney down to the neck. Procure, if possible, a tuning-fork of a dif- ferent pitcli. Set it vibrating, and pour water into the chimney till its sound is i-einforced. Measure the distance between the fork and the top of the water. Is the distance the same as in the first part of the e.xperiment ? In order that the sound may be reinforced, the distance between the prong and the sur- face of the water must be of such a length that, when a condensation is reflected from the surface of the water, it will reach the prong of the tuning-fork in time to coincide with a condensation as the prong moves up- ward, thus making a greater condensation in the air out- side the tube. Fig. 140. 166 SOUND. Experiment 137. — Sound a tuning-fork, and note how long the sound is heard. Sound it again, taking care not to strike it harder or bow it stronger than before, and put the handle down upon the table or desk. Do you notice any difference in intensity of sound ? Is the sound heard for a longer or shorter time than in the first in- stance ? Place a cigar -box, with the cover nailed or glued on, upon two tightly stretched strings. Sprinkle a little fine sand on the top of the box. Sound the tuning-fork, and put the handle down upon the box. What is the effect upon the sand ? The tones given out by stringed instruments, like the violin, guitar, piano, etc., are reinforced by the sovnding- hoard of each instrument. INTERFERENCE OF SOUND. Experiment 138. — Sound a tuning-fork, and hold it near the ear. Revolve it slowly between the thumb and fore-finger. Do you notice any positions where the sound is not heard ? Hold a vibrating tuning-fork horizontally over its reso- nance tube (Exp. 136), and gradually revolve it. Note the four positions in which there is scarcely any sound heard. These positions are when the side faces of the tuning-fork make an angle of about forty-five degrees with the reso- nance tube. When the tuning-fork is in one of these positions, slip over one of the prongs a paper tube (Fig. 141). The sound of the other prong is heard. SUMMARY. 167 We have seen, in Exp. 1.36, that when a condensation of one sound wave unites with the condensation of another wave a reinforcement of sound ensues. In this e X p e ri- inent a condensa- tion unites with a rarefaction, and the sound is near- ly, if not quite, extinguished. The meeting of I two series of sound waves, so that th' ciinden- s at ions of one series coincide FIu. ]41. with the rarefac- tions of the other series, is called Interference of Sound. SUMMARY. Sound is produced by the rapid vibrations of a body. A sonorous body is a body that produces sound. Transverse vibrations are those in which the vibrating body moves at right angles to the direction of the wave. In transverse vibrations a wave length Is the distance from crest to crest, or from hollow to hollow. Longitudinal vib ations are those in which the vibrating body moves Darallel to the direction of the wave. 168 SOUND. In longitudinal vibrations a wave length consists of one condensation and one rarefaction. Sound is not propagated in a vacuunn. At 32° F. sound travels 1 ,090 feet per second. At 60° F. sound travels 1,120 feet per second. In viaXer sound travels about four times as fast as in air, or 4,708 feet per second. In solids sound travels faster than in liquids and gases. Sound waves are composed of an alternate condensation and rarefac- tion of the propagating medium. A sound wave has the form of a constantly enlarging sphere. Sound travels in all directions from a sounding body. An echo is a reflected sound, heard after being reflected. Laws of Intensify of Sound. — I. The intensity of sound varies directly with the density of the propagating medium. II. The intensity of sound varies directly as the square of the amplitude of vibration. III. The intensity of sound decreases inversely as the square of the distance from the sonorous body. A musical tone is produced by a succession of vibrations that follow one another with perfect regularity and so rapidly that one continuous sound, pleasing to the ear, is heard. The pitch of a tone is its degree of highness or lowness, and depends upon the number of vibrations in a given time. The greater the number of vibrations, the higher will be the pitch. The musical scale consists of seven tones, which vibrate in the ratio, 1 : I : t : J : f : f : ¥ : 3. The nodes of a vibrating body are the parts that have the least motion. The ventral segments are the vibrating parts between the nodes. Chladni's figures show the nodes and ventral segments of vibrating plates. The fundamental tone of a vibrating body Is the lowest tone that can be produced by that body without change of length or tension. Overtones, or harmonics, are the tones higher than the fundamental, heard when a sounding body vibrates in two or more ventral seg- ments. QUESTIONS AND PROBLEMS. 169 The quality of sound in any instrument is produced by the combination of overtones with the fundamental. Reinforcement, or resonance, is a strengthening of sound caused by the meeting of two series of sound waves in such a way that the conden- sations of the one series coincide with the condensations of the other series, and the rarefactions of the one with the rarefactions of the other. Interference of sound is the meeting of two series of sound waves in such a way that the condensations of the one series coincide with the rarefactions of the other series. Questions and Problems. 1. Define the terms vibration and amplitude. 2. Explain the difEerence between a transverse and a longitudinal vibration. 3. In what direction do the particles in a water wave move ? In what direction do the particles of air move in a sound wave ? 4. Explain what is meant by a wave length. . 5. What is the velocity of sound per second at the temperature of freezing ? 6. A stone is thrown into a pond. One can count the crests of 16 waves between the shore and where the stone fell, a distance of 37 feet. What is the average length of each water wave ? 7. If the velocity of sound on a mild day is 1,130 feet per second, and a sounding body makes 560 vibrations per second, what is the wave length ? 8. Four seconds elapse from seeing a flash of lightning to hearing the thunder. How- far away was the lightning ? 9. How can you show that when a bell is sounding it is vibrating ? 10. The prong of a tuning-fork makes 513 vibrations per second. When the velocity of sound is 1,130 feet per second, find the length of each sound wave. 11. What is the form of a sound wave ? 18. How does the velocity of sound in air compare with its velocity in liquids and solids ? 13. Two strings are of the same length and have the same tension, but one string is larger in diameter than the other. If they are sounded, in what resp~6ct will the sounds produced differ ? 14. Two strings are of the same size, and are subjected to the same tension, but one string is longer than the other. If sounded, in what respect will their sounds differ ? 170 SOUND. 15. Two strings are of the same length and the same size, but one is subjected to more tension than the other. When set vibrating, -will there be any difference in sound ? What difference ? 16. Explain how the condensation and the rarefaction of a sound wave are produced. 17. Explain why one on the water can hear sounds farther and plainer when the weather is foggy. 18. Explain the meaning of intensity of sound ; the meaning of pitch. 19. Upon what does the intensity of a sound depend ? 20. Upon what does the pitch of a sound depend ? 31. What is an echo ? •23. An echo is heard from a ledge of rocks in 20 seconds. How far distant is the reflecting surface of the rocks ? 23. A person is a quarter of a mile from the whistle of a factory. How does the intensity of the sound he hears compare with the intensity of the sound heard by a person two miles away ? State the law of de- crease in intensity. 24. Show how you can illustrate the difference in intensity and pitch of sound by drawing the thumb-nail over the ridged cover of a book. 25. Distinguish between a noise and a musical sound. 26. A certain note has 126 vibra- tions per second. How many vibra- tions has its octave ? 27. What is meant by the terms a second, a third, a fifth, as inter- vals in the musical scale ? 28. How many vibrations per second has the "Middle C" ? When sound travels 1,120 feet per second, iind the wave length. 29. What is meant by a node ? 30. What is meant by the ventral segment of a vibrating string '.' Make a drawing to illustrate. 31. What is meant by a fundamental tone ? 32. What are overtones, or liarmonics ? 33. What is resonance ? What is interference of sound ? 34. Does a vibrating tuning-fork show nodes ? How would you find by experiment whether it has nodes, and where they are lo- QUESTIONS AND PROBLEMS. 171 cated ? Illustrate by a drawing, the facts learned by your experi- ment. 35. Construct out of cardboard a disk 3 inches in diameter, as shown in Fig. 143. Put a pin through the centre of the disk, so that it may revolve. Make a hollow cone by twisting around the finger a piece of writing-pnper. Fasten the edges with mucilage. Now blow into the large end of the cone, and direct the breath, issuing from the small end, upon the small squares of cardboard that are bent back at small angles from the face of the disk. Why does the pitch of the note rise as you blow ? CHAPTER \^IIT. LIGHT. Luminous, Transparent, Translucent, and Opaque Bodies. — Bodies which emit light, as the sun, a lighted candle, the fire, etc., are called luminous bodies. From observation, we know that light passes through air, glass, water, mica, and many other substances. Anj' substance through which light passes is called a medium. (Plural, media. ) Substances through which objects may be distinctly seen are called transparent. Name four common, transparent substances. Substances through which light passes, but through which objects cannot be distinctly seen, are translucent ; as thin paper, parchment. Name three other translucent substances. Substances through which light does not pass are opaque ; as iron, stone. Light itself Inyisible. — We cannot see light ; it is itself invisible, but falling upon objects that are not luminous it makes them visible. Every point in a luminous body sends out raj-^s of light in every direction. Experiment 139. — Place a candle, as shown in Fig. 143, and set up three cards, through each of which a pin-hole has been made ; a ray of light from the candle will pass SHADOWS. 173 through the three pin-holes only when they are in a straight line. Admit a ray of sunlight into a darkened room, and dis- •<^^ ^^^ juhiiiiiiihuiiir »ii ^iniiiNiili^l^'' iiiJ Fig. 1J3. tribute a little crayon dust through the air. The path of the ray, as shown by the illuminated dust, is jierfectly straight. Light travels in straiglit lines. How Rajs of Light are represented. — In diagrams rays r^ of light are represented by straight lines. One line represents a single ray. Several parallel lines represent a heaia of light (Fig. 144). Pig. 144. Fig. U5. Several lines diverging from a point A, or converging towards a point JB, rejjresent a pencil of light (Figs. 145, 146). SHADOWS. Experiment 140. — Take any opaque cylinder three or four inches in diameter, and jjut it on the table in a dark 174 LIGHT. room. Place two lighted candles four inches away from the cylinder, making \ ,'' , the distance between ■v \ / ,/ the candles ec^ual to the diameter of the cylinder (see Fig. 147). Notice the shad- ows — one d ark, the others light. Is there any part of the space behind the cylinder that does not receive '"^" ' light from either can- dle ? Can you explain "why there are one dark and two light shadows V Kxporiiiient 14:1. — Fit up a piece of apparatus as fol- lows (Fig. 148): Take tlic top of a pasteboard bo.x; about a foot S(juare. Cut out a ])iecc eleven inches square so as to leave a frame. Paste smoothly on the frame and teS; Fia. 148. over this hole a piece of thin white paper. This screen can be set up by placing the side of the box-cover down, and INTENSITY OF LIGHT. 175 putting any convenient weight on the inside. Next, cut out of pasteboard a square, two inches on a side, and mount it as shown in the figure. In a dark room set up the screen, place the square four inches from the screen, and put a lighted candle one inch from the square. You will notice that a part of the shadow on the screen is light, and the larger part dark. Trace carefully with a pencil the outline of the lighter and the darker shadow. The darker shadow is called the umbra, the lighter shadow the penumbra. Now prick pin-holes through the umbra, and see if any part of the candle can be seen. Next prick pin- holes through the penumbra, above, below, and on both sides of the umbra, and note what parts of the flame can be seen. Can you explain how the umbra and the penumbra are produced ? THE INTENSITY OF LIGHT. Experiment 142. — Take a lighted candle and a card two inches square, such as was used in Exp. 141. Fasten to the wall a large sheet of paper for a screen. Place the card eight inches from the screen, and the candle eight inches in front of the card. See that the card is parallel to the screen, and that the middle point of the candle flame is on a level with the middle point of the card. Mark on the screen the outline of the shadow. Measure its sides. You will find that the shadow is four inches on a side. How many square inches does it contain ? If the card is removed, the light that fell upon the card will fall upon sisteei) square inches of the screen. Divide 176 LIGHT. the outline of the shadow on the screen into four square parts. Each part is the size of the cardboard. If the card should be placed sixteen inches from the candle, or twice as far as at first, how much light would it receive compared with the amount it received when eight inches from the candle 'i Next, place the card eight inclies from the screen, and the candle four inches from the card. Mark on the screen the outline of the shadow. You find that it is six inches square, llow many square inches of screen does the shadow cover ? Divide the space covered by the shadow into nine squares the size of the card. If the card is placed twelve inches from the candle, or :-:-'^-- Fig. 149. three times as far as at first, how much light would it receive compared with the amount it received when- four inches from the candle ? Again, place the card twelve inches from the screen, and the candle four inches from tlie card. Mark the outline of the shadow. You find it to lie eight inches square. How many square inches does the shadow cover ? IMAGES FORMED THROUGH SMALL APERTURES. 177 Divide the space into sixteen squares the size of the card. If the card should be placed sixteen inches from the candle, or four times as far as at first, how much of the light it received at first will it then receive ? Study Fig. 149. TJie light falling upon a given area varies inversely as the square of the distance from the source of light. Experiment 143. — Hold near the eye a card with a pin- hole in it. Look through the hole at a pencil three or four inches long, placed at such a distance that its ends may just be seen. Move the pencil farther and farther away, noting that its dimensions apparently diminish as it recedes. Let A (Fig. 150) represent the hole in the card, and _B, C, and D the pencU at different distances. The lines drawn from the ex- tremities of the pencil to the eye represent what is called the visual angle of the pencil. The angle formed by the rays of light coming from the extrem.ities of an object to the centre of the eye is called the Visual Angle. The farther an object is from the eye, the smaller is the visual angle under which it is seen. Pig. 150. IMAGES FORMED THROUGH SMALL APERTURES. Experiment 144. — Procure a pasteboard box seven inches long, three and a half inches wide, and three inches 13 178 LIGHT. deep, or as near these dimensions as possible. Make the inside black by using ink. Construct a little screen (Fig. 151) with pins and pasteboard, and of a size that can be moved from end to end ^ — of the closed box. Over the hole in the screen paste smoothly a piece of white tissue paper. Now in the centre of the ends of the box make two smooth holes the size of a common lead- pencil. Over one of the holes paste a piece of tin foil, and through this make a pin-hole. Set the screen in the middle of the box and put on the cover. Now hold the pin-hole towards some object, as a house, a steeple, or a tree, and look through the other hole. You will see an image of the object. Move the screen backward or forward till you get the clearest image. In what position is the image com- pared with the position of the object ? Take the box into a dark room. Light a candle. Look through the large hole at the image of the candle. In what position is the image compared with the position of the candle ? Can you explain the position of the image by remembering the law of light stated iij Exp. 139 ? Make a diagram to illustrate your explanation. REFLECTION OF LIGHT. Experiment 146. — Hold a piece of looking-glass so that the sun's rays falling upon it will appear as a light spot on the wall. The spot of light on the wall has been reflected there by the bright surface of the looking-glass. The rays of light which fall upon the glass are called REFLECTION OF LIGHT. 179 incident rays ; the rays which the looking-glass sends off to the wall are called the reflected rays. Move the piece of looking-glass so that the angle made by the incident and the reflected rays shall be a very small one. Move the glass so that these rays shall make a very large angle with each other. Fig. 158. Experiment 146. — Make out of cardboard a semicircle having a radius of about twelve inches, like that shown in Fig. 152. Support the cardboard m a vertical position by two pieces of wood in which slots are cut. Directly un- der the centre of the semicircle place a piece of looking-glass. See that the wooden supports are low enough to let the cardboard touch the glass. The radius marked 0° is perpendicular to the plane of the glass. Now with black paint or liquid shoe-blacking, black either the outside or the inside of a piece of glass tubing nine or ten inches long. Set the apparatus where there is plenty of light, though not in the direct rays of the sun. Select a pin with a large bright head. Thrust the pin through the semicircle, near the outer end of one of the lines — for first trial the one making an angle of forty-five degrees with the radius marked 0°, which is a perpendicular to the surface of the glass. Place the tube on the other side of the perpendic- 180 LIGHT. ular, with the lower end at A, and find at what angle it must be held in order to see the image of the pin-head. How do the two angles compare ? Move the pin-head nearer the perpendicular, and find at what angle the tube must be held. How do the two angles compare ? Move the pin-head farther from the perpendicular. Find the angle at which its image may be seen by looking through the tube. How do the two angles compare ? Were we to put a small taper in place of the pin-head, we should obtain the same results. Terms. — The ray from the candle to the mirror is the incident ray, and the angle the ray makes with the perpen- dicular is called the angle of incidence. The ray from the mirror is called the reflected ray, and the angle which this ray makes with the perpendicular is called the angle of reflection. When rays of light are reflected, the angle of inci- dence is always equal to the angle of reflection. DIFFUSION AND ABSORPTION. Experiment 147. — Procure a piece of looking-glass, two pieces of tin, and a piece of unsized white paper. Blacken one piece of tin by smoking its surface with a candle. Darken a room, and allow a small beam of sunlight to shine in. Hold successively in the beam the looking-glass, the bright piece of tin, the piece of drawing jjaper, and the piece of blackened tin, and try to reflect the beam of light from each. VIRTUAL AND REAL IMAGE. 181 Compare the spot of light made upon the wall by the m^irror with that made by the piece of tin. Does the mirror or the piece of tin reflect the more light ? Why ? Try the piece of paper. Can you get a distinct spot on the wall by holding the paper in the beam ? Which can you see the more plainly when held in the beam, the looking-glass or the paper ? Which illuminates the room the more ? Try the piece of blackened tin. Does it reflect any light ? Can you see the piece of blackened tin easily ? The looking-glass and the tin reflected light regularly, and are therefore good reflectors. The paper reflected the light irregularly, or diffused it. The piece of blackened tin absorbed the light. The experiment shows reflection, diffusion, and absorp- tion of light. VIRTUAL AND REAL IMAGE. Experiment 148. — Take the box used in Exp. 144 into a dark room, place a candle in front of the box, and observe the image which appears on the screen inside the box. Next place the candle in front of a looking-glass and observe the image. Wherein do the two images differ ? Is there an image on the screen ? Is there actually an image behind the looking-glass ? An image Ihat may be thrown upon a screen is called a Heal Image. An image that appears behind a reflecting surface, and that cannot be thrown upon a screen, is called a Virtual Image. 182 LIGHT. THE POSITION AND DIRECTION OF AN IMAGE BEHIND A PLANE MIRROR. Experiment 149. — Upon any smooth, flat surface, as the top of a table, draw a line about one foot long. Upon this line, and perpendicular to the surface of the table, support an oblong piece of looking-glass of any conven- ient size. About six inches out from the glass stick a large bright pin, P, into the table so that it shall be per- pendicular to the surface of the table. Six or seven inches from the first pin stick an- other pin, P^, so that it shall make a straight line with the first pin and its image in the mirror. On one side of P and P' place two other pins, P' and P\ so that they shall be in a straight line with the image of the first pin set up. Now remove the mirror, and draw lines through the points where the j)ins stick, producing these lines till they meet behind the mirror. Measure the distance the image -T was behind the mirror, and compare it with the distance the first i>iu was in front of the mirror. The image of an object appears as far behind a plane inirror as the object is in front of it. The image is the same size as the object. Experiment 150. — Set the mirror up again, and also pins P and P". Place the eye at the edge of the table, Fig. iM. INCLINED MIRRORS. 183 and look along the line marked by pins P' and -P*. Remove these pins. The image of pin P is seen behind the mirror, and in the direction along the reflected ray. An object when seen by reflection appears in the direc- tion in which the reflected rarjs enter the eye. INCLINED MIRRORS. Experiment 151. — Take two oblong pieces of looking- glass about three by four inches, and paste carefully upon the back a piece of cloth for a hinge. Set these up on the table so that they open at an angle of ninety degrees. Place a candle or other object between the mirrors. Count the number of images. Set the mirrors at an angle of sixty degrees. How many images are seen ? Set them at an angle of forty-five degrees. How many images are seen ? What relation is there between the number of degrees the mirrors are inclined, the number of images (counting the object as if it were an image), and three hundred and sixty degrees ? Two mirrors are inclined at ' an angle of thirty degrees. How many images are formed ? At an angle of one hundred and twenty degrees, how many images are formed ? How to draw the Image of au Object. — Let A B be an object placed before the mirror MB, and E the place of the eye. Every point of the arrow sends out rays of light in every 184 LIGHT. direction. Select the extre'me points A and £. From the great number of rays which A and B send out, a few only- can enter the eye. Let A C be one ray from the point A, and B Dhe one ray from the point H, falling so that after reflection they will enter the eye. At the points of inci- dence C and D erect the perpendicular C JP and DP. From C draw the line C £J so that the angle of reflection will be equal to the angle of incidence. From D draw the line D IS so that the angle of reflection will be equal to the angle of incidence. Extend the lines beyond the mirror. From the points A and -B draw lines perpendicular to the mirror, and extend them behind, a distance equal to their length from the mirror. The points A' and _B' are the extreme points of the virtual image. Pig. 155. Fig. 156. Construct a diagram to show what effect a plane mirror has upon divergent rays (Fig. 155). Upon convergent rays (Fig. 156). CONCAVE AND CONVEX MIRRORS. Experiment 152. — Procure two watch crystals. Cover evenly the inside of one and the outside of the other with black paint. These are to be used for mirrors. If you can obtain a hollow concave glass reflector, coated inside with quick- silver, such as is placed behind a bracket-lamp, it will answer the purpose better. REFRACTION. 185 Place in front of a concave mirror (the ■watch crystal painted on the convex side) a lighted candle or other object. How does the size of the image compare with the size of the object ? Hold a small piece of white paper in front of the mirror, and see if you can throw an image of the candle upon it. Is the image formed by a concave mirror real or virtual ? Experiment 153. — Place in front and near a convex mirror (the watch crystal painted on the concave side) a lighted candle. How does the image compare in size with the object ? Remove the candle slowly farther and farther away, observing the image all the time. At the greatest distance at which you can see an image of the candle, what changes have taken place ? See if you can throw the image of the candle produced by a convex mirror upon a screen. What kind of an image is formed by a convex mirror, real or virtual ? Notice the images formed in a brightly polished table- spoon, the outside of the bowl being regarded as a convex mirror, and the inside as a concave mirror. REFRACTION. Experiment 154:. — Procure a large rectangular-shaped bottle of uncolored glass. Fill this bottle with water, and place it on a table in a darkened room. With a parte lumiere (see Appendix, § 5) send a beam of light so that it will strike the side of the bottle perpendicularly. Scatter 186 LIGHT. a little crayon dust through the air about the bottle, by clapping together two blackboard erasers, and you will notice that the course of the ray through the water is in a straight line with the course of the ray out- side of the bot- tle (Fig. 157). Turn the. mirror of the ^'"^ ^"''' porte lumiere slightly, so that the ray will fall obliquely upon the side of the bottle. Put a piece of black paper on one side of the bottle, so that the direction of the ray through the water may be more easily seen. You notice now that the ray is bent out of its course. Scatter a little crayon dust about the bottle, and notice the direction of the ray as it leaves the water. Here again it is bent out of its course. Which way was the ray bent as it entered the water, referring to its direction before entering ? Which way after leaving the water, referring to its direction through the water ? When the ray fell perpendicularly upon the side of the bottle, or surface of the water against the side, its course was not changed. When it fell upon the side of the bottle obliquely, it was bent out of its course, or refracted. REFRACTION. 187 Not regarding the thin glass of the bottle, the ray of light passed through two media, air and water. The bending of a ray of light out of its course when it passes from one medium to another is called Refrac- tion. To draw the Directions of Incident and Refracted Bays.— If the experiment was carefully made, RIO Y m Fig. Fig. 168. 158 will represent the direction of the ray. The point I, where the ray fell upon the surface of the water, is the incident point, and R I\s, the incident ray ; I w, the re- fracted ray, and C* I^ is the ray after a second refraction. At the point I and in the medium the ray is entering, draw a perpendicular R I to the surface S W. Had the ray not been refracted out of its course, it would have gone in the direction I A. It took, however, the direction I 0, and was refracted, in going into a more refractive medium, toward the perpendicular P I. At the point and in the medium the ray is entering, erect the perpendicular D to the surface F E. Had 188 LIGHT. the ray not been refracted, it would have continued in the direction B. But upon entering the less refractive medium, it was refracted away from the perpendicular B 0. When a ray of light passes perpendicularly from one medium to another, it is not refracted. When a ray of light passes obliquely from a less re- fractive to a more refractive medium, it is refracted toward the perpendicular. When a ray of light passes obliquely from, a more re- fractive to a less refractive medium, it is refracted from the perpendicular. The incident ray, the refracted ray, arid the perpen- dicular are in the same plane. Experiment 165. — Repeat the previous experiment, using instead of the bottle of water an oblong glass paper- weight, if one can readily be obtained. Stand the paper- weight on end, and let the ray enter one of the narrow sides. Compare the refractive powers of glass and water. Place a coin in a dish ; put the eye in a position so that the farthest edge of the coin may just be seen above the edge of the dish. Hold the eye in the same position, and let some one pour water into the dish, not disturbing the coin. Explain what you observe. TOTAL REFLECTION. Experiment 166. — Take the bottle filled with water that was used in Experiment 154, and support it as shown in Fig. 159. With the parte Iwmiere direct a strong beam of light under the bottle. With a mirror (see Fig. 159) TOTAL EEFLECTION. 189 reflect the beam so that it will strike the side of the bottle perpendicularly. Note that it is not refracted as it passes Fig. 159. through the water. Now turn the mirror so that the beam will strike t) e lower surface of the water obliquely. No- tice how much it is refracted as it passes out of the water into the air. Turn the mirror so that the beam will strike the lower surface of the water m^ore and more obliquely. Notice carefully the amount of refraction. Diffuse a little crayon dust through the air. As the beam strikes more and more obliquely, you will observe that at one point it is refracted almost parallel to the surface of the water. When it strikes a little more obliquely, the beam does not pass through the water, but is reflected from its upper surface. This reflection is called total reflection. 190 LIGHT. Experlmeut 157. — Put a spoon in a glass of water. Raise the glass above the eye, and look at the surface of the water. The surface looks like pol- ished silver, because all the light is reflected. A part of the spoon is dis- tinctly seen by total reflection. A ray of light is totally reflected tvheUf in passing through a more refractive medium to a less refrac- tive one, it strikes the surface at such ail angle as to be reflected back in- stead of being refracted. PRISMS AND LENSES. Experiment 158. — Hold a glass prism in the position shown in Fig. 161, and look at a lighted candle. The candle appears higher up than it actually is. Let us follow one ray from Pig. 160. Pig. 161. the candle to the eye. The ray F P strikes the prism obliquely at the point P. Draw the perpendicular P LENSES. 191 from the incident point and in the refracting medium. The ray in passing from a less refractive to a more re- fractive medium is refracted toward the perpendicular, or in the direction JP D. Position of eye. At D the ray passes again to a less refractive medium, and is refracted away from the perpendicular D V, or in the direction D M The candle appears at H, a prolongation of the direc- tion that the ray entered the eye. Can you now draw Fig. 162 and complete' it, showing where the candle would appear. Fig. 163. LENSES. Different Kinds of Lenses. — A piece of glass bounded by two curved surfaces, or one curved and one plane surface, is a lens. These are six kinds of lenses : Thin-edged, thick in the centre. Fig. 163. 1. Double convex — both surfaces convex. 2. Plano-convex — one plane and one convex surface. 192 LIGHT. 3. Concavo-convex converging — one convex and one con- cave surface. These three lenses are thickest in the middle, and are called converging lenses, because the rays of light after passing through them converge. Thick-edged, thin in the centre. Pie. 164. 4. Double concave lenses — both surfaces concave. 5. Plano-concave — one plane and one concave surface. 6. Concavo-convex diverging — one concave and one con- vex surface. These three lenses are thinnest in the middle, and are called diverging lenses, because rays of light after passing through them diverge. Lenses 3 and 4 are sometimes called meniscus lenses. (Meniscus = moon shaped.) Experiment 169. — Make a temporary collection of lenses by bringing together a reading-glass, the larger lenses from an opera glass or a spy-glass, the lenses from a magic lantern, and the lenses from spectacles. Hold each lens in a horizontal position on a level with the eye, and look across its surface toward the light coming from a win- dow. With a slight movement of the lens, you can tell whether the surface is convex, concave, or plane. Examine both surfaces of each lens, and determine what kind of a lens it is. LENSES. 193 Experiment 160. — Hold a double convex lens in the sun's rays. Scatter a little crayon dust under the lens, and notice how the sun's rays are re- — fracted so Fig. 165. that they converge to one point. This point is called the focus (fire-place). (See F, Fig. 165.) Hold a piece of black paper at this point. What is the effect ? Look through a double convex lens at any object. Does the lens make the object look larger or smaller ? In other words, does the lens magnify or minify? Place the lens near the eye so that the object can be quite clearly seen. Move the lens farther and farther from the eye. Does the object always appear in a vertical position ? Place the lens at such a distance from the eye that the object may be clearly seen. Without changing the dis- tance between the lens and the eye, move the object nearer and then farther away. What is the effect ? Experiment 161. — Darken a room, except a part of one window. Let the day be a bright one. Hold the lens near the wall, and then move it out from the wall till an image of the window and what is beyond is thrown on the wall. Stop the lens at that point where the image is clearest. Measure the distance from the wall to the lens. This distance is called the focal length of the lens.* * The distance thus found is the focal length only if the window is far enough from the lens so that the rays of light from the window are parallel as they fall upon the lens. 13 194 LIGHT. Is the image thrown on the wall a real or a virtual image ? Repeat the experiment, using a plano-convex and a concavo-convex converging lens, if these are in your collec- tion of lenses. Experiment 162. — Darken a room, except a part of one window, as in Exp. 16 1. Hold a double concave lens near the wall, and then move it out slowly from the wall. Can you obtain an image on the wall as you can when using a double convex lens ? Look through the double concave lens at the window or any other object. An image is formed smaller than the object. Is it a real or a virtual image ? The rays of light from the arrow A B (Fig. 166), after passing through the lens, seem to the eye to come from the smaller image a h. COLOR. Experiment 163. — Over the opening in the porte lu- niiere (see Appendix, § 5) put a piece of tin-foil or card- board having an opening three-quarters of an inch long and one thirty-second of an inch wide, with very smooth edges. Direct a beam of sunlight into a darkened room through this slit. COLOR. 195 Place in tlie beam a prism, with the edges of the prism and the slit parallel. If tlie wall is not sufficiently near, jjlace a white screen Vjehind the prism. A band of different colors will appear upon the screen. Such a band of colors is called the solar spectrum. Names of tlie Colors of the Spectrum. — The spectrum is Fig. IBT. composed of a great number of colors, blending one into another. As seven colors c lie clearlj' ma out, it is usual |M|^^ ^ say that the sol H^l spectrum is co] ^flp* posed of 1 1 "^ t seven colors red, orange, y loio, green, cya.iv- blue, ultr a rn a- rine-hlue, and violet. Cyan-blue and ultramarine-blue are Fig. 168. 196 LIGHT. the names of blue pigments which very closely resemble the two blues of the spectrum. Scatter a little crayon dust about the rays, and deter- mine where the sunbeam would have made a bright spot on the screen, if it liad not been refracted. Now deter- mine which of the colors is refracted least ; which is re- fracted most. Experiment 164. — Hold a convex lens in the spectrum (Fig. 168). The colors will be refracted to a focus and recombined to form white light. Cut off the red rays by putting a card between the prism and the lens. What color is the light at the focus ? Cut off the blue rays. What color is the light at the focus ? Or, instead of the convex Fig. 169. , t, i j j • lens, hold a second prism near the first one, with their faces parallel, as shown in Fig. 169. The light coming from the second prism is no longer colored. Experiment 165. — Place a piece of red ribbon or paper in the red rays of the si^ectrum. Is it any brighter in the red ? Now move it towards the blue. You notice that it grows gradually darker, at last appearing black. In what colors does it appear black ? Place a piece of blue ribbon in the blue rays and move it towards the red. In what color is it blue ? In what, black ? Pass a ray of sunlight from the parte lumiere through a piece of red glass ; through a piece of blue glass ; through both the red and the blue glass. Does any light pass MIXING COLORS. 197 through ? The red glass absorbs blue light, and the blue glass absorbs red light. Color of Bodies. — The color of a body is due to its prop- erty of reflecting or transmitting rays of a particular kind, the other rays generally being absorbed. If light were not composed of the different colors, there would be no variety of color in objects. Test this by rolling up a ball the size of a marble out of loose cotton twine. Soak this ball in a strong solution of common salt. Let the ball dry, or squeeze out all the brine possible. Wet the ball with alcohol, place it upon a saucer, and set fire to it in a dark room. In the yellow light that is produced, examine pieces of red, yellow, and blue ribbon or paper, or flowers of dif- ferent colors. They will appear either yellow or black. Why? MIXING COLORS. Experiment 166. — Upon a black background (A, Fig. 170),, place two pieces of colored paper, one yellow and the other ultramarine- ,//# blue. Let them be about one and one- half inches apart, and above them hold a piece of window-glass, as shown in Fig. 170. When the eye is held in the poeition indicated, the light from the yellow paper will be reflected from the nearer side of the glass, and the image will appear close to the blue paper seen f,q. 170. through the glass. By moving the glass a little, the two pieces of colored paper can be made to coincide, but the color then seen will be a grayish white. Caution : if the color has a yellow 198 LIGHT. FiQ. m. tinge, tlie glass is held too high ; if a bluish cast, the glass is held too low. Experiment 167. — Procure a top that is spun by a holder (Fig. 171). Next cut out of stiff paper or card- board seven circular disks four inches in diameter, and color them so that each will represent, as nearly as pos- sible, one of the colors of the spectrum. In the centre of each disk make a hole just large enough to slip snugly over the spindle of the top. Cut a straight slit from the cir- cumference to the centre of each disk. Place together, bj- means of the slits, a yellow and an ultramarine-blue disk, as in Fig. 172. In this position put them on the spindle of the top, and spin the top. The rapid motion causes the sensations produced by the two colors to be blended together in tlie eye, and we see but one color. If the right proportion of each disk has been left visible, this color is grayish white. A>iy two colors that together produce tvhite are called Complementary Colors. Experiment 168. — Put the red, the green, and the vio- let disks together (Fig. 173), allowing less of the green to be exposed than of the other two. Rotate these. You should obtain grayish white. Try the same thing with red, green, and blue disks. Try combining any color with the two that are opposite to it in Fig. 174. Third, combine green and violet, first allowing more of Fig. 172. COLORED PIGMENTS. 199 Fig. 173. Fig. 174. the violet to show, and then more of the green. Can you obtain the colors between them in Fig. 174? In like man- ner mix green and red in varying proportions. What colors result? Now try to make green, red, and violet by mix- ing any of the other colors to- gether. What is the result ? Since we can obtain the whole spectrum from green, violet, and red, but cannot obtain these colors by mixing any others, green, violet, and red are called primary colors, and the others are called second- ary colors. COLORED PIGMENTS. Experiment 169. — Mix a little of the yellow and blue pigments with which you have colored the disks. What is the color of the pigment that you obtain ? Next fill a flat bottle with water, and color it with a little blue vitriol. In a similar bottle color water yellow with bichromate of potash. In a darkened room, throw a solar spectrum upon a screen, and between the prism and the screen intei-pose the bottle containing the blue liquid. The green, cyan-blue, ultramarine-blue, and violet rays are still visible on the screen, but the red, orange, and yellow rays are cut off ; that is, they are absorbed by the blue liquid. Removing the bottle of blue liquid, interpose in the same way the bottle containing the yellow solution. Now the red, 200 LIGHT. orange, yellow, and green rays are allowed to pass, but the cyan-blue, ultramarine-blue, and violet rays are cut off. Next place the two bottles side by side in the spectrum, so that the rays from the prism will have to pass through both in order to reach the screen. What color is visible on the screen ? What rays are now intercepted ? We may express these results as follows : Colors op the Spectrum Absorbed by the blue solution : ^, 0, X, G, U-B, C-B, V. Absorbed by the yellow solution : R, O, Y, G, JB^, J^B", 7^. Absorbed by the two solutions : |i, 0, '^, G, JJ^, jS?^, V. This explains what occurred when we mixed the blue and yellow pigments together. Just as the solutions in the bottle absorbed some rays and transmitted others, so the pigments absorbed some rays and reflected others. The blue pigment absorbed the red, orange, and yellow rays ; the yellow pigment absorbed the cyan-blue, ultra- marine-blue, and violet rays. Hence the only rays not ab- sorbed, namely, the green rays, were reflected to the eye. But in Exp. 168 we did not mix the materials that gave the color, we mixed only the color sensations in the eye. Hence, the very rays absorbed by the blue paper were sup- plied by the reflected image of the yellow paper, and con- sequently we saw all seven rays of the spectrum, or white. THE RAINBOW. How the Rainbow is formed. — A drop of water acts upon a ray of sunlight just as a prism does, decomposing it into the seven prismatic colors. This action of the rain-drop may be understood by Fig. 175, which represents a section THE RAINBOW. 201 of two drops. S S are two parallel rays of sunlight fall- ing upon the drops of rain at a. Each ray is refracted to b, where some of the light passes out of the drop toward X / but the remainder is reflected from the inner surface of the drop and passes out at c, undergoing a second refrac- tion. In its course through the rain-drop, the light is decomposed, the violet rays being refracted more than the red rays (see p. 195). If, then, the red rays from the upper drop enter the eye of an observer at ^, the violet rays from this drop will pass over his head. The violet that he sees comes from a lower drop of water, as shown in Fio. 175. Fie. 176. Fig. 175, while the red rays from this second drop fall at his feet. Other drops of rain between the two represented supply the other colors of the spectrum. When the colors are seen in the order indicated in Fig. 175, the bow is called a, primary bow. A fainter or secondare/ how is often seen above the pri- mary bow. This is caused by other rays of sunlight {S S, Fig. 176), which fall at c? c? on the lower side of rain-drops higher up than those that produce the primary bow. From d the ray is refracted to e / at the point e a part of it is 202 LIGHT. reflected to /, and thence a still smaller portion is reflected to g. At g the ray is again refracted, and the colors into which the ray has been decomposed by the drop pass into the air. Explain from the figure why the colors of the secondary bow are in the reverse order to those of the primary bow. Can you tell why the secondary bow is fainter than the primary bow ? Would an observer standing on the roof of a house and another observer standing on the ground see the same rainbow ? THE VELOCITY OF LIGHT. First diseorered by Ecemer. — The velocity with which light travels through space was first discovered by Roemer, a Danish astronomer, in 1676, in observing the eclipses of the first satellite, or moon, of the planet Jupitei-. This satellite revolves around Jupiter in forty-two hours, twenty- eight minutes, and thirty-six seconds. Now Rcemer ob- served that as the earth moved in its orbit toward Jupiter, or from .Z^to y (Fig. 'ill), the intervals between the suc- cessive times this satellite entered the shadow of Jupiter, or was eclipsed, grew shorter ; and that as the earth was moving away from Jupiter, or from N^ to F, the intervals between successive eclipses grew longer. When the earth was at IST, Roemer found that the eclipse took place sixteen minutes and thirty-six seconds earlier OPTICAL INSTRUMENTS. 203 than if the earth had been at F ; and that when the earth was at F, the eclipse took place sixteen minutes and thirty- six seconds later than if the earth had been at N. He therefore reasoned that this difference was due to the time it takes light to travel from N X^o F, which is the diameter of the earth's orbit. This distance is one hundred and eighty-six million miles. If we reduce sixteen minutes and thirty-six seconds to sec- onds, and divide one hundred and eighty-six million miles by the number, the velocity of light per second will be ob- tained. Other methods for determining the velocity of light have been found. These results show that the velocity of light is about 186,000 miles per second. OPTICAL INSTRUMENTS. The Simple Microscope. — A double convex lens of short focal length is called a simple microscope. The eye is Fig. 178. placed close to the lens and the object at a distance less than the focal length of the lens. This distance varies for different persons. 204 LIGHT. The image is virtual and erect. In Fig. 178 ^4 B is the object, and C D the image. Compound Microscope.- The compound microscope con- sists of two or more convex lenses mounted in a tube, which is blackened inside. One of the lenses is of short focus and is called the objective; the other lens is of longer focal length and is called the eye-inece. The object A B (Fig. 179) is placed just beyond the focus of the objective 0. A real inverted " image C 2> is formed within the tube. The eye-piece E, a lens of greater focal length, is used to examine the image C D. The lens M is so placed that the real image lies between the lens and its focus F. A virtual image M N, great- ly magnified, is seen by the eye. THE ASTRONOMICAL TELE- SCOPE. Experiment 170. — Take a large lens of ten or twelve inches focal length (see Exp. 161). Cut a groove in a cork, and press the edge of the lens into this groove, thus mak- ing a temporary holder for the lens. Stand this lens upon a table in a darkened room, and place at as great a distance Fig. 1' THE ASTRONOMICAL TELESCOPE. as possible a lighted candle (Fig. 180). lens the screen used in Exp. 144. 205 Place behind the Fig. 180. See that the middle part of the candle flame and the centres of the lens and screen are on a level. Now move the screen back from the lens until an inverted real image, smaller than the candle, is formed on the screen. Remove the screen, and put in its place a convex lens of short focal length mounted on a cork. Look through this lens, and a magnified upright virtual image will be seen. If these lenses were placed in tubes blackened inside, and made to slide the one into the other, a common astronomi- cal telescope would be formed. Explanation. — In Fig. 181, ^ ^ is an object very far distant, is the objective. Eays from the extremities of the object are refracted by the objective, and a real in- verted image C D'\s, formed. The eye-piece E magnifies the real image C D, and the eye sees the virtual image Mm M s. Fig. 181. The object A B being at a great distance, an inverted image is seen. This inversion is not found inconvenient when examining the heavenly bodies. 206 LIGHT. Experiment 1 71. — Throw an image upon the screen, as in the previous experiment. Remove the screen, and put in its place a double concave lens mounted on a cork. Now move the concave lens slowly towards the convex lens until the eye sees an erect enlarged image of the candle. It is better to fasten to the cork, with a pin, a piece of paste- board with a hole through it half the diameter of the lens. This piece of pasteboard may be regarded as a diaphragm, and should be on the side of the cork next to the eye. Were these lenses set in sliding tubes of proper size, the Galilean telescope would be formed. The Opera Glass. — The opera glass consists of two small Galilean telescopes placed side by side. Fig. 183. In Fig. 182, ^ ^ is the object, the objective, and E the eye-piece. The rays from the extremities oi A B con- verge after their refraction by the objective 0, and are rendered diverging after their refraction bj the eye-piece E. Thus the rays appear as if they came from C D. The image is erect. THE CAMERA. Explanation. — In Exp. 144, by letting the light from an object shine through a pin-hole in the end of a dark box, THE EYE. 207 an image of that object was obtained on a sliding screen. The photographer's ;\^ camera is like the pin-hole camera (Exp. 144). The only essential differ- ence is that in place of a pin-hole a double convex lens {L, Fig. 183) screen A is made of ground glass, an object form an inverted image, clear in outline, at S, and the movable screen A is pushed up to this point, so that the image may be examined by the eye. To secure a pict- ure of the object, a sensitized plate is put in the place of the screen and exposed to the rays of light that come from the object. These rays produce certain chemical clianges in the plate, imprinting there a picture of the object. is used. The movable The rays coming from THE EYE. The Eye a Camera. — Fig. 184 represents a section of the human eye. It has for its outer wall a tough membrane called the sclerotic coat. Inside of this coat is a delicate mem- brane called the choroid coat. The choroid coat is dark in color, which prevents internal reflection of the rays that enter the eye. The third and inner coat of the eye is the retina. It is a very thin, translucent mem- brane, at the back part of the eye, formed by the expansion of the optic nerve, and is very sensitive to light. Fig. 184. 208 LIGHT. In the front wall of the eye is the cornea, a thin, hard, transparent substance. A little behind the cornea there is a double convex lens, named the crystalline lens. In front of the lens there is a circular curtain, the iris, with a hole in its centre, the pupil, which regulates the amount of light entering the eye. A limpid liquid, called the aqueous humor {A-S, Fig. 184), fills the space between the cornea and the crystalline lens. The ball of the eye behind the lens is filled with a transparent, jelly-like substance called the vitreous humor. By means of the crystalline lens, the rays of light com- ing to the eye from an object are brought to a focus on the retina, where, just as on the screen of the camera, an inverted image is formed. The retina is an expansion of the optic nerve ; and the optic nerve, being stimulated by the rays which form the image, conveys the impression to the brain. In the camera the focus of the lens is adjusted by mov- ing the screen closer to the lens for distant objects, and farther from the lens for near objects. In the eye, by means of a delicate mechanism, which works without our consciousness, the focal distance of the lens is increased and diminished by the flattening and thickening of the lens itself. Thus, for a distant object, the retina does not move forward like the screen in a camera, but the crystalline lens becomes thinner, and so moves the focus back to the retina. On the other hand, for near objects, the crystalline lens becomes thicker, and so brings the focus forward to the retina. Long Sight and Short Sight. — Some eyes are able to see objects at a distance more clearly than objects near by. This condition of vision is known &% far-sightedness. Other eyes see objects clearly only when they are very near the eye, and are said to be near-sighted. The defects are most SUMMARY. 209 commonly due to the shape of the globe of the eye. In the far-sighted eye the globe is too short [F, Fig. 185, repre- senting the retina), and the image, unless the object is very distant, is formed back of the retina, or at It. Converging glasses must be used to correct this defect. -y • _ Fig. 185. In the near-sighted eye the globe is too long [N, Fig. 185), and the image is formed in front of the retina. The correction is made by using diverging glasses. Why ? SUMMARY. Luminous bodies are bodies that emit light. Any substance through which light passes is called a medium. With reference to their ability to transmit light, substances are either transparent, translucent, or opaque. A ray is a single line of light. A beam of light is a collection of rays. A pencil of light is a number of rays diverging from a point or con- verging to a point. If uninterrupted, light always travels in straight lines. The darker part of a shadow is the umbra. The lighter part of a shadow is the penumbra The intensity of illumination varies inversely as the square of the dis- tance from the source of light. The angle made by the incident ray with a perpendicular to the reflect, ing surface at the point of incidence is the angle of incidence. The angle made by the reflected ray with the perpendicular is the angle of reflection. When rays of light are reflected, the angle of incidence is always equa> to the angle of reflection. Light reflected irregularly is said to be diffused. 14 210 LIGHT. When the light falling upon an object is not reflected or diffused, it is said to be absorbed. A real image is an image that is actually formed. A virtual image is an unreal, or apparent, image. Refraction is a change in the direction of a ray of light when it passes obliquely from a less refractive to a more refractive, or from a more refractive to a less refractive, medium. Laws of Refraction. I. — When a ray of light passes perpendicularly from one medium to another, it is not refracted. 11. — When a ray of light passes obliquely from a less refractive to a more refractive medium, it is refracted toward a perpendicular at the point of incidence. III. — When a ray of light passes obliquely from a more refractive to a less refractive medium, it is refracted from the perpendicular. IV. — ^The incident ray, the refracted ray, and the perpendicular are in the same plane. A ray of light is totally reflected when, in passing through a more refractive medium, it strikes the surface of a less refractive medium at such an angle as to be reflected back, instead of being refracted. The convex lenses collect rays of light to a focus, and so magnify the objects seen through them. The concave lenses scatter rays of light, and apparently diminish the size of the object. A prism decomposes a ray of sunlight into seven colored rays: red, orange, yellow, green, cyan-blue, ultramarine-blue and violet. These colors, when thus formed, are called the solar spectrum. The color of a body is due to its property of reflecting, or transmitting, rays of only a particular kind. The primary colors are red, green, and violet. The other colors of the spectrum are called secondary colors. Complementary colors are any two colors that together produce white light. The rainbow is caused by the reflection and the refraction of rays of sunlight by drops of rain. The velocity of light Is about one hundred and eighty-six thousand miles per second. QUESTIONS. 211 Questions. 1. What are luminous bodies ? 3. Distinguish between transparent, translucent, and opaque bodies. 3. What is a shadow ? 4. Explain how the umbra is formed. How the penumbra. 5. Can any part of a luminous body be seen from the penumbra ? 6. What effect has distance upon the intensity of light ? 7. Explain why the images seen through small apertures ara inverted. 3. Show by a diagram what is meant by the statement, the angle of incidence always equals the angle of reflection. 9. In what respect does a real image differ from a virtual image ? 10. Draw a diagram to show the way in which an image is formed by a plane reflecting surface. 11. Explain why several images are sometimes seen when an object is seen by reflection in a thick glass mirror. Which image is the brightest ? Why ? 12. What is diffusion»of light, and how does it differ from reflection and absorption ? 13. What relations does the image formed by a plane mirror bear to the object ? 14. Under what conditions will a ray of sunlight not pass through a prism ? 15. Two mirrors are inclined at an angle of 30° ; how many images of an object placed between them are seen ? At 120° ? 16. Is the image formed by a convex mirror larger or smaller than the object ? 17. Under what conditions is the image formed by a concave mirror larger than the object ? Under what conditions is the image smaller ? 18. What laws are obeyed when a ray of light passes from one medium to another ? 19. Name and classify the different kinds of lenses. 20. How can you ascertain the focal length of a lens ? 21. What is the solar spectrum ? 32. Name in order the colors of the solar spectrum, beginning with the color most refracted. Name the colors beginning with the one least refracted. 23. What are complementary colors ? 24. How did Roemer determine the velocity of light ? 212 LIGHT. 25. Name the colors of the primary rainbow, beginning with the color on the under side of the arch. Name the colors of the secondary bow. 26. How many refractions and reflections in each of the rain-drops trhich produce the secondary bow ? How many in each of the rain- drops producing the primary bow ? 27. Draw a diagram to show how the colors of the spectrum aj .. produced by a drop of dew in the sunlight. 28. What is the most common cause of near-sightedness ? What c.f far-sightedness ? What kind of glasses are required for the former ? What for the latter ? Why ? CHAPTEK IX. MAGNETISM AND ELECTRICITY, Magnets -A body which has the property of attracting iron is called a magnet. There is a particular kind of iron ore called lodestone, found principally in Siberia and Scandinavia, that has the power of attracting iron. These pieces of ore are natural magnets. Lodestone was originally found in Magnesia, Asia Minor, whence the name magnet and magnetism. If a bar of steel be rubbed against lodestone, it will become magnetic. Such a magnet is called an artificial magnet. Experiment 172. — Rub against a bar magnet (see Appendix, § 3) a knife-blade or any other piece of steel. Bring the knife-blade near iron filings or small tacks. Has the blade acquired magnetic properties ? Experiment 173. — Take the bar magnet used in the previous experiment, and lay it down upon a sheet of paper. Sprinkle iron filings over the whole length of the magnet. Now take hold of the middle part of the magnet and lift it out of the iron filings. To what part of the bar magnet do the iron filings cling ? The ends of a magnet where the attrnction is greatest are called its poles. 214 MAGNETISM AND ELECTRICITY Notice that the attraction of the magnet for iron filing diminishes from the poles towards the centre of the bar where it is nothing. This central line is called the equator, or neutral line, of the magnet. Experiment 174. — Sprinkle iron filings evenly over a sheet of paper, one edge of which is fastened to the edge of the table with two pins, while the other edge is held in the hand. Bring the end of the bar magnet against the under surface of the paper, and move the magnet about. Note the results. Now perform the same experiment, using cardboard instead of paper. Next use a piece of glass. Last try a piece of board an inch thick. The power exhibited \>y a magnet at its poles is called magnetic force. The space through which the influence of a magnet extends is called the magnetic field of that magnet. Experiment 175. — Lay a bar magnet upon the table, and put over this a sheet of paper. To keep the surface of the paper level, lay on the desk, about three inches on each side of the magnet, two strips of wood the thickness of the magnet. When you have made the surface of the paper level, sprinkle carefully fine iron filings over the paper. Tap the paper gently. Notice closely the way in which the filings arrange themselves. Make a careful sketch of these. Put the magnet between two books so that it may stand edge up. Lay a sheet of paper over the maj a silk thread C a Ijall of corn-stalk or elder-pith B, made as round as possible. Present an electrified body to the pith-ball. The ball will be instantly attracted and then repelled. Fig. 203. This apparatus is called an tlnetrascope, be- cause it detects the presence of electricity in a body. SIMULTANEOUS DEVELOPMENT OF POSITIVE AND NEGATIVE ELECTRICITY. Experiment 189. — Make a flannel cap about three inches long, so that it will fit over the end of a rod of sealing-wax. Attach a silk thread to the cap, as represented in Fig. 203. Turn the cap around on the rod a few times, remove it by the thread, and pre- sent the rod to the pith-ball of the elec- troscope. The ball is first attracted and then repelled; that is, it becomes charged with the same kind of electricity as the rod. Now present the flannel cap, holding it by the silk thread. It attracts the ball. Present the sealing-wax again. The ball is repelled. What kind of electricity was devel- oped on the sealing-wax ? What kind on the flannel cap ? Fig. INDUCTION. 229 When, two hoilies are ruhbeil together, positive and negative electrifications are developed, equal in amoiin^t. One kind of electrification is never devel- oped without the other. Experiiiient 190. — Suspend a pith-ball by a cotton thread, and hold the thread in the hand. Electrify the rod of sealing-wax, and present it to the pith -ball. The ball is continuously attracted to the sealing-wax, and cannot be charged. The ball does not become charged because the cotton thread conveys or conducts the electricity away from the ball to the hand, while from the hand it is conducted by the body to the earth. All bodies do not conduct elec- tricity equally well. Dry air, shellac, resins, glass, and silk are some of the substances that are poor conductors, or non-conductors. A body separated from other bodies by a non-conductor, so that when the body is charged the electricity is not conveyed to the earth, is said to be insulated. INDUCTION. Experiment 191. — Balance a thin strip of wood an inch wide and fifteen inches long on a square bottle, dry and warm. Under one end of the wood place some bits of tissue paper (Fig. 204), and over the other end hold an electrified rod of glass or sealing-wax, taking care not to 230 MAGISTETISM AND ELECTRICITY. touch the wood. The bits of paper are attracted and repelled. Electrification has been induced in the wood. The electrification of one body when another electri- fied body is brought near it, but not touching, is Induction. Experiment 192. — Balance a small tin pan, warm and dry, upside down on the toja of a bottle, as in Fig. 205. Electrify a warm end of sealing-wax Avith a "'*pi piece of flannel, and charge the pith-ball of the electroscope so that it is repelled. Now bring Pig. 205. ^\^q j.q^ pf ^y^y^ near to one side of the tin, and hold the electroscope near the opposite side. What takes place ? Gradually bring the electroscope around towards the same side of the tin where the sealing-wax is. The ball is attracted to the tin. With what kind of electricity is the sealing-wax charged ? What kind has been induced on that side of the tin next the sealing-wax ? "WTiat kind on the opposite side of the tin ? Can you state the law of induction ? THE ELECTROPHORUS. Experiment 193. — In an ordinary tin pie-plate melt slowly some resin or sealing-wax. Keep the plate as level as possible, taking care that the contents do not catch fire or boil. You should have the resin in the plate about half an inch thick. Now cut out a circular piece of tin a little smaller than the resin in the tin plate. Fasten to this tin disk a bottle for a handle. THE ELECTEOPHOEUS. 231 Place the resin disk on a table, and strike or rub it with a warm piece of flannel, or better, with a piece of cat's fur. By means of the insulating handle set the tin disk on the resin, and touch the upper side of the tin disk with the finger. Remove the finger, and lift the tin disk from the resin. If the apparatus has been kept warm and dry, you ought to be able to draw a spark from the tin disk by placing your knuckle near it. Experiment 194. — Fig. 206 represents a sectional view of the electrophorus, as this instrument is called. D is the sealing-wax or resin, with the tin plate T on the under side ; C is the tin disk to which is fastened the handle S. When we rub\Z> with a cat's fur or flannel, we develop nega- tive electricity on its upper surface. This induces -I- elec- tricity on the upper, or nearer, side of the tin T, while the — electricity is repelled to the under, or farther, side, as in Exp. 192 it was repelled to the farther side of the tin. This — electricity is neutralized by its contact witli the earth, and T remains charged with + electricity. The tin disk C will not touch D at more than one or two points. There will be a thin layer of air between. What kind of electricity is therefore induced on the lower side of C? When you touch the upper side with the finger, what kind passes off to the earth ? When G is finally lifted from D, with what kind of electricity is C charged? 1 H trjp 5 .?■. S:-:;pic I " ~ ~ ' ^ ■ "" '^ ^ + + + + Pig. -H + + T 206. 232 MAGNETISM AND ELECTKICITT. To electrify a conductor by means of this instrument, obtain a charge of + electricity on the tin disk, as described, and then present the tin disk to the body to be electrified. A spark will pass from the one to the other. Repeat the operation several times until the body is elec- trified with the charge desired. DISTRIBUTION. Experiment 195. — Make a cone-shaped muslin bag, and sew the edge to a wire ring, as in Fig. 207. Let the ends of the wire form- ing the ring be bent down- wards, so as to hold the ring upright in the cork of a bottle. Fasten two silk threads to the apex of the bag m such a way that the bag can be drawn inside out. By means of shellac, fasten a copper cent to the side and at the end of a glass rod about six inches long. This little instrument is called a, proof-plane, and is used to test charged bodies. Electrify the muslin bag by means of an electrophorus, and then touch it on the outside with the proof-plane. Upon holding the proof-plane towards the electroscope, you will find that it has received a charge of electri- city from the bag. Touch the proof-plane in order to discharge it, and then test the inside of the bag in the POTENTIAL. 233 same way. Is there any electricity on the inside of the bag? By means of the silk thread, turn the bag inside out, and test both inside and outside as before. Where do you find the electricity ? When a body is charged with frictional electricity the chargS is confined to the outside of the body. If, however, an insulated, electrified body is held within a metallic ves- sel, electricity will be induced on the inner surface of the vessel. POTENTIAL. When a body at a higher temperature comes in contact with a body at a lower temperature, the former loses some of its heat to the latter, and this continues until both bodies are of the same temperature. Just as bodies may be in different states in regard to heat, so we have learned bodies are in different states of electrification. That which corresponds to temperature in heat is called potential in electricity. In order that there may be a flow of elec- tricity from one conductor to another, the first conductor must possess a higher potential than the second. In electricity the earth is taken as the standard. A body positively electrified is at a higher potential than the earth. A body negatively electrified is at a lower poten- tial than the earth. A body that is neutral is at the same potential as the earth. ELECTRICAL MACHINES. Electrical machines are mechanical contrivances for util- izing the principle of the electrophorus, and so obtaining an increased charge of electricity. Fig. 208 shows an 234 MAGNETISM AND ELKCTKICITV. electrical machine of the most improved form, called the Toepler-lloltz machine. CBS— ^ j'=T=Jti'"='- < Fia. 201 CONDENSERS. QO^ ^3-*^S3 Tlie Leyden Jar. — Procure a ulass jar i (Fig. 209), or a lai-tre-iiioutlied Ixittle six or seven iiiclies high. Using a Aveak solution of gum-arabic, cover Ijoth the outside and the inside with tin-foil to "withiu one-third of the lieight from the top. Fasten circular pieces of tin-foil to the bottom of the jar, inside and outside, taking care that each joins tlie coating on tlie snles. Varnish the striji of glass that has p„ j|,_, been left exposed around the top of the jar. Througli a varnished wooden cover ])ass a brass wire five inches h)nL;', and from its lower end snsjiend a brass chain long enouu'li to reach the bottom of the jai-. ]\[ake a hole in a good-sized bullet, and press it on the upper end of the wire. Through the cork of a slim bottle H (Fig. 209), pass aa- EFFECTS OP POINTS. ■ 235 Other wire, bent as shown in the figure. Fasten a bullet on each end. This piece of apparatus is called a discharger. Experiment 196. — Set the Leyden jar on the table, or, better, hold it in the hand, and electrify the ball many times with the electrophorus,* in order to charge the jar, observing the spark. Now touch the tin-foil with one ball of the discharger, and gradually bring the other ball nearer the knob of the jar. How does the spark you obtain com- pare with a spark from the electrophorus ? Evidently the numerous small charges of electricity that you put into the jar were held there, .and united to form one greater discharge. An apparatus such as the Leyden jar, which collects successive charges of electricity and confines them on a smaller surface than they would ordinarily spread over, is called a condenser. When the tin-foil within the jar became charged with -f electricity, what kind of electricity was induced on the inner surface of the tin-foil without the jar ? What kind passed from the outer surface of the tin-foil' through the hand and body to the earth ? Explain the further action of the jar and its discharge. EFFECTS OF POINTS. Experiment 197. — File one end of a piece of wire about three inches long to a point, and twist the other end * A friotional electrical machine, if the school has one, will charge the jar more quickly, though such a machine is not essential. 236 MAGNETISM AND ELECTRICITY. about the knob of the Leyden jar. Try to charge the jar with the electrophorus, as in the previous experiment. After a number of charges, can you obtain any discharge ? Again, remove the wire and charge the jar. 'Now, hold- ing the wire in the hand, with the point directed to the knob of the jar, but not touching it, try to discharge the jar. Do you obtain any spark ? In the first case, the molecules of air coming in contact with the pointed wire, each molecule in turn conveyed away the + electricity from the jar, until the jar was discharged. In the second case, the knob of the jar inducing — electricity on the point of the wire, this — electricity was conveyed by the molecules of air to the knob, and so discharged the jar. From a pointed wire attached to the charged condenser of an electrical machine the molecules of air act in this way fast enough to blow out a lighted candle. Lightning-rods. — The phenomenon known as lightning is only a discharge of frictional electricity. Knowing this, can you explain how lightning-rods protect a house from being struck ? Which has the higher potential, the thun- der-clouds or the house ? Does the action of lightning- rods correspond to the first or to the second of the experi- ments you have just tried with the pointed wire and the Leyden jar ? Questions. 1. What is the general law of electrical attraction and repulsion ? 2. How can you show that positive and negative electrifications are produced simultaneously ? 3. What are conductors ? What, non-conductors ? What othei name is given to non-conductors ? 4. What is an electroscope ? VOLTAIC ELEOTRicrrr. 237 5. Make a diagram of an eleetrophorus, and describe the method ol charging it. 6. Explain electrical induction. 7. Describe the construction of the Leyden jar. 8. How is a Leyden jar charged ? How discharged ? 9. Explain fully what occurs when the jar is charged and dis- charged. 10. Why can a stronger spark be obtained from the Leyden jar than from the machine by which it is charged ? 11. A student, standing upon the floor, once tried to light the gas with a charged Leyden jar. He turned on the gas, and touched the tip of the gas jet with the knob of the jar, but instead of lighting the gas, he received a shock of electricity. Why ? What should he have done 2 12. Why are lightning-rods pointed ? VOLTAIC ELECTRICITY. Experiment 198. — Cut out a piece of sheet zinc four inches long and one and a half inches wide. Place this in a tumbler of dilute sulphuric acid.* Notice the action the acid has upon the zinc. Bubbles of gas collect on the zinc and rise to the surface of the liquid. These bubbles are hydrogen gas. Experiment 199. — After the zinc has remained in acid a few seconds, take it out, and with an old tooth-brush or a piece of cloth rub mercury over both surfaces of the zinc. Take only a tiny drop of mercury for this. When you rub the mercury on the zinc, lay the zinc on a perfectly flat surface, and be very careful not to bend it. * The dilution of sulphuric acid needed in this and other experi- ments is twenty parts of water to one part of sulphuric acid. Always pour the sulphuric acid into the water. Never pour the water upon the sulphuric acid. Be careful not to get any of the pure or dilute acid upon your hands or clothes. 238 MAGNETISM AND ELECTRICITY. Notice that tlie mercury clings to the zinc and gives the surface a bright appearance. Coating zinc with mercury" in this way is called amalgamating the zinc. Now put the amalgamated zinc in the dilute sulphuric acid, and see whether the acid has the same effect upon the zinc as it had before it was amalgamated. If the zinc is thoroughly amalgamated, no bubbles will rise. Experiment 200. — Cut out a piece of sheet copper four inches long and one and a half inches wide (the size of the zinc in Exp. 198). See that the copper has no solder or tin upon it. Put this piece of copper in the tumbler of dilute acid. Has the acid any action upon the copper? Do you see any bubbles col- lecting on the copper and rising to the surface of the acid ? Experiment 201. — Pro- cure a piece of pine about four inches long and one inch square. Tack the piece of copper used in Exp. 200 on one side of the stick, and the piece of zinc used in Exp. 198 on the othei'. Take care that the tacks from either side do not touch each other in the wood. Take two pieces of copper wire, each about two feet long. Fig. 210. VOLTAIC CELLS, 239 Twist the end of each once or twice around a tack, and drive one tack through the zinc into the wood, and the other tack through the copper. One piece of wire is now in contact with the zinc, and one in contact with the cop- per. Put the two strips of metal into the tumbler of acid, as shown in Fig. 210. Connect the ends of the wires A and J3. Notice that hydrogen bubbles collect on the copper strip and rise to the surface of the acid. Disconnect the wires. Do the bubbles continue to rise ? Connect the wires again. Do bubbles rise ? A Current of Electricity and its Direction. — When the wires A and _S in the previous experiment (201) were connected, a current of electricity * was set up. The current of electricity is said to flow from the zinc to the copper through the acid, and along the wire from the copper strip to the wire connected with the zinc. If you connect the wires A and Ji (Exp. 201), and let the current of electricity flow for a few hours, you will find that the zinc wastes away, or is used up, by chemical action. After you have performed Exp. 203, connect A and _B for several hours. THE VOLTAIC CELL. The Terms, Cell, Battery, and Poles. — The combination of zinc, copper, and dilute acid (Fig. 210) is called a voltaic cell. The two metals are called the elements, and the acidulated water the exciting fluid. Several cells consti- tute a battery, though a single cell is often called a battery. * What electricity itself really is, is not known. "We know how to produce it by certain methods, and also what can be done with it when it. has been produced. 240 MAGNETISM AND ELECTBICITT. The extremities of the wire A and £ (Fig. 210) are called poles, or electrodes. The current of electricity in such a cell as we have been using flows from the zinc, or positive element, where the chemical action takes place, through the liquid, to the copper, or negative element, thence through the connected wires to the zinc element. The end of the wire A from the copper element will be a, positive electrode ( +), and the end of the wire leading to the zinc element will be a nega- tive electrode { — ), when the current is flowing. CONDUCTORS. Experiment 202. — Connect the wires of the cell (Fig. 210) so that a current of electricity may flow. Now place between the electrodes a piece of wood. Does the current continue to flow ? Try pasteboard, glass, a piece of gutta percha, a piece of silk or cotton cloth, a piece of wax. Does a current flow through the wires ? Interpose a silver coin between the wires. Does the current flow ? Try a piece of iron ; a piece of brass. Cut a little trough in a piece of wood, put a drop of mercury in this, and place the ends of the wire in the opposite ends of the trough of m.ercury. Does the current flow? Those substances through which electricity passes easily are called, good Conductors. Substances through which electricity passes with great difficulty are called Non-Conductors, or Insulators, Make a list of the good conductors and a list of the non- conductors used in Exp. 202. VOLTAIC CELLS. 241 COMMON VOLTAIC CELLS. Polarization. — In the cell we have been using you noticed that, though bubbles of hydrogen streamed up from the copper plate, there was always a collection of hydrogen bubbles on the surface of the plate. This hydrogen acts in the first place as a non-conductor to the electricity, and so weakens the current ; in the second place, the hydrogen forms a positive element to the copper, and so tends to set up a current with it in an opposite direction to the current set up between the zinc and the copper. The formation of a film of hydrogen on the copper plate, or any other action which tends to set up a cur- rent in an opposite direction, is called Polarization. The Two Classes of Cells. — In order to lessen the polari- zation in a cell, various forms of cells have been devised. These may be divided into two general classes : (1) those which have in each cell one fluid ; (2) those which have in each cell two fluids. Smee's Cell. — A Smee's cell (Fig. 211) consists of two zinc plates, between which is a silver plate. The surface of this silver plate is roughened by being covered over with finely divided plati- num. The bubbles of hydrogen are dis- charged more easily from the points of this roughened sur- face than if the silver The fluid is dilute sulphuric acid. Fig. 211. Fig. 812. plate were smooth. 16 242 MAGNETISM AND ELECTRIOITT. The Bichromate Cell. — One form of the bichromate cell is shown in Fig. 212. It consists of a bottle-shaped glass vessel, containing a zinc plate Z attached to a sliding brass rod, by which it may be drawn up out of the liquid when a current is not needed. Two carbon plates C C are attached to the vulcanite cover. The exciting liquid is a solution of potassium bichromate in sulphuric acid. (For the exact proportions, see Appendix, § 6.) This cell gives a strong current for a short time. -The gravity cell (Fig. 213) has a copper plate placed at the bottom of a glass vessel and the zinc plate near the top. The exciting liquid is a solution of copper sulphate (blue vitriol) in water. This cell is largely used on telegraphic lines. The Le Clanche Cell.— This cell f[~|^(i ! consists of a I ] L I zinc rod Z, and a car- bon plate C, which has on each side a prism of binoxide of manganese B, £ (Fig. 214). The exciting fluid is a solution of ammo- nium chloride (sal ammoniac) in water. The prisms of binoxide of manganese prevent polarization by uniting with the hydrogen bubbles. This cell is used where a current is needed for a brief interval of time, as in working telephones and electric bells. Fig. 213. Fig. 214. THE ELECTRO-MAGNET. 243 Fig. 815. The Bunsen's Cell. — The Bunsen's cell '(Fig. 215) is a two-fluid battery. The outer glass vessel contains dilute sulphuric acid. In this acid is placed a zinc cylinder Z, having a slot cut in it lengthwise. In- side the zinc is put a porous, unglazed, earthenware cup E, con- t a i n i n g strong nitric acid, in which is immersed a rod of car- bon C. THE ELECTRO-MAGNET. Experiment 203. — Wind around a piece of soft iron a quarter of an inch in diameter (a carriage bolt will answer) eighteen feet of Xo. 20 insu- lated copper wire.* This will make four turns of wire around the bolt. Connect the ends of the wire with the plates of the cell used in Exp. 202, or any other cell that you may have. Xow jjut the ends of the iron rod into iron filings. The iron filings cling to the ends of the rud, showing that the I'od has acquired magnetic projjerties. Such a magnet is called an electro- magnet. The coil of wire is called a helix, and the iron in the helix is called its core. Disconnect one wire from the battery. The iron filings fall. Connect the wire again with the battery. Will iron filings now cling to the rod ? The rod becomes an electro-magnet when the current flows through the helix, and ceases to be a magnet when * Wire covered with any of the non-conductors, cotton, silk, or India rubber, is called insulated wire. 244 MAGNETISM AND ELECTRICITY. IJiiiiiBi^HlliiliPl! A Fig. 216. the current is not allowed to flow. Connecting the wires so that the electricity will flow through the helix, and then separating them so as to stop the flow, is called making and break- ing, or closing and opening, tlic circuit. An electro-magnet is a bar of soft iron, generally in the shape of a horseshoe (Fig. 216), round which a coil of insulated copper wire is wound. A current of electricity is passed through the coil. The bar A is the armature. Electro-magnets are much more powerful than ordinary peiTaanent magnets, and are used to magnetize bars of steel. THE TELEGRAPH. Principle Involved and Instruments Used. — You have now learned how an electro-magnet may be produced. The electric telegraph works by alternately making and unmak- ing an electro-magnet at a distance. For telegraphing, it is necessary to have a battery to generate the currents, a wire to connect the two places, a key, and a sounder. Two lines were formerly employed in telegraphing ; one l)2ing used for the current to pass to the receiving station, and the other for the current to return to the battery. Now only one wire is used. The zinc plate at one end of tiie line and the copper plate at the other end are each con- nected with a large plate of metal buried in the earth. The earth takes the place of a return wire. The Key. — The telegraph key is shown in Fig. 217. The instrument is made of brass, excejjt that the knob JET of the lever and the knob i8 of the switch are of vulcanite. The THE TELEGltAFH. 245 switch is closed in Fit;'. i.'17, ami allows the current to jiass through the key and along the main wire. The wires are wound around lhe posts TV TV. "When a message is to be sent, the switch is first thrown open. Then by pressing down on the knob //of the lever, a little ])oint under tiie lever touclies the eml of the post W, and the circuit is closed. When the pressure is removed, the spring /? raises the lever, ami the circuit is broken. The Sounder. — ^Vhen the circuit is clnsed, the armature ^1 of the .so/(;/r/- er (Fig. il.s) is drawn d o w n by the electro- m a g n e t /?/ ami when the circuit is lirok- cn, i' is no longer au elec- tro-m a g net, and the arma- t u r e A is thrown np a little waj^ by a spring under the lever X. As often as the operator presses down ou the lever of 246 MAGNETISM AND ELECTRICITY. his key and allows it to spring back, the same action takes place in the sounder, and a click is heard. The operator receiving a message reads the letters by the duration, number, and arrangement of the clicks. Formerly messages were printed by the receiving instru- ment, or sounder. A point attached to the armature lever pressed down upon paper, and made dots and dashes. Be- cause of this, the telegraphic alphabet is represented by printed dots and dashes. TELEGRAPHIC ALPHABET. A B C D E P G H I J K L M X P Q E S T u V w X Y Z & ._:_ ? FIGURES. 1 2 3 4 5 6 7 8 9 At most offices, messages are now read by the sound of the clicks. The R(4ii.Y. — With a given battery, if the current is to be sent over a very long wire it will be too weak to draw the armature of the sounder down. To remedy this a relay is introduced. The relay has a helix of very fine wire, and when the weak current reaches it, the electro-magnet attracts its armature, and thus brings into use a local battery which works the sounder. Fig. 219 shows the arrangement of relays, sounders, and keys. In Fig. 219 the switch ^4 is open at Pliiladelphia. Press the key at Philadelphia. What effect will this have upon THE ELECTEIC BELL. 247 the relay at Philadelphia and the relay at New York ? When the armature E of the relay at New York is drawn to the magnet, what takes place at C ? How does this operate the sounder G f How do the relay and the sounder G Earth. Fig. 219. Earth. at Philadelphia work at the same time ? Does the direc- tion in which the message is sent make any diiference in the working of the instruments ? THE ELECTRIC BELL. Explanation. — One form of the electric bell is indicated in Fig. 220. The hammer S is attached to a piece of soft iron D, which acts as an armature of the electro-mag- net C. To the armature JD is attached a spring which brings it back to the position shown in Fig. 220, after it has been attracted by the electro-magnet. ^ is a spring so adjusted that when the armature is drawn to the electro- 248 MAGNETISM AND ELECTRICITY. Fig. 230. magnet there is no electrical connection between E and D. As soon as the circuit is closed, the current passes through the insulated wire If, which is wound around the soft iron, then through the wire M, up the spring and the armature D, to the point where the spring E touches the armature, down the spring E to the binding- post P, and thence back to the battery. As soon as the current completes its circuit, G becomes an electro-magnet, and attracts the armature D, causing the clapper to strike the bell. The instant the armature is drawn to the electro- magnet, contact is broken with the spring E, the electro- magnet is unmade, and the spring carries the armature back to the spring E, remaking contact with the battery. In this way a vibrating motion of the clapper is set up, and the bell rings. The battery usually consists of two or three Le Clanche cells, and contact is made at a distance by means of a puih-hutton. Examine any electric bell you may have, and show how the current passes through the bell from one binding-post to the other. EFFECT OF THE CURRENT UPON A MAGNETIC NEEDLE. Experiment 204. — File a knitting needle in two and magnetize both halves at the same time. Mark the north EFFECT OF CUERENT UPON A MAGNETIC NEEDLE. 249 end of one half, and put it aside for use in Exp. 206 (the astatic needle). Suspend the other half bj an untwisted silk thread.* When the needle has come to rest, hold above the needle, and as near as possible without touching it, an insulated copper wire through which a current of electricity is passing. What is the effect upon the needle ? Arrange the cell used so that the current shall flow from south to north above the needle. Which way is the north end of the needle now deflected? Hold the wire so that the cur- rent shall flow from south to north below the needle. Which way is the north end of the needle now deflected ? Turn cell about so that the current in the wire shall flow from north to south, and hold the wire above the needle. Which way is the north end of the needle de- flected? Hold the wire below the needle. Which way is the north end of the needle now deflected ? FlQ. ^1. Ampere's Rule. — Ampere formulated a rule for readily determining tlie deflection of a magnetic needle, due to an electric current passing through a wire parallel to the needle. * For an untwisted silk thread unravel the end of a piece of ribbon. 250 MAGNETISM AND ELECTRICITY. Fig. 222. Let the observer imagine himself swimming in the direction the current moves, nnd facing the needle, then the north end of the needle tvill always be de- flected to his left hand. Experiment 205. — Suspend the same needle used in the previous experiment. Bend insulated copper wire into the form of an oblong, place it near the needle, as shown in Fig. 222. Pass a current through the wire from the same cell used in the preceding experi- ment. Explain why the needle is de- flected more than when one wire was used, as in Exp. 204. Make several turns of wire, as shown in Fig. 223, and see if the needle is deflected more than with one turn. THE ASTATIC NEEDLE. Experiment 206.— Twist copper wire about each of the two halves of the knit- ting needle magnetized in Exp. 204 so that the north pole of one half shall be above the south pole of the other. Suspend the pair by an untwisted silk thread so that they will move in a horizontal plane (Pig. 224). Does the pair point as strongly in one direction as a single needle ? Why ? Fig. 223. THE GALVANOMETEE. 251 A combination of two tnagnetic needles ivitli their poles so arranged that the earth's tnagnetism has no directive influence upon them is called an Astatic NeedZe* ^ Pass a current of electricity around the astatic needle, as shown in Fig. 225. Can you explain why it is deflected more than a single needle was with one turn of wire in Exp. 204 ? ^m Fig. 834. THE GALVANOMETER. Its Use. — We have learned that when a wire through which a cur- rent is passing is placed above a magnetic needle, the needle is Pio.225. deflected (Exp. 204) ; that when several turns of wire are made about the needle, the needle is still more deflected (Exp. 205). If, then, we wish to ascertain whether a current is pass- ing through a wire, we can introduce into the circuit several turns of wire wound about a magnetic needle, and a deflection of this needle will indicate the presence of a current in the wire. A galvanometer is constructed upon this principle. It can be used not only to detect the presence of a cur- rent, but also to show its direction, and roughly to indicate its intensity. * It is very difficult to make a perfect astatic pair, for the two nee- dles must be of equal length and of exactly the same magnetic strength. They must be parallel and their axes must lie in the same vertical plane. 252 MAGNETISM AND ELECTRICITY. Fia. 236. How to construct a Galvanometer. — Construct a galvanom- eter by winding about a wood- en frame six inches long, one and three quar- ter inches wide, seven -eighths of an inch deep, about twenty- eight turns of insulated cop- per wire No. 20 (fourteen turns on each side of the mag- netic needle). Suspend by an untwisted silk thread the magnetic needle used in Exp. 204, so that it may move in a hori- zontal plane be- tween the upper and lower wind- ing of the wire. Put a pin at C, to prevent the needle from turning more than half way. Fig. 226 is a sketch of the instrument when completed. Fig. 227 gives I 1 a side plan of the wooden frame, and Fig. 228 a top or bottom plan. No iron must enter into the construc- tion. Next make on paper a graduated circle two inches in diameter, which is to be placed under the galvanometer, so that the centre 1^ Fig. a-T. Fig. 238. EXTERNAL RESISTANCE, 253 of the circle may lie directly below the point where the needle is suspended. In using this galvanometer it must be set so that the sides shall be parallel with the needle. Both will point in the direction of the magnetic meridian. To prevent a disturbance of the galvanometer when set, place a heavy piece of wood over the wires A and £ (Fig. 226), which should extend out a foot from the frame. Connectors, or binding screws (Fig. 229), are fastened to A and -S. (For home- made connectors, see Appendix, § '7.) EXTERNAL RESISTANCE. Experiment 207. — Take one hundred feet of No. 16 insulated copper wire, and make a coil about six inches in diameter. Make a second coil of one hundred feet of insulated copper wire No. 30 ; a third coil of fifty feet of insu- lated copper wire No. 30 ; a fourth coil of fifty feet of insulated German silver wire No. 30. Introduce into the circuit of the galvanometer a gravity cell, and note the deflection of the needle. Now with the same cell introduce into the circuit one hundred feet of No. 16 copper wire. Note exactly how much the needle is deflected. Is it deflected more or less than it was without the wire ? Introduce into the circuit the one hundred feet of No. 30 copper wire. Note again the deflection of the needle. Is it deflected more or less than when the same length of No. 16 copper wire was in the circuit ? Introduce into the circuit the fifty feet of No. 30 254 MAGNETISM AND ELBCTRICITV. oopper wire. Notice the deflection of the needle. How does the amount of deflection compare with that when one hundred feet of the same kind of wire was in the circuit ? Lastly, introduce into the circuit the fifty feet of No. 30 German silver wire. Note, the deflection of the needle. How does the amount of deflection compare with that when fifty feet of copper wire of the same size was used ? We found, in comparing the deflection of the needle, when there was no coil in circuit with its deflection when any of the coils was in circuit, that the current acted as if it met with a certain amount of opposition in the wire. This apparent opposition to the flow of the current is called resistance. AYhich has the more resistance, one hundred feet of No. 16 copper wire or one hundred feet of No. 30 copper wire ? Since the material is the same and the length the same, to what is the difference due ? "Which has the greater resistance, one hundred feet of No. 30 copper wire or fifty feet of No. 30 copper wire? To what is the difference in this instance due ? Which has the greater resistance, fifty feet of No. 30 copper wire or fifty feet of No. 30 German silver wire ? To what is the difference in this instance due ? The resistance of a conductor increases tvith its length, and decreases as its cross-section increases. The nature of the material has also to be taken into account in estimating resistance. INTEENAL EESISTANOE. 255 Resistance is measured in Ohms.* An ohm is the resistance of two hundred and fifty feet of No. 16 copper wire or ten feet of No. 30 copper wire. INTERNAL RESISTANCE. Experiment 208. — Connect a gravity battery, in which the plates are as far apart as possible, with the galvanom- eter, and note the deflection of the needle. Then bring the plates as near together as possible, and note the deflec- tion of the needle. In which instance did the galvanometer show the greater strength of current ? The resistance which the liquid offers to the passage of the current is called Internal Resistance. ELECTRO-MOTIVE FORCE. JExperiment 209. — Cut out a copper and a zinc plate four and three-quarters by three and one-quarter inches. Araalgam^ate the zinc. Nail each plate to a strip of wood, fastening at the same time a copper wire to each. Now put them into a jar of dilute sulphuric acid. The dilute acid should come almost to the top of the plates. Connect the wires from the plates with the galvanometer, and note how much the needle is deflected. Remove the zinc plate, and put in its place a plate of sheet lead of the same size. Connect the wires from each plate with the galvanometers, and note how much it is deflected. With which pair, copper and zinc, or copper and lead, was the needle deflected the more ? * The ohm is named from a German investigator, G. S. Ohm, who first stated the laws which determine the strength of currents. 256 MAGNETISM AND ELECTRICITY. If possible, try a carbon plate with the zinc plate, and compare the deflection with the deflection of the needle when the zinc and copper plates were in the acid. Electro-motiTe Force and its Unit of Measure. — To account for a greater deflection of the needle when zinc and copper are used than when lead and copper, we say that a cell with zinc and copper plates has a greater electro-motive force (written E. M. F.) than a cell with lead and copper plates. A cell with zinc and carbon plates has a greater E. M. F. than a cell with zinc and copper plates. The unit of measurement of electro- motive force is called a Volt* The E. M. F. of a current from one gravity cell is about one volt. A Grenet cell has an E. M. F. of about two volts ; a Le Clanche cell has an E. M. F. of about one and a half volts. STRENGTH OF CURRENT. Experiment 210. — Construct another galvanometer according to directions given (p. 252), but instead of using No. 20 insulated copper wire use No. 12 insulated copper wire, and wind but two turns around the frame on each side of the needle. Procure an empty copper cartridge-shell one inch long and about half an inch in diameter, and twist tightly around it a copper wire. Cut a strip of zinc one-eighth of * Volt from Volta, an Italian scientist, who In the very first part of this century made important discoveries in Electricity. Galvani was another Italian investigator, who made some important discoveries a few years previous to those made by Volta. STEKNGTH OF CURRENT. 257 an inch wide and one and one-quarter inches long. Fasten a copper wire to one end of the zinc. Next bore a small hole through the zinc near the wire and three-quarters of an inch from the end, so that a piece of match may be put through the hole. Fill the copper shell with di- lute sulphuric acid, put in the strip of zinc, and push the piece of match through the hole so that the zinc will not touch Fig. 230. the bottom of the shell (Fig. 230). Connect the wire from this little cell with the galva- nometer made of No. 20 copper wire, and note carefully the deflection. Now connect with the same galvanometer the cell used in Exp. 209, having its zinc and copper plates four and three-quarters by three and one-quarter inches. Note carefully the deflection of the needle. The deflection is the same in each case. Connect the wire from the small cartridge battery with the galvanometer made of No. 12 wire, and note carefully the amount the needle is deflected. Now connect the cell having the large plates with this galvanometer, and note the deflection of the needle. With the large cell, you noticed that there was a much greater deflection than with the little cell. This shows that the larger cell, though its E. M. F. is the same as that of the smaller cell, is able to yield a much greater quantity of electricity. When, however, these cells were connected with the gal- vanometer made of much smaller wire (No. 20), the needle was deflected the same amount by each, because only a 14 258 MAGNETISM AND ELECTKICITY. small quantity of electricity could pass through the fine wire galvanometer, on account of its resistance, and the smaller cell yielded that quantity as well as the larger one. THE DIFFERENCE BETWEEN STRENGTH OF THE CURRENT AND ELECTRO-MOTIVE FORCE. Upon wliat Electro-motive Force depends. — The electro- motive force of the large cell used in Exp. 210 was no greater than the electro-motive force of the small cell. The electro-motive force set up by the two elements of a battery depends n])on what is termed their difference of potential, and is wholly independent of the size of the plates. Although the electro-motive force of the large cell was the name as that of the small cell, there was considerable difference in the quantity of electricity generated by each. The quantity of electricity which passes through a con- ductor in :i given time determines the strength of the current. The Ampere. — The measure of strength of current is called an ampere. It is the strength of current maintained by an E. M. F. of one volt, through a resistance of one ohm. Ohm's Law. — The relation existing between the strength of a current, the electro-motive force, and the resistance was tirst stated by Ohm in the following law : The stfoifftJi of a ciiri-enf varies directlji as the electro-motive force, and inversely as the resistance. This law is expressed by the formula : « = 1 Where G is the strength of current in amperes, E is the electro-motive force in volts, and R is the entire resistance in the circuit in ohms. AEEANGEMENT OF CELLS. 259 If we wish to distinguish between the external and the internal resistance, the formula may be written : in which R represents external, and r internal, resis- tance. Application of the Formula. — Suppose the E. M. F. of a cell is two volts, the external resistance B, three ohms, and the internal resistance r one ohm, what is the strength of the current ? Solution : E _ 2 volts _ 2 _ 1 " K + r ~ 3 ohms + lohm"~4~3' ^ When the E. M. F. is eight volts,* and the resistance is three ohms, what is the strength of current ? With an E. M. F. of fifteen volts, and a resistance of twenty -five ohms, what is the strength of current ? Find the resistance in a circuit when an E. M. F. of fif- teen volts gives a current of one and a half amperes. What is the E. M. F. when the current strength is seven amperes, and the resistance eight ohms ? Arrangement of Cells. — The cells of a battery may be arranged in a variety of ways. The copper or carbon of one cell may be joined to the zinc of the next, and (^^ Ct[m\ (M^ l^2i) so on, as shown in Fig. |? | \\ 'ij | 11 ¥~. ''| 23^- Fig. 231. This is called coupling, or aiTanging the cells in series. When four cells are * As a single cell can give at most but two and a half volts, let it be understood, for the purpose of the problems, that the E. M. F. comes from several cells. 260 MAGNETISM AND ELECTRICITY. Fio. 232. joined in series, the total current generated by the bat- tery passes through the liquid of each cell, or, in other words, through a liquid conductor four times as long as the liquid conductor of one cell. This increases the internal resistance fourfold. The E. M. F. of the battery, how- ever, is four times as great as that of one cell. So that if it is desired to send a current through a long and small wire — in other words, if the external resistance be great as compared with the internal re- sistance—the cells sho'uld be coupled in series. Instead of joining copper to zinc, or in series, all the copper plates may be joined, and all the zinc plates be joined, as in Fig. 232. This is called arranging the cells parallel, or w multiple arc, or abreast. When cells are arranged parallel (Fig. 232) the current divides itself between the cells, and the internal resistance of the four cells is one-quarter that of a single cell, because the liquid conductor of the current is four times as wide as that of one cell. The E. M. F. of cells joined parallel is only equal to that of one cell. If the internal resistance be great as compared with the external resistance, the cells should be joined parallel. The strongest current is obtained when the internal resistance of the battery is equal to that of the external j.,g 233. resistance of the circuit. To secure this it often becomes necessary to arrange the cells partly in series and partly parallel, as shown in Fig. 233. Suppose, for example, that we have eight cells, each hav BEST AEEANGEMENT OF CELLS. 261 ing an E. M. F. of two volts, and a resistance of eight ohms, and that the external circuit has a resistance of six- teen ohms. How shall the cells be arranged to produce the strongest current ? First, let us arrange the cells in series, and then calcu- late the strength of the current hj Ohm's law. E = 2 volts X 8 (number of cells) = 16 volts. R = 8 ohms (internal resistance of each cell) x 8 (number of cells) = 64 ohms + 16 ohms (resistance of external circuit) = 80 ohms. Then as i Tp 1 fi 1 C = =, we have 5^ == ampere, the strength of the current. Second, arrange the cells parallel. E = 3 volts. R = 1 ohm (internal resistance being I of what it is in one cell) + 16 ohms (resistance of external circuit) = 17 ohms. r Then as E 2 C = rp, we have =-=■ ampere, the strength of the current. Third, arrange the cells in two rows, four cells being in series in each row, and the two rows being joined parallel. E = 2 volts X 4 = 8 volts. R = 8 ohms (internal resistance of each cell) x 4 (number of cells) = 32 ohms. 33 ohms -v- 2 (the two rows being parallel) = 16 ohms (in- ternal resistance) + 16 ohms (external resistance) = 32 ohms. Then as 17 8 1 C = =■, we have 57; = 7 arapi-re, the strength of the current. 262 MAGNETISM AND ELECTRICITY. Under the conditions stated, the strongest current is produced by arranging the cells in two rows, with four cells of each row in series. COMPOUND CIRCUITS. Experiment 211. — With No. 16 insulated copper wire, make a compound circuit, as shown in Fig. 234. Insert a galvanometer (the first one made) in one branch at G, Fig. 234. When the current reaches B, it divides, one portion going by the branch S G A, and the other portion by the branch BRA. Either branch may be called a shunt (by-path) to the other. Introduce at R, in the branch 13 A, the coil of one hun- dred feet of No. 16 copper wire used in Exp. 207. Note the deflection of the galvanometer. How does its deflec- tion compare with its deflection before the coil was intro- duced at R ? Put in place of the coil of No. 16 wire the coil of No. 30 copper wire, also used in Exp. 207. Compare the deflection of the galvanometer with its deflection when the coil of No. 16 wire was at R. If when the coil of No. 30 wire was at R the resistance ELECTROLYSIS. 263 oi jB It A was two ohms, and the resistance oi J3 G A was one ohm, two-thirds of the current would flow through the circuit B G A, and one-third tlirough B R A. A compound or divided circuit may have more than two branches. In a divided circuit the amount of current that will flow tlirough different branches is invevseli/ as their resistances. CHEMICAL EFFECTS OF THE ELECTRIC CURRENT. Experiment 212. — Procure two strips of platinum foil one inch long by three-eighths of an inch wide. Solder these to the ends of two pieces of insulated copper wire. Arrange the wires with the platinum electrodes at the ends, as shown in Fig. 235. Fill the glass vessel with water, and into this water pour a little sulphuric acid to increase its conducting power. Over each elec- trode invert a test-tube tilled with the acidulated water (Fig. 235). Connect the wires Z and C with two bichromate cells (see Appendix, § 6). You notice that bubbles of gas rise from each electrode, causing the water to sink in the test- tubes. Fio. S35. 264 MAGNETISM AND ELEOTBIOITT. The gases are hydrogen and oxygen, the elements which are in combination in the molecules of water. The hydro- gen will be found in the tube containing the more gas. The element given off front the positive ( + ) electrode is the electro-negative element ; the element given off from the negative (— ) electrode is the electro-positive element. Determine (see p. 239) which is the + electrode, and which the — electrode. Which gas is an electro-positive element ? Which, an electro-negative element ? Insert a galvanometer in the circuit. Is the needle deflected ? Has a current passed through the acidulated water ? Various other compounds in solution, as sulphate of cop- per, potassium iodide, may be decomposed by the current. The decomposition of compounds by the electric cur- rent is called Electrolysis. The compound itself is called an Electrolyte. Experiment 213. — Put into a tumbler a strong solu- tion of copper sulphate (blue vitriol). Couple two gravity cells in series. Attach to the wire leading from the copper element a strip of sheet copper an inch wide and two inches long. Attach to the end of the wire leading from the zinc element a small piece of iron, as an eight-penny cut-nail, thoroughly cleaned. See that all connections are thor- oughly made. Immerse both the iron and the strip of copper in the solution of copper sulphate. Put the copper and the iron very near each other. Let them remain in MAGNETIC CONDITION OF A WIBE. 265 the solution for twenty-four houi-s. Upon removing the iron, you will find that a film of copper has been deposited upon the iron. The iron has been electroplated with copper. The current of electricity passing through the solution of copper sulphate decomposes it, and the metallic copper is deposited on the body attached to the negative electrode. As fast as copper was taken from the solution and deposited on the iron, copper was dissolved from the sheet copper, thus keeping up the strength of the solution. Nickel, gold, and silver may be deposited in the same manner, provided the bath used is a solution of a salt of the metal to be deposited. MAGNETIC CONDITION OF A WIRE THROUGH WHICH A CURRENT IS PASSING. Experiment 214. — Pass a piece of copper wire one- eighth of an inch in diameter through a hole in a piece of cardboard, as shown in Pig. 236. Use wire that is not insulated, as more obvious results are obtained. Sprinkle iron filings on the cardboard around the wire. Connect this piece of copper with wire lead- ing from a battery of five bichromate cells. Tap lightly the cardboard, and ti I Fig. 336. notice that the filings arrange themsel v^es in circles round the wire. 266 MAGNETISM AND ELECTRICITY. A conductor throuffh which a current is passing is surrounded by Lines of Force* just as a magnet is. Let the current pass in the direction indicated by the arrow (Fig. 236). Place a very small magnetic needle (see Appen- dix, § 7) near the copper. Notice which way the north jjole is deflected. Place the nuedle in other positions round the copper, keeping it at the same distance from the copper. Notice each time which way the north pole is deflected. Reverse the current (see Appendix, § 7), and put the needle in the same positions as in the former trial. Notice which way the north pole of the needle is deflected. * The phenomena of a steady electric current are not confined to the conducting wire, for the space surrounding the wire bearing a current is found to be in a peculiar condition — a condition which can be explained as due to displacement of the ether or other dielectric or medium filling that space, and one which it seems impossible physi- cally to account for on any other satisfactory basis. The condition of a space immediately surrounding a ciirrent may be explored by means of iron filings. If a conducting wire be passed vertically through a hole in a piece of card adjusted to a horizontal position, and if iron filings be then sprinkled upon the card, and if the card be gently tapped downwards so that the filings may leap into positions spontaneously assumed by them, they will be found to range themselves in concentric circles round the current, while each filing becomes, for the time being, a little magnet. The space round the current is therefore an Mectro-magiwfic Field of Force, permeated by concentric circular Lines of Force. The Lines of Force mark the direction in which an ordinary magnet, such as a small compass needle, when placed within the field, tends to place itself. The one end of the magnet is driven in one direction, the other end equally in the opposite direction, along these lines of force ; the magnet is acted upon by a couple, which acts upon the two extrem- ities, or poles, like the hands on the handles of a copying press — one pole being pushed or repelled, the other being pulled or attracted, until the magnet lies along a line of force. The direction in which a current tends to throw the positive, or DIRECTION OF LINES OF FORCE ROUND A WIRE. 267 ^y A Pig. 237. Deterinlningr the Direction of tlie Lines of Force around a Wire. — If we apply Ampere's rule, we can determine the direction in which the lines of force set round a wire con- veying a current. If we suppose the obsei^ver /i^ ^ to be swimming in the current, he would pass, according to the direction we first gave the current, through the plane of the paper, head foremost ; if, then, he looks at the needle JV S, the north pole will turn in the direction of his left hand, as shown in Fig. 237. In Fig. 238 A £ represents the copper wire, and the arrow indicates the direction of the current. When the current flows along the wire from A to B, the direction of the lines of force was right-handed, or in the direction of the motion of the hands of a clock, as shown by the curved arrow ; when the current flowed from B to A, the direction of the lines of force was left- handed, or counter clock-wise. Or, to state it differently, if a person looks along a conductor ui the directio?i in B B Pig. 238. O north-seeking, pole of a magnet placed in its neighborhood is shown by Fig. 289. This direction is called the Positive Direction of the lines of force. The current in the figure passes vertically upwards ; the positive pole is thrown to the left hand of the current. This expres- sion, "left hand of the current," is obtained by sup- posing the current to be replaced by a person whose head is at B, and feet at A, and who turns so as con- stantly to keep the magnet-pole in full view. The relation between the direction of the current and the positive direction of the lines of force is always the same as that between the. propulsion of the point and the twist of the hand in the ordinary use of a corkscrew.— P«?M;*p^es of Phjjsics. Da/niell. Fig. 239. 268 MAGNETISM AND ELECTBICITT. n which the current is moving, the lines of force will bt directed positively round the conductor in the same direc- tion as that in which the hands of a watch turn. Experiment 315.— Take two pieces of No. 16 insulated copper wire thirty inches long. File one end of each to a sharp point, and then bend each wire as shown in Figure 240. Procure two small copper cartridge-s hells. Fill these with mercury and place them on the edge of a table. Hang the two wires in the shells so that the wires shall be parallel and less than a quarter of an inch apart. See that the copper shells do not touch each other. Let the wires dip half an inch into a tumbler of dilute sulphuric acid, shell lead out a wire A and B. V^ > Fig. 340. Fig. 241. From each Connect the ends of the wires A and B with a battery of five or six bichromate cells (see Appendix, § 6). The current now flows down one wire, through the acidulated water, and up the other wire. We have then two parallel currents moving in oppo- site directions. Make and break the connection quickly. The wires repel each other. Place the two shells upon a piece of sheet copper, so that tJiey may be electrically connected, and lead one wire from SPIRALS AND HELICES. 269 the battery to the tumbler of acidulated water, and let the other wire from the battery dip into mercury in one ol the shells. We have now two parallel currents flowing through the wires in the same direction. Make and break the connec- tion quickly. The wires attract each other. If the direction of the lines of force round the wires is examined, we find that when the current in the wires is passing in opposite directions, the adjacent lines of force (Fig. 242) set in the same direction ; and when the cur- rent in the wires is passing in the same direction, the adjacentYwiQ^ of force (Fig. 243) set in oppo- site directions. We ^ , have learned that _ „.„ Fig. 242. Fio. 243. like magnetic poles repel, and that unlike poles attract, each other. In the same manner, lines of force directed alike repel, and those directed oppositely attract, each other,. ^ SPIRALS AND HELICES. Experiment 216. — Procure a piece of iron wire one- eighth of an inch in diameter and nine inches long. Wind around this No. 16 insulated copper wire to within half an inch of each end. Let the wire be wound as if it were the thread of a right-hand screw (see p. 26). You have now formed a right-hand spiral or helix. The iron ■within the helix is the lore. Turn the helix end for end. 270 MAGNETISM AND ELECTEICITr, Is it still a right-hand helix ? Would the turn of the right hand and wrist from you propel the spiral forward, no matter which end of the spiral was held in the hand ? Connect the ends of the wires of this right-hand helix with the battery of one or two bichromate cells. The core inside the helix becomes a magnet. Now, knowing the direction the current is flowing through the helix, deter- mine, by applying Ampere's rule, the polarity of the ends of the core. Remember that taking any particular part of the wire you imagine yourself swimming in the current so as to look at the core, then the north jjole will be towards your left hand. When you have determined the j)olarity, prove whether you have reasoned correctly, by bringing the pole of a magnetic needle near the poles of the core. Jji, a right-hand helix the north pole of the core is at the end ivhere the current leaves the helix. Experiment 217. — With the same piece of iron for a core and insulated wire of the same size, make a left-hand spiral. Determine by Amji^re's rule the polarity of the ends of the core, and prove that your reasoning is coi-rect, by pass- ing a current through the spiral, and bringing the poles of a magnetic needle near the poles of the core. Write out the law for determining the poles of a left-hand helix. Test. — Examine any electro-magnet, ascertain the kind of helix, fix the direction the current is to take, and then determine the poles of the core. THE SOLENOID. 271 THE SOLENOID. ^WfSMTfff^f^' Fig. 244. Experiment 218. — Make a solenoid three inches long, similar to that shown in Fig. 244, by winding insulated copper wire No. 20 around a lead-pen- cil. The wire must be wound so that one turn llf s close to the next, not sepa- rated as in Fig. 244. Put a strip of copper and one of amalgamated zinc, each one and a half inches long by half an inch wide, through a large cork or piece of light wood, and connect the ends of the spirals to the strips, as shown in Fig. 244. Float the apparatus in a bowl of dilute sulphuric acid. A current passes through the solenoid in the direction indicated by the arrows. Prove this by Ampere's rule. Present the north pole of a bar magnet to the end of the solenoid marked JV. What is the result ? Present the south pole of the bar magnet to the end of the solenoid marked JV. What is the result ? The solenoid acts as a magnet. To what is this due ? Before we can understand it fully we must perform the following experiment : Experiment 319. — In Exp. 214 we saw that a con- ductor through which a current is passing is surrounded by lines of force. Fit up a piece of apparatus like that shown in Fig. 245, but instead of one wire, as in Exp. 214, use two 272 MAGNETISM AND ELECTRICITY. wires, letting them pass through the card a quarter to three-eighths of an inch apart. Connect the wires with a strong bat- tery, and the current will flow through each wire in the same direction. Sprinkle iron filings around the wires, and gently tap the cardboard. The filings will be found to arrange them- selves in a curve around both wires, as shown in Pig. 245, showing that the tendency of the lines of force is to surround both wires. Should we use three or more wires the lines of force FlQ. 345. would tend to surround them all. Explanation. — Suppose we should cut down through the Fig. 246. solenoid used in Exp. 218 with a plane, the small circles LINES OF FORCE AROUND A SOLENOID. 273 AAA and £ JB Ji would represent sections of the -wire (Fig. 246). The lines of force, instead of surrounding each wire separately (Exp. 218), unite and surround all the wires. The lines of force then are crowded together through the inside of the solenoid, and spread through a larger field outside the solenoid. If we apply Ampere's rule we shall find the lines of force entering at S and leaving at JST (Pig. 246). Verify this by applying the rule. The crowding together of the lines of force gives the great strength of the field within the solenoid. Within the solenoid the lines of force are conducted through the air. The conducting power of iron is seven hundred times as great as that of air, and so were we to put a piece of iron, of the same length as the solenoid, in the solenoid as a core, the number of lines of force pro- duced by the current in the wire would be increased, and the solenoid would exhibit stronger magnetic properties at its ends. Experiment 220. — Procure a piece of wood three by four inches and one-eighth of an inch thick (a piece of cigar box will answer), and cut two slits in the centre half an inch apart. Into these put the ends of a piece of copper and a piece of amalgamated zinc, each piece one and a half inches long by half an inch wide. Make a coil ^RSf^; of twenty turns of No, 30 insu- "^ ss^B S lated copper wire, fastening the '-^^ 1 1 1 M Fig. S4r. wire together, and also the coil to the wood, with sealing-wax. Connect one end of the coil with the zinc and the other end with the copper strip (Pig. 18 274 MAGNETISM AJSTD ELECTElOITr. 247). Float the coil upon a bowl of dilute sulphuric acid. Present the north pole of the bar magnet and then the south pole to the same side of the coil, and explain in each instance the movement of the floating coil. INDUCED CURRENTS. Experiment 231. — Wind around a piece of quarter-inch round iron, three and a half inches long, one hundred and eighty turns of No. 30 insulated copper wire (see C, Fig. 248). — -T#i^ Place two bar magnets on the edge of the table, letting them project two inches over the edge. Across the other ends lay a piece of soft iron to act as an armature. Connect the ends of the wire coil with a sensitive galvanometer G (see Appendix, § 8). Now, when the armature ^ ^ is in contact with the poles JV and /?, quickly draw it away, and notice that the needle of the gal- vanometer is deflected. A current is pro- duced in the coil 0. Be careful that cur- rents of air caused by quick movements in drawing the armature away do not affect the galva- nometer. Next hold the armature A A sufficiently far away from the poles A^and S to be beyond the lines of force which pass from one pole to the other. Move the armature A quickly towards the poles, bringing it as near as possible without letting the iron touch the poles.* * A piece of paper folded and placed over the ends of the polea will aid in performing the experiment. ESDirCED OITERENTS. 275 Note that the needle of the galvanometer is deflected. A current is again produced in the coil C. Was the needle deflected in the same direction as when the iron was drawn away from the poles iV^and S? I \\ Explanation. — Between the poles N j ii. and S lines of force are passing ■ f . (Fig. '249). When the armature ^ is ' — '^ against these poles there are more lines of force passing between the poles, because iron as a conductor of them is much better than air. The lines of force which pass through the armature A A also pass through the coil C. ISTow, as the coil is drawn away from N" and S, the number of lines of force passing through it constantly diminishes or changes. As the coil is moved towards the poles N and S the number of lines of force constantly increases or changes. In either case there is a change in the number of lines of force passing through the coil. A change in the number of lines of force passing through the space enclosed by a coil of wire produces a current of electricity in the wire. • Experiment 222. — ^Make a hollow cylinder two and a half inches long, having an inner diameter of one and one- eighth inches, by rolling up a strip of manila paper, and fastening it with mucilage. Slip over the ends of this hollow cylinder two flat rings of cardboard one and one- eighth inches inside diameter, and one and a half inches outside diameter, and fasten them with glue. Next wind 276 MAGNETISM AND ELECTBICITT. about the cylinder nine hundred turns of No. 30 insulated copper wire. The rings projecting beyond the cylinder - ; ; ,. ,, prevent the wire from slipping off. Shel- '..///; ^,;';y /.■!;',;:;!-.■:.";" lac each layer of wire to hold it in place. Connect the ends of the wire coil with the galvanometer used in the preceding experiment. Place the two bar magnets side by side f/i/ with their north poles together, and quickly plunge the magnets, north pole first, into the coil, as shown in Fig. 250. Notice that a current is produced. Determine by the deflection of the gal- vanometer needle in which direction the current is passing in the helix. Withdraw the bar magnet quickly. Again a current is produced in the helix. What is its direction when compared with the direction of the current produced by plunging in the magnets ? Put the south poles of the magnets together. Plunge these quickly into the helix. How does the direction of the current compare with the direction caused by plunging in the north poles ? Withdraw the south poles. Compare the direction of the current with its direction when the north poles were quickly withdrawn. What must be the movement of the north poles to give a current in the same direction as that produced when the south poles are withdrawn ? What, when the south poles are plunged in ? Lay the helix on its side, thrust into one end either pole Fig. 2S0. A CURRENT OF ELECTRICITY BY LINES OF FORCE 277 of the magnet, and then suddenly take it outby pulling it through the coil. What is the result ? Explanation. — As the bar magnet goes down into the helix (Fig. 251), the number of lines of force passing through the space enclosed by the coils is increased, because, as the magnet enters, lines of force pass through the area enclosed by the first coil, then through the area enclosed by the second coil, while continuing to pass through the area of the first, and so on for the third coil, and the other coils below it. As the bar magnet is removed, the number of lines of force passing through the space enclosed by all the coils is de- creasing, because the lines of force first cease to pass through the area enclosed by the last coil, then cease to pass through the area enclosed by the coil next above, and so on. Fig. 251. Increasing the number of lines of force passing through the space enclosed by a coil of wire produces a current in the coil in one direction ; decreasing the number of lines of force, produces a current in the op- posite direction. In each instance the direction of the current depends upon the direction of the lines of force. Experiment 223. — Make a helix of coarser wire to fit inside of the helix used in Exp. 222, by winding about a hollow paper cylinder one hundred and fifty turns of No. 24 insulated copper wire. 278 MAGNETISM AND ELECTEICITT. Connect the ends of the coil of coarser wire A and 3 with three bichromate cells. Lines of force are produced around the coarser coil, as shown by the dotted lines in Fig. 252. Con- nect the ends of the coil of finer wire with the galvanometer. Plunge the coil of coarser wire quickly into the coil of finer wire. Note the deflec- tion of the galvanometer needle. A current is produced in the coil of finer wire. "Withdraw the coil quickly. A current is produced in the opposite direction. The current produced in this way is known as an induced current, as is also that when a magnet is thrust into a coil. The coil through which the electric current is passing is called the primary coil ; the coil in which a current of electricity is induced is called the secondary coil. Determine, by the direction of the current in the primary coil, the polarity of the ends of this coil, and see whether the currents produced by putting the primary coil into the secondary coil and taking it out agree in direction with the currents produced in the secondary coil by putting in and taking out the same poles of the bar magnet. Reverse the current in the primary coils (see Appen- dix, § 7), quickly plunge it into the secondary coil, and then withdraw it. In what direction do the currents flow in the secondary coil ? Fig. 262. THE INDUCTION COIL. 279 Pack the inside of the primary coil with pieces of iron wire. Pass a current through tlie coil, and plunge it into the secondary coil. A stronger current is produced in the secondarjr coil. Can you explain why? The Induction Coil. — Fig. i;.53 shows a Ruhmkorff's in- duction coil. In the induction coil the object is to secure an induced cur- rent of high E. JM. F. as com- ]iared with the E. jM. F. of the battery current. The coil con- sists of coarse wire coiled a few times around a bundle of soft iron wires. This is called the prhnarn coil (see Exp. 22-3). Outside the primary coil is the secondary coil (see Exp. Fin. 253. Fig. 254. 223), which consists of a great number of turns of fine wire. 280 MAGNETISM AND ELECTRICITY. Fig. 254 represents a section of the coil. The arrows in- dicate the direction of the primary and of the secondary- current at the instant the circuit is closed. When the current flows through the primary coil, the end G of the core becomes a strong electro-magnetic pole, and attracts the little hammer-shaped piece of soft iron A attached to a stiff spring 8. Contact is made with this spring by an adjustable screw J3. As the electro-magnetic pole G attracts A, the circuit is broken. Instantly the core ceases to be an electro-magnet, and the hamrner is carried back by the stiff spring S, and the circuit is again closed. In this way the circuit is rapidly made and broken, and a current is induced in the secondary coil (Exp. 223). The condenser J" J?' consists of sheets of tin-foil 'separated from each other by silk or paper soaked in paraffine. The alternate sheets of tin-foil are connected with the opposite terminals of the contact-breaker A, S, and £. The object of the condenser is to prevent the current from leaping across from the screw to the spring each time the contact is broken. The induced currents produced by an induction coil are capable, like frictional electricity, of yielding sparks by overcoming the resistance of the air between the knobs marked + and — in Fig. 254. The electricity produced is of high tension or E. M. F. (measured in volts) as compared with the E. M. F. of the primary current. On the other hand the strength of cur- rent in the secondary coil (measured in amp&res) is much less than that of the primary coil. THE TELEPHONE. Experiment 224. — ^Make a weak bar magnet A (Fig. 255) by magnetizing a piece of steel a quarter of an inch in diameter and three inches long. Around one pole wind THE TELEPHONE. 281 a hundred turns of No. 30 insulated copper wire £, and connect one end of the wire with two gravity cells, so that the current will flow in the direction that will strengthen Fig. 365. the magnet ; that is, if the wire is wound around the south pole of the magnet, the direction of the flow of the current when one looks at the face of the south pole must be the same as the direction in which the hands of a watch move. In front of the magnet suspend by a thread or strip of paper a disk of tin (or sheet iron) JS, the size of a dime. Connect the other battery wire with a piece of tin S about two inches square, and the remaining wire C" from the coil with a strip of tin V. On S pile up a little dry lamp-black L, and press it with the strip V, being careful not to allow the two pieces of tin to come into contact. The lamp- black will be compacted into a thin cake. If the disk E be the right distance from the magnet A, it will answer to every pressure of the finger upon V by a quick movement towards the magnet. This experiment illustrates the prin- ciple that the power of carbon to transmit a current of electricity varies directly with the pressure upon the car- bon. What causes the movement of the disk E^ Description of the Telephone. — In Exp. 224 we have the essential parts of the telephone. The only difference is 282 MAGNETISM AND ELECTEICITT. ■5^:x:?^c Pig. 256. that the plate V, instead of being pressed by the finger, is caused to vibrate by the sound waves of the voice striking against it ; and the disk -E', instead of being sus- pended, is firmly fixed close to the magnet, so that it vibrates in exact accordance with 1^, and reproduces the sound waves that set V into vibration. Fig. 256 is a sectional view of the mouth-piece, or transmitter, of the tele- phone, now generally used. T^ is the vibrating metallic diaphragrd ; I is an ivory button, which conveys the motion to J'. P, the white strip in Fig. -'f 256, is a piece of platinum foil'rest- ing upon a but- ton of compressed lamp-b lack X. One wii"e C is attached to the platinum, and the which is in contact with L. Fig. 257 represents a sec- tional view of the receiver, shown in Fig. 258. A is the permanent mag- net ; a is the coil ; C O are the wires from B to the binding-screws I> D. JS is. a, vibrat- ing metallic disk. The corresponding parts in the figures are marked with the same letters. Fig. 287. Other wire C to the standard S, Pig. 358. THE DYNAMO. 283 THE DYNAMO. We have seen (p. 275) that a change in the number of lines of force that pass through the area of a coil of conduct ing material will produce a current of electricity in the wire constituting the coil. The dynamo-electric machine gener- ates electricity upon this principle, in causing, by mechanical means, a change in the number of lines of force that pass through the area enclosed by coils of electric conductors. Fig. 259 shows a rectangular coil of wire G, that may be revolved about the axis JVT between the grooved poles of Fig. 259. two powerful magnets iV" and S. The ends of the coil are connected to the galvanometer G. The lines of force that exist between the poles N and S are indicated by the dotted lines. Fig. 260 shows an end view of Fig. 259. Here the coil C is shown in four different positions [A, B 0, and D) of its rotation about its axis .X "When in the position A E the greatest number of lines of force pass through the area of the coil. When the coil has 284 MAGNETISM AND ELECTRICITY. /V''vlillil'i'^^li;l:!;'' 1 ',' .1 "' s Fig. 260. moved to the position £, one-eightli of the way around, a less number of lines passes through this area. When turned to the position C, one-quarter of a revolution, no lines pass through the area of the coil, because its plane is parallel to the lines of force. But the strength of the current de- pends upon the rapidity of the change in the number, of lines of force passing through the coil. If the lines of force represented in Fig. 260 are examined, it will be seen that the number of lines that the coil passes out of in the first eighth of its revolution, or from A E to Ji, is less than the number it passes out of in the next eighth of a revolution, or from Ji to C. Or, to state this fact generally, in the revolution of the coil from A E to G, the number of lines of force that the coil passes out of at each successive moment is greater than the number it passed out of during the preceding moment. Hence, the strength of the current in the coil constantly increases during the revolution from A E to G. In the passage of the coil from C to E A the conditions are the reverse, and the strength of the current in the coil constantly decreases. The fact that during this quarter of a revolution the number of lines of force is increasing, and not decreasing as in the revolution from A E to G, would produce a cur- rent in the opposite direction, were it not that the lines of force pass through the coil from the opposite side. Thus we see that the current in the coil flows in the same direc' tion during the first half of its revolution. But as the coil moves on from E A through the third quarter of a revolution, the lines of force again decrease in number, continuing, however, to pass through the coil from THE DTNAMO. 285 the same side. Hence, as the coil leaves position £J A, the current changes to the opposite direction. This direction is maintained throughout the second half revolution, for the same reasons that the current is maintained in one direction during the first half revolution. Now, if the galvanometer G (Fig. 259) is watched while the coil C is revolved clockwise from position A E (Fig. 260) to position E A, or one hundred and eighty degrees, the needle will indicate that a current of electricity is gener- ated, which at the start is nothing at position A E, and con- stantly increases till position C is reached, and then again decreases till position E A is reached, where it is again noth- ing. If the rotation is continued through the other half revolu- tion, the needle will indicate that a cur- rent, increasing and Fig. S61. then decreasing again, is generated in the coil, but in an op- posite direction to that generated in the first half revolution. With the construction shown in Fig. 259 it is impossible to revolve the coil C without twisting the wires leading to the galvanometer G ; hence the constructions shown in Fig. 261. In Fig. 261 the ends of the coil Care connected to two 286 MAGNETISM AJSTD ELECTKIOITT. semicircular metal bars ; the end 1 being connected to bar E, and the end 2 to bar H. These bars lie in a circle, through the centre of which the line O X passes. The bars ^and JT a,re sepa- rated by a small space, and two brushes J3 and D, made of thin sheets of copper, are prepared to lie on the circular bars at the opposite ends of a horizontal diameter. With the brushes a gal- vanometer G is connect- ed. With this arrange- FiG. 262. ment the coil G can be revolved about the line A", and yet an electrical connection be maintained be- tween the coil and the galvanometer. Fig. 262 is an end view of Fig. 261, with the coil in a position G at right angles to a line between the centres of the poles iV and *S'. In this position the brushes B and D lie across the open spaces between the bars E and H, touching both bars. Now rapidly revolve the coil clockwise until the plane of the coil includes the line drawn between the centres of the poles. Fig. 263 (C) shows this position of the coil. Here the brush D lies on bar £1, Fig. 363. THE DYNAMO. 287 and brush B on bar H. Following the law for the direc- tion of the current produced by this motion (see p. 277), a current will be pro- duced in the coil, which wi 1 1 flow out of the brush D on bar E, and into brush _B on bar H. Continuing the revolu- tion of the coil through the next ninety degrees, the position shown in Fig. 264 is reached, the first half revolution having been completed. During this motion the current generated will continue to flow from the brush D and into the brush B, according to the law above referred to. But the current produced becomes weaker and weaker through this second quarter of the revolu- tion, until at the posi- tion shown in Fig. 264 no current is produced, and the brushes again rest on the open space between the bars. Then continue the motion through the third quar ter, when the coil stands in the position shown in Fig. 265. And again, if the law for the direction of the current pro- duced be followed, it will be found that, as before, the cur- FlG. 265. 288 magitetism: and electricitt, rent flows from brush D, which is now on bar H, and into brush a, which rests on bar E ; that is, the current flows in the same direction through the fourth quarter of the revo- lution. Hence, by means of the stationary brushes resting on the two bars, which revolve with the coil, the current, alternating in direction once for every half revolution of the coil, is conveyed to the two brushes, so that it always flows out of one brush D, called the positive brush, and into the other J3, called the negative brush. This changing of the alternating current in the coil to a continuous one in the circuit to which the brushes lead is called commututlun., and the bars E and JS are called the commutator. When the coil O is wrapped about an iron cylinder, the wire and iron together constitute the armature ; the iron is the armature core, and the toire is the armature winding. The coil Cis, in this case, an armature coil of one turn. When there is but one coil on the armature, the current obtained from the brushes, although always in one direc- tion, will vary in strength from nothing when the plane of Pig. 266 the coil is in the position A E (Fig. 260), to the maximuns strength of current when the plane of the coil is in the position G (Pig. 260). THE DYHAMO. 289 This varying strength in the current would be nearly- removed by wrapping another coil M (Fig. 266) about the armature core, with its plane at right angles to the plane of coil C. The end 1 of coil G would be connected to bar E, and the end 2 to bar S ; the end 3 of coil Jtf would be connected to bar E, and the end 4 to bar S. Furthermore, four coils evenly distributed around the armature core, and properly connected to a four-bar com- mutator, would give a still smoother current than the two- coil armature, illustrated in Fig. 266. The plan of connect- ing a four-coil armature to its commutator is shown in Fig. 267, where coils A, B, C, and D have their ends 1 and 1', 2 and 2', 3 and 3', 4 and 4', respectively, connected to bars iV and F; F and W, W and M, J!f and W. The front end only of the armature is shown in Fig. 267. By increasing the number of armature coils and the number of commutator bars, a still smoother current could be obtained from the brushes. In machines of any consider- able size, the practice is to have from thirty to iifty coils of from one to twelve turns each on an armature. The commutator bars are insulated from each other by mica. The magnets between whose poles the armature rotates are called the field magnets of the dynamo. The first dynamos, or so called magneto-electric machines, had per- manent magnets for the field magnets, but now these are never used when a considerable current is desired. The practice is to use electro-magnets for the field, and the cur- rent to excite these magnets is usually furnished by the Id 290 MAGNETISM AND ELEOTRICITT. dynamo itself. The two simplest forms of connecting the field winding to the armature winding are shown in Figs. 268 and 269. In Fig. 268 the positive brush is connected with one end of the field windings, and the other end of the field windings is con- nected to one end of the working circuit, which, in the case illustrated, consists of arc lamps ; the negative brush is connected with the other end of the working circuit. The field magnet is mag- netized at first at the factory, and after a dynamo is run for the first time, the field cores retain suflicient mag- netism, so that the machine when started again generates a small current as a mag- neto-electric machine. This current passes through the field windings, and thus, by increasing the pole strength, in- creases the current generated. This increased current also passes around the field cores, and thus the current generated increases until the full capacity of the machine is reached. The dynamo illustrated in Fig. 268 is known as series inound, because the field windings are in series with the armature and working circuit. Another type is illustrated in Fig. 269. Here the positive brush is connected to one end of the field winding, and the negative brush to the other end of the field winding. These brushes are each connected to an end of the working circuit, which in this case consists of incandescent lamps. This ifi known as the shunt wound dynamo, because the Fig. 268. THE DYNAMO. 291 Kg. 268. field winding forms a shunt, or otlier path, for the current from the armature than the working circuit. In other words, at the armature ter- minals the current divides, part going through the field windings, and part through the working circuit. From what has been writ- ten, we see that in a dynamo there are two principal parts : first, the field, which includes the field cores and connecting metal, the field poles and the field windings ; second, the armature, which includes the armature core, armature wind- ings, and commutator. The office of the field of the dynamo is to produce a field of magnetic force, so placed that the armature may revolve in it. A modern type of dy- namo is illustrated in Fig. 270. Here A is the armature ; C the commutator ; B, and B' on the opposite side, the brushes ; iVand the cor- responding piece at the base, the poles ; and F and F', the field wind- ings, or coils. Now we have seen (p. 266) that a wire conveying a current of electricity is surrounded by a field of magnetic lines of force. In the Via.«ta. 292 MAGNETISM AND ELECTRICITY. machine here shown, the lines of force surrounding the current in tliat part of tlie tield windings next to tlie armature include the armature in their circuit from pole to pole. This is the useful part of the field windings. But, as indicated by the dotted curved lines (Fig. 270), there is a part of each turn of the field coils, the current in which does not aid in producing a useful field, for the negative whirls about this part of the field windings complete their circuits through the air, and hence do not pass through the armature. Fig. STl. In the Eickemeyer dynamo (Fig. -'"71), the current in the field coils is quite completely used for furnishing a useful field. Fig. 2 72 shows the machine with one side and one half THE DYNAMO. 293 of the field windings removed. Here it will be seen that the field coils encircle the armature, so that the armature core is not only the core for the armature windings, but also serves the sanre jiurpose as tlie field cores in the style of machine illustrated in Fig. SYO. Therefore, all of the lines of force generated by the current in the field windings must include the armature core in their circuits, and thus all of them aid in producing the magnetic field of force for the machine. The field coils of the Eickemej-er dj'namo are so sur- FlG. 272, rounded by iron that the lines of force produced by the current in the field are quite entirely confined within the machine, and the dynamo when working at full capacity shows very little external evidence of magnetism. 294 MAGNETISM AND ELECTRICITY. MOTORS. The opposite of tlie dynamo-electric machine is the elec- tric motor. In the dynamo a current of electricity is generated by mechanically revolving the ai-mature ; in the motor the armature is revolved by the action of a cur- rent of electricity sent through the machine. A simple way of treating the action of the currents in an electric motor is illustrated in Fig. 273, where a current generated by a dynamo enters the field coils, passing through them enters the armature at the negative brush, and passing around the armature, through its several coils, leaves the positive motor brush, and so returns to the generator. Fig. 273. The current in the field coils produces poles JV and *S respectively, above and below the armature, and the cur- rent around the armature produces poles in the armature HEATING AND LUMINOUS EFFECTS. 295 core JV and S respectively, at the left hand and right hand sides of the armature. The south pole in the armature and the north pole in the field mutually attract each other, while the south pole in the armature and the south pole in the field mutually repel each other (see p. 273). The north pole in the armature and the south pole in the field mutually attract each other, while the north pole in the armature and the north pole in the field mutually repel each other. These attractive and repellent forces all tend to produce rotation in the armature, in a counter clockwise direction, as indicated by the curved arrow. And as the armature revolves, the armature poles advance and recede through the space occupied by one armature coil, as the commutator bars successively pass under the brushes. As the armature revolves, power is taken by a crank or a pulley from the shaft 'If. HEATING AND LUMINOUS EFFECTS OF ELECTRICITY. Experiment 225. — ^Put a piece of No. 30 insulated German silver wire, about seven inches long, in circuit with three bichromate cells arranged in series. Let the current pass through the wire for three or four minutes. Take the wire in the hand ; note whether there is any change in the temperature of the wire as compared with its temperature just before it was introduced into the circuit. An electric current passing through a conductor of small diameter and great resistance will generate intense heat in the conductor. The Incandescent Lamp. — Lamps constructed on the prin- ciple stated above are called incandescent lamps. In the globe of the lamp there is a conductor of small diameter. 296 MAGNETISM AND ELECTRICITY. which is heated white hot, or to incandescence, by the cur- rent. Fig. 274 shows an Edison incandescent lamp. In the glass bulb, Ji is the filament of carbon, which is fas- tened at its lower end to two small platinum wires, one of which is connected with the brass sci'cw D, and the other with the brass disk M The brass screw D and the disk E are insulated from each other by a layer of plaster of paris. When the globe is screwed into its socket, connec- tion is made with the current by turning the thumb-screw shown in the figure at the right of the socket. The filament in the Edison lamp is made of carbonized bamboo, and has a high resistance. All air must be removed from the bulb, other- wise when the filament is heated to incandes- cence it will quickly burn out because of the oxygen contained in the air. Accordingly the air is exhausted from the globe by means of a mercury pump. The standard incandes- cent lamp requires from one-half to three- fourths of an ampere of current, and an E. M. F. of one hundred volts to produce a light of sixteen candle power. Incandescent lamps are usually connected parallel, as shown in Fig. 269. The Arc Lamp. — If a conductor conveying FiQ 274 ^ current of electricity from a powerful dy- namo or battery be cut in two, and the ends separated about an eighth of an inch, the current will leap across the space between the ends of the wires. The air between the two ends offers great resistance to the passage of the cui'rent, and intense heat is generated. So great is the heat that the ends of the wire become white hot and melt away. If to the end of each wire a stick of carbon is attached and the ends of the carbon separated a very short THE AEC LAMP. 297 distance, the ends of the carbon will be heated to incandes- cence. A small quantity of carbon is volatilized, and this carbon vapor, in- tensely heated, forms a luminous arc of great brilliancy (called the voltaic arc) between the carbons. The daz- zling light is due to the white-hot ends of the carbon and to the luminous arc. The arc lamp, shown in Fig. 275, involves the principle stated above. The carbons are called electrodes. The positive electrode is the one from which the current flows, and is usually the upper carbon in arc lamps. The lower carbon is the negative electrode. The carbons burn away, and there is a mechanical device working automat- ically that keeps the car- bons the proper distance apart. j,jg g75 The positive electrode is hollow at the end (Fig. 276), and burns away twice as fast as the negative electrode. Arc lamps are connected in series (see Fig. 268). An ordinary lamp requires from forty to fifty volts E. M. F., and from five to ten amperes of current. A circuit of fifty such lamps would require from, two thousand to twenty-five hundred volts E. M. F. THE STORAGE BATTERY. Experiment 226. — Cut out of sheet lead two plates six inches by four inches, and punch smoothly in each as many Pig. 276. 298 MAGNETISM AND ELECTEICITY. quarter-inch holes as possible. Make a paste of litharge and twenty per cent, solution of sulphuric acid, and fill the holes of the plate with this paste, spreading it with a brush on both sides of the plate. Now make another paste of red lead and twenty per cent, solution of the acid, and with it fill the holes and cover both sides of the other plate. After letting the plates dry thoroughly, bind them to- gether with stout cord, keeping them about half an inch apart by means of a strip of wood placed at the top and at the bottom. Wires should be attached to the top of the plates. To store this battery, immerse the plates, bound together as described, into a twenty per cent, solution of sulphuric acid, and connect the wires with the poles of a strong battery. After letting the apparatus stand for an hour or so, disconnect the battery cells from the plates, and connect the wires leading from the plates with the astatic galva- nometer. A perceptible current is obtained. After the storage battery has been charged a few times, the current it returns becomes stronger. SUMMARY. A magnet is a body which has the property of attracting iron. The magnetic field is the space through which the attractive influence of a magnet extends. The like poles of different magnets repel each other; the unlike poles attract each other. The magnetic needle is a slender bar magnet balanced so that it will revolve in a horizontal plane. The declination of the magnetic needle is the angle which the needle makes with the geographical meridian. Induced magnetism is the magnetism which exists temporarily in soft SUMMARY. 299 iron when in contact with a magnet, or when brought within the magnetic field. The dipping needle is a slender bar magnet suspended so that it will move in a vertical plane, with a graduated arc attached to measure the angle the needle makes with the horizon. Frictional or static electricity is that produced upon bodies by friction. There are two kinds of electrification, positive and negative. Bodies charged with the same kind of electricity repel each other ; bodies charged with unlike kinds of electricity attract each other. An electroscope is an instrument used to determine whether a body is electrified or not. Positive and negative electrifications are developed equal in amount when two bodies are rubbed together. Induction is the electrification of one body when another electrified body is brought near it. A body is insulated when it is separated from other bodies by a non- conducting substance, so that it is able to retain the electricity it acquires. The electrophorus is an apparatus for obtaining a series of small charges of elfectricity from a single charge. A charge of electricity, resides on the surface of a body. The Leyden jar is an apparatus for accumulating by induction charges of electricity. A voltaic cell consists of two elements and an exciting fluid. Good conductors are those substances through which electricity passes easily. Non-conductors or insulators are substances through which elec- tricity passes with great difficulty. Polarization of a cell is any action which tends to set up a current in an opposite direction to the current of the cell. An electro-magnet consists of a bar of soft iron given magnetic proper- ties by being placed within a coil of insulated wire, through which a cur- rent of electricity is passing. Anastatic needle isa combination of two magnetic needles so arranged that the earth's magnefsm has no directive influence upon them. External resistance is the opposition a current of electricity meets in the conductor outside the battery. Internal resistance is the opposition which a current of electricity meets in the exciting fluid of the battery. An ohm is the unit of measurement of resistance. 800 MAGNETISM AND ELECTRICITY. A volt is the unit of measurement of E. M. F. An ampere is the unit of measurement of the strength of the current. Ohm's law: The strength of a current varies directly as the E. M. F.. and inversely as the resistance In arranging cells, the strongest current is obtained when the internal resistance of the battery is equal to the external resistance of the circuit. In divided circuits the amount of current that will flow through differ- ent branches is inversely as their resistances. Electrolysis is the decomposition (by the electric current) of compounds in solution. Magnetic lines of force surround a wire through which a current of electricity is passing. Lines of force directed alike repel each other ; lines of force directed oppositely attract each other. The north pole of the core of a right-hand helix is at the end where the current leaves the helix. An induced current is an instantaneous current produced in a closed circuit by the influence of a magnet or another current of electricity. An induced current is produced by a change in the number of lines of force passing through the space enclosed by a coil of wire. An increase in the number of lines of force passing through the area enclosed by a coil of wire produces a current in the coil in one direction. A decrease in the number of lines of force produces a current in an op- posite direction. The direction of the current depends upon the direction of the lines of force. An induction coil is an instrument for securing an induced current of electricity of high E. M. P., as compared with the E. M. F. of the battery current. The telephone involves the principle that the power of carbon to trans- mit a current of electricity varies directly with the pressure upon it. The dynamo is a machine for generating electricity by causing, by mechanical means, a change in the number of lines of force that pass through the area of closed coils of electric conductors. An electric motor is a machine in which an armature is revolved by the action of a current of electricity sent through the machine. A storage battery is an apparatus which yields a current of electricity due to a chemical change of its constituents produced by a current of electricity. QUESTIONS AND PROBLEMS. 301 Questions and Problems. 1. What is meant by amalgamating zinc ? Why should the zinc of a voltaic cell be amalgamated ? 3. What is an electrode ? What, a, pole ? 3. What are insulators ? 4. What is meant by polarization ? 5. Which way is the current said to flow in a voltaic cell of coppei and zinc, or carbon and zinc ? 6. What device is used in Smee's cell to prevent polarization ? 7. What are the essential parts of a voltaic cell ? 8. Examine the liquid in a gravity cell that has been working two or three days, and tell why it is named a gravity cell. 9. What is an electro-magnet ? Name its parts. 10. Upon what principle does the electric telegraph work ? 11. Of what use are relays on telegraph lines ? 12. Make a drawing of an electric bell on the blackboard, and explain how it works. 13. What effect has a wire conveying a current of electricity upon a magnetic needle when held parallel, and near the wire ? 14. What is an astatic needle ? Why has the earth's magnetism so little influence upon an astatic needle ? 15. State Ampere's rule for determining the deflection of a magnetic needle. 16. What is meant by external resistance ? State three conditions that affect external resistance. 17. What is meant by internal resistance ? 18. What is an ohm ? What is a volt ? 19. To what is the strength of a current of electricity due ? 20. What is an ampere ? 21. State Ohm's law. Write the formula expressing this law. 22. Draw diagrams on the blackboard to show how four cells may be arranged in series, how parallel, and how partly in series and partly parallel. 23. What is the current strength in a circuit with an E. M. F. of 7.5 volts, and a resistance of 9 ohms ? 24. Required the current strength in a circuit with an E. M. P. of 11 volts, and a resistance of 33 ohms. 25. What is the resistance in a circuit when an B. M. F. of 9 volts gives a current 1.8 ampSres ? 20. Required the B. M. F. when the current strength in a circuit is 8.2 amperes, and the resistance 12 ohms. 302 MAGNETISM AND ELECTRICITT. 27. Pour cells, each having an E. M. P. of 1.5 volts, an internal resistance of 1.5 ohms, an external resistance of 8 ohms, are arranged in series. Find the strength of the current. 28. Find the strength of the current when these four cells are arranged parallel. 29. What is the strength of the current when these cells are coupled in two rows, two cells being in series and the two rows being parallel ? 30. Which is the best arrangement, in series or parallel, of six cells, when the E. M. F. of each cell is 1.35 volts, the internal resistance 1.5 ohms, and the external resistance 20 ohms ? 31. What was the strength of the current in Exp. 207, when the cell gave an E. 51. P. of 3 volts, and the 50 feet of No. 30 copper wire offered a resistance of 3 ohms ? 33. The plates of a cell having an E. M. F. of 1.5 volts are connected by two wires A and B. The resistance of A is 5 ohms ; the resistance of 5 is 3 ohms. Find the strength of current in each branch. 33. Make a drawing of the apparatus you set up for electrolysis, indicate the direction of the current by arrows, mark the positive and negative electrodes + and — , and state what element is given off at each electrode. 34. In what direction do the lines of force set round a wire convey- ing a current of electricity ? 35. Under what conditions will parallel wires, conveying a current of electricity, repel each other ? Why ? 36. Under what condition will the wires attract each other ? Why ? 37. If you wish to electroplate copper with silver, to which pole would j'ou attach the copper when immersed in a solution of the proper silver salt ? 38. A piece of wire is wound around a rod of iron so as to form a left-hand heUs. Determine the north and south poles of the core when a current of electricity is sent through the helix. Make a draw- ing on the blackboard to illustrate. 39. What would be the effect upon a magnetic needle suspended by a sOt thread, if a wire carrying a current were placed parallel to the needle and alongside it ? 40. What is meant by the terms electro-motive force, and current strength ? 41. A piece of copper wire through which a current is passing, de- creases in diameter, so that the end where the current leaves is one-third the diameter of the end where the current enters. Is there any differ- THE X-RAYS 303 ence in the strength ol the current at the two ends of the wire ? Any difference in the temperature ? 42. How would you determine in what direction a current is flowing iu a telegraph wire that passes by your dwelling ? 43. Why is a galvanometer with an astatic pair more sensitive than the same coils would be if a single needle were used ? 44. Explain why wires conveying parallel currents in the same direction are attracted towards each other ? 45. Explain why wires conveying parallel currents in opposite direc- tions separate. 46. Explain the Ruhmkoril coil. 47. How may a current of electricity be induced ? THE X-RAYS. Electrical Discharges and X-Rays. In Experiments 221 to 224 we learned that the induced currents produced by an induction coil are capable, like frictional electricity, of yielding sparks. A powerful Ruhmkorflf coil (see page 279) is the most convenient means of effecting electrical discharges, as it can be made to send a continuous spark from two to fourteen inches in length. These induced or secondary currents are alternate currents (see Exp. 223), being opposite in direction to the-primary current at the making of the contact, and of the same direction with the primary current at the break- ing of the contact. The current induced on breaking the contact is, however, of much greater intensity than that on making it, and plays the principal part in electrical dis- charge. The electrodes or terminals of the secondary coil are spoken of as anode and cathode. The anode is the positive terminal, the cathode the negative terminal with reference to the current induced on breaking the contact. When an electrical discharge is effected between anode and cathode under ordinary atmospheric pressure, two dif- ferent light effects may be observed : first, the spark proper 304 MAGNETISM AND ELECTRICITY. forms a clearly defined straight or zig-zag line between the two terminals, and second, a glow of light of a reddish color surrounds this spark. A conduction of electricity through the air seems to take place in this glow or aureole, while the spark itself is due to the violent breaking through of the current. Heat effects are especially noticed at the negative terminal, and if this consists of a thin iron wire it will begin to glow or even melt away. These phenomena become more brilliant when the dis- charge between anode and cathode is effected through tubes containing rarefied gases. Gases when highly rarefied pos- sess a greater conducting power than the same gases under ordinary atmospheric pressure. It is the degree to winch the gases have been rarefied that is the important fact, for it is probable that an electrical discharge could not be made to pass through an absolute vacuum. Tubes containing rarefied gases were constructed by Geissler, a German mechanic, more than a century ago. These tubes, known as Geissler tubes, are of various shapes, and contain rarefied gases through which an electrical dis- charge can be made to pass by means of two platinum wires sealed in the glass at the two ends and projecting a little into the interior of the tube. In these tubes the spark proper usually disappears, while the aureole or glow displays color effects of great brilliancy. The light coming from the cathode is usually of a bluish color, while that which comes from the anode is reddish. William Crookes, an English physicist, made tubes of a globular or pear-like shape. In these Crookes tubes the cathode consisted of an aluminum plate into the centre of which the connecting wire was riveted. The Crookes tube has two aluminum electrode disks. One is used as the cathode, and the lother as the anode. Crookes secured a very high degree of exhaustion in some of the tubes he con- structed, and found that in reducing the pressure inside the THE X-RAYS. 305 tube the glow became fainter and fainter, and finally dis- appeared when rarefaction was carried to about one-mil- lionth of an atmosphere. He noticed, however, at this stage, that a peculiar influence proceeded from the cathode, which, as he thought, consisted in electrified particles pro- jected in straight lines from the cathode. These cathode rays, be it especially noted, may be deflected by a magnet. They also produce heat and fluorescence where they strike the wall of the glass tube or some object placed within the tube. Hertz, a German scientist, found that the cathode rays penetrated thin sheets of metal placed within the tube, but that the rays did not pass through the glass of the tube. In order to establish a passage for the rays to the outer air. Prof. Paul Lenard in 1893 made a small window in the vacuum-tube opposite to the cathode, and closed this tiny window with a thin sheet of aluminum. Through this aluminum window the cathode rays easily passed to a dis- tance of about three inches. They could be deflected from their course by a magnet, and like ordinary light they could be reflected, refracted, and polarized ; fluorescent sub- stances held in their path became luminous, and photo- graphic effects were obtained from them. Professor Roentgen, in 1895, while experimenting with an excited Crookes tube without such a window, made the discovery that the cathode beam was accompanied by a new form of radiance — the X-rays, as he called it. He had his tube covered with a thin mantle of black cardboard, and observed that in a completely darkened room a paper screen washed with barium-platinocyanide lighted up brilliantly and fluoresced equally well whether the side washed with the chemical or the other side was turned towards the discharge tube. This influence emanating from the tube was still observable at a distance of over six feet. He soon found out that these rays passed not 20 306 MAGNETISM AND ELECTRICITT. only through cardboard, but more or less through all bodies. They penetrated a bound volume of 1,000 pages, thick blocks of wood, several sheets of tin-foil, a film of aluminum about 15 mm. thick, and thin plates of other metals. Glass plates offered more or less resistance to the rays, according as the glass contained lead or not. When the hand was held between the tube and the screen the dark shadow of the bones was clearly visible within the lighter shadow of the flesh. With increasing thickness and density all bodies became less transparent. The X-rays are not visible to the eye, but they exhibit a very powerful action upon sensitized photographic plates or films. They cannot be concentrated by lenses, or other- wise refracted or deflected. They produce no heating- effect, are incapable of polarization, and cannot, like the cathode rays, be deflected by a magnet. In an ordinary Crookes tube they start from the point at which the cath- ode rays impinge upon the wall of the glass tube. In order, however, to avoid overheating of the glass by the cathode rays, the so-called focus tubes have been constructed. In these focus tubes the cathode consists of a concave metallic mirror which focuses the cathodic stream upon a small plate of platinum supported in the centre of the bulb, which may, at the same time, be used as the anode. This arrangement has the further advantage that the X-rays start from a smaller surface, thus giving sharper outlines to the shadows on the fluorescent screen. An improvement on this tube is Prof. Elihu Thomson's standard tube, where two cup-shaped cathodes and one V-shaped anode are used. By this arrangement the X-rays are made to cover a very large field. The Fluoroscope. Instead of Roentgen's original fluorescent screen, an ip- vestigator named Salvioni devised a tube over one end of THE X-EAYS. 307 whicli he put a cover of cardboard spread with a layer of fish glue and calcium sulphide. "When the X-rays fell upon this end it became fluorescent. At the other end he placed an eye-piece so that he might look through this at the shadow cast when a dense object was placed be- tween the Crookes tube and the fluorescent end of the observer's tube. Mr. Edison later devised an instrument for this purpose, which he termed a fluoroscope. It consists of a dark chamber in the shape of a stereopticon, the front wall of which is made of cardboard coated with fine crystals of tungstate of calcium ; this substance was found to fluo- resce more brilliantly under the influence of the X-rays than the platinocyanide flrst used by Roentgen. By placing the hand or the part of the body to be examined close to the screen with the X-rays beyond, one can look through the smaller end of the fluoroscope and see the bones or any foreign body that may be lodged in the flesh. The fluoroscope is designed to obviate the necessity in some cases of making photographs of the part to be examined. Several theories have been advanced concerning the nature of the X-rays. Roentgen himself was inclined to consider them as longitudinal vibrations of the ether ; other investigators tried to explain them as a stream of infinitesimal electrified particles projected from the tube with enormous velocity ; others as transversal ether waves of an exceedingly brief period, similar to the rays of the ultra-violet light. None of these theories is as yet proven, but the majority of investigators seem to be in favor of the ultra-violet theory. Thus far we have only spoken of the Ruhmkorff induc- tion coil as a means of producing the X-rays. We saw that these rays are pi-oduced when electrical discharges of a high potential take place in a Crookes tube, the air of which is exhausted to about one one-hundred-thousandth of 308 MAGN"ETISM AND ELECTRICITY. THE X-EATS. 809 an atmosphere. Such discharges may also be obtained from a powerful static machine or from a Tesla coil. In using a static machine it is sufficient to connect the negative pole of the machine with the cathode of the Crookes tube, and the positive pole with the anode. As a condenser two small Leyden jars may be used, one between the positive pole and anode, and the other between the negative pole and cathode. To regulate the strength of the current, the discharging rods of the prime conductors have to be so adjusted that a dangerous overcharge may pass between them, and thus prevent the Crookes tube from being cracked. The Tesla coil is, in its construction, similar to an ordi- nary induction coil. The primary coil contains only a few well-insulated turns of heavy wire. The number of sec- ondary turns is about twenty-four times the number of those of the primary coil. Oil is generally used as insulat- ing medium. Through the primary coil are passed the rapid discharges from two Leyden jars, which, in their turn, may be charged by the secondary current of an ordinary induction coil, or by a static machine. The results are exceedingly rapid, alternating discharges in the secondary coil. The Crookes tube, in this case, must be fitted with two electrodes that can both act as cathodes. Photography by X-Rays. The X-rays have opened a new field to photography. The shadows cast on the fluorescent screen by different objects may be photographed in the usual way. Or the direct rays may be used for the same purpose. In this case a camera is not needed, because the lens has no refractory power upon the rays. It is sufficient to place the object to be photographed on a dry plate wrapped in paper, or placed in an ordinary plate-holder in order to protect it from other 310 MAGNETISM AND ELECTBICITY. Fis. S78. THE X-EATS. 311 sources of light. The time of exposure varies, according to the opacity of the object and to the efficiency of the appa- ratus, from a few seconds to half an hour and more. Fig. 277 shows a common arrangement of apparatus for photographing. The muscles and other soft tissues of the body obstruct the passage of the rays very little and so give light shadows on the plate, while the harder tissue, the bones, obstruct the passage much more and so give darker shadows. See photograph of frog. Fig. 278. Bullets, pins, and other dense foreign substances may by this means be located, and after such location, be easily removed by the surgeon. Fractures of the bones and other malformations are revealed by a photograph made by the X-rays. And broken bones near the joints may be distinguished from disloca- tions, in cases where swelling would prevent examination by the surgeon. Whatever revelations the X-rays may make in the future for pure science, its practical applications in the domain of medicine give guiding and definite information where formerly there was only conjecture. The Roentgen rays, moreover, are likely to cause a revo- lution in electric lighting. Edison has constructed a lamp in which the fluorescence produced by the Roentgen rays is used as a source of light. This lamp consists of a Crookes tube whose inner walls are coated with a highly fluorescing matter. When excited in the ordinary way by a powerful induction coil it emits a mild, but effective, light, which may be used for illuminating purposes like any other source of light. APPENDIX. § 1. To make a Pair of Scale§.— The parts and dimensions of a pair of scales are shown in the aocompanying sketch. The base and upright pieces are made of pine. The beam in the scales repre- sented in Fig. 379 was made of a maple ruler planed off at the edges. .4 is a piece of sheet iron or brass. A piece of knitting needle driven XlZi lilllCHES. FiH. 279. through the beam rests in the V of the iron pieces. The scale-pans are constructed so as to be removed from the wire hooks. In making the scales, see that all the bearings are very smooth. § 2. To cut. Glass Tubing. — Lay the tube upon the table, and give a strong, steady cut with a three-cornered file. If the tube is large and thick, continue the cut around the tube. Then, holding the tube in the hands, with the cut between the thumbs, press outward with the thumbs, and the tube will break ofC as desired. 314 APPENDIX. To bend Glass Tnbing. — Warm gradually that part of the tube that is to be bent, by passing it through the flame of an alcohol lamp or Bunsen's burner. Then hold the tube constantly in the flame just above the blue cone of the flame. As soon as the flame burns yellow, and two or three inches of the tube are found to be soft, gently bend the tube to the required shape. If an ordinary gas flame is used and the tube held lengthwise of the flame, a less abrupt bend can be made. To smootb the Ends of a Olass Tube. — Warm the end of the tube by passing it through the flame. Then hold the tube obliquely in the flame and slowly turn the tube. When the flame begins to be colored yellow by the sodium of the glass, the edges are rounded. To close tbe End of a Olass Tube. — Heat the tube grad- ually and then hold in the flame, turning the tube till the end is closed. Or, hold the end in the flame till it becomes soft, and then pull the end out by touching it wth another piece of glass. The glass will be drawn out to a, long thread. Break off this thread, and hold the small end in the flame. The small opening will soon close. To bore a Hole in Olass. — Make a spot on the glass with ink where the hole is to be drilled. Moisten the glass with a drop of tur- pentine and add a few particles of camphor. Break off the end of an old round or three-cornered file, so that it will have a cutting edge. Take the file in the hand, and bore upon the spot marked. Do not press too hard. Break ofE a small particle from the end of the file from time to time. Water may be used instead of turpentine and camphor. Large holes may be drilled by using a piece of brass tubing and emery powder moistened with water. To cut off the Bottom of a Bottle or to cut a Lamp Chimney in two. — Pile a little groove in the glass, and touch this groove with a hot iron or a piece of lighted "punk" till a crack is started. Then, by holding the hot iron just ahead of the crack, it may be led in any direction. Rub the edges with a file to smooth them, or, what is better, use a whetstone upon them. APPENDIX. 315 § 3. L.lst of Apparatus. — The following articles may be or- dered of Messrs. Eimer & Amend, 209 and 311 Third Avenue, New York, or other dealers in chemical and physical apparatus and chemi- cals, at the prices here given : 1 ounce pure rubber tissue $ .25 3 rubber stoppers, two holes each, No. 4, No. 7, and No. 10 . .75 5 fe^t rubber tubing (J inch inside diameter), catalogue No. 8,012 50 1 eight-ounce flask .15 1 four-ounce flask .13 Ignition tube .15 2 test-tubes .10 i pound (16 feet) glass tubing (J inch outside diameter) . .25 1 pound mercury .75 1 Fahrenheit thermometer (370°, paper scale inside of tube) . .90 (A Fahrenheit thermometer, 300°, engraved on stem, cost- ing $1.70, is rather more convenient.) Wire gauze (fine) .25 .25 1.50 .50 .50 Crown glass prism (four-inch) . 3 bar magnets (eight-inch) Platinum foil (1 square inch) . Spring balance The following may be ordered of J. H. Bunnell & Co., 76 Cortlandt Street, New York, or any dealer in elec- trical supplies. The prices given are at New York rates : 2 gravity batteries (not crow-foot pattern) 2 pounds of copper sulphate (blue vitriol) 1 pound bichromate of potash 3 zinc rods (Le Clanche cell pattern) .... 3 electric light carbons (uncoppered, 12 inches by f inch) 1^ pounds*No. 16 copper wire (double cotton covered) . i ounce No. 30 German silver wire (single cotton covered) i pound No. 30 copper wire (double cotton covered) 3 ounces No. 30 copper wire (single silk covered, about 40 feet) 1 ounce No. 27 copper wire (single silk covered) Amovmt forward 1.50 .30 .25 .30 .35 .90 .15 .55 .35 .15 $11.32 318 APPENDIX. Amount forward . . $11.33 The hollow cylindrical cross (Exp. 59), 6 inches long, IJ inches in diameter, arms projecting out li inches, can be made by any tinsmith, and ought not to cost more than .30 Tuning-fork .25 Sulphuric acid .13 Miscellaneous . . 8.00 $15.00 No account is made of strips of wood, bottles, pieces of lead, strips of old sheet copper, and similar articles that are always at hand. § 4. How to make a Siren. — Cut out of smooth, stiff card- board a disk 10 inches in diameter. Prom the centre draw concentric Fiu. aso. circles, having for their radii 3f , 3}, 4i, and 4^ inches respectively. Next draw two diameters A B and C D (Pig. 380), at right angles to APPENDIX 317 each other. From ^ as a centre, with a radius of 5 inches, draw the arcs E and F. From /) as a centre, with the same radius, draw the arc //, and from C draw the arc G. From the points where each of these arcs cuts the circuml'erence draw diameters. Six diameters have now been drawn. Midway between these draw six other diameters as shown in Fig. 280. Now punch smooth holes through the cardboard, as shown by the figure, 48 holes iu the inner circle, 60 holes in the next, 72 holes in the nest, and 90 holes in the outer circle. Cut out of wood a wheel 1^ inches in diameter, and groove the wheel so that it may be used as a pulley. Tack the cardboard disk upon this small pulley wheel, centre for centre. Make a large pulley wheel 13 inches in diameter, as shown in Fig. 127, page 154. In place of the handle .1, use a spool fastened to the pulley wheel by a screw. Complete the apparatus, as shown in Fig. 127, by driving a wire nail through the centres of each of the pulley wheels into the upright strip of wood. Let the centres be 21 inches apart. Put on a tight cord for a pulley band, and revolve the siren disk. -The sonometer (Fig. 128) HoAV to make a Sonometer. consists of a wooden box 4 feet long, 6 inches wide, and 2 inches deep. It should be made of wood not thicker than half, an inch. A hole should be bored in the top about 2 inches in diameter. Very good results can be ob- tained without this sonometer, by following the suggestion at the foot of page 157. § 5. How to make a Porie L,uniiere. — Cut out a board A A (Fig. 281) ten inches square and about three- quarters of an inch thick. Draw a line through the middle from end to end. On this line, Pig. 281. three and five-eighths inches from the top, bore a hole H one and a half inches in diameter. 318 APPENDIX. Next cut out the following pieces of wood : 5, 3i x 1^ x f inches ; (7, 4 X 2 X i inches ; D, 5J x f x 1 inch ; ^, 6^ x J x f inches ; F, to which the mirror is to be fastened, 6 x 3 x | inches. Nail B flat upon A A, making the end flush with the lower end of the board. To this cleat nail C, as shown in the figure. In the end of D, cut a slit three-eighths of an inch wide, and one and a half inches deep. At the upper end of this insert E, fastening it at its central point with a wire nail P for an axis. Secure D to (7, as shown in the figure. Cut off the heads of two wire nails G G, and one inch from the point bend them at a right angle. Make a hole in the centre of each end of the piece of wood F, and insert the shorter arms of the naUs. Then drive the longer arras of the nails into the upper edge of E, so that the mirror board F will turn easily. On each side of D drive a small staple, as shown in the figure. Make a small hole Ftwo inches from the central line. Fasten a string K to E one inch from the end, pass it through the staple, through the hole V, along behind the board, as shown by the dotted line, through a cor- responding hole and staple on the other side, and up {K') to the other end of E. This string should be drawn tight. By moving it back and forth behind the board, the mirror may be made to turn on its axis P. At the top of ^ ^, one and a half inches each side of the central line bore two more holes. Secure a string N N' to the centre of one side of F, put it through one hole, pass it behind the board, then put it through the other hole, and fasten it to the other side of F. By moving this string back and forth, the mirror may be made to turn on its axis G G. Make a dark shutter to fit any window into which the sun shines, and put the parte lumUre in a square hole cut to receive it in the dark shutter. If there are other windows in the room, darken these so that no sunlight can come in. By moving from the inside the strings of the parte lumibre, a hori- zontal beam of sunlight may be directed through the hole H. A piece of tin with holes of different sizes drilled through it may be placed over H when it is necessary to make smaller the aperture through which the sunlight comes. § 6. How to make a Bichromate Cell. — Cut each of the three electric light carbons (see § 3) into two parts. Pile a little groove around the top of each, twist the end of a piece of copper wire about six inches long around the carbon in the groove. Make a mould APPENDIX. 319 of wood or plaster of pans, and run lead around the carbon where the wire is wound (see L, Fig. 383). Cut out a strip of wood long enough to extend across the top erf a, tumbler or other glass vessel that may be used, bore three holes through this piece of wood, and drop a carbon into each outer hole. Between the carbons place a rod of zinc (see § 3), as shown in Fig. 382. There should not be more than half an inch between the zinc and the carbon. The exciting fluid is made by pouring fiye fluid ounces of sulphuric acid into three pints of cold water, and, when this has cooled, adding six ounces (or as much as the solution wOl dissolve) of pulverized bichromate of potash (see § 3). Be careful to stir well. Pour this solution into the glass vessels that are to be used for battery jars, and put in the carbons and zinc. When this cell is not in use, lift the elements out of tJie exciting fluid. Three cells can be made of the three carbons and three zincs mentioned in §3. § 7. Hoiv to make a Small Magnetic Hfeedle. — Heat a piece of watch-spring to redness, and allow it to cool. Then cut off a piece half an inch long, and straighten it. In the centre make an indentation with a Pi sharp punch. Thrust a pin through a piece of cigar box half an inch square for a standard, tation made in the piece of watch-spring upon the point of the pin, and see whether the piece of watch-spring will balance. If it does not, then file off one end till the piece balances exactly. Now magnetize the piece. Should there be any dip, file off the needle till it balances exactly. Home-made Connectors. — Connectors that will answer every purpose may be made by taking a piece of sheet copper one inch long, and three-eighths of an inch wide, and drilling a hole in each end to carry a small brass screw. Put the piece of copper upon a small strip of wood, place a copper washer under the head of each screw, and wind the wires around each screw respectively. Then Put the inden- 320 APPENDIX drive the screws into the wood through the holes in the strip of copper. See that the wire is held firmly between the washer and the strip ol copper, and a. good connection will be formed. Current Reverser. — Draw a square one inch on a side upon a piece of wood. At each corner of the square bore a quarter-inch hole to the depth of half an inch. Fill each hole with mercury. Kext make of insulated copper wire two staples of such a length that they may be used to connect any two holes of the square either diagonally or along the side. Remove the insulation from the ends of these staples. Bend the ends of the wires from the battery down at right angles a quarter of an inch, and dip these into two holes of the square numbered 1 and 2, Fig. 283. In like manner bend the ends of the wires from the circuit, and dip these ends into holes 3 and 4. Now by connecting with one staple, holes 1 and , and with the other staple, holes 2 and 4, the current is not reversed in the circuit. But by Fig. »83. connecting with one staple, holes 1 and 4, and with the other staple, holes 2 and 8, the current is reversed in the circuit. § 8. How to make a Oalvaiioineter. — Cut out of a block of wood a frame A A, Fig. 284, four inches long, two and three-quarters inches wide, and three-eighths of an inch thick, with the hole three inches by two inches. Make the outside edges of the frame round, as shown in the figure. Around this frame, about one and five-eighths inches from the end, wind as evenly as possible forty-five turns of No. 27 insulated copper wire, in such a manner that there will be in the coil B B fifteen strands of wire in width and three layers in thickness. Bind this coil together with a piece of string K, and carrying the wire on in the same direction, make another similar coil D D, one-eighth of an inch from the first. Secure the ends of both coils by tying them, or by means of double-pointed tacks of brass wire driven into the wooden frame. Give the wire a thick coat of shellac to hold the strands firmly together. Next, pass a sixteenth of an inch brass wire through a cork C, and bending the ends ff H down at right angles, as in Pig. 284, drive them into the upper side of the frame between the two coils. Pass a smaller brass wire E, one and a half inches long, vertically through the cork C, and make a loop at each end. APPENDIX 321 Place two needles one and five-eighths inches long, side by side, with their points together, and holding them in this position magnetize them equally and as strongly as possible with a bar magnet. Remove the insulation from a, piece of No. 30 copper wire >F(Fig. 284), and fasten the needles so as to form an astatic pair five-sixteenths of an inch apart in the position indicated, by twisting the wire a few turns about each needle. In one end of this wire make a small loop, and suspend the astatic pair from the wire E bj a piece of untwisted silk, T. By turning the wire H, and moving it up or down, the needles FicSSl. may be brought into the proper position. One needle should be above the coils, and the other below the upper turns of the coil. The needles should be parallel to each other and to the edges of the coils, and they should turn without obstruction. In order to make the wire W (Fig. 284) pass through the centre of the narrow slit between the coils, the apparatus may be balanced by pieces of wood or paper placed under the corners of the frame. In order that the needles may not be moved by currents of air, the galvanometer should be placed in a box with a glass cover ; the points of attachment P P should pass through the sides of the box. ' INDEX. Absorption and diffusion of liglit, 180, 181. Absorption of heat, 136. Adhesion, 46. Air exerts pressure, 96-99. " has weight, 107. " transmits pressure, 82, 83. Air-pnmp, 105-107. Air thermometer, 120. Alphabet, telegraphic, 246. Amalgamizing zinc, 237, 238. Amorphous substances, 46. Ampere, the, 258. Ampere's rule, 250. Amplitude of a vibration. 147. Affects intensity of sound, 154, 155. Angle of incidence, 180. " " reflection, 180. Apparatus, list of, 815, 316. Appendix, 313-^21. Arc lamp, 296, 297. Armature of a magnet, 222, 223. Armature of the dynamo and its parts, 288. Arrangement of cells, 259-262. Artificial cold, 133, 134. Astatic needle, 250, 251. Astronomical telescope, 204-206. Atmosphere, weight of one, 99. Atom, 32-34. Definition of the, 34. Attraction, capillary, 48. Attraction and repulsion of charged bodies, 226-228. Attraction and repulsion of parallel cur- rents, 268, 269. Attraction and repulsion of lines of force, 269. Attraction between molecules, S6, 46. Attraction between poles o'' magnets, 213i 217. Attraction of gravitation (see Gravitation), 50, 78. Barker's mill, 92, 93. Barometer, 99, 100. Battery (see Cells), 239. The storage. 297, 298. Beam of light, 173. Bell, electric, 247, 248. Bichromate cell, 242. How to make a, 318, 319. Block and fall, 21, 22. Boiling point of water, 118, 126, 127. Effect of pressure upon the, 127, 128. Boyle's law, 107-109. Brahma's press, 85, 86. Brittleness, 42. Bunsen's cell, 243. Button, the electric, or push, 248. Camera, 206, 207. Capillary attraction, 48. Cartesian diver and imp, 91, 92. Cell, bichromate, 242. " Bunsen's, 243. " gravity, 242. " La Clanche, 242. " Smee's, 241. Cells, arrangement of, 259, 268. " two classes of, 241. " voltaic, 239, 241-243. Centrifugal tendency, 52, 53. Influences, weight, 53. Charged, or electrified, bodies, 226. Effect of points upon, 2.35, 886. Chemical changes, 34, 35. Chemical effects of the electric current. 263-266. INDEX. 323 Chladni'B figures, 161, 162. Circuit, electric, 244. Compound, 262, 263. Cohesion, 46. Cold, artificial, 133, 184. Color, 194-200. Color of bodies, cause of the, 196, 197. Colored pigments, 190, 200. Colors, complementary, 198. " mixing, 197, 199. " of the solar spectrum, 195. " primary and secondary, 199. Common pump, 100, 102. Commutators and commutation, 288, 289. Compass, 218, 219. On iron ships, 223. Complementary colors, 198. Complete vibration, 147. Compound circuits, 262, 263. " microscope, 209. Compressibility, 40. Of water, 81. Concave lenses, 192-194. " mirrors, 183, 184. Condensation as part of a wave, 151. Condensers, electrical, 234, 235, 280. Conduction of heat, 122-124. Conductors of electricity, 229, 240. " " heat, 122, 123. Connector, 253. Home made, 319, .320. Convection, 120, 121. Convex lenses, 191-194. " mirrors, 183, 184. Core of a spiral, or helix, 269. " " an armature, 288. Crystalline bodies, 46. " lens, 208. Current of electricity (see Electric current) 239. Current reverser, 320. Cutting glass, directions for, 313, 314. Declination, 219. Diffusion and absorption of light, 180, 181. Dipping needle, 223-226. Distillation, 129, 130. Distribution of frictional electricity, Si2, Diver, Cartesian, 91, 92. Divisibility, 39. Double convex lenses, 191, 193. Ductility, 42. Dynamic electricity, 237-303, Summary of chapter on, 298-300. Dynamo, principle of the, 283-289. Dynamos, kinds of, 289-293. Ebullition, 125-129. Definition of, 126. Echoes, 163, 164. Elasticity, 41. Electric bell, 247, 248. " circuit, 244. Compound, 262, 263. Electric current, 239. Chemical effects of the, 263-265. Difference between the strength oi the, and electro-motive force, 258, 259. Direction of induced, 277. Effect of, upon the magnetic condition of a wire, 265.-269. Effect of, upon the magnetic needle, 248- 251, 267, 268. Induced, 274-280. Resistance to the, 253-256. Strength of the, 256-268. Electric lights, 296-297. Two ways of connecting with the dy- namo, 290, 291. Llectric motors, 294, 295. Electrical machines, 233, 234. Magneto-electrical machines, 289-293. Electricity, 226-303. Conductors and nnn-conductors of, 229 240. ■ Dynamic (same as voltaic). Frictional (which see), 226-237. Heating and limiinous effects of, 2% 297. Questions and problems on, 301-30.3. Summary of chapter on. 298-300. Voltaic (which see), 2.37-303. Electrification, two kinds of, 226-228. Electrified, or charged, bodies, 226. Electro-inagnet, 243. 244. Electro-magnetic field of force, 266. Electro-motive force, 256, 256. Electroly.iis, 263, 264. 324 INDEX. Electrolyte, 264. Electrophonis, explanation and nse of the, 231, 232. Electrophonis, to make an, 230, 231. Electroplating, 264, 265. Electroscope, pith-ball, 228. Elements of the voltaic cell, 239. Energy, 7^75. Conservation of, 74, 75. Mechanical, converted into heat, 141. Equator of earth, magnetic, 226. *' of a magnet, 214. Equllibrinm, indifferent or neutral, 56. " questions and problems on, 57. Equilibrium, stable, 55, 56. " unstable, 56. Evaporation, 124, 125. Conditions aifecting, 1S9. Exciting fluid of the voltaic cell, 239. Extension, or magnitude, 88. External resistance to the electric current, 253-265. Eye, the, 207-209. Falling bodies, 57-61. In a vacuum, 58. Laws of, 60, 61. Questions and problems on, 61, 77, 78. Field, electro-magnetic, 266. " magnetic, 214. " magnets, 289. " of dynamo, 291. Flat, as a term in music, 160. Focal length of lenses, 193. Focus, 193. Foot-pound, 69. Force, definition of, 58. " electro-motive, 255, 256. *' lines of magnetic (which see), 214, 215. Force, magnetic, 214. Force-pump, 102, 103. Forces, composition of, 63-66. " example of attractive and repel- lent, 86. Forces, molecular, 35-37, 46-48. Freezing point, 134, Friction, 19, 20. " producing heat, 141. Prictional electricity, 226-237. Condensers of, 234, 235. Distribution of, 232, 233. Effect of points in, 236, 236. Induction of, 229, 230. Positive and negative, 226-228. Potential of, 233. Questions on, 236, 237. Simultaneous development of positive and negative, 228, 229. Summary of chapter on, 298, 299. Fulcrum, definition of, 12. Fundamental tone, 163. Galilean telescope, 206. Galvanometer, 251. How to construct a, 252, 253, 320, 321. Gases, 44, 45. Volume of, 107-109. Glass, to work in, 313, 314. Gravitation, 60-78. Description of, 50. Law of, 51. Summary of chapter on, 75, 76. Terrestrial, Bl. Gravity, 51. Centre of, 54, 55. Laws of weight, or, 52. Problems on, 58, 54, 57, 77, 78. Specific, 88-91. Gravity cell, 242. Hardness, 42. Harmonics, or overtones, 162-165. Heat, 112-145. Absorption of, 136. Definition of, 112. General effect of, 114-116. Latent and sensible, 1.S0, 131. Questions and problems on, 142-14F Eadiation of, 134-186. Eeflection of, 186, 137. Some sources of, 140, 141. Specific, 131-133. Summary of chapter on, 141, 142. Heating and luminous effects of eleo tricity, 295-297. Heating by conduction, 122. " " convection, 120, 121. Horseshoe magnet, 222. Hydraulic jack, 86. INDEX 325 Hydraulic press, 84-86. Principle of the, 83, 84. Hydrostatics, 79-95. Questions and problems on, 94, 95. Summary of chapter on, 94. Image, real and virtual, 181. " to draw the, of an object, 183-185. Images formed by concave and convex mirrors, 184, 185. Images formed by plane mirrors, 182, 183. *' '* through small apertures, 177, 178. Impenetrability, 38. Incandescent lamp, 295, 296. Incident ray, 179. Inclination of the dipping needle, 225. Inclined mirrors, 183. Inclined plane, 23, 24. Indestructibility, 39. Indifferent equilibrium, 56. Induced currents, 274-280. Direction of, 375-278. Induction coil, Euhmkorff's, 279, 280. " of frictional electricity, 229. 230. Insulated bodies, 229. " wire, 243. Intensity of sound, 147, 154-156. Interference of sound waves, 166, 167. Internal resistance of the electric current, 255. Intervals of the musical scale, 159. Jack, hydraulic, 86. Keeper, or armature, of magnets, 222, 233. Key, the telegraphic, 344, 345. Kinetic energy, 73-74. La Clanche cell, 342. Lamp, arc, 296, 397. " incandescent, 395, 296. Latent heat, 130, 131. Lens, crystalline, 308. Lenses, 191-194. Convex, action of, 193. Concave, " " 194. To find the focal length of, 193. Lever, 9-18. Advantages derived from the, 16, 17. Definition of the, 12. Lever, 9-18. Of fiist class, 9-12. Of second class, 13-15. Of third class, 15, 16. Lever, principle of the, 12. '* questions and problems on the, 13 17, 18, 27-39. Lever, terms connected with the, 10, 13. Light, 173-313. Decomposition of, 194, 195. Diffusion andabsorption of, 180, 181. Blectric (which see). Forms images through apertures, 177, 178. Intensity of, 175-177. Is invisible, 173. Questions on, 311, 213. Rays of, how represented, 173. Heflection of (see Mirrors), 178-180, 183- 185. Eefraction of (see Prism and lenses), 185-194. Summary of chapter on, 309, 210. Travels in straight lines, 173. * Velocity of, 202, 203. Lightning and lightning rods, 236. Line of direction, 56, 57. Lines of magnetic force, 314, 215. A change in the number of, produces a current, 274, 275. Around a conductor, 265, 266. *' " solenoid, 273. Attraction and repulsion of, 369. Direction of, around a wire, 267, 268 Liquids, 44. List of apparatus, 315, 316. Long sight, 308, 209. Longitudinal vibration, 147. Loudness of sounds, 147, 154-156. Luminous bodies, 173. Luminous effects of electricity, 295-:i97. Magnet, definition of a, 213. " effect of cutting in two, 220, 221. " effect of heat upon a, 223. *' effect of one, upon the force of another, 219, 220. Magnet, electro, 243, 344. " equator of a, 214. field of the, 214. " horseshoe, 233. 326. INDEX. Magnet, lines of force of the, 314, 215. " natural, 313. " permanent, 333. poles of the, 313, 316, 317. Magnetic condition of an electric wire, a65-369. Magnetic ec^uator of the earth, 335. field, 314. Electro-, 266. Magnetic force {see Lines of magnetic force), 214. Magnetic needle, 318, 319, 233-325. Effect of electric current upon the, 248- 851, 267, 268. How to make a small, 319. Magnetic poles of the earth, 219. Magneti!?m (see Magnet), 213-326. Questions on, 335, 236. Summary of chapter on, 398-300. Magnetism, induced, 331, 323. Magneto-electric machine (see Dynamo). Magnifying glass, 193, 303, 304. Magnitude, or extension, 38. JUalleability, 43. Mariotte'8 law, 107-109. Matter, 30-49. Definition of, 37. May be changed from one state to an- other, 45. Minuteness of the particles of, 33. Properties of, 37-43. Questions on, 49. Subdivisions of, 33-34. Summary of chapter on, 48, 49. Three states of, 43-45. Mechanical powers, 9-39. Questions and problems on, 12, 17, 18, 27-29. Summary of chapter on, 27. Microscope, 203, 204. Mirrors, concave and convex, 184, 185. " inclined, 183. " plane, 183, 183. Mixing colors, 197-199. Molecule, 30-33. Definition of the, 33. Not the smallest division of matter, 34. Molecular forces, 35-37, 46-48. Momentum, 59. JVlotlon, 58-63, 71. Definition of, 68. Motion, 58-62, 71. Newton's laws of, 61-64, 71. Problems on, 66, 77, 78. Uniformly accelerated or retarded, 58. Motors, electric, 394, 295. Musical scale, 158-160. sounds, 166-158. Natural magnets, 313. Near-sightedness, 308, 209. Needle, astatic, 260,251. " dipping, 223-325. magnetic, 218, 219, 223, 335. Negative electricity, 226-228. " electrode, 240. " pole of a magnet (see pole), 218 Neutral equilibrium, 56. " line of a magnet, 214. Newton's laws of motion, 61-64, 71. Nodes of a vibrating body. 160-168. Noise, 166. Non-conductors of electricity, 239, 340. Octave, 159. Ohm, the, 355. Ohm's law, 368. Applications of, 359-262. Opaque bodies, 174. Opera glass, 206. Overtones, or harmonics, 162-165. Pencil of light, 173. Pendulum, 66-70. Law of the, 70. Of a clock, 67, 68. Questions and problems on the, 70, 77 78. Penumbra, 175. Percussion, production of heat by, 140. Permanent magnet, 323. Physical changes, 34, 35. Pigments, colored, 199, 200. Pitch of sound, 166-168. Pit^i-ball electroscope, 328. Plane mirrors, 163, 183. Plano-convex lens, 191. Plating by electricity. 2B4, 365. Pneumatics, 96-111. Questions and problems on, 110, 111. Summary of chapter on, 109, 110. Points, effect of, in frictional electricity 335,236. INDEX 327 Polarization, 241. Poles, magnetic, ot tlie eartli, 319. " of a magnet, 213, 216, 217. " of spirals and lielices, 269, 270. " of tlie solenoid, 271-274. " of the voltaic cell, 239. " positive and negative, 218. Porosity, 35, 40, Porte lumi^re, how to make a, 317, 318. Positive and negative electricity, 226- 228. Positive electrode, 240. Potential, 233. Potential energy, 73, 74. Power arm of the lever, 12. Power, definition of, with reference to the lever, 12. Powers, mechanical (which see), 9-29. Press, Brahma's, 85, 86. " hydranlic, 84-86. *' principle of the hydraulic, 83, 81. Pressure, effect of, on the boiling point, 127, 128. Pressure exerted by air, 96-99, 103. Amount of the, 99. Pressure exerted by water, 79, 80. " of a column of water, 86, 87. To calculate the, 88. Pressure transmitted by air, 82, 83. " " water, 81-84. Primary coil, 278. " colors, 199. Prisms, 190, 191, 195, 196. Proof-plane, 232. Properties of matter, 37-43. General definition of, 38. Questions on the, 43. Specific, definition of, 38. Pulley, 18-82. Advantages gained by use of the, 21. Combination of fixed and movable, 21. Fixed, 18, 19. Movable, 20. Principle of the, 23. Questions and problems on the, 27-29. Pomp, air, 105-107. " common, 100-102. " force, 102, 103. Push-button, 248. Quality of sound, 158, 165. Radiation of heat, 134-136. Effect of rough and smooth surfaces upon, 135, 136. Rainbow, 200-303. Rarefaction, as part of a wave, 151. Ray of light, 173. Incident and reflected, 179. Real image, 181. Reflection ot heat, 136, 137. " of light (see Mirror), 178-180i 183-186. Total, 188-190. Reflection of sound, 153, 153. Refracted rays, to draw the direction of incident and, 187, 188. Refraction of light (see Prisms and lenses), 185-194. Reinforcement of sound, or resonance, 165, 166. Relay, 346, 347. Repulsion and attraction (see Attraction and repulsion). Repulsion between molecules, 36. " " poles of magnets, 31it 317. Resistance, external, 253-355. " internal, 355. Resonance, 165, 166. Resultant of forces, 65, 66. Ruhmkorff's coil, 279, 280. Safety lamp, 123, 124. Scale, the musical, 158-160. Scales, to make a pair of, 313. Screw, definition of the, 25. " right hand^and left hand, 36. " questions on the, 26-39. Secondary coil, 278. " colors, 199. Segments, ventral, 160, 163. Sensible heat, 130, 131. Series wound dynamo, 390. Shadows, 173-175. Sharp, as a term in music, 160. Shunt, 363. Shunt wound dynamo, 290, 291. Sight, long and short, 308, 209. Simple microscope, 203, 204. Simultaneous development of positive and negative electricity, 228, 339. Siphon, 104, 105. 328 IKDEX, Siren, how to make a. 316, 317. Smee's cell, 241. Solenoid, 271-274. SolidB, 43. Sonometer, how to make a, 31T. Sonorous bodies, 147. Sound, 146^in. Interference of, 166, 167. Loudness or intensity of, 148,149,154-156. Pitch of, 156-158. Production of, 146, 147. Propagation of, 148. Eeflection of, 151-154. Reinforcement of, 165, 166. Quality of, 168, 165. Questions and problems on, 169-171. Summary of chapter on, 167-169. Velocity of, 149, 150. Sound-waves, 151, 152. Sounds, musical, 156-158. And noise, 156. Sounder, the telegraphic, 245. Sounding boards, 166. Specific gravity, 88-91. Definition and standards of, 90. Of some substances, 91. Problems on, 94, 95. Specific heat, 131, 133. Of some substances, 133. Spectrum, the solar, 194, 195. Spirals and helices, 269, 270. Stable equilibrium, 55, 56. States of matter, three, 43-45. How changed, 145. Steam engine, 137-140. Storage battery, 297, 298. Strength of the electric current, 256, 257. And electro-motive force, 258, 259. Thermometer, 116, 117. Air, 120. Different kinds of, 117-119. Limits of the, 119, 120. Selection of a, 119. Telegraph, 244-247. Principle of the, 244. Telephone, 280-282. Telescope, astronomical, 204-206. " Galilean, 206. Temperature, 113, 114. Effect of an evaporation, 130. Tenacity, 41. Torricelli's experiment, 97, 98^ Total reflection, 188-190. Translucent bodies, 173. Transparent bodies, 172. Transverse vibration, 147. Tubing, to work with glass, 205, 206. Umbra, 175. Uniformly accelerated motion, 58. " retarded motion, 68. Unstable equilibrium, 56. Vacuum, 100. Falling bodies in a, 58. Variation of the magnetic needle, 218, 213 Line of no, 219. Velocity, 60. Of light, 202, 203. Of sound, 149, 150. Vibrating bodies, 160-166. Vibrations producing sound, 146, 147. Virtual image, 181. Visual angle, 177. Volt, 256. Voltaic arc, 297. Voltaic cells (see Cell), 239, 211-243. Voltaic electricity (see Electric current), 237, Production of, 237-239. Water, boiling point of, 118, 126, 127. " exerts pressure, 79, 80. " freezing point of, 134. " level of, 81. " point of maximum density of, 134. '' pressure of a column of, 86, 87. " slightly compressible, 80,_81. '* to calculate pressure exerted by, 88 *' transmits pressure, 81-84. Wave length, 150, 152. Waves in general, 150, 151. Of sound, 161, 152. Wedge, 24, 26. Weight, 62. Definition of, with reference to the lever, 12. Laws of, 52. Questions and problems on,63, 54, 77, 78. Weight arm of the lever, 12. Wheel and axle, 22, 23. Work, definition and measurement of, SO. ** questions and problems on, 60, 78. ?/^