Fig. 95.Explain why the breakers are white and the sea green or blue.

Fig. 95.Fig. 95.Explain why the breakers are white and the sea green or blue.

How we can tell what the sun and stars are made of.When a gas or vapor becomes hot enough to give off light (when it is incandescent), it does not give off white light but light of different colors. An experiment will let you see this for yourself.

Experiment 53.Sprinkle a little copper sulfate (bluestone) in the flame of a Bunsen burner. What color does it make the flame?

Copper vapor always gives this greenish-blue light when it is heated. The photographer's mercury-vapor light gave a greenish-violet glow. When you burn salt or soda in a gas flame, you remember that you get a clear yellow light. By breaking up these lights, somewhat as you broke up the sunlight with the prism, chemists and astronomers can tell what kind of gas is glowing. The instrument they use to break up the light into its different colors is called aspectroscope, and the band of colors formed is called thespectrum. With the spectroscope they examine the light that comes from the sun and stars and by the colors of the spectra they can tell what these far-distant bodies are made of.

Application 39.If you were going to the tropics, would it be better to wear outside clothes that were white or black?Application 40.A dancer was to dance in a spotlight on the stage. The light was to change colors constantly. She wanted her robe to reflect each color that was thrown on it. Should she have worn a robe of red, yellow, white, green, or blue?Application 41.If you looked through a red glass at a purple flower (purple is red mixed with blue), would the flower look red, blue, purple, black, or white?

Application 39.If you were going to the tropics, would it be better to wear outside clothes that were white or black?

Application 40.A dancer was to dance in a spotlight on the stage. The light was to change colors constantly. She wanted her robe to reflect each color that was thrown on it. Should she have worn a robe of red, yellow, white, green, or blue?

Application 41.If you looked through a red glass at a purple flower (purple is red mixed with blue), would the flower look red, blue, purple, black, or white?

Explain the following:241. Mercury is separated from its ore by heating the ore so strongly that the mercury rises from it as a vapor.242. Hothouses are built of glass.243. A "rainbow" is sometimes seen in the spray of a garden hose.244. Your feet become hot when your shoes are being polished.245. Doors into offices usually have windows of ground glass or frosted glass.246. Opera glasses are of value to those sitting at a distance from the stage.247. In order to see clearly through opera glasses, you have to adjust them.248. It is warm inside an Eskimo's hut although it is built of ice and snow.249. It is usually cooler on a lawn than on dry ground.250. Black clothes are warmer in the sunlight than clothes of any other color.

Explain the following:

241. Mercury is separated from its ore by heating the ore so strongly that the mercury rises from it as a vapor.

242. Hothouses are built of glass.

243. A "rainbow" is sometimes seen in the spray of a garden hose.

244. Your feet become hot when your shoes are being polished.

245. Doors into offices usually have windows of ground glass or frosted glass.

246. Opera glasses are of value to those sitting at a distance from the stage.

247. In order to see clearly through opera glasses, you have to adjust them.

248. It is warm inside an Eskimo's hut although it is built of ice and snow.

249. It is usually cooler on a lawn than on dry ground.

250. Black clothes are warmer in the sunlight than clothes of any other color.

Section 28.What sound is.

What makes a dictaphone or a phonograph repeat your words?What makes the wind howl when it blows through the branches of trees?Why can you hear an approaching train better if you put your ear to the rail?

What makes a dictaphone or a phonograph repeat your words?

What makes the wind howl when it blows through the branches of trees?

Why can you hear an approaching train better if you put your ear to the rail?

If you were to land on the moon tonight, and had with you a tank containing a supply of air which you could breathe (for there is no air to speak of on the moon), you mighttryto speak. But you would find that you had lost your voice completely. You could not say a word. You would open and close your mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that would not work. You could not make a sound. "Am I deaf and dumb?" you might wonder.

The whole trouble would lie in the fact that the moon has practically no air. And sound is usually a kind of motion of the air,—hundreds of quick, sharp puffs in a second, so close together that we cannot feel them with anything less sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick, sharp puffs of air, or to use a more scientific term,vibrationsof air, it will be easy for you to understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly simple principle. When you talk, the breath passing out of your throat makes thevocal cords vibrate. These and your tongue and lips make the air in front of you vibrate.

When you talk into a dictaphone horn, the vibrating air causes the needle at the small end of the horn to vibrate so that it traces a wavy line in the soft wax of the cylinder as the cylinder turns. Then when you run the needle over the line again it follows the identical track made when you talked into the horn, and it vibrates back and forth just as at first; this makes the air in the horn vibrate exactly as when you talked into the horn, and you have the same sound.

All this goes back to the fundamental principle that sound is vibrations of air; different kinds of sounds are simply different kinds of vibrations. The next experiments will prove this.

Experiment 54.Turn the rotator rapidly, holding the corner of a piece of stiff paper against the holes in the disk. As you turn faster, does the sound become higher or lower? Keep turning at a steady rate and move your paper from the inner row of holes to the outer row and back again. Which row has the most holes in it? Which makes the highest sound? Hold your paper against the teeth at the edge of the disk. Is the pitch higher or lower than before? Blow through a blowpipe against the different rows of holes while the disk is being whirled. As the holes make the air vibrate do you get any sound?

This experiment shows that by making the air vibrate you get a sound.

The next experiment will show that when you have sound you are getting vibrations.

Experiment 55.Tap a tuning fork against the desk, then hold the prongs lightly against your lips. Can you feelthem vibrate? Tap it again, and hold the fork close to your ear. Can you hear the sound?

Fig. 96.Fig. 96.An interesting experiment in sound.

The experiment which follows will show that we usually must have air to do the vibrating to carry the sound.

Experiment 56.Make a pad of not less than a dozen thicknesses of soft cloth so that you can stand an alarm clock on it on the plate of the air pump. The pad is to keep the vibrations of the alarm from making the plate vibrate. A still better way would be to set a tripod on the plate of the air pump and to suspend the alarm clock from the tripod by a rubber band. Set the alarm so that it will ring in 3 or 4 minutes, put it under the bell jar, and pump out the air. Before the alarm goes off, be sure that the air is almost completely pumped out of the jar. Can you hear the bell ring? Distinguish between a dull trillingsound caused by the jarring of the air pump when the alarm is on, and the actualringingsound of the bell.

Fig. 97.Fig. 97.When the air is pumped out of the jar, you cannot hear the bell ring.

The experiment just completed shows how we know there would be no sound on the moon, since there is practically no air around it. The next experiment will show you more about the way in which phonographs work.

Fig. 98.Fig. 98.Making a phonograph record on an old-fashioned phonograph.Experiment 57.Put a blank cylinder on the dictaphone, adjust the recording (cutting) needle and diaphragm at the end of the tube, start the motor, and talk into the dictaphone. Shut off the motor, remove the cutting needle, and put on the reproducing needle (the cutting needle, being sharp, would spoil the cylinder). Start the reproducing needle where the recording needle started, turn on the motor, and listen to your own voice.Notice that in the dictaphone the air waves of your voice are all concentrated into a small space as they go down the tube. At the end of the tube is a diaphragm, a flat disk which is elastic and vibrates back and forth very easily. The air waves from your voice would not vibrate the needle itself enough to make any record; but they vibrate the diaphragm, and the needle, being fastened rigidly to it, vibrates with it.In the same way, when the reproducing needle vibrates as it goes over the track made by the cutting needle, it would make air vibrations too slight for you to hear if it were not fastened to the diaphragm. When the diaphragm vibrates with the needle, it makes a much larger surface of air vibrate than the needle alone could. Then the tube, like an ear trumpet, throws all the air vibrations in one direction, so that you hear the sound easily.

Fig. 98.Fig. 98.Making a phonograph record on an old-fashioned phonograph.

Experiment 57.Put a blank cylinder on the dictaphone, adjust the recording (cutting) needle and diaphragm at the end of the tube, start the motor, and talk into the dictaphone. Shut off the motor, remove the cutting needle, and put on the reproducing needle (the cutting needle, being sharp, would spoil the cylinder). Start the reproducing needle where the recording needle started, turn on the motor, and listen to your own voice.Notice that in the dictaphone the air waves of your voice are all concentrated into a small space as they go down the tube. At the end of the tube is a diaphragm, a flat disk which is elastic and vibrates back and forth very easily. The air waves from your voice would not vibrate the needle itself enough to make any record; but they vibrate the diaphragm, and the needle, being fastened rigidly to it, vibrates with it.In the same way, when the reproducing needle vibrates as it goes over the track made by the cutting needle, it would make air vibrations too slight for you to hear if it were not fastened to the diaphragm. When the diaphragm vibrates with the needle, it makes a much larger surface of air vibrate than the needle alone could. Then the tube, like an ear trumpet, throws all the air vibrations in one direction, so that you hear the sound easily.

Notice that in the dictaphone the air waves of your voice are all concentrated into a small space as they go down the tube. At the end of the tube is a diaphragm, a flat disk which is elastic and vibrates back and forth very easily. The air waves from your voice would not vibrate the needle itself enough to make any record; but they vibrate the diaphragm, and the needle, being fastened rigidly to it, vibrates with it.

In the same way, when the reproducing needle vibrates as it goes over the track made by the cutting needle, it would make air vibrations too slight for you to hear if it were not fastened to the diaphragm. When the diaphragm vibrates with the needle, it makes a much larger surface of air vibrate than the needle alone could. Then the tube, like an ear trumpet, throws all the air vibrations in one direction, so that you hear the sound easily.

Experiment 58.Put a clean white sheet of paper around the recording drum, pasting the two ends together to hold it in place. Put a small piece of gum camphor on a dish just under the paper, light it, and turn the drum so that all parts will be evenly smoked. Be sure to turn it rapidly enough to keep the paper from being burned.

Fig. 99.Fig. 99.A modern dictaphone.

Melt a piece of glass over a burner and draw it out into a thread. Break off about 8 inches of this glass thread and tie it firmly with cotton thread to the edge of one prong of a tuning fork. Clamp the top of the tuning fork firmly above the smoked drum, adjusting it so that the point of the glass thread rests on the smoked paper. Turn the handle slightly to see if the glass is making a mark. If it is not, adjust it so that it will. Now let some one turn the cylinder quickly and steadily. While it is turning, tap the tuning fork on the prong which hasnotthe glass thread fastened to it. The glass point should trace a white, wavy line through the smoke on the paper. If it does not, keep on trying, adjusting the apparatus until the point makes a wavy line.

Making a record in this way is, on a large scale, almost exactly like the making of a phonograph record. The smoked paper on which a tracing can easily be madeas it turns is like the soft wax cylinder. The glass needle is like the recording needle of a phonograph. The chief difference is that you have struck the tuning fork to make it and the needle vibrate, instead of making it vibrate by air waves set in motion by your talking. It is because these vibrations of the tuning fork are more powerful and larger than are those of the recording needle of a phonograph that you can see the record on the recording drum, while you cannot see it clearly on the phonograph cylinder.

Fig. 100.Fig. 100.How the apparatus is set up.

In all ordinary circumstances, sound is the vibration ofair. But in swimming we can hear with our earsunder water, and fishes hear without any air. So, to be accurate, we should say that sound is vibrations of any kind of matter. And the vibrations travel better in most other kinds of matter than they do in air. Vibrations move rather slowly in air, compared with the speed at which they travel in other substances. It takes sound about 5 seconds to go a mile in air; in other words, it would go 12 miles while an express train went one. But it travels faster in water and still faster in anything hard like steel. That is why you can hear the noise of an approaching train better if you put your ear to the rail.

Fig. 101.Fig. 101.When the tuning fork vibrates, the glass needle makes a wavy line on the smoked paper on the drum.

Why we see steam rise before we hear a whistle blow.But even through steel, sound does not travel withanything like the speed of light. In the time that it takes sound to go a mile, light goes hundreds of thousands of miles, easily coming all the way from the moon to the earth. That is why we see the steam rise from the whistle of a train or a boat before we hear the sound. The sound and the light start together; but the light that shows us the steam is in our eyes almost at the instant when the steam leaves the whistle; the sound lags behind, and we hear it later.

Application 42.Explain why a bell rung in a vacuum makes no noise; why the clicking of two stones under water sounds louder if your head is under water, than the clicking of the two stones in the air sounds if your head is in the air; why you hear a buzzing sound when a bee or a fly comes near you; how a phonograph can reproduce sounds.

Explain the following:251. The paint on woodwork blisters when hot.252. You can screw a nut on a bolt very much tighter with a wrench than with your fingers.253. When a pipe is being repaired in the basement of a house, you can hear a scraping noise in the faucets upstairs.254. Sunsets are unusually red after volcanic eruptions.255. Thunder shakes a house.256. Shooting stars are really stones flying through space. When they come near the earth, it pulls them swiftly down through the air. Explain why they glow.257. At night it is difficult to see out through a closed window of a room in which a lamp is lighted.258. When there is a breeze you cannot see clear reflections in a lake.259. Rubbing with coarse sandpaper makes rough wood smooth.260. A bow is bent backward to make the arrow go forward.

Explain the following:

251. The paint on woodwork blisters when hot.

252. You can screw a nut on a bolt very much tighter with a wrench than with your fingers.

253. When a pipe is being repaired in the basement of a house, you can hear a scraping noise in the faucets upstairs.

254. Sunsets are unusually red after volcanic eruptions.

255. Thunder shakes a house.

256. Shooting stars are really stones flying through space. When they come near the earth, it pulls them swiftly down through the air. Explain why they glow.

257. At night it is difficult to see out through a closed window of a room in which a lamp is lighted.

258. When there is a breeze you cannot see clear reflections in a lake.

259. Rubbing with coarse sandpaper makes rough wood smooth.

260. A bow is bent backward to make the arrow go forward.

Section 29.Echoes.

When you put a sea shell to your ear, how is it that you hear a roar in the shell?Why can you sometimes hear an echo and sometimes not?

When you put a sea shell to your ear, how is it that you hear a roar in the shell?

Why can you sometimes hear an echo and sometimes not?

If it were not for the fact that sound travels rather slowly, we should have no echoes, for the sound would get back to us practically at the instant we made it. An echo is merely a sound, a series of air vibrations, bounced back to us by something at a distance. It takes time for the vibration which we start to reach the wall or cliff that bounces it back, and it takes as much more time for the returning vibration to reach our ears. So you have plenty of time to finish your shout before the sound bounces back again. The next experiment shows pretty well how the waves, or vibrations, of sound are reflected; only in the experiment we use waves of water because they go more slowly and we can watch them.

Experiment 59.Fill the long laboratory sink (or the bathtub at home) half full of water and start a wave from one end. Watch it move along the side of the sink. Notice what happens when it reaches the other end.

Air waves do the same thing; when they strike against a flat surface, they bounce back like a rubber ball. If you are far enough away from a flat wall or cliff, and shout, the sound (the air vibrations you start) is reflected back to you and you hear the echo. But if you are close to the walls, as in an empty room, the soundreverberates; it bounces back and forth from one wallto the other so rapidly that no distinct echo is heard, and there is merely a confusion of sound.

Fig. 102.Fig. 102.When the wave reaches the end of the sink, it is reflected back. Sound waves are reflected in the same way.

When you drop a pebble in water, the ripples spread in all directions. In the same way, when you make a sound in the open air, the air waves spread in all directions. But when you shout through a megaphone the air waves are all concentrated in one direction and consequently they are much stronger in that direction. However, while the megaphone intensifies sound, the echoing from the sides of the megaphone makes the sound lose some of its distinctness.

Why it is hard to understand a speaker in an empty hall.A speaker can be heard much more easily in a room full of people than in an empty hall. The sound does not reflect well from the soft clothes of the audienceand the uneven surfaces of their bodies, just as a rubber ball does not bounce well in sand. So the sound does not reverberate as in an empty hall.

Application 43.Explain why a carpeted room is quieter than one with a bare floor; why you shout through your hands when you want to be heard at a distance.

Explain the following:261. It is harder to walk when you shuffle your feet.262. The air over a lamp chimney, or over a register in a furnace-heated house, is moving upward rapidly.263. The shooting of a gun sounds much louder within a room than it does outdoors.264. A drum makes a loud, clear sound when the tightened head is struck.265. The pull of the moon causes the ocean tides.266. Sand is sometimes put in the bottom of vases to keep them from falling over.267. It is difficult to understand clearly the words of one who is speaking in an almost empty hall.268. The ridges in a washboard help to clean the clothes that are rubbed over them.269. One kind of mechanical toy has a heavy lead wheel inside. When you start this to whirling, the toy runs for a long time.270. If you raise your finger slightly after touching the surface of water, the water comes up with your finger.

Explain the following:

261. It is harder to walk when you shuffle your feet.

262. The air over a lamp chimney, or over a register in a furnace-heated house, is moving upward rapidly.

263. The shooting of a gun sounds much louder within a room than it does outdoors.

264. A drum makes a loud, clear sound when the tightened head is struck.

265. The pull of the moon causes the ocean tides.

266. Sand is sometimes put in the bottom of vases to keep them from falling over.

267. It is difficult to understand clearly the words of one who is speaking in an almost empty hall.

268. The ridges in a washboard help to clean the clothes that are rubbed over them.

269. One kind of mechanical toy has a heavy lead wheel inside. When you start this to whirling, the toy runs for a long time.

270. If you raise your finger slightly after touching the surface of water, the water comes up with your finger.

Section 30.Pitch.

What makes the keys of a piano give different sounds?Why does the moving of your fingers up and down on a violin string make it play different notes?Why is the whistle of a peanut roaster so shrill, and why is the whistle of a boat so deep?

What makes the keys of a piano give different sounds?

Why does the moving of your fingers up and down on a violin string make it play different notes?

Why is the whistle of a peanut roaster so shrill, and why is the whistle of a boat so deep?

Did you ever notice how tiresome the whistle on a peanut roaster gets? Well, suppose that whenever youspoke you had to utter your words in exactly that pitch; that every time a car came down the street its noise was like the whistle of the peanut roaster, only louder; that every step you took sounded like hitting a bell of the same pitch; that when you went to the moving-picture theater the orchestra played only the one note; that when any one sang, his voice did not rise and fall; in short, that all the sounds in the world were in one pitch. That is the way it would be if different kinds of air vibrations did not make different kinds of notes,—if there were no differences in pitch.

Pitch due to rapidity of vibration.When air vibrations are slow,—far apart,—the sound is low; when they are faster, the sound is higher; when they are very quick indeed, the sound is very shrill and high. In various ways, as by people talking and walking and by the running of street cars and automobiles, all sorts of different vibrations are started, giving us a pleasant variety of high and low and medium pitches in the sounds of the world around us.

An experiment will show how pitch varies and how it is regulated:

Fig. 103.Fig. 103.When the prongs of the tuning fork are made longer or shorter, the pitch of the sound is changed.Experiment 60.Move the slide of an adjustable tuning fork well up from the end of the prongs, tap one prong lightly on the desk, and listen. Move the slide somewhat toward the end of the prongs, and repeat. Is a higher or a lower sound produced as the slide shortens the length of the prongs?Whistle a low note, then a high one. Notice what you do with your lips; when is the opening the smaller? Sing a low note, then a high one. When are the cords in your throat looser? Fill a drinking glass half full of water, andstrike it. Now pour half the water out, and strike the glass again. Do you get the higher sound when the column of water is shorter or when it is longer? Stretch a rubber band across your thumb and forefinger. Pick the band as you make it tighter, not making it longer, but pulling it tighter with your other fingers. Does it make a higher or a lower sound as you increase the tightness? Stretch the band from your thumb to your little finger and pick it; now put your middle finger under the band so as to divide it in halves, and pick it again. Does a short strand give a higher or lower pitch than a long strand?

Fig. 103.Fig. 103.When the prongs of the tuning fork are made longer or shorter, the pitch of the sound is changed.

Experiment 60.Move the slide of an adjustable tuning fork well up from the end of the prongs, tap one prong lightly on the desk, and listen. Move the slide somewhat toward the end of the prongs, and repeat. Is a higher or a lower sound produced as the slide shortens the length of the prongs?Whistle a low note, then a high one. Notice what you do with your lips; when is the opening the smaller? Sing a low note, then a high one. When are the cords in your throat looser? Fill a drinking glass half full of water, andstrike it. Now pour half the water out, and strike the glass again. Do you get the higher sound when the column of water is shorter or when it is longer? Stretch a rubber band across your thumb and forefinger. Pick the band as you make it tighter, not making it longer, but pulling it tighter with your other fingers. Does it make a higher or a lower sound as you increase the tightness? Stretch the band from your thumb to your little finger and pick it; now put your middle finger under the band so as to divide it in halves, and pick it again. Does a short strand give a higher or lower pitch than a long strand?

Whistle a low note, then a high one. Notice what you do with your lips; when is the opening the smaller? Sing a low note, then a high one. When are the cords in your throat looser? Fill a drinking glass half full of water, andstrike it. Now pour half the water out, and strike the glass again. Do you get the higher sound when the column of water is shorter or when it is longer? Stretch a rubber band across your thumb and forefinger. Pick the band as you make it tighter, not making it longer, but pulling it tighter with your other fingers. Does it make a higher or a lower sound as you increase the tightness? Stretch the band from your thumb to your little finger and pick it; now put your middle finger under the band so as to divide it in halves, and pick it again. Does a short strand give a higher or lower pitch than a long strand?

A violinist tunes his violin by tightening the strings; the tighter they are and the thinner they are, the higher the note they give. Some of the strings are naturally higher than others; the highest is a smaller, finer string than the lowest. When the violinist plays, he shortens the strings by holding them down with his fingers, and the shorter he makes them the higher the note. A bass drum is much larger than a high-pitched kettledrum. The pipes of an organ are long and large for the low notes, shorter and smaller for the high ones.

In general, the longer or larger the object is that vibrates, the slower the rate of vibration and consequently the lower the pitch. But the shorter or finer the object is that vibrates, the higher the rate of vibration and the higher the pitch.

All musical instruments contain devices which can be made to vibrate,—either strings or columns of air, or other things that swing to and fro and start waves in the air. And by tightening them, or making them smaller or shorter, the pitch can be made higher; that is, the number of vibrations to each second can be increased.

Application 44.Explain why a steamboat whistle is usually of much lower pitch than is a toy whistle; why a banjo player moves his fingers toward the drum end of the banjo when he plays high notes; why the sound made by a mosquito is higher in pitch than that made by a bumblebee.Application 45.A boy had a banjo given him for Christmas. He wanted to tune it. To make a string give a higher note, should he have tightened or loosened it? Or could he have secured the same result by moving his finger up and down the string to lengthen or shorten it?Application 46.A man was tuning a piano for a concert. The hall was cold, yet he knew it would be warm at the time of the concert. Should he have tuned the piano to a higher pitch than he wanted it to have on the concert night, to the exact pitch, or to a lower pitch?

Application 45.A boy had a banjo given him for Christmas. He wanted to tune it. To make a string give a higher note, should he have tightened or loosened it? Or could he have secured the same result by moving his finger up and down the string to lengthen or shorten it?

Application 46.A man was tuning a piano for a concert. The hall was cold, yet he knew it would be warm at the time of the concert. Should he have tuned the piano to a higher pitch than he wanted it to have on the concert night, to the exact pitch, or to a lower pitch?

Explain the following:271. A cowboy whirls his lasso around and around his head before he throws it.272. Furnaces are always placed in the basements of buildings, never on top floors.273. A rather slight contraction of a muscle lifts your arm a considerable distance.274. A player on a slide trombone changes the pitch of the notes by lengthening and shortening the tube while he blows through it.275. Rain runs off a tar roof in droplets, while on shingles it soaks in somewhat and spreads.276. There is a sighing sound as the wind blows through the branches of trees, or through stretched wires or ropes.277. Sometimes a very violent noise breaks the membrane in the drum of a person's ear.278. As a street car goes faster and faster, the hum of its motor is higher and higher.279. If a street is partly dry, the wet spots shine more than the dry spots do.280. Molten type metal, when poured into a mold, becomes hard, solid type when it cools.

Explain the following:

271. A cowboy whirls his lasso around and around his head before he throws it.

272. Furnaces are always placed in the basements of buildings, never on top floors.

273. A rather slight contraction of a muscle lifts your arm a considerable distance.

274. A player on a slide trombone changes the pitch of the notes by lengthening and shortening the tube while he blows through it.

275. Rain runs off a tar roof in droplets, while on shingles it soaks in somewhat and spreads.

276. There is a sighing sound as the wind blows through the branches of trees, or through stretched wires or ropes.

277. Sometimes a very violent noise breaks the membrane in the drum of a person's ear.

278. As a street car goes faster and faster, the hum of its motor is higher and higher.

279. If a street is partly dry, the wet spots shine more than the dry spots do.

280. Molten type metal, when poured into a mold, becomes hard, solid type when it cools.

Section 31.Magnets; the compass.

What makes the needle of a compass point north?What causes the Northern Lights?

What makes the needle of a compass point north?

What causes the Northern Lights?

For many hundreds of years sailors have used the compass to determine directions. During all this time men have known that one point of the needle always swings toward the north if there is no iron near to pull it some other way, but until within the past century they did not know why. Now we have found the explanation in the fact that the earth is a great big magnet. The experiment which follows will help you to understand why the earth's being a magnet should make the compass needle point north and south.

Experiment 61.Lay a magnetic compass flat on the table. Notice which point swings to the north. Now hold a horseshoe magnet, points down, over the compass. Turn the magnet around and watch the compass needle; see which end of the magnet attracts the north point; hold that end of it toward the south point and note the effect. Hold the magnet, ends up, under the table directly below the compass and turn the magnet, watching the compass needle.

The earth is a magnet, and it acts just as your magnet does: one end attracts one point of the compass, and the other end attracts the other point. That ought to make it clear why the compass points north. But how is the compass made? The next experiment will show this plainly.

Experiment 62.Take a long shoestring and make a loop in one end of it. Slip the magnet through the loop andsuspend it, ends down. Fasten the shoestring to the top of a doorway so that the magnet can swing easily. Steady the magnet and let it turn until it comes to a rest. Mark the end that swings to the north. Turn this end around to the south; let go and watch it. Place the magnet the other way around in the loop so that you can be sure that it is not twisting of the shoestring that makes the magnet turn in this direction.Fig. 104.Fig. 104.The compass needle follows the magnet.Now stroke a needle several times along one arm of the magnet,always in the same direction, as shown in Figure 105. Hold the needle over some iron filings or touch any bit of iron or steel with it. What has the needle become? Lay it on a cardboard milk-bottle top of the flat kind, and on that float it in the middle of a glass or earthenware dish of water. Notice which end turns north. Turn this end to the south and see what happens. Hold your magnet, ends up, under the dish, and turn the magnet. What does the needle do?

Fig. 104.Fig. 104.The compass needle follows the magnet.

Now stroke a needle several times along one arm of the magnet,always in the same direction, as shown in Figure 105. Hold the needle over some iron filings or touch any bit of iron or steel with it. What has the needle become? Lay it on a cardboard milk-bottle top of the flat kind, and on that float it in the middle of a glass or earthenware dish of water. Notice which end turns north. Turn this end to the south and see what happens. Hold your magnet, ends up, under the dish, and turn the magnet. What does the needle do?

Now it should be easy to understand why the compass points north. One end of any magnet pulls ononeend of another magnet and drives theotherend away. The earth is a big magnet. So if you make a magnet and balance it in such a way that it is free to swing, the north end of the big earth magnet pulls one end of the little magnet toward it and pushes the other end away. Therefore one end of your compass always points north.

Other effects of the earth's magnetism.Another interesting effect of the earth's being a big magnet is to be seen if you lay a piece of steel so that it points north and south, and then pound it on one end. It becomes magnetized just as your needle became magnetized when it was rubbed on the small magnet.

Fig. 105.Fig. 105.Magnetizing a needle.

Fig. 106.Fig. 106.A compass made of a needle and a piece of cardboard.

And still another effect of the earth's magnetism is this: Tiny particles of electricity, calledelectrons, are probably shooting through space from the sun. It is believed that as they come near the earth, the magnetism of the north and south polar regions attracts them toward the poles, and that as they rush through the thin, dry upper air, they make it glow. And this is probably what causes the Northern Lights or Aurora Borealis.

What happens when a needle is magnetized.The reason that a needle becomes magnetic if it is rubbedover a magnet is probably this: Every molecule of iron may be an extremely tiny magnet; if it is, each molecule has a north and south pole like the needle of a compass. In an ordinary needle (or in any unmagnetized piece of iron or steel) these molecules would be facing every way, as shown in Figure 107.

Fig. 107.Fig. 107.Diagram of molecules in unmagnetized iron. The north and south poles of the molecules are supposed to be pointing in all directions.Fig. 108.Fig. 108.Diagram of magnetized iron. The north and south poles of the molecules are all supposed to point in the same direction.

Fig. 107.Fig. 107.Diagram of molecules in unmagnetized iron. The north and south poles of the molecules are supposed to be pointing in all directions.

Fig. 108.Fig. 108.Diagram of magnetized iron. The north and south poles of the molecules are all supposed to point in the same direction.

But when a piece of steel or iron that is already magnetized is brought near the unmagnetized needle, all the north poles of the molecules of the needle are pulledin the same direction—it is almost like combing tangled hair to stroke a needle over a magnet. Then the molecules are arranged more as shown in Figure 108. When all the molecules, each of which is a tiny magnet, pull in the same direction, they make a strong magnet, and they magnetize any iron that comes near them just as they were magnetized.

Steel will stay magnetized a long time; but ordinary soft iron loses magnetism almost as soon as another magnet is taken away from it,—the molecules become all disarranged again.

In a later section you will find that whenever electricity flows through a wire that is coiled around a piece of iron, the iron becomes magnetized just as when it is rubbed with a magnet.

Application 47.An explorer lost his compass. In clear weather he could tell the directions by the sun and stars, but in cloudy weather he was badly handicapped. He had with him a gun, plenty of ammunition, a sewing kit, a hunting knife, and some provisions. How could he have made a compass?

Explain the following:281. Snow turns to water in the first warm weather.282. A person's face looks ghastly by the greenish light of a mercury-vapor lamp.283. If a red-hot coal is touched with a cold poker, the coal turns black at the place touched.284. Stereopticon slides are put in upside down, yet the picture on the screen is right side up.285. If the vocal cords of your throat did not vibrate, you could not talk out loud.286. A watch is sometimes put out of order if it is held near a magnet.287. The water will be no higher on the inside of a leaky boat than it is on the outside.288. A bass viol is considerably larger than a violin.289. Ships that are used by men testing the earth's magnetism carry very sensitive compasses. Explain why such ships are made entirely of wood and brass.290. Thunder rolls; that is, after the first peal there is a reverberating sound that becomes less and less distinct.

Explain the following:

281. Snow turns to water in the first warm weather.

282. A person's face looks ghastly by the greenish light of a mercury-vapor lamp.

283. If a red-hot coal is touched with a cold poker, the coal turns black at the place touched.

284. Stereopticon slides are put in upside down, yet the picture on the screen is right side up.

285. If the vocal cords of your throat did not vibrate, you could not talk out loud.

286. A watch is sometimes put out of order if it is held near a magnet.

287. The water will be no higher on the inside of a leaky boat than it is on the outside.

288. A bass viol is considerably larger than a violin.

289. Ships that are used by men testing the earth's magnetism carry very sensitive compasses. Explain why such ships are made entirely of wood and brass.

290. Thunder rolls; that is, after the first peal there is a reverberating sound that becomes less and less distinct.

Section 32.Static electricity.

What is electricity?What makes thunder and lightning?Why does the barrel or cap of a fountain pen pick up small bits of paper after it has been rubbed on your coat sleeve?Why do sparks fly from the fur of a cat when you stroke it in the dark?

What is electricity?

What makes thunder and lightning?

Why does the barrel or cap of a fountain pen pick up small bits of paper after it has been rubbed on your coat sleeve?

Why do sparks fly from the fur of a cat when you stroke it in the dark?

The Greeks, 2000 years ago, knew that there was such a thing as electricity, and they used to get it by rubbing amber with silk. In the past century men have learned how to make electricity do all sorts of useful work: making boats and cars and automobiles go, ringing bells, furnishing light, and, in the telephone and telegraph, carrying messages. But no one knew what electricity really was until, within the last 25 years, scientists found out.

Atoms and electrons.When we talked about molecules, we said that they were as much smaller than a germ as a germ is smaller than a mountain. Well, a molecule is made up, probably, of some things that are much smaller still,—so small that people thought that nothing could be smaller. Those unthinkably tiny things are calledatoms; you will hear more about them when you come to the parts of this book that tell about chemistry.

But if you took the smallest atom in the world and divided it into 1700 pieces, each one of these would be about the size of a piece of electricity.

Electricity is made up of the tiniest things known to man—things so small that nobody really can think of their smallness. These little pieces of electricity are calledelectrons, and for all their smallness, scientists have been able to find out a good deal about them. They have managed to get one electron all by itself on a droplet of oil and they have seen how it made the oil behave. Of course they could not see the electron, but they could tell from various experiments that they had just one. Scientists know how many trillions of electrons flow through an incandescent electric lamp in a second and how many quadrillions of them it would take to weigh as much as a feather. They know what the electrons do when they move, how fast they can move, and what substances let electrons move through them easily and what substances hold them back; and they know perfectly well how to set them in motion. How the scientists came to know all these things you will learn in the study of physics; it is a long story. But you can find out some things about electrons yourself. The first experiment is a simple one such as the Greeks used to do with amber.

Fig. 109.Fig. 109.When the comb is rubbed on the coat, it becomes charged with electricity.Experiment 63.Rub a hard rubber comb on a piece of woolen cloth. The sleeve of a woolen coat or sweater will do. Rub the comb quickly in the same direction several times. Now hold it over some small bits of paper or sawdust. What does it do to them? Hold it over some one's hair. The rest of this experiment will work well only oncool, clear days. Rub the comb again, a dozen or more times in quick succession. Now touch it gently to the lobe of your ear. Do you hear the snap as the small spark jumps from the comb to your ear?Pull a dry hair out of your head and hold it by one end. Charge your comb by rubbing it again, and bring it near the loose end of the hair. If the end of the hair clings to the comb at first, leave it clinging until it flies off. Now try to touch the hair with the comb. Next, pinch the end of the hair between your thumb and finger and again bring the charged comb near it. Is the hair attracted or repelled? After touching the comb what does it do?You can get the same effects by rubbing glass or amber on silk.

Fig. 109.Fig. 109.When the comb is rubbed on the coat, it becomes charged with electricity.

Experiment 63.Rub a hard rubber comb on a piece of woolen cloth. The sleeve of a woolen coat or sweater will do. Rub the comb quickly in the same direction several times. Now hold it over some small bits of paper or sawdust. What does it do to them? Hold it over some one's hair. The rest of this experiment will work well only oncool, clear days. Rub the comb again, a dozen or more times in quick succession. Now touch it gently to the lobe of your ear. Do you hear the snap as the small spark jumps from the comb to your ear?Pull a dry hair out of your head and hold it by one end. Charge your comb by rubbing it again, and bring it near the loose end of the hair. If the end of the hair clings to the comb at first, leave it clinging until it flies off. Now try to touch the hair with the comb. Next, pinch the end of the hair between your thumb and finger and again bring the charged comb near it. Is the hair attracted or repelled? After touching the comb what does it do?You can get the same effects by rubbing glass or amber on silk.

Pull a dry hair out of your head and hold it by one end. Charge your comb by rubbing it again, and bring it near the loose end of the hair. If the end of the hair clings to the comb at first, leave it clinging until it flies off. Now try to touch the hair with the comb. Next, pinch the end of the hair between your thumb and finger and again bring the charged comb near it. Is the hair attracted or repelled? After touching the comb what does it do?

You can get the same effects by rubbing glass or amber on silk.

Objects negatively and positively charged with electricity.There are probably electrons in everything. But when there is just the usual number of electrons in an object, it acts in an ordinary way and we say that it is not charged with electricity. If there are more than the usual number of electrons on an object, however, we say that it isnegatively charged, or that it has a negative charge of electricity on it. But if there are fewer electrons than usual in an object, we say that it has a positive charge of electricity on it, or that it ispositively charged.

Fig. 110.Fig. 110.The charged comb picks up pieces of paper.

You might expect a "negative charge" to indicate fewer electrons than usual, not more. But people called the charge "negative" long before they knew anything about electrons; and it is easier to keep the old name than to change all the books that have been written about electricity. So we still call a charge "negative" when there are unusuallymanyelectrons, and we call it "positive" when there are unusuallyfew. Anegative chargemeans that more electrons are present than usual. Apositive chargemeans that fewer electrons are present than usual.

Before you rubbed your comb on wool, neither the comb nor the wool was charged; both had just the usual number of electrons. But when you rubbed them together, you rubbed some of the electrons off the wool on to the comb. Then the comb had a negative charge; that is, it had too many electrons—too many little particles of electricity.

When you brought the comb near the hair, the hair had fewer electrons than the comb. Whenever one object has more electrons on it than another, the twoobjects are pulled toward each other; so there was an attraction between the comb and the hair, and the hair came over to the comb. As soon as it touched the comb, some of the extra electrons jumped from the comb to the hair. The electrons could not get off the hair easily, so they stayed there. Electrons repel each other—drive each other away. So when you had a number of electrons on the end of the comb and a number on the end of the hair, they pushed each other away, and the hair flew from the comb. But when you pinched the hair, the electrons could get off it to your moist hand, which lets electricity through it fairly easily. Then the comb had extra electrons on it and the hair did not; so the comb pulled the hair over toward it again.

When you brought the charged comb near your ear, some of the electrons on the comb pushed the others off to your ear, and you heard them snap as they rushed through the air, making it vibrate.

How lightning and thunder are caused.In thunderstorms the strong currents of rising air blow some of the forming raindrops in the clouds into bits of spray. The tinier droplets get more than their share of electrons when this happens and are carried on up to higher clouds. In this way clouds become charged with electricity. One cloud has on it many more electrons than another cloud that is made, perhaps, of lower, larger droplets. The electricity leaps from the cloud that has the greater number of electrons to the cloud that has the less number, or it leaps from the heavily charged cloud down to a tree or house or the ground. You see the electricityleap and call itlightning. Much more leaps, however, than leaped from the comb to your ear, and so it makes a very much louder snap. The snap is caused in this way: As the electric spark leaps through the air, it leaves an empty space or vacuum immediately behind it. The air from all sides rushes into the vacuum and collides there; then it bounces back. This again leaves a partial vacuum; so the air rushes in once more, coming from all sides at once, and again bounces back. This starts the air vibrations which we callsound. Then the sound is echoed from cloud to cloud and from the clouds to the earth and back again, and we call itthunder.The electricity you have been reading about and experimenting with in this section is calledstatic electricity. "Static" means standing still. The electricity you rubbed up to the surface of the comb or glass stayed still until it jumped to the bit of paper or hair; then it stayed still on that. This was the only kind of electricity most people knew anything about until the nineteenth century; and it is not of any great use. Electricity must be flowing through things to do work. That is why people could not invent electric cars and electric lights and telephones before they knew how to make electricity flow steadily rather than just to stand still on one thing until it jumped across to another and stood there. In the next chapter we shall take up the ways in which electrons are made to flow and to do work.

The electricity you have been reading about and experimenting with in this section is calledstatic electricity. "Static" means standing still. The electricity you rubbed up to the surface of the comb or glass stayed still until it jumped to the bit of paper or hair; then it stayed still on that. This was the only kind of electricity most people knew anything about until the nineteenth century; and it is not of any great use. Electricity must be flowing through things to do work. That is why people could not invent electric cars and electric lights and telephones before they knew how to make electricity flow steadily rather than just to stand still on one thing until it jumped across to another and stood there. In the next chapter we shall take up the ways in which electrons are made to flow and to do work.

Application 48.Explain why the stroking of a cat's back will sometimes cause sparks and make the cat's hairs stand apart; why combing sometimes makes your hairs fly apart.Both of these effects are best secured on a dry day, because on a damp day the water particles in the air will let the electrons pass to them as fast as they are rubbed up to the surface of the hair.

Explain the following:291. If you shuffle your feet on a carpet in clear, cold weather and then touch a person's nose or ear, a slight spark passes from your finger and stings him.292. If you stay out in the cold long, you get chilled through.293. The air and earth in a greenhouse are warmed by the sun through the glass even when it is cold outside and when the glass itself remains cold.294. When you hold a blade of grass taut between your thumbs and blow on it, you get a noise.295. Shadows are usually black.296. Some women keep magnets with which to find lost needles.297. You can grasp objects much more firmly with pliers than with your fingers.298. If the glass in a mirror is uneven, the image of your face is unnatural.299. A sweater clings close to your body.300. Kitchens, bathrooms, and hospitals should have painted walls.

Explain the following:

291. If you shuffle your feet on a carpet in clear, cold weather and then touch a person's nose or ear, a slight spark passes from your finger and stings him.

292. If you stay out in the cold long, you get chilled through.

293. The air and earth in a greenhouse are warmed by the sun through the glass even when it is cold outside and when the glass itself remains cold.

294. When you hold a blade of grass taut between your thumbs and blow on it, you get a noise.

295. Shadows are usually black.

296. Some women keep magnets with which to find lost needles.

297. You can grasp objects much more firmly with pliers than with your fingers.

298. If the glass in a mirror is uneven, the image of your face is unnatural.

299. A sweater clings close to your body.

300. Kitchens, bathrooms, and hospitals should have painted walls.

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