CHAPTER I MAGNETS AND MAGNETISMOver two thousand years ago, in far-away Asia Minor, a shepherd guarding his flocks on the slope of Mount Ida suddenly found the iron-shod end of his staff adhering to a stone. Upon looking further around about him he found many other pieces of this peculiar hard black mineral, the smaller bits of which tended to cling to the nails and studs in the soles of his sandals.This mineral, which was an ore of iron, consisting of iron and oxygen, was found in a district known as Magnesia, and in this way soon became widely known as the "Magnesstone," or magnet.This is the story of the discovery of the magnet. It exists in legends in various forms. As more masses of this magnetic ore were discovered in various parts of the world, the stories of its attractive power became greatly exaggerated, especially during the Middle Ages. In fact, magnetic mountains which would pull the iron nails out of ships, or, later, move the compass needle far astray, did not lose their place among the terrors of the sea until nearly the eighteenth century.For many hundreds of years the magnet-stone was of little use to mankind save as a curiosity which possessed the power of attracting small pieces of iron and steel and other magnets like itself. Then some one, no one knows who, discovered that if a magnet-stone were hung by a thread in a suitable manner it would always tend to point North and South; and so the "Magnes-stone" became also called the "lodestone," or "leading-stone."These simple bits of lodestone suspended by a thread were the forerunners of the modern compass and were of great value to the ancient navigators, for they enabled them to steer ships in cloudy weather when the sun was obscured and on nights when the pole-star could not be seen.The first realcompasseswere calledgnomons, and consisted of a steel needle which had been rubbed upon a lodestone until it acquired its magnetic properties. Then it was thrust through a reed or short piece of wood which supported it on the surface of a vessel of water. If the needle was left in this receptacle, naturally it would move against the side and not point a true position. Therefore it was given a circular movement in the water, and as soon as it came to rest, the point on the horizon which the north end designated was carefully noted and the ship’s course laid accordingly.The modern mariners’ compass is quite a different arrangement. It consists of three parts, thebowl, thecard, and theneedle. The bowl, which contains the card and needle, is usually a hemispherical brass receptacle, suspended in a pair of brass rings, calledgimbals, in such a manner that the bowl will remain horizontal no matter how violently the ship may pitch and roll. The card, which is circular, is divided into 32 equal parts called thepoints of the compass. The needles, of which there are generally from two to four, are fastened to the bottom of the card.Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."In the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl, so that the card hanging on the pin turns freely around its center. On shipboard, the compass is so placed that a black mark, called thelubber’s line, is fixed in a position parallel to the keel. The point on the compass-card which is directly against this line indicates the direction of the ship’s head.Experiments with MagnetismThe phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus.Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation.Artificial Magnetsare those made from steel by the application of a lodestone or some other magnetizing force.The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is calledMagnetization.Fig. 2.—A Bar MagnetFig. 2.—A Bar MagnetSmall horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments.Fig. 3.—A Horseshoe MagnetFig. 3.—A Horseshoe MagnetStroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet.Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.This experiment shows that the attractive power of a magnet exists intwo oppositeplaces. These are called the poles.There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space.The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is calledMagnetic Force.The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away.Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*.Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.Then try some nails in place of the tacks. The nails are heavier than the tacks, and it will require a greater force to lift them. The magnet will have to be brought much closer to the nails than to the tacks before they are lifted, showing that the force exerted by the magnet is strongest nearest to it.Magnetize a needle and lay it on a piece of cork floating in a glass vessel of water. It will then be seen that the needle always comes to rest lying nearly in a north and south line, with the same end always toward the north.Fig. 6.—A Simple Compass.Fig. 6.—A Simple Compass.The pole of the magnet which tends to turn towards the north is called thenorth-seeking poleand the opposite one is called thesouth-seeking pole.The name is usually abbreviated to simply the north and south poles. The north pole of a magnet is often indicated by a straight line or a letter N stamped into the metal.A magnetized needle floating on a cork in a basin of water is a simple form ofFig. 7.—Several Different Methods of Making a Simple Compass.Fig. 7.—Several Different Methods of Making a Simple Compass.Compass.Figure 7 shows several other different ways of making compasses. The first method is to suspend a magnetized needle from a fine silk fiber or thread.The second method illustrates a very sensitive compass made from paper. Two magnetized needles are stuck through the sides with their north poles both at the same end. The paper support is mounted upon a third needle stuck through a cork.A compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring.The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.Balance the needle on the end of a pin stuck through the bottom of a pill-box.Magnetic Substancesare those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. Ordinary magnets attract but very few substances. Iron, steel, cobalt, and nickel are about the only ones worthy of mention.Attraction through Bodies.A magnet will attract a nail or a tack through a piece of paper, just as if nothing intervened.Fig. 8.—The Attraction of an Iron Nail through Glass.Fig. 8.—The Attraction of an Iron Nail throughGlass.It will also attract through glass, wood, brass, and all other substances. Through an iron plate, however, the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail.A number of carpet-tacks may be supported from a magnet in the form of a chain. Each individual tack in the series becomes atemporarymagnet byinduction.If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart.Fig. 9.—A Magnetic Chain.Fig. 9.—A Magnetic Chain.It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if asecondmagnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks.The reason for this is that the magnetism in the tacks is increased by induction.Magnets will Attract or Repeleach other, depending upon which poles are nearest.Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.This fact gives rise to the general law of magnetism:Like poles repel each other and unlike poles attract each other.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.Fig. 11.—A Magnetic Boat.Fig. 11.—A Magnetic Boat.If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.The needles will then arrange themselves in different symmetrical groups, according to their number.A bar magnet thrust among them will attract or repel them depending upon its polarity.The upper ends of the needles should all have the same polarity, that is, all be either north or south poles.Magnetism flows along certain lines calledLines of Magnetic Force.These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called thefield of force, and the path through which they flow is called theMagnetic Circuit.The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles.The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15.Fig. 15.—Lines of Force between Like and Unlike Poles.Fig. 15.—Lines of Force between Like and Unlike Poles.A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place.It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves.The Earth is a Great Magnet.The direction assumed by a compass needle is called themagnetic meridian.The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.Magnetic Dip.If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that thenorthend points slightly downward.Fig. 16.—A Simple Dipping Needle.Fig. 16.—A Simple Dipping Needle.This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called aDipping Needle.A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.Permanent Magnetshave a number of useful applications in the construction of scientific instruments, voltmeters, ammeters, telephone receivers, magnetos and a number of other devices.In order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately, and then riveted together. A magnet made in this way is called a compound magnet, and may have either a bar or a horse-shoe shape.Magnets are usually provided with a soft piece of iron called an armature or "keeper." The "keeper" is laid across the poles of the magnet when the latter is not in use and preserves its magnetism.A blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it. The most powerful magnet becomes absolutely demagnetized at a red heat, and remains so after cooling.Therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet, do not allow it to receive rough usage.CHAPTER II STATIC ELECTRICITY
CHAPTER I MAGNETS AND MAGNETISMOver two thousand years ago, in far-away Asia Minor, a shepherd guarding his flocks on the slope of Mount Ida suddenly found the iron-shod end of his staff adhering to a stone. Upon looking further around about him he found many other pieces of this peculiar hard black mineral, the smaller bits of which tended to cling to the nails and studs in the soles of his sandals.This mineral, which was an ore of iron, consisting of iron and oxygen, was found in a district known as Magnesia, and in this way soon became widely known as the "Magnesstone," or magnet.This is the story of the discovery of the magnet. It exists in legends in various forms. As more masses of this magnetic ore were discovered in various parts of the world, the stories of its attractive power became greatly exaggerated, especially during the Middle Ages. In fact, magnetic mountains which would pull the iron nails out of ships, or, later, move the compass needle far astray, did not lose their place among the terrors of the sea until nearly the eighteenth century.For many hundreds of years the magnet-stone was of little use to mankind save as a curiosity which possessed the power of attracting small pieces of iron and steel and other magnets like itself. Then some one, no one knows who, discovered that if a magnet-stone were hung by a thread in a suitable manner it would always tend to point North and South; and so the "Magnes-stone" became also called the "lodestone," or "leading-stone."These simple bits of lodestone suspended by a thread were the forerunners of the modern compass and were of great value to the ancient navigators, for they enabled them to steer ships in cloudy weather when the sun was obscured and on nights when the pole-star could not be seen.The first realcompasseswere calledgnomons, and consisted of a steel needle which had been rubbed upon a lodestone until it acquired its magnetic properties. Then it was thrust through a reed or short piece of wood which supported it on the surface of a vessel of water. If the needle was left in this receptacle, naturally it would move against the side and not point a true position. Therefore it was given a circular movement in the water, and as soon as it came to rest, the point on the horizon which the north end designated was carefully noted and the ship’s course laid accordingly.The modern mariners’ compass is quite a different arrangement. It consists of three parts, thebowl, thecard, and theneedle. The bowl, which contains the card and needle, is usually a hemispherical brass receptacle, suspended in a pair of brass rings, calledgimbals, in such a manner that the bowl will remain horizontal no matter how violently the ship may pitch and roll. The card, which is circular, is divided into 32 equal parts called thepoints of the compass. The needles, of which there are generally from two to four, are fastened to the bottom of the card.Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."In the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl, so that the card hanging on the pin turns freely around its center. On shipboard, the compass is so placed that a black mark, called thelubber’s line, is fixed in a position parallel to the keel. The point on the compass-card which is directly against this line indicates the direction of the ship’s head.Experiments with MagnetismThe phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus.Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation.Artificial Magnetsare those made from steel by the application of a lodestone or some other magnetizing force.The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is calledMagnetization.Fig. 2.—A Bar MagnetFig. 2.—A Bar MagnetSmall horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments.Fig. 3.—A Horseshoe MagnetFig. 3.—A Horseshoe MagnetStroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet.Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.This experiment shows that the attractive power of a magnet exists intwo oppositeplaces. These are called the poles.There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space.The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is calledMagnetic Force.The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away.Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*.Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.Then try some nails in place of the tacks. The nails are heavier than the tacks, and it will require a greater force to lift them. The magnet will have to be brought much closer to the nails than to the tacks before they are lifted, showing that the force exerted by the magnet is strongest nearest to it.Magnetize a needle and lay it on a piece of cork floating in a glass vessel of water. It will then be seen that the needle always comes to rest lying nearly in a north and south line, with the same end always toward the north.Fig. 6.—A Simple Compass.Fig. 6.—A Simple Compass.The pole of the magnet which tends to turn towards the north is called thenorth-seeking poleand the opposite one is called thesouth-seeking pole.The name is usually abbreviated to simply the north and south poles. The north pole of a magnet is often indicated by a straight line or a letter N stamped into the metal.A magnetized needle floating on a cork in a basin of water is a simple form ofFig. 7.—Several Different Methods of Making a Simple Compass.Fig. 7.—Several Different Methods of Making a Simple Compass.Compass.Figure 7 shows several other different ways of making compasses. The first method is to suspend a magnetized needle from a fine silk fiber or thread.The second method illustrates a very sensitive compass made from paper. Two magnetized needles are stuck through the sides with their north poles both at the same end. The paper support is mounted upon a third needle stuck through a cork.A compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring.The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.Balance the needle on the end of a pin stuck through the bottom of a pill-box.Magnetic Substancesare those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. Ordinary magnets attract but very few substances. Iron, steel, cobalt, and nickel are about the only ones worthy of mention.Attraction through Bodies.A magnet will attract a nail or a tack through a piece of paper, just as if nothing intervened.Fig. 8.—The Attraction of an Iron Nail through Glass.Fig. 8.—The Attraction of an Iron Nail throughGlass.It will also attract through glass, wood, brass, and all other substances. Through an iron plate, however, the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail.A number of carpet-tacks may be supported from a magnet in the form of a chain. Each individual tack in the series becomes atemporarymagnet byinduction.If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart.Fig. 9.—A Magnetic Chain.Fig. 9.—A Magnetic Chain.It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if asecondmagnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks.The reason for this is that the magnetism in the tacks is increased by induction.Magnets will Attract or Repeleach other, depending upon which poles are nearest.Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.This fact gives rise to the general law of magnetism:Like poles repel each other and unlike poles attract each other.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.Fig. 11.—A Magnetic Boat.Fig. 11.—A Magnetic Boat.If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.The needles will then arrange themselves in different symmetrical groups, according to their number.A bar magnet thrust among them will attract or repel them depending upon its polarity.The upper ends of the needles should all have the same polarity, that is, all be either north or south poles.Magnetism flows along certain lines calledLines of Magnetic Force.These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called thefield of force, and the path through which they flow is called theMagnetic Circuit.The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles.The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15.Fig. 15.—Lines of Force between Like and Unlike Poles.Fig. 15.—Lines of Force between Like and Unlike Poles.A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place.It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves.The Earth is a Great Magnet.The direction assumed by a compass needle is called themagnetic meridian.The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.Magnetic Dip.If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that thenorthend points slightly downward.Fig. 16.—A Simple Dipping Needle.Fig. 16.—A Simple Dipping Needle.This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called aDipping Needle.A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.Permanent Magnetshave a number of useful applications in the construction of scientific instruments, voltmeters, ammeters, telephone receivers, magnetos and a number of other devices.In order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately, and then riveted together. A magnet made in this way is called a compound magnet, and may have either a bar or a horse-shoe shape.Magnets are usually provided with a soft piece of iron called an armature or "keeper." The "keeper" is laid across the poles of the magnet when the latter is not in use and preserves its magnetism.A blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it. The most powerful magnet becomes absolutely demagnetized at a red heat, and remains so after cooling.Therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet, do not allow it to receive rough usage.CHAPTER II STATIC ELECTRICITY
CHAPTER I MAGNETS AND MAGNETISMOver two thousand years ago, in far-away Asia Minor, a shepherd guarding his flocks on the slope of Mount Ida suddenly found the iron-shod end of his staff adhering to a stone. Upon looking further around about him he found many other pieces of this peculiar hard black mineral, the smaller bits of which tended to cling to the nails and studs in the soles of his sandals.This mineral, which was an ore of iron, consisting of iron and oxygen, was found in a district known as Magnesia, and in this way soon became widely known as the "Magnesstone," or magnet.This is the story of the discovery of the magnet. It exists in legends in various forms. As more masses of this magnetic ore were discovered in various parts of the world, the stories of its attractive power became greatly exaggerated, especially during the Middle Ages. In fact, magnetic mountains which would pull the iron nails out of ships, or, later, move the compass needle far astray, did not lose their place among the terrors of the sea until nearly the eighteenth century.For many hundreds of years the magnet-stone was of little use to mankind save as a curiosity which possessed the power of attracting small pieces of iron and steel and other magnets like itself. Then some one, no one knows who, discovered that if a magnet-stone were hung by a thread in a suitable manner it would always tend to point North and South; and so the "Magnes-stone" became also called the "lodestone," or "leading-stone."These simple bits of lodestone suspended by a thread were the forerunners of the modern compass and were of great value to the ancient navigators, for they enabled them to steer ships in cloudy weather when the sun was obscured and on nights when the pole-star could not be seen.The first realcompasseswere calledgnomons, and consisted of a steel needle which had been rubbed upon a lodestone until it acquired its magnetic properties. Then it was thrust through a reed or short piece of wood which supported it on the surface of a vessel of water. If the needle was left in this receptacle, naturally it would move against the side and not point a true position. Therefore it was given a circular movement in the water, and as soon as it came to rest, the point on the horizon which the north end designated was carefully noted and the ship’s course laid accordingly.The modern mariners’ compass is quite a different arrangement. It consists of three parts, thebowl, thecard, and theneedle. The bowl, which contains the card and needle, is usually a hemispherical brass receptacle, suspended in a pair of brass rings, calledgimbals, in such a manner that the bowl will remain horizontal no matter how violently the ship may pitch and roll. The card, which is circular, is divided into 32 equal parts called thepoints of the compass. The needles, of which there are generally from two to four, are fastened to the bottom of the card.Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."In the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl, so that the card hanging on the pin turns freely around its center. On shipboard, the compass is so placed that a black mark, called thelubber’s line, is fixed in a position parallel to the keel. The point on the compass-card which is directly against this line indicates the direction of the ship’s head.Experiments with MagnetismThe phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus.Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation.Artificial Magnetsare those made from steel by the application of a lodestone or some other magnetizing force.The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is calledMagnetization.Fig. 2.—A Bar MagnetFig. 2.—A Bar MagnetSmall horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments.Fig. 3.—A Horseshoe MagnetFig. 3.—A Horseshoe MagnetStroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet.Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.This experiment shows that the attractive power of a magnet exists intwo oppositeplaces. These are called the poles.There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space.The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is calledMagnetic Force.The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away.Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*.Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.Then try some nails in place of the tacks. The nails are heavier than the tacks, and it will require a greater force to lift them. The magnet will have to be brought much closer to the nails than to the tacks before they are lifted, showing that the force exerted by the magnet is strongest nearest to it.Magnetize a needle and lay it on a piece of cork floating in a glass vessel of water. It will then be seen that the needle always comes to rest lying nearly in a north and south line, with the same end always toward the north.Fig. 6.—A Simple Compass.Fig. 6.—A Simple Compass.The pole of the magnet which tends to turn towards the north is called thenorth-seeking poleand the opposite one is called thesouth-seeking pole.The name is usually abbreviated to simply the north and south poles. The north pole of a magnet is often indicated by a straight line or a letter N stamped into the metal.A magnetized needle floating on a cork in a basin of water is a simple form ofFig. 7.—Several Different Methods of Making a Simple Compass.Fig. 7.—Several Different Methods of Making a Simple Compass.Compass.Figure 7 shows several other different ways of making compasses. The first method is to suspend a magnetized needle from a fine silk fiber or thread.The second method illustrates a very sensitive compass made from paper. Two magnetized needles are stuck through the sides with their north poles both at the same end. The paper support is mounted upon a third needle stuck through a cork.A compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring.The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.Balance the needle on the end of a pin stuck through the bottom of a pill-box.Magnetic Substancesare those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. Ordinary magnets attract but very few substances. Iron, steel, cobalt, and nickel are about the only ones worthy of mention.Attraction through Bodies.A magnet will attract a nail or a tack through a piece of paper, just as if nothing intervened.Fig. 8.—The Attraction of an Iron Nail through Glass.Fig. 8.—The Attraction of an Iron Nail throughGlass.It will also attract through glass, wood, brass, and all other substances. Through an iron plate, however, the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail.A number of carpet-tacks may be supported from a magnet in the form of a chain. Each individual tack in the series becomes atemporarymagnet byinduction.If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart.Fig. 9.—A Magnetic Chain.Fig. 9.—A Magnetic Chain.It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if asecondmagnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks.The reason for this is that the magnetism in the tacks is increased by induction.Magnets will Attract or Repeleach other, depending upon which poles are nearest.Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.This fact gives rise to the general law of magnetism:Like poles repel each other and unlike poles attract each other.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.Fig. 11.—A Magnetic Boat.Fig. 11.—A Magnetic Boat.If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.The needles will then arrange themselves in different symmetrical groups, according to their number.A bar magnet thrust among them will attract or repel them depending upon its polarity.The upper ends of the needles should all have the same polarity, that is, all be either north or south poles.Magnetism flows along certain lines calledLines of Magnetic Force.These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called thefield of force, and the path through which they flow is called theMagnetic Circuit.The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles.The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15.Fig. 15.—Lines of Force between Like and Unlike Poles.Fig. 15.—Lines of Force between Like and Unlike Poles.A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place.It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves.The Earth is a Great Magnet.The direction assumed by a compass needle is called themagnetic meridian.The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.Magnetic Dip.If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that thenorthend points slightly downward.Fig. 16.—A Simple Dipping Needle.Fig. 16.—A Simple Dipping Needle.This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called aDipping Needle.A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.Permanent Magnetshave a number of useful applications in the construction of scientific instruments, voltmeters, ammeters, telephone receivers, magnetos and a number of other devices.In order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately, and then riveted together. A magnet made in this way is called a compound magnet, and may have either a bar or a horse-shoe shape.Magnets are usually provided with a soft piece of iron called an armature or "keeper." The "keeper" is laid across the poles of the magnet when the latter is not in use and preserves its magnetism.A blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it. The most powerful magnet becomes absolutely demagnetized at a red heat, and remains so after cooling.Therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet, do not allow it to receive rough usage.CHAPTER II STATIC ELECTRICITY
Over two thousand years ago, in far-away Asia Minor, a shepherd guarding his flocks on the slope of Mount Ida suddenly found the iron-shod end of his staff adhering to a stone. Upon looking further around about him he found many other pieces of this peculiar hard black mineral, the smaller bits of which tended to cling to the nails and studs in the soles of his sandals.
This mineral, which was an ore of iron, consisting of iron and oxygen, was found in a district known as Magnesia, and in this way soon became widely known as the "Magnesstone," or magnet.
This is the story of the discovery of the magnet. It exists in legends in various forms. As more masses of this magnetic ore were discovered in various parts of the world, the stories of its attractive power became greatly exaggerated, especially during the Middle Ages. In fact, magnetic mountains which would pull the iron nails out of ships, or, later, move the compass needle far astray, did not lose their place among the terrors of the sea until nearly the eighteenth century.
For many hundreds of years the magnet-stone was of little use to mankind save as a curiosity which possessed the power of attracting small pieces of iron and steel and other magnets like itself. Then some one, no one knows who, discovered that if a magnet-stone were hung by a thread in a suitable manner it would always tend to point North and South; and so the "Magnes-stone" became also called the "lodestone," or "leading-stone."
These simple bits of lodestone suspended by a thread were the forerunners of the modern compass and were of great value to the ancient navigators, for they enabled them to steer ships in cloudy weather when the sun was obscured and on nights when the pole-star could not be seen.
The first realcompasseswere calledgnomons, and consisted of a steel needle which had been rubbed upon a lodestone until it acquired its magnetic properties. Then it was thrust through a reed or short piece of wood which supported it on the surface of a vessel of water. If the needle was left in this receptacle, naturally it would move against the side and not point a true position. Therefore it was given a circular movement in the water, and as soon as it came to rest, the point on the horizon which the north end designated was carefully noted and the ship’s course laid accordingly.
The modern mariners’ compass is quite a different arrangement. It consists of three parts, thebowl, thecard, and theneedle. The bowl, which contains the card and needle, is usually a hemispherical brass receptacle, suspended in a pair of brass rings, calledgimbals, in such a manner that the bowl will remain horizontal no matter how violently the ship may pitch and roll. The card, which is circular, is divided into 32 equal parts called thepoints of the compass. The needles, of which there are generally from two to four, are fastened to the bottom of the card.
Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."
Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."
In the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl, so that the card hanging on the pin turns freely around its center. On shipboard, the compass is so placed that a black mark, called thelubber’s line, is fixed in a position parallel to the keel. The point on the compass-card which is directly against this line indicates the direction of the ship’s head.
Experiments with MagnetismThe phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus.Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation.Artificial Magnetsare those made from steel by the application of a lodestone or some other magnetizing force.The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is calledMagnetization.Fig. 2.—A Bar MagnetFig. 2.—A Bar MagnetSmall horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments.Fig. 3.—A Horseshoe MagnetFig. 3.—A Horseshoe MagnetStroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet.Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.This experiment shows that the attractive power of a magnet exists intwo oppositeplaces. These are called the poles.There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space.The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is calledMagnetic Force.The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away.Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*.Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.Then try some nails in place of the tacks. The nails are heavier than the tacks, and it will require a greater force to lift them. The magnet will have to be brought much closer to the nails than to the tacks before they are lifted, showing that the force exerted by the magnet is strongest nearest to it.Magnetize a needle and lay it on a piece of cork floating in a glass vessel of water. It will then be seen that the needle always comes to rest lying nearly in a north and south line, with the same end always toward the north.Fig. 6.—A Simple Compass.Fig. 6.—A Simple Compass.The pole of the magnet which tends to turn towards the north is called thenorth-seeking poleand the opposite one is called thesouth-seeking pole.The name is usually abbreviated to simply the north and south poles. The north pole of a magnet is often indicated by a straight line or a letter N stamped into the metal.A magnetized needle floating on a cork in a basin of water is a simple form ofFig. 7.—Several Different Methods of Making a Simple Compass.Fig. 7.—Several Different Methods of Making a Simple Compass.Compass.Figure 7 shows several other different ways of making compasses. The first method is to suspend a magnetized needle from a fine silk fiber or thread.The second method illustrates a very sensitive compass made from paper. Two magnetized needles are stuck through the sides with their north poles both at the same end. The paper support is mounted upon a third needle stuck through a cork.A compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring.The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.Balance the needle on the end of a pin stuck through the bottom of a pill-box.Magnetic Substancesare those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. Ordinary magnets attract but very few substances. Iron, steel, cobalt, and nickel are about the only ones worthy of mention.Attraction through Bodies.A magnet will attract a nail or a tack through a piece of paper, just as if nothing intervened.Fig. 8.—The Attraction of an Iron Nail through Glass.Fig. 8.—The Attraction of an Iron Nail throughGlass.It will also attract through glass, wood, brass, and all other substances. Through an iron plate, however, the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail.A number of carpet-tacks may be supported from a magnet in the form of a chain. Each individual tack in the series becomes atemporarymagnet byinduction.If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart.Fig. 9.—A Magnetic Chain.Fig. 9.—A Magnetic Chain.It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if asecondmagnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks.The reason for this is that the magnetism in the tacks is increased by induction.Magnets will Attract or Repeleach other, depending upon which poles are nearest.Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.This fact gives rise to the general law of magnetism:Like poles repel each other and unlike poles attract each other.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.Fig. 11.—A Magnetic Boat.Fig. 11.—A Magnetic Boat.If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.The needles will then arrange themselves in different symmetrical groups, according to their number.A bar magnet thrust among them will attract or repel them depending upon its polarity.The upper ends of the needles should all have the same polarity, that is, all be either north or south poles.Magnetism flows along certain lines calledLines of Magnetic Force.These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called thefield of force, and the path through which they flow is called theMagnetic Circuit.The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles.The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15.Fig. 15.—Lines of Force between Like and Unlike Poles.Fig. 15.—Lines of Force between Like and Unlike Poles.A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place.It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves.The Earth is a Great Magnet.The direction assumed by a compass needle is called themagnetic meridian.The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.Magnetic Dip.If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that thenorthend points slightly downward.Fig. 16.—A Simple Dipping Needle.Fig. 16.—A Simple Dipping Needle.This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called aDipping Needle.A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.Permanent Magnetshave a number of useful applications in the construction of scientific instruments, voltmeters, ammeters, telephone receivers, magnetos and a number of other devices.In order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately, and then riveted together. A magnet made in this way is called a compound magnet, and may have either a bar or a horse-shoe shape.Magnets are usually provided with a soft piece of iron called an armature or "keeper." The "keeper" is laid across the poles of the magnet when the latter is not in use and preserves its magnetism.A blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it. The most powerful magnet becomes absolutely demagnetized at a red heat, and remains so after cooling.Therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet, do not allow it to receive rough usage.CHAPTER II STATIC ELECTRICITY
The phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus.
Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation.
Artificial Magnetsare those made from steel by the application of a lodestone or some other magnetizing force.
The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is calledMagnetization.
Fig. 2.—A Bar MagnetFig. 2.—A Bar Magnet
Fig. 2.—A Bar Magnet
Small horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments.
Fig. 3.—A Horseshoe MagnetFig. 3.—A Horseshoe Magnet
Fig. 3.—A Horseshoe Magnet
Stroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet.
Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle.
Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.
Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.
This experiment shows that the attractive power of a magnet exists intwo oppositeplaces. These are called the poles.
There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space.
The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is called
Magnetic Force.The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away.
Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*.Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.
Fig. 5.—The Lifting Power of a Bar Magnet.It must be brought closer to the nails than the tacks because they are heavier.
Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.
Then try some nails in place of the tacks. The nails are heavier than the tacks, and it will require a greater force to lift them. The magnet will have to be brought much closer to the nails than to the tacks before they are lifted, showing that the force exerted by the magnet is strongest nearest to it.
Magnetize a needle and lay it on a piece of cork floating in a glass vessel of water. It will then be seen that the needle always comes to rest lying nearly in a north and south line, with the same end always toward the north.
Fig. 6.—A Simple Compass.Fig. 6.—A Simple Compass.
Fig. 6.—A Simple Compass.
The pole of the magnet which tends to turn towards the north is called thenorth-seeking poleand the opposite one is called thesouth-seeking pole.
The name is usually abbreviated to simply the north and south poles. The north pole of a magnet is often indicated by a straight line or a letter N stamped into the metal.
A magnetized needle floating on a cork in a basin of water is a simple form of
Fig. 7.—Several Different Methods of Making a Simple Compass.Fig. 7.—Several Different Methods of Making a Simple Compass.
Fig. 7.—Several Different Methods of Making a Simple Compass.
Compass.Figure 7 shows several other different ways of making compasses. The first method is to suspend a magnetized needle from a fine silk fiber or thread.
The second method illustrates a very sensitive compass made from paper. Two magnetized needles are stuck through the sides with their north poles both at the same end. The paper support is mounted upon a third needle stuck through a cork.
A compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring.
The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.
Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.
Balance the needle on the end of a pin stuck through the bottom of a pill-box.
Magnetic Substancesare those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. Ordinary magnets attract but very few substances. Iron, steel, cobalt, and nickel are about the only ones worthy of mention.
Attraction through Bodies.A magnet will attract a nail or a tack through a piece of paper, just as if nothing intervened.
Fig. 8.—The Attraction of an Iron Nail through Glass.Fig. 8.—The Attraction of an Iron Nail throughGlass.
Fig. 8.—The Attraction of an Iron Nail throughGlass.
It will also attract through glass, wood, brass, and all other substances. Through an iron plate, however, the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail.
A number of carpet-tacks may be supported from a magnet in the form of a chain. Each individual tack in the series becomes atemporarymagnet byinduction.
If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart.
Fig. 9.—A Magnetic Chain.Fig. 9.—A Magnetic Chain.
Fig. 9.—A Magnetic Chain.
It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if asecondmagnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks.
The reason for this is that the magnetism in the tacks is increased by induction.
Magnets will Attract or Repeleach other, depending upon which poles are nearest.
Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.
This fact gives rise to the general law of magnetism:Like poles repel each other and unlike poles attract each other.
Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.
Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.
Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.
Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.
Fig. 11.—A Magnetic Boat.Fig. 11.—A Magnetic Boat.
Fig. 11.—A Magnetic Boat.
If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.
The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.
Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.
Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.
The needles will then arrange themselves in different symmetrical groups, according to their number.
A bar magnet thrust among them will attract or repel them depending upon its polarity.
The upper ends of the needles should all have the same polarity, that is, all be either north or south poles.
Magnetism flows along certain lines called
Lines of Magnetic Force.These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called thefield of force, and the path through which they flow is called the
Magnetic Circuit.The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths.
Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.
Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.
The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet.
Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.
Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.
Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles.
The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15.
Fig. 15.—Lines of Force between Like and Unlike Poles.Fig. 15.—Lines of Force between Like and Unlike Poles.
Fig. 15.—Lines of Force between Like and Unlike Poles.
A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place.
It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves.
The Earth is a Great Magnet.The direction assumed by a compass needle is called themagnetic meridian.
The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.
The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.
Magnetic Dip.If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that thenorthend points slightly downward.
Fig. 16.—A Simple Dipping Needle.Fig. 16.—A Simple Dipping Needle.
Fig. 16.—A Simple Dipping Needle.
This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.
A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called a
Dipping Needle.A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.
A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.
Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.
Permanent Magnetshave a number of useful applications in the construction of scientific instruments, voltmeters, ammeters, telephone receivers, magnetos and a number of other devices.
In order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately, and then riveted together. A magnet made in this way is called a compound magnet, and may have either a bar or a horse-shoe shape.
Magnets are usually provided with a soft piece of iron called an armature or "keeper." The "keeper" is laid across the poles of the magnet when the latter is not in use and preserves its magnetism.
A blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it. The most powerful magnet becomes absolutely demagnetized at a red heat, and remains so after cooling.
Therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet, do not allow it to receive rough usage.
CHAPTER II STATIC ELECTRICITY