CHAPTER II STATIC ELECTRICITYIf you take a glass rod and rub it with a piece of flannel or silk, it will be found to have acquired a property which it did not formerly possess: namely, the power of attracting to itself such light bodies as dust or bits of thread and paper.Hold such a rod over some small bits of paper and watch them jump up to meet it, just as if the glass rod were a magnet attracting small pieces of iron instead of paper.The agency at work to produce this mysterious power is calledelectricity, from the Greek word "Elektron," which meansamber. Amber was the first substance found to possess this property.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.The use of amber begins with the dawn of civilization. Amber beads have been found in the royal tombs at Mycenae and at various places throughout Sardinia, dating from at least two thousand years before our era.Amber was used by the ancient world as a jewel and for decoration.The ancient Syrian woman used distaffs made of amber for spinning. As the spindle whirled around it often rubbed against the spinner’s garments and thus becameelectrified, as amber always does when it is rubbed. Then on nearing the ground it drew to itself the dust or bits of chaff or leaves lying there, or sometimes perhaps attracted the fringe of the clothing.The spinner easily saw this, because the bits of chaff which were thus attracted would become entangled in her thread unless she were careful. The amber spindle was, therefore, called the "harpaga" or "clutcher," for it seemed to seize such light bodies as if it had invisible talons, which not only grasped but held on.This was probably the first intelligent observation of an electrical effect.In the eighteenth century, when Benjamin Franklin performed his famous kite experiment, electricity was believed to be a sort of fiery atmospheric discharge which could be captured in small quantities and stored in receptacles such as Leyden jars.Franklin was the first to prove that the lightning discharges taking place in the heavens are electrical.The story of his experiment is very interesting.He secured two light strips of cedar wood, placed cross-wise and covered with a silk handkerchief for a kite. To the top of the upright stick of the kite was fastened a sharp wire about a foot long. The twine was of the usual kind, but was provided with a short piece of silk ribbon and a key. The purpose of the ribbon was possible protection against the lightning running through his body, silk being a "non-conductor," as will be explained a little farther on. The key was secured to the junction of the silk ribbon and the twine, to serve as a convenient conductor from which to draw the sparks—if they came. He did not have to wait long for a thunderstorm, and as he saw it gathering he went out with his son, then a young man twenty-two years of age. The great clouds rolled up from the horizon, and the gusts of wind grew fitful and strong. The kite felt a swishing blast and began to rise steadily, swooping this way and that as the breeze caught it. The thunder muttered nearer and nearer and the rain began to patter on the grass as the kite flew higher.The rain soon began to fall heavily, compelling Franklin and his son to take refuge under a near-by shed. The heavy kite, wet with water, was sailing sluggishly when suddenly a huge low-lying black cloud traveling overhead shot forth a forked flame and the flash of thunder shook the very earth. The kite moved upward, soaring straight into the black mass, from which the flashes began to come rapidly.Franklin watched the silk ribbon and the key. There was not a sign. Had he failed? Suddenly the loose fibers of the twine erected themselves. The moment had come. Without a tremor he advanced his knuckle to the key. And between his knuckle and the key passed a spark! then another and another. They were the same kind of little sparks that he had made hundreds of times with aglass tube.And then as the storm abated and the clouds swept off towards the mountains and the kite flew lazily in the blue, the face of Franklin gleamed in the glad sunshine. The great discovery was complete, his name immortal.The cause of lightning is the accumulation of the electric charges in the clouds, the electricity residing on the surface of the particles of water in the cloud. These charges grow stronger as the particles of water join together and become larger. As the countless multitude of drops grows larger and larger the "potential" is increased, and the cloud soon becomes heavily charged.Through the effects of a phenomenon calledinduction,and which we have already stumbled against in the experiment with the tacks and the magnetic chain, the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath. These charges continually strive to burst across the intervening air.As soon as the charge grows strong enough a vivid flash of lightning, which may be from one to ten miles long, takes place. The heated air in the path of the lightning expands with great force; but immediately other air rushes in to fill the partial vacuum, thus producing the terrifying sounds calledthunder.In the eighteenth century, electricity was believed to be a sort of fiery atmospheric discharge, as has been said. Later it was discovered that it seemed to flow like water through certain mediums, and so was thought to be a fluid. Modern scientists believe it to be simply a vibratory motion, either between adjacent particles or in the ether surrounding those particles.It was early discovered that electricity would travel through some mediums but not through others. These were termed respectively "conductors" and "non-conductors" or insulators. Metals such as silver, copper, gold, and other substances like charcoal, water, etc., are good conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or insulators, while many other substances, like wood, marble, paper, cotton, etc., are partial conductors.There seems to be two kinds of electricity, one called "static" and the other "current" electricity. The former is usually produced by friction while the latter is generated by batteries or dynamos.A very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet. The body will then becomecharged, and if the knuckles are presented to some metallic object, such as a gas-jet or radiator, a stinging little spark will jump out to meet it.A Double Lightning Discharge from a Cloud to the Earth.From the author's "Wireless Telegraphy and Telephony" by permission.A Double Lightning Discharge from a Cloud to the Earth.The electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified.Warm a piece of writing-paper, then lay it on a wooden table and rub it briskly with the hand. It soon will become stuck to the table and will not slide along as it did at first. If one corner is raised slightly it will tend to jump right back. If the paper is lifted off the table it will tend to cling to the hands and the clothing. If held near the face it will produce a tickling sensation. All these things happen because the paper is electrified. It is drawn to the other objects because they areneutral, that is, do not possess an electrical charge.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.All experiments with static electricity perform better in the winter time, when it is cool and clear, than in the summer. The reason is that the air in winter is drier than in summer. Summer air contains considerable moisture and water vapor. Water vapor is apartialconductor of electricity, and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time.Fig. 20.—A Surprise for the Cat.Fig. 20.—A Surprise for the Cat.Some day during the winter time, when it is cool and clear, and the cat is near a fire or a stove, stroke the cat rapidly with the hand. The fur will stand up towards the hand and a faint crackling noise will be heard. The crackling is caused by small sparks passing between the cat and the hand. If the experiment is performed in a dark room, the sparks may be plainly seen. If you present your knuckle to the cat's nose a spark will jump to your knuckle and somewhat surprise the cat.If the day is brisk and cool, so that everything outside is frozen and dry, try combing the hair with a rubber comb. Your hair will stand up all over your head instead of lying down flat, and the faint crackling noise, showing that sparking is taking place as the comb passes through the hair, will be plainly heard. The electricity is produced by the friction between the hair and the comb.Electricity may be produced by friction between a number of substances. A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry, warm flannel.Electroscopesare devices for detecting the presence of static electricity.Fig. 21.—A Paper Electroscope.Fig. 21.—A Paper Electroscope.A very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter. It may be cut out of writing-paper and mounted on a pin stuck through a cork. If an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it.The Pith-Ball Electroscopeis a very simple device, in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support. A suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball. When the electrified rod is presented to the pith ball, it will fly out towards the rod.Fig. 22.—A Pith-Ball Electroscope.Fig. 22.—A Pith-Ball Electroscope.If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again.This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.When the rod is first presented to the pith ball, the latter is neutral and does not possess an electrical charge. When the rod has touched the ball, however, some of the electricity from the rod passes to the ball, and after this they will repel each other.The reason is that the rod and the ball aresimilarlycharged andsimilarly charged bodies will repel each other.Fig. 23.—A Double Pith-Ball Electroscope.Fig. 23.—A Double Pith-Ball Electroscope.If you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper, that some of the little papers were first attracted and flew upwards to the rod, but having once touched it, were quickly repelled.The repulsion between two similarly electrified bodies may be shown by a double electroscope.A double electroscope is made by hanging two pith balls on two silk threads from the same support.Electrify a glass rod and touch it to the pith balls. They will immediately fly apart because they are electrified with the same kind of electricity.The Gold-leaf Electroscopeis one of the most sensitive means which can be employed to detect small amounts of static electricity.Fig. 24.—A Gold-Leaf Electroscope.Fig. 24.—A Gold-Leaf Electroscope.It is a very simple instrument and is easily made in a short time. A couple of narrow strips of the thinnest tissue paper, or, better still, two strips of gold leaf, are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air.The mouth of the jar is closed by a plug of paraffin wax, through the center of which passes a small glass tube. A stiff copper wire passes through the tube. The lower end of the wire is bent at right angles to furnish support for the strips of gold leaf. A round sheet metal disk about the size of a quarter is soldered to the upper end of the rod.If an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope, the strips will repel each other very strongly. If the instrument is sensitive, the strips should begin to diverge some time before the rod reaches the disk. It is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge.There are two kinds of static electricity.Rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in Figure 25. Excite a second rod also with a piece of silk and bring it near one end of the suspended one. The suspended rod isrepelledand will swing away from the one held in the hand.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Now rub a stick ofsealing-waxwith a piece offlanneluntil the sealing-wax is electrified. Then bring the stick of sealing-wax near the end of the suspended rod. The rod will beattractedto the sealing-wax.If you experiment further you will find that two sticks of sealing-wax will repel each other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.This experiment indicates that there are two kinds of electrification: one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel.In the first instance, the glass rod is said to bepositivelyelectrified, and in the latter case the sealing-wax isnegativelyelectrified.The same law that applies to magnetism also holds true in the case of static electricity, and similarly electrified bodies will repel each other and dissimilar ones attract.The Electrophorusis an instrument devised by Volta in 1775 for the purpose of obtaining static electricity.Fig. 27.—The ElectrophorousFig. 27.—The ElectrophorousIt is easily constructed and will furnish a source of electricity for quite a number of interesting experiments. An electrophorus consists of two parts, a round cake of resinous material cast in a metal dish or pan, and a round metal disk which is provided with an insulating handle.To make an electrophorus, first procure an old cake or pie tin, and fill it with bits of resin or sealing-wax. Place the pan in a warm spot upon the stove where the resin will melt, taking care not to overheat or it will spatter and possibly take fire. As the resin melts, add more until the pan is nearly full. When all is melted, remove from the fire and set it away where it may cool and harden in the pan without being disturbed.Cut a circular disk out of sheet tin, zinc, or copper, making the diameter about two inches less than that of the pie pan. Solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle. The latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long, placed in the center of the cylinder and secured with molten sealing-wax.In order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel. Then place the disk on the cake holding the insulating handle with the right hand. Touch the cover or the disk momentarily with the forefinger of the left hand. After the finger is removed, raise the disk from the cake by picking it up with the glass insulating handle. The disk will now be found heavily charged with positive electricity, and if the knuckles are presented to the edge, a spark will jump out to meet them.Fig. 28.—An Electric Frog-Pond.Fig. 28.—An Electric Frog-Pond.The cover may then be replaced, touched, and once more removed. It will yield any number of sparks, the resinous cake only needing to be recharged by rubbing once in a long while.An Electric Frog-Pondmay be experimented with by cutting out some small tissue-paper frogs. Moisten them a little and lay them on the cover of the electrophorus. Touch the electrophorus with the finger and then raise it with the insulating handle. If the "frogs" are not too wet they will jump from the cover upon the table as soon as the cover is raised.STATIC ELECTRIC MACHINES
CHAPTER II STATIC ELECTRICITYIf you take a glass rod and rub it with a piece of flannel or silk, it will be found to have acquired a property which it did not formerly possess: namely, the power of attracting to itself such light bodies as dust or bits of thread and paper.Hold such a rod over some small bits of paper and watch them jump up to meet it, just as if the glass rod were a magnet attracting small pieces of iron instead of paper.The agency at work to produce this mysterious power is calledelectricity, from the Greek word "Elektron," which meansamber. Amber was the first substance found to possess this property.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.The use of amber begins with the dawn of civilization. Amber beads have been found in the royal tombs at Mycenae and at various places throughout Sardinia, dating from at least two thousand years before our era.Amber was used by the ancient world as a jewel and for decoration.The ancient Syrian woman used distaffs made of amber for spinning. As the spindle whirled around it often rubbed against the spinner’s garments and thus becameelectrified, as amber always does when it is rubbed. Then on nearing the ground it drew to itself the dust or bits of chaff or leaves lying there, or sometimes perhaps attracted the fringe of the clothing.The spinner easily saw this, because the bits of chaff which were thus attracted would become entangled in her thread unless she were careful. The amber spindle was, therefore, called the "harpaga" or "clutcher," for it seemed to seize such light bodies as if it had invisible talons, which not only grasped but held on.This was probably the first intelligent observation of an electrical effect.In the eighteenth century, when Benjamin Franklin performed his famous kite experiment, electricity was believed to be a sort of fiery atmospheric discharge which could be captured in small quantities and stored in receptacles such as Leyden jars.Franklin was the first to prove that the lightning discharges taking place in the heavens are electrical.The story of his experiment is very interesting.He secured two light strips of cedar wood, placed cross-wise and covered with a silk handkerchief for a kite. To the top of the upright stick of the kite was fastened a sharp wire about a foot long. The twine was of the usual kind, but was provided with a short piece of silk ribbon and a key. The purpose of the ribbon was possible protection against the lightning running through his body, silk being a "non-conductor," as will be explained a little farther on. The key was secured to the junction of the silk ribbon and the twine, to serve as a convenient conductor from which to draw the sparks—if they came. He did not have to wait long for a thunderstorm, and as he saw it gathering he went out with his son, then a young man twenty-two years of age. The great clouds rolled up from the horizon, and the gusts of wind grew fitful and strong. The kite felt a swishing blast and began to rise steadily, swooping this way and that as the breeze caught it. The thunder muttered nearer and nearer and the rain began to patter on the grass as the kite flew higher.The rain soon began to fall heavily, compelling Franklin and his son to take refuge under a near-by shed. The heavy kite, wet with water, was sailing sluggishly when suddenly a huge low-lying black cloud traveling overhead shot forth a forked flame and the flash of thunder shook the very earth. The kite moved upward, soaring straight into the black mass, from which the flashes began to come rapidly.Franklin watched the silk ribbon and the key. There was not a sign. Had he failed? Suddenly the loose fibers of the twine erected themselves. The moment had come. Without a tremor he advanced his knuckle to the key. And between his knuckle and the key passed a spark! then another and another. They were the same kind of little sparks that he had made hundreds of times with aglass tube.And then as the storm abated and the clouds swept off towards the mountains and the kite flew lazily in the blue, the face of Franklin gleamed in the glad sunshine. The great discovery was complete, his name immortal.The cause of lightning is the accumulation of the electric charges in the clouds, the electricity residing on the surface of the particles of water in the cloud. These charges grow stronger as the particles of water join together and become larger. As the countless multitude of drops grows larger and larger the "potential" is increased, and the cloud soon becomes heavily charged.Through the effects of a phenomenon calledinduction,and which we have already stumbled against in the experiment with the tacks and the magnetic chain, the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath. These charges continually strive to burst across the intervening air.As soon as the charge grows strong enough a vivid flash of lightning, which may be from one to ten miles long, takes place. The heated air in the path of the lightning expands with great force; but immediately other air rushes in to fill the partial vacuum, thus producing the terrifying sounds calledthunder.In the eighteenth century, electricity was believed to be a sort of fiery atmospheric discharge, as has been said. Later it was discovered that it seemed to flow like water through certain mediums, and so was thought to be a fluid. Modern scientists believe it to be simply a vibratory motion, either between adjacent particles or in the ether surrounding those particles.It was early discovered that electricity would travel through some mediums but not through others. These were termed respectively "conductors" and "non-conductors" or insulators. Metals such as silver, copper, gold, and other substances like charcoal, water, etc., are good conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or insulators, while many other substances, like wood, marble, paper, cotton, etc., are partial conductors.There seems to be two kinds of electricity, one called "static" and the other "current" electricity. The former is usually produced by friction while the latter is generated by batteries or dynamos.A very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet. The body will then becomecharged, and if the knuckles are presented to some metallic object, such as a gas-jet or radiator, a stinging little spark will jump out to meet it.A Double Lightning Discharge from a Cloud to the Earth.From the author's "Wireless Telegraphy and Telephony" by permission.A Double Lightning Discharge from a Cloud to the Earth.The electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified.Warm a piece of writing-paper, then lay it on a wooden table and rub it briskly with the hand. It soon will become stuck to the table and will not slide along as it did at first. If one corner is raised slightly it will tend to jump right back. If the paper is lifted off the table it will tend to cling to the hands and the clothing. If held near the face it will produce a tickling sensation. All these things happen because the paper is electrified. It is drawn to the other objects because they areneutral, that is, do not possess an electrical charge.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.All experiments with static electricity perform better in the winter time, when it is cool and clear, than in the summer. The reason is that the air in winter is drier than in summer. Summer air contains considerable moisture and water vapor. Water vapor is apartialconductor of electricity, and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time.Fig. 20.—A Surprise for the Cat.Fig. 20.—A Surprise for the Cat.Some day during the winter time, when it is cool and clear, and the cat is near a fire or a stove, stroke the cat rapidly with the hand. The fur will stand up towards the hand and a faint crackling noise will be heard. The crackling is caused by small sparks passing between the cat and the hand. If the experiment is performed in a dark room, the sparks may be plainly seen. If you present your knuckle to the cat's nose a spark will jump to your knuckle and somewhat surprise the cat.If the day is brisk and cool, so that everything outside is frozen and dry, try combing the hair with a rubber comb. Your hair will stand up all over your head instead of lying down flat, and the faint crackling noise, showing that sparking is taking place as the comb passes through the hair, will be plainly heard. The electricity is produced by the friction between the hair and the comb.Electricity may be produced by friction between a number of substances. A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry, warm flannel.Electroscopesare devices for detecting the presence of static electricity.Fig. 21.—A Paper Electroscope.Fig. 21.—A Paper Electroscope.A very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter. It may be cut out of writing-paper and mounted on a pin stuck through a cork. If an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it.The Pith-Ball Electroscopeis a very simple device, in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support. A suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball. When the electrified rod is presented to the pith ball, it will fly out towards the rod.Fig. 22.—A Pith-Ball Electroscope.Fig. 22.—A Pith-Ball Electroscope.If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again.This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.When the rod is first presented to the pith ball, the latter is neutral and does not possess an electrical charge. When the rod has touched the ball, however, some of the electricity from the rod passes to the ball, and after this they will repel each other.The reason is that the rod and the ball aresimilarlycharged andsimilarly charged bodies will repel each other.Fig. 23.—A Double Pith-Ball Electroscope.Fig. 23.—A Double Pith-Ball Electroscope.If you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper, that some of the little papers were first attracted and flew upwards to the rod, but having once touched it, were quickly repelled.The repulsion between two similarly electrified bodies may be shown by a double electroscope.A double electroscope is made by hanging two pith balls on two silk threads from the same support.Electrify a glass rod and touch it to the pith balls. They will immediately fly apart because they are electrified with the same kind of electricity.The Gold-leaf Electroscopeis one of the most sensitive means which can be employed to detect small amounts of static electricity.Fig. 24.—A Gold-Leaf Electroscope.Fig. 24.—A Gold-Leaf Electroscope.It is a very simple instrument and is easily made in a short time. A couple of narrow strips of the thinnest tissue paper, or, better still, two strips of gold leaf, are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air.The mouth of the jar is closed by a plug of paraffin wax, through the center of which passes a small glass tube. A stiff copper wire passes through the tube. The lower end of the wire is bent at right angles to furnish support for the strips of gold leaf. A round sheet metal disk about the size of a quarter is soldered to the upper end of the rod.If an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope, the strips will repel each other very strongly. If the instrument is sensitive, the strips should begin to diverge some time before the rod reaches the disk. It is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge.There are two kinds of static electricity.Rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in Figure 25. Excite a second rod also with a piece of silk and bring it near one end of the suspended one. The suspended rod isrepelledand will swing away from the one held in the hand.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Now rub a stick ofsealing-waxwith a piece offlanneluntil the sealing-wax is electrified. Then bring the stick of sealing-wax near the end of the suspended rod. The rod will beattractedto the sealing-wax.If you experiment further you will find that two sticks of sealing-wax will repel each other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.This experiment indicates that there are two kinds of electrification: one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel.In the first instance, the glass rod is said to bepositivelyelectrified, and in the latter case the sealing-wax isnegativelyelectrified.The same law that applies to magnetism also holds true in the case of static electricity, and similarly electrified bodies will repel each other and dissimilar ones attract.The Electrophorusis an instrument devised by Volta in 1775 for the purpose of obtaining static electricity.Fig. 27.—The ElectrophorousFig. 27.—The ElectrophorousIt is easily constructed and will furnish a source of electricity for quite a number of interesting experiments. An electrophorus consists of two parts, a round cake of resinous material cast in a metal dish or pan, and a round metal disk which is provided with an insulating handle.To make an electrophorus, first procure an old cake or pie tin, and fill it with bits of resin or sealing-wax. Place the pan in a warm spot upon the stove where the resin will melt, taking care not to overheat or it will spatter and possibly take fire. As the resin melts, add more until the pan is nearly full. When all is melted, remove from the fire and set it away where it may cool and harden in the pan without being disturbed.Cut a circular disk out of sheet tin, zinc, or copper, making the diameter about two inches less than that of the pie pan. Solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle. The latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long, placed in the center of the cylinder and secured with molten sealing-wax.In order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel. Then place the disk on the cake holding the insulating handle with the right hand. Touch the cover or the disk momentarily with the forefinger of the left hand. After the finger is removed, raise the disk from the cake by picking it up with the glass insulating handle. The disk will now be found heavily charged with positive electricity, and if the knuckles are presented to the edge, a spark will jump out to meet them.Fig. 28.—An Electric Frog-Pond.Fig. 28.—An Electric Frog-Pond.The cover may then be replaced, touched, and once more removed. It will yield any number of sparks, the resinous cake only needing to be recharged by rubbing once in a long while.An Electric Frog-Pondmay be experimented with by cutting out some small tissue-paper frogs. Moisten them a little and lay them on the cover of the electrophorus. Touch the electrophorus with the finger and then raise it with the insulating handle. If the "frogs" are not too wet they will jump from the cover upon the table as soon as the cover is raised.STATIC ELECTRIC MACHINES
CHAPTER II STATIC ELECTRICITYIf you take a glass rod and rub it with a piece of flannel or silk, it will be found to have acquired a property which it did not formerly possess: namely, the power of attracting to itself such light bodies as dust or bits of thread and paper.Hold such a rod over some small bits of paper and watch them jump up to meet it, just as if the glass rod were a magnet attracting small pieces of iron instead of paper.The agency at work to produce this mysterious power is calledelectricity, from the Greek word "Elektron," which meansamber. Amber was the first substance found to possess this property.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.The use of amber begins with the dawn of civilization. Amber beads have been found in the royal tombs at Mycenae and at various places throughout Sardinia, dating from at least two thousand years before our era.Amber was used by the ancient world as a jewel and for decoration.The ancient Syrian woman used distaffs made of amber for spinning. As the spindle whirled around it often rubbed against the spinner’s garments and thus becameelectrified, as amber always does when it is rubbed. Then on nearing the ground it drew to itself the dust or bits of chaff or leaves lying there, or sometimes perhaps attracted the fringe of the clothing.The spinner easily saw this, because the bits of chaff which were thus attracted would become entangled in her thread unless she were careful. The amber spindle was, therefore, called the "harpaga" or "clutcher," for it seemed to seize such light bodies as if it had invisible talons, which not only grasped but held on.This was probably the first intelligent observation of an electrical effect.In the eighteenth century, when Benjamin Franklin performed his famous kite experiment, electricity was believed to be a sort of fiery atmospheric discharge which could be captured in small quantities and stored in receptacles such as Leyden jars.Franklin was the first to prove that the lightning discharges taking place in the heavens are electrical.The story of his experiment is very interesting.He secured two light strips of cedar wood, placed cross-wise and covered with a silk handkerchief for a kite. To the top of the upright stick of the kite was fastened a sharp wire about a foot long. The twine was of the usual kind, but was provided with a short piece of silk ribbon and a key. The purpose of the ribbon was possible protection against the lightning running through his body, silk being a "non-conductor," as will be explained a little farther on. The key was secured to the junction of the silk ribbon and the twine, to serve as a convenient conductor from which to draw the sparks—if they came. He did not have to wait long for a thunderstorm, and as he saw it gathering he went out with his son, then a young man twenty-two years of age. The great clouds rolled up from the horizon, and the gusts of wind grew fitful and strong. The kite felt a swishing blast and began to rise steadily, swooping this way and that as the breeze caught it. The thunder muttered nearer and nearer and the rain began to patter on the grass as the kite flew higher.The rain soon began to fall heavily, compelling Franklin and his son to take refuge under a near-by shed. The heavy kite, wet with water, was sailing sluggishly when suddenly a huge low-lying black cloud traveling overhead shot forth a forked flame and the flash of thunder shook the very earth. The kite moved upward, soaring straight into the black mass, from which the flashes began to come rapidly.Franklin watched the silk ribbon and the key. There was not a sign. Had he failed? Suddenly the loose fibers of the twine erected themselves. The moment had come. Without a tremor he advanced his knuckle to the key. And between his knuckle and the key passed a spark! then another and another. They were the same kind of little sparks that he had made hundreds of times with aglass tube.And then as the storm abated and the clouds swept off towards the mountains and the kite flew lazily in the blue, the face of Franklin gleamed in the glad sunshine. The great discovery was complete, his name immortal.The cause of lightning is the accumulation of the electric charges in the clouds, the electricity residing on the surface of the particles of water in the cloud. These charges grow stronger as the particles of water join together and become larger. As the countless multitude of drops grows larger and larger the "potential" is increased, and the cloud soon becomes heavily charged.Through the effects of a phenomenon calledinduction,and which we have already stumbled against in the experiment with the tacks and the magnetic chain, the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath. These charges continually strive to burst across the intervening air.As soon as the charge grows strong enough a vivid flash of lightning, which may be from one to ten miles long, takes place. The heated air in the path of the lightning expands with great force; but immediately other air rushes in to fill the partial vacuum, thus producing the terrifying sounds calledthunder.In the eighteenth century, electricity was believed to be a sort of fiery atmospheric discharge, as has been said. Later it was discovered that it seemed to flow like water through certain mediums, and so was thought to be a fluid. Modern scientists believe it to be simply a vibratory motion, either between adjacent particles or in the ether surrounding those particles.It was early discovered that electricity would travel through some mediums but not through others. These were termed respectively "conductors" and "non-conductors" or insulators. Metals such as silver, copper, gold, and other substances like charcoal, water, etc., are good conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or insulators, while many other substances, like wood, marble, paper, cotton, etc., are partial conductors.There seems to be two kinds of electricity, one called "static" and the other "current" electricity. The former is usually produced by friction while the latter is generated by batteries or dynamos.A very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet. The body will then becomecharged, and if the knuckles are presented to some metallic object, such as a gas-jet or radiator, a stinging little spark will jump out to meet it.A Double Lightning Discharge from a Cloud to the Earth.From the author's "Wireless Telegraphy and Telephony" by permission.A Double Lightning Discharge from a Cloud to the Earth.The electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified.Warm a piece of writing-paper, then lay it on a wooden table and rub it briskly with the hand. It soon will become stuck to the table and will not slide along as it did at first. If one corner is raised slightly it will tend to jump right back. If the paper is lifted off the table it will tend to cling to the hands and the clothing. If held near the face it will produce a tickling sensation. All these things happen because the paper is electrified. It is drawn to the other objects because they areneutral, that is, do not possess an electrical charge.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.All experiments with static electricity perform better in the winter time, when it is cool and clear, than in the summer. The reason is that the air in winter is drier than in summer. Summer air contains considerable moisture and water vapor. Water vapor is apartialconductor of electricity, and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time.Fig. 20.—A Surprise for the Cat.Fig. 20.—A Surprise for the Cat.Some day during the winter time, when it is cool and clear, and the cat is near a fire or a stove, stroke the cat rapidly with the hand. The fur will stand up towards the hand and a faint crackling noise will be heard. The crackling is caused by small sparks passing between the cat and the hand. If the experiment is performed in a dark room, the sparks may be plainly seen. If you present your knuckle to the cat's nose a spark will jump to your knuckle and somewhat surprise the cat.If the day is brisk and cool, so that everything outside is frozen and dry, try combing the hair with a rubber comb. Your hair will stand up all over your head instead of lying down flat, and the faint crackling noise, showing that sparking is taking place as the comb passes through the hair, will be plainly heard. The electricity is produced by the friction between the hair and the comb.Electricity may be produced by friction between a number of substances. A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry, warm flannel.Electroscopesare devices for detecting the presence of static electricity.Fig. 21.—A Paper Electroscope.Fig. 21.—A Paper Electroscope.A very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter. It may be cut out of writing-paper and mounted on a pin stuck through a cork. If an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it.The Pith-Ball Electroscopeis a very simple device, in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support. A suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball. When the electrified rod is presented to the pith ball, it will fly out towards the rod.Fig. 22.—A Pith-Ball Electroscope.Fig. 22.—A Pith-Ball Electroscope.If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again.This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.When the rod is first presented to the pith ball, the latter is neutral and does not possess an electrical charge. When the rod has touched the ball, however, some of the electricity from the rod passes to the ball, and after this they will repel each other.The reason is that the rod and the ball aresimilarlycharged andsimilarly charged bodies will repel each other.Fig. 23.—A Double Pith-Ball Electroscope.Fig. 23.—A Double Pith-Ball Electroscope.If you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper, that some of the little papers were first attracted and flew upwards to the rod, but having once touched it, were quickly repelled.The repulsion between two similarly electrified bodies may be shown by a double electroscope.A double electroscope is made by hanging two pith balls on two silk threads from the same support.Electrify a glass rod and touch it to the pith balls. They will immediately fly apart because they are electrified with the same kind of electricity.The Gold-leaf Electroscopeis one of the most sensitive means which can be employed to detect small amounts of static electricity.Fig. 24.—A Gold-Leaf Electroscope.Fig. 24.—A Gold-Leaf Electroscope.It is a very simple instrument and is easily made in a short time. A couple of narrow strips of the thinnest tissue paper, or, better still, two strips of gold leaf, are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air.The mouth of the jar is closed by a plug of paraffin wax, through the center of which passes a small glass tube. A stiff copper wire passes through the tube. The lower end of the wire is bent at right angles to furnish support for the strips of gold leaf. A round sheet metal disk about the size of a quarter is soldered to the upper end of the rod.If an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope, the strips will repel each other very strongly. If the instrument is sensitive, the strips should begin to diverge some time before the rod reaches the disk. It is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge.There are two kinds of static electricity.Rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in Figure 25. Excite a second rod also with a piece of silk and bring it near one end of the suspended one. The suspended rod isrepelledand will swing away from the one held in the hand.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Now rub a stick ofsealing-waxwith a piece offlanneluntil the sealing-wax is electrified. Then bring the stick of sealing-wax near the end of the suspended rod. The rod will beattractedto the sealing-wax.If you experiment further you will find that two sticks of sealing-wax will repel each other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.This experiment indicates that there are two kinds of electrification: one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel.In the first instance, the glass rod is said to bepositivelyelectrified, and in the latter case the sealing-wax isnegativelyelectrified.The same law that applies to magnetism also holds true in the case of static electricity, and similarly electrified bodies will repel each other and dissimilar ones attract.The Electrophorusis an instrument devised by Volta in 1775 for the purpose of obtaining static electricity.Fig. 27.—The ElectrophorousFig. 27.—The ElectrophorousIt is easily constructed and will furnish a source of electricity for quite a number of interesting experiments. An electrophorus consists of two parts, a round cake of resinous material cast in a metal dish or pan, and a round metal disk which is provided with an insulating handle.To make an electrophorus, first procure an old cake or pie tin, and fill it with bits of resin or sealing-wax. Place the pan in a warm spot upon the stove where the resin will melt, taking care not to overheat or it will spatter and possibly take fire. As the resin melts, add more until the pan is nearly full. When all is melted, remove from the fire and set it away where it may cool and harden in the pan without being disturbed.Cut a circular disk out of sheet tin, zinc, or copper, making the diameter about two inches less than that of the pie pan. Solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle. The latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long, placed in the center of the cylinder and secured with molten sealing-wax.In order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel. Then place the disk on the cake holding the insulating handle with the right hand. Touch the cover or the disk momentarily with the forefinger of the left hand. After the finger is removed, raise the disk from the cake by picking it up with the glass insulating handle. The disk will now be found heavily charged with positive electricity, and if the knuckles are presented to the edge, a spark will jump out to meet them.Fig. 28.—An Electric Frog-Pond.Fig. 28.—An Electric Frog-Pond.The cover may then be replaced, touched, and once more removed. It will yield any number of sparks, the resinous cake only needing to be recharged by rubbing once in a long while.An Electric Frog-Pondmay be experimented with by cutting out some small tissue-paper frogs. Moisten them a little and lay them on the cover of the electrophorus. Touch the electrophorus with the finger and then raise it with the insulating handle. If the "frogs" are not too wet they will jump from the cover upon the table as soon as the cover is raised.STATIC ELECTRIC MACHINES
If you take a glass rod and rub it with a piece of flannel or silk, it will be found to have acquired a property which it did not formerly possess: namely, the power of attracting to itself such light bodies as dust or bits of thread and paper.
Hold such a rod over some small bits of paper and watch them jump up to meet it, just as if the glass rod were a magnet attracting small pieces of iron instead of paper.
The agency at work to produce this mysterious power is calledelectricity, from the Greek word "Elektron," which meansamber. Amber was the first substance found to possess this property.
Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.
Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.
The use of amber begins with the dawn of civilization. Amber beads have been found in the royal tombs at Mycenae and at various places throughout Sardinia, dating from at least two thousand years before our era.
Amber was used by the ancient world as a jewel and for decoration.
The ancient Syrian woman used distaffs made of amber for spinning. As the spindle whirled around it often rubbed against the spinner’s garments and thus becameelectrified, as amber always does when it is rubbed. Then on nearing the ground it drew to itself the dust or bits of chaff or leaves lying there, or sometimes perhaps attracted the fringe of the clothing.
The spinner easily saw this, because the bits of chaff which were thus attracted would become entangled in her thread unless she were careful. The amber spindle was, therefore, called the "harpaga" or "clutcher," for it seemed to seize such light bodies as if it had invisible talons, which not only grasped but held on.
This was probably the first intelligent observation of an electrical effect.
In the eighteenth century, when Benjamin Franklin performed his famous kite experiment, electricity was believed to be a sort of fiery atmospheric discharge which could be captured in small quantities and stored in receptacles such as Leyden jars.
Franklin was the first to prove that the lightning discharges taking place in the heavens are electrical.
The story of his experiment is very interesting.
He secured two light strips of cedar wood, placed cross-wise and covered with a silk handkerchief for a kite. To the top of the upright stick of the kite was fastened a sharp wire about a foot long. The twine was of the usual kind, but was provided with a short piece of silk ribbon and a key. The purpose of the ribbon was possible protection against the lightning running through his body, silk being a "non-conductor," as will be explained a little farther on. The key was secured to the junction of the silk ribbon and the twine, to serve as a convenient conductor from which to draw the sparks—if they came. He did not have to wait long for a thunderstorm, and as he saw it gathering he went out with his son, then a young man twenty-two years of age. The great clouds rolled up from the horizon, and the gusts of wind grew fitful and strong. The kite felt a swishing blast and began to rise steadily, swooping this way and that as the breeze caught it. The thunder muttered nearer and nearer and the rain began to patter on the grass as the kite flew higher.
The rain soon began to fall heavily, compelling Franklin and his son to take refuge under a near-by shed. The heavy kite, wet with water, was sailing sluggishly when suddenly a huge low-lying black cloud traveling overhead shot forth a forked flame and the flash of thunder shook the very earth. The kite moved upward, soaring straight into the black mass, from which the flashes began to come rapidly.
Franklin watched the silk ribbon and the key. There was not a sign. Had he failed? Suddenly the loose fibers of the twine erected themselves. The moment had come. Without a tremor he advanced his knuckle to the key. And between his knuckle and the key passed a spark! then another and another. They were the same kind of little sparks that he had made hundreds of times with aglass tube.
And then as the storm abated and the clouds swept off towards the mountains and the kite flew lazily in the blue, the face of Franklin gleamed in the glad sunshine. The great discovery was complete, his name immortal.
The cause of lightning is the accumulation of the electric charges in the clouds, the electricity residing on the surface of the particles of water in the cloud. These charges grow stronger as the particles of water join together and become larger. As the countless multitude of drops grows larger and larger the "potential" is increased, and the cloud soon becomes heavily charged.
Through the effects of a phenomenon calledinduction,and which we have already stumbled against in the experiment with the tacks and the magnetic chain, the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath. These charges continually strive to burst across the intervening air.
As soon as the charge grows strong enough a vivid flash of lightning, which may be from one to ten miles long, takes place. The heated air in the path of the lightning expands with great force; but immediately other air rushes in to fill the partial vacuum, thus producing the terrifying sounds calledthunder.
In the eighteenth century, electricity was believed to be a sort of fiery atmospheric discharge, as has been said. Later it was discovered that it seemed to flow like water through certain mediums, and so was thought to be a fluid. Modern scientists believe it to be simply a vibratory motion, either between adjacent particles or in the ether surrounding those particles.
It was early discovered that electricity would travel through some mediums but not through others. These were termed respectively "conductors" and "non-conductors" or insulators. Metals such as silver, copper, gold, and other substances like charcoal, water, etc., are good conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or insulators, while many other substances, like wood, marble, paper, cotton, etc., are partial conductors.
There seems to be two kinds of electricity, one called "static" and the other "current" electricity. The former is usually produced by friction while the latter is generated by batteries or dynamos.
A very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet. The body will then becomecharged, and if the knuckles are presented to some metallic object, such as a gas-jet or radiator, a stinging little spark will jump out to meet it.
A Double Lightning Discharge from a Cloud to the Earth.From the author's "Wireless Telegraphy and Telephony" by permission.A Double Lightning Discharge from a Cloud to the Earth.
From the author's "Wireless Telegraphy and Telephony" by permission.A Double Lightning Discharge from a Cloud to the Earth.
The electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified.
Warm a piece of writing-paper, then lay it on a wooden table and rub it briskly with the hand. It soon will become stuck to the table and will not slide along as it did at first. If one corner is raised slightly it will tend to jump right back. If the paper is lifted off the table it will tend to cling to the hands and the clothing. If held near the face it will produce a tickling sensation. All these things happen because the paper is electrified. It is drawn to the other objects because they areneutral, that is, do not possess an electrical charge.
Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.
Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.
All experiments with static electricity perform better in the winter time, when it is cool and clear, than in the summer. The reason is that the air in winter is drier than in summer. Summer air contains considerable moisture and water vapor. Water vapor is apartialconductor of electricity, and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time.
Fig. 20.—A Surprise for the Cat.Fig. 20.—A Surprise for the Cat.
Fig. 20.—A Surprise for the Cat.
Some day during the winter time, when it is cool and clear, and the cat is near a fire or a stove, stroke the cat rapidly with the hand. The fur will stand up towards the hand and a faint crackling noise will be heard. The crackling is caused by small sparks passing between the cat and the hand. If the experiment is performed in a dark room, the sparks may be plainly seen. If you present your knuckle to the cat's nose a spark will jump to your knuckle and somewhat surprise the cat.
If the day is brisk and cool, so that everything outside is frozen and dry, try combing the hair with a rubber comb. Your hair will stand up all over your head instead of lying down flat, and the faint crackling noise, showing that sparking is taking place as the comb passes through the hair, will be plainly heard. The electricity is produced by the friction between the hair and the comb.
Electricity may be produced by friction between a number of substances. A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry, warm flannel.
Electroscopesare devices for detecting the presence of static electricity.
Fig. 21.—A Paper Electroscope.Fig. 21.—A Paper Electroscope.
Fig. 21.—A Paper Electroscope.
A very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter. It may be cut out of writing-paper and mounted on a pin stuck through a cork. If an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it.
The Pith-Ball Electroscopeis a very simple device, in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support. A suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball. When the electrified rod is presented to the pith ball, it will fly out towards the rod.
Fig. 22.—A Pith-Ball Electroscope.Fig. 22.—A Pith-Ball Electroscope.
Fig. 22.—A Pith-Ball Electroscope.
If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again.
This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.
When the rod is first presented to the pith ball, the latter is neutral and does not possess an electrical charge. When the rod has touched the ball, however, some of the electricity from the rod passes to the ball, and after this they will repel each other.
The reason is that the rod and the ball aresimilarlycharged andsimilarly charged bodies will repel each other.
Fig. 23.—A Double Pith-Ball Electroscope.Fig. 23.—A Double Pith-Ball Electroscope.
Fig. 23.—A Double Pith-Ball Electroscope.
If you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper, that some of the little papers were first attracted and flew upwards to the rod, but having once touched it, were quickly repelled.
The repulsion between two similarly electrified bodies may be shown by a double electroscope.
A double electroscope is made by hanging two pith balls on two silk threads from the same support.
Electrify a glass rod and touch it to the pith balls. They will immediately fly apart because they are electrified with the same kind of electricity.
The Gold-leaf Electroscopeis one of the most sensitive means which can be employed to detect small amounts of static electricity.
Fig. 24.—A Gold-Leaf Electroscope.Fig. 24.—A Gold-Leaf Electroscope.
Fig. 24.—A Gold-Leaf Electroscope.
It is a very simple instrument and is easily made in a short time. A couple of narrow strips of the thinnest tissue paper, or, better still, two strips of gold leaf, are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air.
The mouth of the jar is closed by a plug of paraffin wax, through the center of which passes a small glass tube. A stiff copper wire passes through the tube. The lower end of the wire is bent at right angles to furnish support for the strips of gold leaf. A round sheet metal disk about the size of a quarter is soldered to the upper end of the rod.
If an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope, the strips will repel each other very strongly. If the instrument is sensitive, the strips should begin to diverge some time before the rod reaches the disk. It is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge.
There are two kinds of static electricity.Rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in Figure 25. Excite a second rod also with a piece of silk and bring it near one end of the suspended one. The suspended rod isrepelledand will swing away from the one held in the hand.
Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.
Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.
Now rub a stick ofsealing-waxwith a piece offlanneluntil the sealing-wax is electrified. Then bring the stick of sealing-wax near the end of the suspended rod. The rod will beattractedto the sealing-wax.
If you experiment further you will find that two sticks of sealing-wax will repel each other.
Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.
Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.
This experiment indicates that there are two kinds of electrification: one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel.
In the first instance, the glass rod is said to bepositivelyelectrified, and in the latter case the sealing-wax isnegativelyelectrified.
The same law that applies to magnetism also holds true in the case of static electricity, and similarly electrified bodies will repel each other and dissimilar ones attract.
The Electrophorusis an instrument devised by Volta in 1775 for the purpose of obtaining static electricity.
Fig. 27.—The ElectrophorousFig. 27.—The Electrophorous
Fig. 27.—The Electrophorous
It is easily constructed and will furnish a source of electricity for quite a number of interesting experiments. An electrophorus consists of two parts, a round cake of resinous material cast in a metal dish or pan, and a round metal disk which is provided with an insulating handle.
To make an electrophorus, first procure an old cake or pie tin, and fill it with bits of resin or sealing-wax. Place the pan in a warm spot upon the stove where the resin will melt, taking care not to overheat or it will spatter and possibly take fire. As the resin melts, add more until the pan is nearly full. When all is melted, remove from the fire and set it away where it may cool and harden in the pan without being disturbed.
Cut a circular disk out of sheet tin, zinc, or copper, making the diameter about two inches less than that of the pie pan. Solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle. The latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long, placed in the center of the cylinder and secured with molten sealing-wax.
In order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel. Then place the disk on the cake holding the insulating handle with the right hand. Touch the cover or the disk momentarily with the forefinger of the left hand. After the finger is removed, raise the disk from the cake by picking it up with the glass insulating handle. The disk will now be found heavily charged with positive electricity, and if the knuckles are presented to the edge, a spark will jump out to meet them.
Fig. 28.—An Electric Frog-Pond.Fig. 28.—An Electric Frog-Pond.
Fig. 28.—An Electric Frog-Pond.
The cover may then be replaced, touched, and once more removed. It will yield any number of sparks, the resinous cake only needing to be recharged by rubbing once in a long while.
An Electric Frog-Pondmay be experimented with by cutting out some small tissue-paper frogs. Moisten them a little and lay them on the cover of the electrophorus. Touch the electrophorus with the finger and then raise it with the insulating handle. If the "frogs" are not too wet they will jump from the cover upon the table as soon as the cover is raised.
STATIC ELECTRIC MACHINES