Part 37

A low voltage of current is then passed through the coil, a sufficient length of time, to thoroughly dry out and bake the coating. This renders the magnet absolutely fireproof, eliminating all danger of short circuiting of the coil.

When finished it is well taped to protect the outside wire from becoming chafed.

The coil is made slightly smaller than the inside dimensions of the shell and the remaining space is filled with an impregnating compound, which hardens to the consistency of pitch.

This renders the coil thoroughly waterproof; also forms a cushion to prevent injury from the severe jars and shocks, received when dropping a magnet on its load.

_A Controller._--The rapidity with which it is necessary to turn current on and off while operating a magnet, creates what is called a “back kick.” Unless this is dissipated quickly it is very destructive to the coil.

A special controller dissipates this back kick through a set of resistance coils placed in the controller. By means of an automatic arrangement, connection with these coils is made instantly upon breaking the current between the magnet and generator.

A system of control used prevents undue heating of the coil. This enables the magnet to lift as large a load after a long steady run as at the start.

What Is a Lodestone?

A lodestone is a variety of the mineral named magnetite which is a natural magnet. The name magnet comes from the name of the mineral magnetite and this in turn derived its name from the fact that it was first discovered in Magnesia. The word magnet really means the “Stone of Magnesia.”

A lodestone is one of the mysteries of nature. Its properties can more nearly be understood if we examine an artificial magnet, which is generally made in the form of either a straight bar or a shoe. An artificial magnet is made of iron. If you drop a bar magnet into a box of iron filings, the filings attach themselves to the bar. If you examine it closely you observe that most of the filings attach themselves to the ends of the bar. Therefore we call the ends of the bar the poles of the magnet.

If you suspend a magnetic needle at its center of gravity so that it is absolutely free to turn, you will soon find one end of the needle pointing north and the other south of course. The end which is pointed toward the north is called the north pole and the other the south pole. If you have a horse-shoe magnet, you can demonstrate this for yourself. Rub the end of your magnet over a sewing needle and oil the needle so that when you lay it on the surface of a glass of water it will float. Then look at it closely. You will see the needle slowly turn until finally it becomes quite still. If you have a compass at hand so that you know surely which is north and which is south, you will find one end of the needle pointing north and the other south. You can then place the end of your magnet against the outside of the glass and draw the needle toward your magnet. Your horse-shoe magnet has its north and south poles close together.

If you have a bar magnet and the end of the needle with the eye in it is pointing north, you can drive the needle on the surface of the water away from you by touching the outside of the glass opposite that end of the needle with the north pole of your magnet. On the other hand, if you reverse the experiment and place the south pole of your magnet to the side of the glass, the needle will come toward the magnet. In other words then the like poles of a magnet repel each other and the unlike poles attract each other.

Another interesting way to show this is to take two lodestones or two magnets and let a lot of iron filings attach themselves to the ends of them. Then when you have done this, point the two north poles of the magnets or lodestones at each other close together. You will be intensely interested in seeing how quickly the mysterious something that is in the magnets makes the filings on the two ends of the magnet try to get away from each other. On the other hand when you put a north and south pole together, they form a union of the iron filings.

Another strange thing about a magnet is that if you break it in two, each half will be a complete magnet in itself with a north and south pole also, and this is true no matter how many times you break it into pieces. From this we learn that each tiny particle or molecule throughout the bar is a magnet by itself.

[Illustration: WHAT A LODESTONE IS

This is a picture of a complete electro magnet. The magnet is attached to the arm of a crane by the loop in the center and when the magnet then comes in contact with any kind of iron or steel it lifts it as soon as the current is turned on. By making the electric current stronger, greater weight can be lifted. Many tons of material can be lifted at one time. An electro magnet will do the work of many men at much less cost.]

[Illustration: In this picture we see the magnet lifting a great weight of miscellaneous pieces of scrap iron. As many as twenty tons can be lifted and transferred from one place to another at one time.]

Some things can be magnetized while others cannot. Many substances have not the property of magnetizing other substances when they have once been attracted by a magnet. These are called magnetic substances. They remain magnetized only as long as they are in touch with the magnet; other substances when once magnetized become permanent magnets. Steel and lodestone have this faculty. A compass needle is an artificial magnet which becomes a permanent magnet when rubbed with a magnet.

What Is Electricity?

If you pass a hard rubber comb through your hair, in frosty weather, a crackling sound is produced, and the individual hairs show a tendency to stick to the comb. After being drawn through your hair a few times, you may notice that the comb has become charged with electricity. This electricity is produced by friction. Not only rubber but many other substances become electrified by friction, such as a bar of sealing wax rubbed with flannel, or a glass rod rubbed with silk, will show the same qualities, and these simple experiments teach us many of the fundamental facts about electricity.

Some simple experiments will be found instructive and interesting. Rub with flannel a stick of sealing wax until it is electrified and then bring it close to a pith ball which should be hung by a silk thread. The pith ball will at once be attracted to the sealing wax, and, if brought quite close, the ball will adhere to the wax for a few moments, and then fly away from it. The ball will now be repelled by the sealing wax instead of being drawn toward it. Now take a glass rod, rub it with a silk cloth after drying it thoroughly. When the pith ball is brought close to the glass rod it also will at first be attracted toward the glass and, if brought in contact with the glass, the pith ball will adhere as before. It will also then fly away in the same way it did from the sealing wax. Repeat these experiments with the sealing wax now and you will find the ball will be attached, as it was at first, but if it touches the wax it will again adhere for a moment and then fly away. By using the sealing wax and glass rod alternately and bringing them into contact with the pith ball, you discover that when it is attracted by one, it is repelled by the other, and that, after it has been in contact with either for a few moments it is no longer attracted by it.

We learn thus that the electricity in the glass and the sealing wax are not the same. To distinguish the two kinds of attraction, we say the glass is charged with positive, or vitreous electricity, while the charge on the sealing wax is called negative, or resinous electricity.

When the pith ball was touched with the sealing wax, it became filled with negative electricity, and was then no longer attracted by the wax, but was repelled by it and attracted by the glass rod; but when the ball had been filled with positive electricity, it was repelled by the glass and attracted by the wax. We conclude from these facts that bodies filled with the same kind of electricity repel each other, while bodies filled with opposite kinds of electricity attract each other.

When two substances are charged, as we say, with electricity of opposite kinds and are brought into contact, and left so for some time, the two charges disappear, one appearing to neutralize the other. From this, we conclude, and rightly, that any substance not electrified, contains equal amounts both positive and negative electricity. When, therefore, we rub a piece of glass with silk, we are not creating electricity, but only separating the different kinds. The positive electricity adheres to the glass, and the negative remains behind, on the silk. In the same manner, when we electrify sealing wax with flannel the negative kind remains in the sealing wax and the flannel becomes charged with the positive. Whenever a body is electrified by friction, both kinds of electricity are produced; it is impossible to produce one kind without the other.

[Illustration: WHAT ELECTRICITY IS

Magnets are particularly valuable in lifting raw material in a steel mill. The red-hot pig-iron, from which steel is made, can be handled easily in this way, whereas it would be impossible to handle same by hand. Sometimes great quantities of iron are broken up by the magnet. A weight of many tons is lifted by the magnet and allowed to fall on the material to be broken up. The weight falls as soon as the current is turned off.]

[Illustration:

Weight of wheel, 8160 lbs.

Pieces of machinery which cannot be lifted by men on account of their great weight and shape are handled easily.]

You must rub the entire glass rod or bar of sealing wax to electrify the whole of it. If only a part of the glass rod or sealing wax is rubbed, only that part becomes electrified, as may be shown by trying to attract a pith ball with the part that has not been rubbed.

~WHAT GOOD AND BAD CONDUCTORS OF ELECTRICITY ARE~

If, however, the charged part of the sealing wax is brought into contact with a metal rod resting on, say, a drinking glass, the rod becomes charged, not only where it is brought into contact, but all over its surface. Substances over which electricity flows readily are called conductors of electricity. All metals are of this kind. Things like glass and sealing wax over which electricity does not flow readily, are called non-conductors, or insulators. Water, the human body, and the earth are good conductors and rubber, porcelain, most resins, and dry air are non-conductors.

You have already learned that substances charged with opposite kinds of electricity attract each other, and substances charged with the same kind repel each other. We will try to discover why substances charged with either kind of electricity attract small light objects, such as pith balls, when these latter are not charged with electricity. As we have discovered, all substances which have remained undisturbed have both kinds of electricity present in them, in equal amounts. Now, when an uncharged body is brought near a charged body, the two kinds of electricity in the uncharged body have a tendency to separate. The kind opposite in character, to that on the charged body, is attracted toward the charged body, and the other kind is repelled. Thus, if our bar of sealing wax, charged with, let us say, negative electricity, is brought near a pith ball, the positive electricity in the ball is attracted to the side nearest the scaling wax, and the negative electricity is repelled to the farther side. As the positive electricity on the pith is nearer to the scaling wax than the negative, its attraction for the negative charge, on the sealing wax, is stronger than the repulsion between the negative electricities of the two objects, and consequently, the ball is attracted to the sealing wax. If the charged sealing wax is brought near a good conductor, which is supported on some non-conducting substance, such as glass, silk, or rubber, over which electricity will not flow, a much more complete separation of the two kinds of electricity occurs on the conductor than on the pith ball. If the charged sealing wax is brought near one end of a metal rod so placed, the charge of negative electricity upon the sealing wax will attract the positive electricity on the metal, to that end, and will repel the negative electricity to the other end. When a pith ball, hung by the silk thread, is brought close to either end of the metal rod, when the charged sealing wax is near the other end, the pith ball will be attracted toward the rod; but will not be attracted if placed close to the middle of the rod. This proves that the metal rod is electrified only in the parts nearest to and farthest away from the charged body. The two kinds of electricity neutralize each other at the parts in between.

If now we take two conductors and place them end to end, we have for all practical purposes, a single conductor. It has the decided advantage, however, of being easily separated into two parts. When an electrified substance is brought close to one end of such a conductor, a charge of one kind is attracted to the near portion of the conductor, and a charge of the opposite kind is repelled to the farther part. By separating the two parts of the conductor, we learn that one of the ends, which have been in contact, is charged with positive and the other with negative electricity.

This act of separating the two kinds of electricity upon a conductor by means of a charge upon another body which is not permitted to come into contact with the conductor, is called induction, and two charges of electricity produced in this way are known as induced charges.

There are other ways in which a charge of electricity may be induced upon a conductor. One end of the conductor may be connected with the earth by means of some good conducting material, and the charged substance brought close to the other end. A charge, opposite in character to the initial charge, is attracted to the end of the conductor that is near the charged body, and the electricity of the opposite kind is repelled, through the conductor to the earth. By securing the connection with the earth, while the charged body is near the conductor, a charge is obtained upon the conductor, that is opposite in character to the initial charge. This method of charging conductors, by induction, is practically the same as the one first described, for the earth is a conductor of electricity, and corresponds to the more distant part of the two-piece conductor.

An instrument, known as the electrophorus, is especially designed for the production of electric charges by induction in the manner just described. This instrument consists of a brass plate, on an insulating handle of glass, and a disk of sealing wax, fitted into a brass dish, whose edges rise somewhat higher than the surface of the wax. In using the electrophorus the brass dish, or sole, is placed upon some support that will conduct electricity, and the sealing wax disk is then rubbed vigorously with a piece of flannel, or catskin, which electrifies the sealing wax, with negative electricity. The brass plate is then taken by the glass handle and brought close to the charged sealing wax. The charge of negative electricity on the wax attracts a charge of positive electricity to the under surface of the plate and repels a negative charge to its upper surface. If the charged plate is now brought into contact with the edge of the brass dish the negative charge, on the back of the plate, flows away, through the legs of the dish, to the earth, but the positive charge remains on the under surface, where it is bound, by the attraction of the negative charge on the disk of sealing wax. If the brass plate is now removed, it will be found to be charged with positive electricity.

The negative charge upon the sealing wax is not reduced or diminished by its action in charging the brass plate, and it is possible to charge the plate an indefinite number of times by means of one charge on the sealing wax.

The charges of electricity, produced in any of the ways that have been described, are necessarily small, and the disturbance produced, when they are destroyed by bringing oppositely charged conductors together, is very slight, merely a little snapping noise and, perhaps, a small spark, that seems to leap from the positively charged conductor to the negatively charged one, when they come very close together. By the use of electrical machines of various kinds, in some of which the electricity is produced by friction, and in others by induction, conductors may be charged with much larger quantities of electricity, and the disturbance produced by their discharge is greatly increased. The noise produced is louder and the spark much brighter, and leaps from one conductor to the other, while they are much farther apart. It is possible to produce still larger charges of electricity upon conductors if they are arranged so as to form what are called condensers.

What Is a Leyden Jar?

One of the commonest forms of condenser is the Leyden jar, which is so named because it was invented at Leyden, in Holland. This is a glass jar, upon the outside of which is fastened a coating of tinfoil, that covers the bottom of the jar and extends two-thirds of the way up the sides. Inside the jar there is a similar coating of tinfoil, and through the top of the jar, which is usually made of wood, extends a metal rod. On the upper end of the rod, there is a metal ball, and, at the lower end, is attached a chain which runs down to the bottom of the jar and rests upon the inner tinfoil coating.

In using the Leyden jar, the ball on the metal rod that runs through the top of the jar is connected with an electrical machine, and the jar is supported upon some conducting material, through which electricity may be conveyed from the outer coating of tinfoil to the earth. If the inner coating of tinfoil is now charged with positive electricity, by means of the electrical machine, it induces, upon the outer coating of foil, a charge of negative electricity, which is bound by the attraction of the positive charge on the inside of the jar. At the same time, the positive electricity, on the outer coating of foil, is repelled, through the conducting support, to the earth.

The charge that can be communicated to the coating of the foil, inside the Leyden jar, is greatly increased by the presence of a charge of the opposite kind of electricity, on the coating on the outside of the jar. Each of these charges attracts the other, through the glass of the jar, and serves to bind or hold it. If either coating of foil is removed, the charge on the other coating tends to fly off the tinfoil, and will immediately do so, if a conductor is brought near. It is because the negative effects of the initial charge, inside the jar, and of the induced charge outside the jar, make it possible to communicate, to each coating of foil, a larger charge than it could otherwise be made to receive, that a Leyden jar is called a condenser.

When a Leyden jar is disconnected from the electrical machine, two opposite charges of electricity are present on it, one inside and the other on the outside. If the two coats of tinfoil are now connected, by means of a condenser, they will at once neutralize each other, and the jar will be discharged. A jar may be discharged, by simply taking hold of the tinfoil on the outside of the jar, with one hand, and touching the metal rod, running through the top of the jar, with the other. If you do this, there will be a sudden flow of electricity through your body, your muscles will give a sudden jerk, and you will feel a peculiar tingling sensation. In other words, you will have received a shock.

It is not necessary, for the hand that does not grasp the jar, actually to touch the rod that runs through the top. If the hand is brought toward the rod, rather slowly, you will see a spark leap across the space between the rod and your hand, while your hand is still some distance from the rod. The greater the distance, across which the spark leaps, the brighter will be the spark, and the stronger the shock produced. This distance is sometimes spoken of as the length of the spark, and it indicates the size of the charges on the tinfoil coatings of the jar.

Who Discovered Electricity?

It may seem difficult to believe, that the tiny spark and weak snapping noise that are produced when a Leyden jar is discharged, are, in many respects, the same as lightning and thunder, but it is nevertheless true. This was proved by Benjamin Franklin, about the middle of the 18th century, in the following way. One afternoon, when a thunder shower was approaching, he sent up a kite, to the string of which he fastened a large metal key; and to the key, a ribbon of non-conducting silk, which he held in his hand. When the rain had been falling long enough to wet the string thoroughly, it become a good conductor of electricity, and Franklin found that the key had become charged with electricity transmitted from the clouds, along the wet kite string. The non-conducting silk ribbon, that formed the continuation of the kite string, from the key to his hand, was employed to prevent him from receiving shocks from the passage of the electricity, through his body, to the earth.

Up to this point, your attention has been directed in charges of electricity. You have been told how they may be produced, what some of their leading properties are, and what effects they produce, when they are discharged. The subject that will now be explained to you is that of electric currents.

What Is an Electric Current?

By an electric current, is meant a flow of electricity along a conductor. The flow of electricity, through your body, when you receive an electric shock, is a current, but it lasts only for an instant, and it is difficult to learn much about its nature. By the use of various devices, it is possible to produce currents, that will continue as long as we want them, so that we are enabled to study their properties quite thoroughly.