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.
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.
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.