simple science

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Simple Science

281. Electricity as first Obtained by Man
Until modern times the only electricity known to us was that of the lightning flash, which man could neither hinder nor make. But in the year 1800, electricity in the form of a weak current was obtained by Volta of Italy in a very simple way; and even now our various electric batteries and cells are but a modification of that used by Volta and called a voltaic cell. A strip of copper and a strip of zinc are placed in a glass containing dilute sulphuric acid, a solution composed of oxygen, hydrogen, sulphur, and water. As soon as the plates are immersed in the acid solution, minute bubbles of gas rise from the zinc strip and it begins to waste away slowly. The solution gradually dissolves the zinc and at the same time gives up some of the hydrogen which it contains; but it has little or no effect on the copper, since there is no visible change in the copper strip.

If, now, the strips are connected by means of metal wires, the zinc wastes away rapidly, numerous bubbles of hydrogen pass over to the copper strip and collect on it, and a current of electricity flows through the connecting wires. Evidently, the source of the current is the chemical action between the zinc and the liquid.

Mere inspection of the connecting wire will not enable us to detect that a current is flowing, but there are various ways in which the current makes itself evident. If the ends of the wires attached to the strips are brought in contact with each other and then separated, a faint spark passes, and if the ends are placed on the tongue, a twinge is felt.
282. Experiments which grew out of the Voltaic Cell
Since chemical action on the zinc is the source of the current, it would seem reasonable to expect a current if the cell consisted of two zinc plates instead of one zinc plate and one copper plate. But when the copper strip is replaced by a zinc strip so that the cell consists of two similar plates, no current flows between them. In this case, chemical action is expended in heat rather than in the production of electricity and the liquid becomes hot. But if carbon and zinc are used, a current is again produced, the zinc dissolving away as before, and bubbles collecting on the carbon plate. By experiment it has been found that many different metals may be employed in the construction of an electric cell; for example, current may be obtained from a cell made with a zinc plate and a platinum plate, or from a cell made with a lead plate and a copper plate. Then, too, some other chemical, such as bichromate of potassium, or ammonium chloride, may be used instead of dilute sulphuric acid.

Almost any two different substances will, under proper conditions, give a current, but the strength of the current is in some cases so weak as to be worthless for practical use, such as telephoning, or ringing a door bell. What is wanted is a strong, steady current, and our choice of material is limited to the substances which will give this result. Zinc and lead can be used, but the current resulting is weak and feeble, and for general use zinc and carbon are the most satisfactory.

FIG. - A simple electric cell.
283. Electrical Terms
The plates or strips used in making an electric cell are called electrodes; the zinc is called the negative electrode (-), and the carbon the positive electrode (+); the current is considered to flow through the wire from the + to the-electrode. As a rule, each electrode has attached to it a binding post to which wires can be quickly fastened.

The power that causes the current is called the electromotive force, and the value of the electromotive force, generally written E.M.F., of a cell depends upon the materials used.

When the cell consists of copper, zinc, and dilute sulphuric acid, the electromotive force has a definite value which is always the same no matter what the size or shape of the cell. But the E.M.F. has a decidedly different value in a cell composed of iron, copper, and chromic acid. Each combination of material has its own specific electromotive force.
284. The Disadvantage of a Simple Cell
When the poles of a simple voltaic cell are connected by a wire, the current thus produced slowly diminishes in strength and, after a short time, becomes feeble. Examination of the cell shows that the copper plate is covered with hydrogen bubbles. If, however, these bubbles are completely brushed away by means of a rod or stick, the current strength increases, but as the bubbles again gather on the + electrode the current strength diminishes, and when the bubbles form a thick film on the copper plate, the current is too weak to be of any practical value. The film of bubbles weakens the current because it practically substitutes a hydrogen plate for a copper plate, and we saw in Section 282 that a change in any one of the materials of which a cell is composed changes the current.

This weakening of the current can be reduced mechanically by brushing away the bubbles as soon as they are formed; or chemically, by surrounding the copper plate with a substance which will combine with the free hydrogen and prevent it from passing onward to the copper plate.

In practically all cells, the chemical method is used in preference to the mechanical one. The numerous types of cells in daily use differ chiefly in the devices employed for preventing the formation of hydrogen bubbles, or for disposing of them when formed. One of the best-known cells in which weakening of the current is prevented by chemical means is the so-called gravity cell.
285. The Gravity Cell
A large, irregular copper electrode is placed in the bottom of a jar, and completely covered with a saturated solution of copper sulphate. Then a large, irregular zinc electrode is suspended from the top of the jar, and is completely covered with dilute sulphuric acid which does not mix with the copper sulphate, but floats on the top of it like oil on water. The hydrogen formed by the chemical action of the dilute sulphuric acid on the zinc moves toward the copper electrode, as in the simple voltaic cell. It does not reach the electrode, however, because, when it comes in contact with the copper sulphate, it changes places with the copper there, setting it free, but itself entering into the solution. The copper freed from the copper sulphate solution travels to the copper electrode, and is deposited on it in a clean, bright layer. Instead of a deposit of hydrogen there is a deposit of copper, and falling off in current is prevented.

The gravity cell is cheap, easy to construct, and of constant strength, and is in almost universal use in telegraphic work. Practically all small railroad stations and local telegraph offices use these cells.

FIG. - The gravity cell.
286. Dry Cells
The gravity cell, while cheap and effective, is inconvenient for general use, owing to the fact that it cannot be easily transported, and the dry cell has largely supplanted all others, because of the ease with which it can be taken from place to place. This cell consists of a zinc cup, within which is a carbon rod; the space between the cup and rod is packed with a moist paste containing certain chemicals. The moist paste takes the place of the liquids used in other cells.

FIG. - A dry cell.
287. A Battery of Cells
The electromotive force of one cell may not give a current strong enough to ring a door bell or to operate a telephone. But by using a number of cells, called a battery, the current may be increased to almost any desired strength. If three cells are arranged as in Figure 200, so that the copper of one cell is connected with the zinc of another cell, the electromotive force of the battery will be three times as great as the E.M.F. of a single cell. If four cells are arranged in the same way, the E.M.F. of the battery is four times as great as the E.M.F. of a single cell; when five cells are combined, the resulting E.M.F. is five times as great.

FIG. - A battery of three cells.
288. Heat
Some Uses of Electricity:
Any one who handles electric wires knows that they are more or less heated by the currents which flow through them. If three cells are arranged as in Figure 200 and the connecting wire is coarse, the heating of the wire is scarcely noticeable; but if a shorter wire of the same kind is used, the heat produced is slightly greater; and if the coarse wire is replaced by a short, fine wire, the heating of the wire becomes very marked. We are accustomed to say that a wire offers resistance to the flow of a current; that is, whenever a current meets resistance, heat is produced in much the same way as when mechanical motion meets an obstacle and spends its energy in friction. The flow of electricity along a wire can be compared to the flow of water through pipes: a small pipe offers a greater resistance to the flow of water than a large pipe; less water can be forced through a small pipe than through a large pipe, but the friction of the water against the sides of the small pipe is much greater than in the large one.

So it is with the electric current. In fine wires the resistance to the current is large and the energy of the battery is expended in heat rather than in current. If the heat thus produced is very great, serious consequences may arise; for example, the contact of a hot wire with wall paper or dry beams may cause fire. Insurance companies demand that the wires used in wiring a building for electric lights be of a size suitable to the current to be carried, otherwise they will not take the risk of insurance. The greater the current to be carried, the coarser is the wire required for safety.
289. Electric Stoves
Some Uses of Electricity:
It is often desirable to utilize the electric current for the production of heat. For example, trolley cars are heated by coils of wire under the seats. The coils offer so much resistance to the passage of a strong current through them that they become heated and warm the cars.

Some modern houses are so built that electricity is received into them from the great plants where it is generated, and by merely turning a switch or inserting a plug, electricity is constantly available. In consequence, many practical applications of electricity are possible, among which are flatiron and toaster.

Within the flatiron, is a mass of fine wire coiled as shown in Figure; as soon as the iron is connected with the house supply of electricity, current flows through the fine wire which thus becomes strongly heated and gives off heat to the iron. The iron, when once heated, retains an even temperature as long as the current flows, and the laundress is, in consequence, free from the disadvantages of a slowly cooling iron, and of frequent substitution of a warm iron for a cold one. Electric irons are particularly valuable in summer, because they eliminate the necessity for a strong fire, and spare the housewife intense heat. In addition, the user is not confined to the laundry, but is free to seek the coolest part of the house, the only requisite being an electrical connection.

The toaster is another useful electrical device, since by means of it toast may be made on a dining table or at a bedside. The small electrical stove, shown in Figure, is similar in principle to the flatiron, but in it the heating coil is arranged as shown in Figure. To the physician electric stoves are valuable, since his instruments can be sterilized in water heated by the stove; and that without fuel or odor of gas.

A convenient device is seen in the heating pad, a substitute for a hot water bag. Embedded in some soft thick substance are the insulated wires in which heat is to be developed, and over this is placed a covering of felt.

FIG. - An electric iron on a metal stand.

FIG. - The fine wires are strongly heated by the current which flows through them.

FIG. - Bread can be toasted by electricity.

FIG. - An electric stove.

FIG. - The heating element in the electric stove.

FIG. - An electric pad serves the same purpose as a hot water bag.
290. Electric Lights
Some Uses of Electricity:
The incandescent bulbs which illuminate our buildings consist of a fine, hairlike thread inclosed in a glass bulb from which the air has been removed. When an electric current is sent through the delicate filament, it meets a strong resistance. The heat developed in overcoming the resistance is so great that it makes the filament a glowing mass. The absence of air prevents the filament from burning, and it merely glows and radiates the light.

FIG. - An incandescent electric bulb.

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