Suppose a note of 800 vibrations per second is sung. Then 800 pulses of air will reach the ear each second, and the ear drum, being flexible, will respond and will vibrate at the same rate. The vibration of the ear drum will be transmitted by the three bones and the fluid to the fibers of the auditory nerves. The impulses imparted to the auditory nerve reach the brain and in some unknown way are translated into sound.

278. Care of the Ear. Most catarrhal troubles are accompanied by an oversupply of mucus which frequently clogs up the Eustachian tube and produces deafness. For the same reason, colds and sore throat sometimes induce temporary deafness.

The wax of the ear is essential for flexibility of the ear drum; if an extra amount accumulates, it can be got rid of by bathing the ear in hot water, since the heat will melt the wax. The wax should never be picked out with pin or sharp object except by a physician, lest injury be done to the tympanic membrane.

279. The Phonograph. The invention of the phonograph by Edison in 1878 marked a new era in the popularity and dissemination of music. Up to that time, household music was limited to those who were rich enough to possess a real musical instrument, and who in addition had the understanding and the skill to use the instrument. The invention of the phonograph has brought music to thousands of homes possessed of neither wealth nor skill. That the music reproduced by a phonograph is not always of the highest order does not, in the least, detract from the interest and wonder of the instrument. It can reproduce what it is called upon to reproduce, and if human nature demands the commonplace, the instrument will be made to satisfy the demand. On the other hand, speeches of famous men, national songs, magnificent opera selections, and other pleasing and instructive productions can be reproduced fairly accurately. In this way the phonograph, perhaps more than any other recent invention, can carry to the "shut-ins" a lively glimpse of the outside world and its doings.



The phonograph consists of a cylinder or disk of wax upon which the vibrations of a sensitive diaphragm are recorded by means of a fine metal point. The action of the pointer in reporting the vibrations of a diaphragm is easily understood by reference to a tuning fork. Fasten a stiff bristle to a tuning fork by means of wax, allowing the end of the point to rest lightly upon a piece of smoked glass. If the glass is drawn under the bristle a straight line will be scratched on the glass, but if the tuning fork is struck so that the prongs vibrate back and forth, then the straight line changes to a wavy line and the type of wavy line depends upon the fork used.

In the phonograph, a diaphragm replaces the tuning fork and a cylinder (or a disk) coated with wax replaces the glass plate. When the speaker talks or the singer sings, his voice strikes against a delicate diaphragm and throws it into vibration, and the metal point attached to it traces on the wax of a moving cylinder a groove of varying shape and appearance called the "record." Every variation in the speaker's voice is repeated in the vibrations of the metal disk and hence in the minute motion of the pointer and in the consequent record on the cylinder. The record thus made can be placed in any other phonograph and if the metal pointer of this new phonograph is made to pass over the tracing, the process is reversed and the speaker's voice is reproduced. The sound given out in the this way is faint and weak, but can be strengthened by means of a trumpet attached to the phonograph.



CHAPTER XXX

ELECTRICITY

280. Many animals possess the five senses, but only man possesses constructive, creative power, and is able to build on the information gained through the senses. It is the constructive, creative power which raises man above the level of the beast and enables him to devise and fashion wonderful inventions. Among the most important of his inventions are those which relate to electricity; inventions such as trolley car, elevator, automobile, electric light, the telephone, the telegraph. Bell, by his superior constructive ability, made possible the practical use of the telephone, and Marconi that of wireless telegraphy. To these inventions might be added many others which have increased the efficiency and production of the business world and have decreased the labor and strain of domestic life.



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.

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 (Fig. 198), 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.



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.



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.



CHAPTER XXXI

SOME USES OF ELECTRICITY

288. Heat. 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. 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 (Fig. 201), is a mass of fine wire coiled as shown in Figure 202; 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 (Fig. 203) 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 204, is similar in principle to the flatiron, but in it the heating coil is arranged as shown in Figure 205. 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 (Fig. 206), 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.



290. Electric Lights. 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.



291. Blasting. Until recently, dynamiting was attended with serious danger, owing to the fact that the person who applied the torch to the fuse could not make a safe retreat before the explosion. Now a fine wire is inserted in the fuse, and when everything is in readiness, the ends of the wire are attached to the poles of a distant battery and the heat developed in the wire ignites the fuse.



292. Welding of Metals. Metals are fused and welded by the use of the electric current. The metal pieces which are to be welded are pressed together and a powerful current is passed through their junction. So great is the heat developed that the metals melt and fuse, and on cooling show perfect union.

293. Chemical Effects. The Plating of Gold, Silver, and Other Metals. If strips of lead or rods of carbon are connected to the terminals of an electric cell, as in Figure 208, and are then dipped into a solution of copper sulphate, the strip in connection with the negative terminal of the cell soon becomes thinly plated with a coating of copper. If a solution of silver nitrate is used in place of the copper sulphate, the coating formed will be of silver instead of copper. So long as the current flows and there is any metal present in the solution, the coating continues to form on the negative electrode, and becomes thicker and thicker.



The process by which metal is taken out of solution, as silver out of silver nitrate and copper out of copper sulphate, and is in turn deposited as a coating on another substance, is called electroplating. An electric current can separate a liquid into some of its various constituents and to deposit one of the metal constituents on the negative electrode.



Since copper is constantly taken out of the solution of copper sulphate for deposit upon the negative electrode, the amount of copper remaining in the solution steadily decreases, and finally there is none of it left for deposit. In order to overcome this, the positive electrode should be made of the same metal as that which is to be deposited. The positive metal electrode gradually dissolves and replaces the metal lost from the solution by deposit and electroplating can continue as long as any positive electrode remains.



Practically all silver, gold, and nickel plating is done in this way; machine, bicycle, and motor attachments are not solid, but are of cheaper material electrically plated with nickel. When spoons are to be plated, they are hung in a bath of silver nitrate side by side with a thick slab of pure silver, as in Figure 209. The spoons are connected with the negative terminal of the battery, while the slab of pure silver is connected with the positive terminal of the same battery. The length of time that the current flows determines the thickness of the plating.

294. How Pure Metal is obtained from Ore. When ore is mined, it contains in addition to the desired metal many other substances. In order to separate out the desired metal, the ore is placed in some suitable acid bath, and is connected with the positive terminal of a battery, thus taking the place of the silver slab in the last Section. When current flows, any pure metal which is present is dissolved out of the ore and is deposited on a convenient negative electrode, while the impurities remain in the ore or drop as sediment to the bottom of the vessel. Metals separated from the ore by electricity are called electrolytic metals and are the purest obtainable.

295. Printing. The ability of the electric current to decompose a liquid and to deposit a metal constituent has practically revolutionized the process of printing. Formerly, type was arranged and retained in position until the required number of impressions had been made, the type meanwhile being unavailable for other uses. Moreover, the printing of a second edition necessitated practically as great labor as did the first edition, the type being necessarily set afresh. Now, however, the type is set up and a mold of it is taken in wax. This mold is coated with graphite to make it a conductor and is then suspended in a bath of copper sulphate, side by side with a slab of pure copper. Current is sent through the solution as described in Section 293, until a thin coating of copper has been deposited on the mold. The mold is then taken from the bath, and the wax is replaced by some metal which gives strength and support to the thin copper plate. From this copper plate, which is an exact reproduction of the original type, many thousand copies can be printed. The plate can be preserved and used from time to time for later editions, and the original type can be put back into the cases and used again.



CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS

296. An Electric Current acts like a Magnet. In order to understand the action of the electric bell, we must consider a third effect which an electric current can cause. Connect some cells as shown in Figure 200 and close the circuit through a stout heavy copper wire, dipping a portion of the wire into fine iron filings. A thick cluster of filings will adhere to the wire (Fig. 210), and will continue to cling to it so long as the current flows. If the current is broken, the filings fall from the wire, and only so long as the current flows through the wire does the wire have power to attract iron filings. An electric current makes a wire equivalent to a magnet, giving it the power to attract iron filings.



Although such a straight current bearing wire attracts iron filings, its power of attraction is very small; but its magnetic strength can be increased by coiling as in Figure 211. Such an arrangement of wire is known as a helix or solenoid, and is capable of lifting or pulling larger and more numerous filings and even good-sized pieces of iron, such as tacks. Filings do not adhere to the sides of the helix, but they cling in clusters to the ends of the coil. This shows that the ends of the helix have magnetic power but not the sides.

If a soft iron nail (Fig. 212) or its equivalent is slipped within the coil, the lifting and attractive power of the coil is increased, and comparatively heavy weights can be lifted.



A coil of wire traversed by an electric current and containing a core of soft iron has the power of attracting and moving heavy iron objects; that is, it acts like a magnet. Such an arrangement is called an electromagnet. As soon as the current ceases to flow, the electromagnet loses its magnetic power and becomes merely iron and wire without magnetic attraction.

If many cells are used, the strength of the electromagnet is increased, and if the coil is wound closely, as in Figure 213, instead of loosely, as in Figure 211, the magnetic strength is still further increased. The strength of any electromagnet depends upon the number of coils wound on the iron core and upon the strength of the current which is sent through the coils.



To increase the strength of the electromagnet still further, the so-called horseshoe shape is used (Fig. 214). In such an arrangement there is practically the strength of two separate electromagnets.

297. The Electric Bell. The ringing of the electric bell is due to the attractive power of an electromagnet. By the pushing of a button (Fig. 215) connection is made with a battery, and current flows through the wire wound on the iron spools, and further to the screw P which presses against the soft iron strip or armature S; and from S the current flows back to the battery. As soon as the current flows, the coils become magnetic and attract the soft iron armature, drawing it forward and causing the clapper to strike the bell. In this position, S no longer touches the screw P, and hence there is no complete path for the electricity, and the current ceases. But the attractive, magnetic power of the coils stops as soon as the current ceases; hence there is nothing to hold the armature down, and it flies back to its former position. In doing this, however, the armature makes contact at P through the spring, and the current flows once more; as a result the coils again become magnets, the armature is again drawn forward, and the clapper again strikes the bell. But immediately afterwards the armature springs backward and makes contact at P and the entire operation is repeated. So long as we press the button this process continues producing what sounds like a continuous jingle; in reality the clapper strikes the bell every time a current passes through the electromagnet.



298. The Push Button. The push button is an essential part of every electric bell, because without it the bell either would not ring at all, or would ring incessantly until the cell was exhausted. When the push button is free, as in Figure 216, the cell terminals are not connected in an unbroken path, and hence the current does not flow. When, however, the button is pressed, the current has a complete path, provided there is the proper connection at S. That is, the pressure on the push button permits current to flow to the bell. The flow of this current then depends solely upon the connection at S, which is alternately made and broken, and in this way produces sound.



The sign "Bell out of order" is usually due to the fact that the battery is either temporarily or permanently exhausted. In warm weather the liquid in the cell may dry up and cause stoppage of the current. If fresh liquid is poured into the vessel so that the chemical action of the acid on the zinc is renewed, the current again flows. Another explanation of an out-of-order bell is that the liquid may have eaten up all the zinc; if this is the case, the insertion of a fresh strip of zinc will remove the difficulty and the current will flow. If dry cells are used, there is no remedy except in the purchase of new cells.

299. How Electricity may be lost to Use. In the electric bell, we saw that an air gap at the push button stopped the flow of electricity. If we cut the wire connecting the poles of a battery, the current ceases because an air gap intervenes and electricity does not readily pass through air. Many substances besides air stop the flow of electricity. If a strip of glass, rubber, mica, or paraffin is introduced anywhere in a circuit, the current ceases. If a metal is inserted in the gap, the current again flows. Substances which, like an air gap, interfere with the flow of electricity are called non-conductors, or, more commonly, insulators. Substances which, like the earth, the human body, and all other moist objects, conduct electricity are conductors. If the telephone and electric light wires in our houses were not insulated by a covering of thread, or cloth, or other non conducting material, the electricity would escape into surrounding objects instead of flowing through the wire and producing sound and light.

In our city streets, the overhead wires are supported on glass knobs or are closely wrapped, in order to prevent the escape of electricity through the poles to the ground. In order to have a steady, dependable current, the wire carrying the current must be insulated.

Lack of insulation means not only the loss of current for practical uses, but also serious consequences in the event of the crossing of current-bearing wires. If two wires properly insulated touch each other, the currents flow along their respective wires unaltered; if, however, two uninsulated wires touch, some of the electricity flows from one to the other. Heat is developed as a result of this transference, and the heat thus developed is sometimes so great that fire occurs. For this reason, wires are heavily insulated and extra protection is provided at points where numerous wires touch or cross.

Conductors and insulators are necessary to the efficient and economic flow of a current, the insulator preventing the escape of electricity and lessening the danger of fire, and the conductor carrying the current.

300. The Telegraph. Telegraphy is the process of transmitting messages from place to place by means of an electric current. The principle underlying the action of the telegraph is the principle upon which the electric bell operates; namely, that a piece of soft iron becomes a magnet while a current flows around it, but loses its magnetism as soon as the current ceases.

In the electric bell, the electromagnet, clapper, push button, and battery are relatively near,—usually all are located in the same building; while in the telegraph the current may travel miles before it reaches the electromagnet and produces motion of the armature.



The fundamental connections of the telegraph are shown in Figure 217. If the key K is pressed down by an operator in Philadelphia, the current from the battery (only one cell is shown for simplicity) flows through the line to New York, passes through the electromagnet M, and thence back to Philadelphia. As long as the key K is pressed down, the coil M acts as a magnet and attracts and holds fast the armature A; but as soon as K is released, the current is broken, M loses its magnetism, and the armature is pulled back by the spring D. By a mechanical device, tape is drawn uniformly under the light marker P attached to the armature. If K is closed for but a short time, the armature is drawn down for but a short interval, and the marker registers a dot on the tape. If K is closed for a longer time, a short dash is made by the marker, and, in general, the length of time that K is closed determines the length of the marks recorded on the tape. The telegraphic alphabet consists of dots and dashes and their various combinations, and hence an interpretation of the dot and dash symbols recorded on the tape is all that is necessary for the receiving of a telegraphic message.

The Morse telegraphic code, consisting of dots, dashes, and spaces, is given in Figure 218.



The telegraph is now such a universal means of communication between distant points that one wonders how business was conducted before its invention in 1832 by S.F.B. Morse.



301. Improvements. The Sounder. Shortly after the invention of telegraphy, operators learned that they could read the message by the click of the marker against a metal rod which took the place of the tape. In practically all telegraph offices of the present day the old-fashioned tape is replaced by the sounder, shown in Figure 219. When current flows, a lever, L, is drawn down by the electromagnet and strikes against a solid metal piece with a click; when the current is broken, the lever springs upward, strikes another metal piece and makes a different click. It is clear that the working of the key which starts and stops the current in this line will be imitated by the motion and the resulting clicks of the sounder. By means of these varying clicks of the sounder, the operator interprets the message.



The Relay. When a telegraph line is very long, the resistance of the wire is great, and the current which passes through the electromagnet is correspondingly weak, so feeble indeed that the armature must be made very thin and light in order to be affected by the makes and breaks in the current. The clicks of an armature light enough to respond to the weak current of a long wire are too faint to be recognized by the ear, and hence in such long circuits some device must be introduced whereby the effect is increased. This is usually done by installing at each station a local battery and a very delicate and sensitive electromagnet called the relay. Under these conditions the current of the main line is not sent through the sounder, but through the relay which opens and closes a local battery in connection with the strong sounder. For example, the relay is so arranged that current from the main line runs through it exactly as it runs through M in Figure 217. When current is made, the relay attracts an armature, which thereby closes a circuit in a local battery and thus causes a click of the sounder. When the current in the main line is broken, the relay loses its magnetic attraction, its armature springs back, connection is broken in the local circuit, and the sounder responds by allowing its armature to spring back with a sharp sound.

302. The Earth an Important Part of a Telegraphic System. We learned in Section 299 that electricity could flow through many different substances, one of which was the earth. In all ordinary telegraph lines, advantage is taken of this fact to utilize the earth as a conductor and to dispense with one wire. Originally two wires were used, as in Figure 217; then it was found that a railroad track could be substituted for one wire, and later that the earth itself served equally well for a return wire. The present arrangement is shown in Figure 220, where there is but one wire, the circuit being completed by the earth. No fact in electricity seems more marvelous than that the thousands of messages flashing along the wires overhead are likewise traveling through the ground beneath. If it were not for this use of the earth as an unfailing conductor, the network of overhead wires in our city streets would be even more complex than it now is.

303. Advances in Telegraphy. The mechanical improvements in telegraphy have been so rapid that at present a single operator can easily send or receive forty words a minute. He can telegraph more quickly than the average person can write; and with a combination of the latest improvements the speed can be enormously increased. Recently, 1500 words were flashed from New York to Boston over a single wire in one second.

In actual practice messages are not ordinarily sent long distances over a direct line, but are automatically transferred to new lines at definite points. For example, a message from New York to Chicago does not travel along an uninterrupted path, but is automatically transferred at some point, such as Lancaster, to a second line which carries it on to Pittsburgh, where it is again transferred to a third line which takes it farther on to its destination.



CHAPTER XXXIII

MAGNETS AND CURRENTS

304. In the twelfth century, there was introduced into Europe from China a simple instrument which changed journeying on the sea from uncertain wandering to a definite, safe voyage. This instrument was the compass (Fig. 221), and because of the property of the compass needle (a magnet) to point unerringly north and south, sailors were able to determine directions on the sea and to steer for the desired point.



Since an electric current is practically equivalent to a magnet (Section 296), it becomes necessary to know the most important facts relative to magnets, facts simple in themselves but of far-reaching value and consequences in electricity. Without a knowledge of the magnetic characteristics of currents, the construction of the motor would have been impossible, and trolley cars, electric fans, motor boats, and other equally well-known electrical contrivances would be unknown.

305. The Attractive Power of a Magnet. The magnet best known to us all is the compass needle, but for convenience we will use a magnetic needle in the shape of a bar larger and stronger than that employed in the compass. If we lay such a magnet on a pile of iron filings, it will be found on lifting the magnet that the filings cling to the ends in tufts, but leave it almost bare in the center (Fig. 222). The points of attraction at the two ends are called the poles of the magnet.



If a delicately made magnet is suspended as in Figure 223, and is allowed to swing freely, it will always assume a definite north and south position. The pole which points north when the needle is suspended is called the north pole and is marked N, while the pole which points south when the needle is suspended is called the south pole and is marked S.

A freely suspended magnet points nearly north and south.

A magnet has two main points of attraction called respectively the north and south poles.



306. The Extent of Magnetic Attraction. If a thin sheet of paper or cardboard is laid over a strong, bar-shaped magnet and iron filings are then gently strewn on the paper, the filings clearly indicate the position of the magnet beneath, and if the cardboard is gently tapped, the filings arrange themselves as shown in Figure 224. If the paper is held some distance above the magnet, the influence on the filings is less definite, and finally, if the paper is held very far away, the filings do not respond at all, but lie on the cardboard as dropped.

The magnetic power of a magnet, while not confined to the magnet itself, does not extend indefinitely into the surrounding region; the influence is strong near the magnet, but at a distance becomes so weak as to be inappreciable. The region around a magnet through which its magnetic force is felt is called the field of force, or simply the magnetic field, and the definite lines in which the filings arrange themselves are called lines of force.



The magnetic power of a magnet is not limited to the magnet, but extends to a considerable distance in all directions.

307. The Influence of Magnets upon Each Other. If while our suspended magnetic needle is at rest in its characteristic north-and-south direction another magnet is brought near, the suspended magnet is turned; that is, motion is produced (Fig. 225). If the north pole of the free magnet is brought toward the south pole of the suspended magnet, the latter moves in such a way that the two poles N and S are as close together as possible. If the north pole of the free magnet is brought toward the north pole of the suspended magnet, the latter moves in such a way that the two poles N and N are as far apart as possible. In every case that can be tested, it is found that a north pole repels a north pole, and a south pole repels a south pole; but that a north and a south pole always attract each other.



The main facts relative to magnets may be summed up as follows:—

a. A magnet points nearly north and south if it is allowed to swing freely.

b. A magnet contains two unlike poles, one of which persistently points north, and the other of which as persistently points south, if allowed to swing freely.

c. Poles of the same name repel each other; poles of unlike name attract each other.

d. A magnet possesses the power of attracting certain substances, like iron, and this power of attraction is not limited to the magnet itself but extends into the region around the magnet.

308. Magnetic Properties of an Electric Current. If a current-bearing wire is really equivalent in its magnetic powers to a magnet, it must possess all of the characteristics mentioned in the preceding Section. We saw in Section 296 that a coiled wire through which current was flowing would attract iron filings at the two ends of the helix. That a coil through which current flows possesses the characteristics a, b, c, and d of a magnet is shown as follows:—

a, b. If a helix marked at one end with a red string is arranged so that it is free to rotate and a strong current is sent through it, the helix will immediately turn and face about until it points north and south. If it is disturbed from this position, it will slowly swing back until it occupies its characteristic north and south position. The end to which the string is attached will persistently point either north or south. If the current is sent through the coil in the opposite direction, the two poles exchange positions and the helix turns until the new north pole points north.



c. If a coil conducting a current is held near a suspended magnet, one end of the helix will be found to attract the north pole of the magnet, while the opposite end will be found to repel the north pole of the magnet. In fact, the helix will be found to behave in every way as a magnet, with a north pole at one end and a south pole at the other. If the current is sent through the helix in the opposite direction, the north and south poles exchange places.



If the number of turns in the helix is reduced until but a single loop remains, the result is the same; the single loop acts like a flat magnet, one side of the loop always facing northward and one southward, and one face attracting the north pole of the suspended magnet and one repelling it.

d. If a wire is passed through a card and a strong current is sent through the wire, iron filings will, when sprinkled upon the card, arrange themselves in definite directions (Fig. 227). A wire carrying a current is surrounded by a magnetic field of force.

A magnetic needle held under a current-bearing wire turns on its pivot and finally comes to rest at an angle with the current. The fact that the needle is deflected by the wire shows that the magnetic power of the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by Oersted of Denmark less than a hundred years ago; but since that time practically all important electrical machinery has been based upon one or more of the magnetic properties of electricity. The motors which drive our electric fans, our mills, and our trolley cars owe their existence entirely to the magnetic action of current electricity.



309. The Principle of the Motor. If a close coil of wire is suspended between the poles of a strong horseshoe magnet, it will not assume any characteristic position but will remain wherever placed. If, however, a current is sent through the wire, the coil faces about and assumes a definite position. This is because a coil, carrying a current, is equivalent to a magnet with a north and south face; and, in accordance with the magnetic laws, tends to move until its north face is opposite the south pole of the horseshoe magnet, and its south face opposite the north pole of the magnet. If, when the coil is at rest in this position, the current is reversed, so that the north pole of the coil becomes a south pole and the former south pole becomes a north pole, the result is that like poles of coil and magnet face each other. But since like poles repel each other, the coil will move, and will rotate until its new north pole is opposite to the south pole of the magnet and its new south pole is opposite the north pole. By sending a strong current through the coil, the helix is made to rotate through a half turn; by reversing the current when the coil is at the half turn, the helix is made to continue its rotation and to swing through a whole turn. If the current could be repeatedly reversed just as the helix completed its half turn, the motion could be prolonged; periodic current reversal would produce continuous rotation. This is the principle of the motor.



It is easy to see that long-continued rotation would be impossible in the arrangement of Figure 228, since the twisting of the suspending wire would interfere with free motion. If the motor is to be used for continuous motion, some device must be employed by means of which the helix is capable of continued rotation around its support.

In practice, the rotating coil of a motor is arranged as shown in Figure 229. Wires from the coil terminate on metal disks and are securely soldered there. The coil and disks are supported by the strong and well-insulated rod R, which rests upon braces, but which nevertheless rotates freely with disks and coil. The current flows to the coil through the thin metal strips called brushes, which rest lightly upon the disks.

When the current which enters at B flows through the wire, the coil rotates, tending to set itself so that its north face is opposite the south face of the magnet. If, when the helix has just reached this position, the current is reversed—entering at B' instead of B—the poles of the coil are exchanged; the rotation, therefore, does not cease, but continues for another half turn. Proper reversals of the current are accompanied by continuous motion, and since the disk and shaft rotate with the coil, there is continuous rotation.

If a wheel is attached to the rotating shaft, weights can be lifted, and if a belt is attached to the wheel, the motion of the rotating helix can be transferred to machinery for practical use.

The rotating coil is usually spoken of as the armature, and the large magnet as the field magnet.

310. Mechanical Reversal of the Current. The Commutator. It is not possible by hand to reverse the current with sufficient rapidity and precision to insure uninterrupted rotation; moreover, the physical exertion of such frequent reversals is considerable. Hence, some mechanical device for periodically reversing the current is necessary, if the motor is to be of commercial value.



The mechanical reversal of the current is accomplished by the use of the commutator, which is a metal ring split into halves, well insulated from each other and from the shaft. To each half of this ring is attached one of the ends of the armature wire. The brushes which carry the current are set on opposite sides of the ring and do not rotate. As armature, commutator, and shaft rotate, the brushes connect first with one segment of the commutator and then with the other. Since the circuit is arranged so that the current always enters the commutator through the brush B, the flow of the current into the coil is always through the segment in contact with B; but the segment in contact with B changes at every half turn of the coil, and hence the direction of the current through the coil changes periodically. As a result the coil rotates continuously, and produces motion so long as current is supplied from without.

311. The Practical Motor. A motor constructed in accordance with Section 309 would be of little value in practical everyday affairs; its armature rotates too slowly and with too little force. If a motor is to be of real service, its armature must rotate with sufficient strength to impart motion to the wheels of trolley cars and mills, to drive electric fans, and to set into activity many other forms of machinery.

The strength of a motor may be increased by replacing the singly coiled armature by one closely wound on an iron core; in some armatures there are thousands of turns of wire. The presence of soft iron within the armature (Section 296) causes greater attraction between the armature and the outside magnet, and hence greater force of motion. The magnetic strength of the field magnet influences greatly the speed of the armature; the stronger the field magnet the greater the motion, so electricians make every effort to strengthen their field magnets. The strongest known magnets are electromagnets, which, as we have seen, are merely coils of wire wound on an iron core. For this reason, the field magnet is usually an electromagnet.

When very powerful motors are necessary, the field magnet is so arranged that it has four or more poles instead of two; the armature likewise consists of several portions, and even the commutator may be very complex. But no matter how complex these various parts may seem to be, the principle is always that stated in Section 309, and the parts are limited to field magnet, commutator, and armature.



The motor is of value because by means of it motion, or mechanical energy, is obtained from an electric current. Nearly all electric street cars (Fig. 232), are set in motion by powerful motors placed under the cars. As the armature rotates, its motion is communicated by gears to the wheels, the necessary current reaching the motor through the overhead wires. Small motors may be used to great advantage in the home, where they serve to turn the wheels of sewing machines, and to operate washing machines. Vacuum cleaners are frequently run by motors.



CHAPTER XXXIV

HOW ELECTRICITY MAY BE MEASURED

312. Danger of an Oversupply of Current. If a small toy motor is connected with one cell, it rotates slowly; if connected with two cells, it rotates more rapidly, and in general, the greater the number of cells used, the stronger will be the action of the motor. But it is possible to send too strong a current through our wire, thereby interfering with all motion and destroying the motor. We have seen in Section 288 that the amount of current which can safely flow through a wire depends upon the thickness of the wire. A strong current sent through a fine wire has its electrical energy transformed largely into heat; and if the current is very strong, the heat developed may be sufficient to burn off the insulation and melt the wire itself. This is true not only of motors, but of all electric machinery in which there are current-bearing wires. The current should not be greater than the wires can carry, otherwise too much heat will be developed and damage will be done to instruments and surroundings.

The current sent through our electric stoves and irons should be strong enough to heat the coils, but not strong enough to melt them. If the current sent through our electric light wires is too great for the capacity of the wires, the heat developed will injure the wires and may cause disastrous results. The overloading of wires is responsible for many disastrous fires.

The danger of overloading may be eliminated by inserting in the circuit a fuse or other safety device. A fuse is made by combining a number of metals in such a way that the resulting substance has a low melting point and a high electrical resistance. A fuse is inserted in the circuit, and the instant the current increases beyond its normal amount the fuse melts, breaks the circuit, and thus protects the remaining part of the circuit from the danger of an overload. In this way, a circuit designed to carry a certain current is protected from the danger of an accidental overload. The noise made by the burning out of a fuse in a trolley car frequently alarms passengers, but it is really a sign that the system is in good working order and that there is no danger of accident from too strong a current.

313. How Current is Measured. The preceding Section has shown clearly the danger of too strong a current, and the necessity for limiting the current to that which the wire can safely carry. There are times when it is desirable to know accurately the strength of a current, not only in order to guard against an overload, but also in order to determine in advance the mechanical and chemical effects which will be produced by the current. For example, the strength of the current determines the thickness of the coating of silver which forms in a given time on a spoon placed in an electrolytic bath; if the current is weak, a thin plating is made on the spoon; if the current is strong, a thick plating is made. If, therefore, the exact value of the current is known, the exact amount of silver which will be deposited on the spoon in a given time can be definitely calculated.



Current-measuring instruments, or galvanometers, depend for their action on the magnetic properties of current electricity. The principle of practically all galvanometers is as follows:—

A closely wound coil of fine wire free to rotate is suspended as in Figure 233 between the poles of a strong magnet. When a current is sent through the coil, the coil becomes a magnet and turns so that its faces will be towards the poles of the permanent magnet. But as the coil turns, the suspending wire becomes twisted and hinders the turning. For this reason, the coil can turn only until the motion caused by the current is balanced by the twist of the suspending wire. But the stronger the current through the coil, the stronger will be the force tending to rotate the coil, and hence the less effective will be the hindrance of the twisting string. As a consequence, the coil swings farther than before; that is, the greater the current, the farther the swing. Usually a delicate pointer is attached to the movable coil and rotates freely with it, so that the swing of the pointer indicates the relative values of the current. If the source of the current is a gravity cell, the swing is only two thirds as great as when a dry cell is used, indicating that the dry cell furnishes about 1-1/2 times as much current as a gravity cell.

314. Ammeters. A galvanometer does not measure the current, but merely indicates the relative strength of different currents. But it is desirable at times to measure a current in units. Instruments for measuring the strength of currents in units are called ammeters, and the common form makes use of a galvanometer.

A current is sent through a movable coil (the field magnet and coil are inclosed in the case) (Fig. 234), and the magnetic field thus developed causes the coil to turn, and the pointer attached to it to move over a scale graduated so that it reads current strengths. This scale is carefully graduated by the following method.

If two silver rods (Fig. 208) are weighed and placed in a solution of silver nitrate, and current from a single cell is passed through the liquid for a definite time, we find, on weighing the two rods, that one has gained in weight and the other has lost. If the current is allowed to flow twice as long, the amount of silver lost and gained by the electrodes is doubled; and if twice the current is used, the result is again doubled.

As a result of numerous experiments, it was found that a definite current of electricity will deposit a definite amount of silver in a definite time, and that the amount of silver deposited on an electrode in one second might be used to measure the current of electricity which has flowed through the circuit in one second.

A current is said to be one ampere strong if it will deposit silver on an electrode at the rate of 0.001118 gram per second.



In marking the scale, an ammeter is placed in the circuit of an electrolytic cell and the position of the pointer is marked on the blank card which lies beneath and which is to serve as a scale (Fig. 235). After the current has flowed for about an hour, the amount of silver which has been deposited is measured. Knowing the time during which the current has run, and the amount of deposit, the strength of the current in amperes can be calculated. This number is written opposite the place at which the pointer stood during the experiment.

The scale may be completed by marking the positions of the pointer when other currents of known strength flow through the ammeter.



All electric plants, whether for heating, lighting, or for machinery, are provided with ammeters, such instruments being as important to an electric plant as the steam gauge is to the boiler.

315. Voltage and Voltmeters. Since electromotive force, or voltage, is the cause of current, it should be possible to compare different electromotive forces by comparing the currents which they produce in a given circuit. But two voltages of equal value do not give equal currents unless the resistances met by the currents are equal. For example, the simple voltaic cell and the gravity cell have approximately equal voltages, but the current produced by the voltaic cell is stronger than that produced by the gravity cell. This is because the current meets more resistance within the gravity cell than within the voltaic cell. Every cell, no matter what its nature, offers resistance to the flow of electricity through it and is said to have internal resistance. If we are determining the voltages of various cells by a comparison of the respective currents produced, the result will be true only on condition that the resistances in the various circuits are equal. If a very large external resistance of fine wire is placed in circuit with a gravity cell, the total resistance of the circuit (made up of the relatively small resistance in the cell and the larger resistance in the rest of the circuit) will differ but little from that of another circuit in which the gravity cell is replaced by a voltaic cell, or any other type of cell.

With a high resistance in the outside circuit, the deflections of the ammeter will be small, but such as they are, they will fairly accurately represent the electromotive forces which produce them.

Voltmeters (Fig. 236), or instruments for measuring voltage, are like ammeters except that a wire of very high resistance is in circuit with the movable coil. In external appearance they are not distinguishable from ammeters.



The unit of electromotive force is called the volt. The voltage of a dry cell is approximately 1.5 volts, and the voltage of a voltaic cell and of a gravity cell is approximately 1 volt.

316. Current, Voltage, Resistance. We learned in Section 287 that the strength of a current increases when the electromotive force increases, and diminishes when the electromotive force diminishes. Later, in Section 288, we learned that the strength of the current decreases as the resistance in circuit increases.

The strength of a steady current depends upon these two factors only, the electromotive force which causes it and the resistance which it has to overcome.

317. Resistance. Since resistance plays so important a role in electricity, it becomes necessary to have a unit of resistance. The practical unit of resistance is called an ohm, and some idea of the value of an ohm can be obtained if we remember that a 300-foot length of common iron telegraph wire has a resistance of 1 ohm. An approximate ohm for rough work in the laboratory may be made by winding 9 feet 5 inches of number 30 copper wire on a spool or arranging it in any other convenient form.

In Section 299 we learned that substances differ very greatly in the resistance which they offer to electricity, and so it will not surprise us to learn that while it takes 300 feet of iron telegraph wire to give 1 ohm of resistance, it takes but 39 feet of number 24 copper wire, and but 2.2 feet of number 24 German silver wire, to give the same resistance.

NOTE. The number of a wire indicates its diameter; number 30, for example, being always of a definite fixed diameter, no matter what the material of the wire.

If we wish to avoid loss of current by heating, we use a wire of low resistance; while if we wish to transform electricity into heat, as in the electric stove, we choose wire of high resistance, as German silver wire.



CHAPTER XXXV

HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE

318. The Dynamo. We have learned that cells furnish current as a result of chemical action, and that the substance usually consumed within the cell is zinc. Just as coal within the furnace furnishes heat, so zinc within the cell furnishes electricity. But zinc is a much more expensive fuel than coal or oil or gas, and to run a large motor by electricity produced in this way would be very much more expensive than to run the motor by water or steam. For weak and infrequent currents such as are used in the electric bell, only small quantities of zinc are needed, and the expense is small. But for the production of such powerful currents as are needed to drive trolley cars, elevators, and huge machinery, enormous quantities of zinc would be necessary and the cost would be prohibitive. It is safe to say that electricity would never have been used on a large scale if some less expensive and more convenient source than zinc had not been found.

319. A New Source of Electricity. It came to most of us as a surprise that an electric current has magnetic properties and transforms a coil into a veritable magnet. Perhaps it will not surprise us now to learn that a magnet in motion has electric properties and is, in fact, able to produce a current within a wire. This can be proved as follows:—



Attach a closely wound coil to a sensitive galvanometer (Fig. 237); naturally there is no deflection of the galvanometer needle, because there is no current in the wire. Now thrust a magnet into the coil. Immediately there is a deflection of the needle, which indicates that a current is flowing through the circuit. If the magnet is allowed to remain at rest within the coil, the needle returns to its zero position, showing that the current has ceased. Now let the magnet be withdrawn from the coil; the needle is deflected as before, but the deflection is in the opposite direction, showing that a current exists, but that it flows in the opposite direction. We learn, therefore, that a current may be induced in a coil by moving a magnet back and forth within the coil, but that a magnet at rest within the coil has no such influence.

An electric current transforms a coil into a magnet. A magnet in motion induces electricity within a coil; that is, causes a current to flow through the coil.

A magnet possesses lines of force, and as the magnet moves toward the coil it carries lines of force with it, and the coil is cut, so to speak, by these lines of force. As the magnet recedes from the coil, it carries lines of force away with it, this time reducing the number of the lines which cut the coil.



320. A Test of the Preceding Statement. We will test the statement that a magnet has electric properties by another experiment. Between the poles of a strong magnet suspend a movable coil which is connected with a sensitive galvanometer (Fig. 237). Starting with the coil in the position of Figure 228, when many lines of force pass through it, let the coil be rotated quickly until it reaches the position indicated in Figure 238, when no lines of force pass through it. During the motion of the coil, a strong deflection of the galvanometer is observed; but the deflection ceases as soon as the coil ceases to rotate. If, now, starting with the position of Figure 238, the coil is rotated forward to its starting point, a deflection occurs in the opposite direction, showing that a current is present, but that it flows in the opposite direction. So long as the coil is in motion, it is cut by a varying number of lines of force, and current is induced in the coil.

The above arrangement is a dynamo in miniature. By rotation of a coil (armature) within a magnetic field, that is, between the poles of a magnet, current is obtained.

In the motor, current produces motion. In the dynamo, motion produces current.

321. The Dynamo. As has been said, the arrangement of the preceding Section is a dynamo in miniature. Every dynamo, no matter how complex its structure and appearance, consists of a coil of wire which can rotate continuously between the poles of a strong magnet. The mechanical devices to insure easy rotation are similar in all respects to those previously described for the motor.



The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction. Such alternating currents are unsatisfactory for many purposes, and to be of service are in many cases transformed into direct currents; that is, current which flows steadily in one direction. This is accomplished by the use of a commutator. In the construction of the motor, continuous motion in one direction is obtained by the use of a commutator (Section 310); in the construction of a dynamo, continuous current in one direction is obtained by the use of a similar device.

322. Powerful Dynamos. The power and efficiency of a dynamo are increased by employing the devices previously mentioned in connection with the motor. Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up of many coils wound on soft iron. The speed with which the armature is rotated influences the strength of the induced current, and hence the armature is run at high speed.



A small dynamo, such as is used for lighting fifty incandescent lamps, has a horse power of about 33.5, and large dynamos are frequently as powerful as 7500 horse power.

323. The Telephone. When a magnet is at rest within a closed coil of wire, as in Section 319, current does not flow through the wire. But if a piece of iron is brought near the magnet, current is induced and flows through the wire; if the iron is withdrawn, current is again induced in the wire but flows in the opposite direction. As iron approaches and recedes from the magnet, current is induced in the wire surrounding the magnet. This is in brief the principle of the telephone. When one talks into a receiver, L, the voice throws into vibration a sensitive iron plate standing before an electromagnet. The back and forth motion of the iron plate induces current in the electromagnet c. The current thus induced makes itself evident at the opposite end of the line M, where by its magnetic attraction, it throws a second iron plate into vibrations. The vibrations of the second plate are similar to those produced in the first plate by the voice. The vibrations of the far plate thus reproduce the sounds uttered at the opposite end.



324. Cost of Electric Power. The water power of a stream depends upon the quantity of water and the force with which it flows. The electric power of a current depends upon the quantity of electricity and the force under which it flows. The unit of electric power is called the watt; it is the power furnished by a current of one ampere with a voltage of one volt.

One watt represents a very small amount of electric power, and for practical purposes a unit 1000 times as large is used, namely, the kilowatt. By experiment it has been found that one kilowatt is equivalent to about 1-1/3 horse power. Electric current is charged for by the watt hour. A current of one ampere, having a voltage of one volt, will furnish in the course of one hour one watt hour of energy. Energy for electric lighting is sold at the rate of about ten cents per kilowatt hour. For other purposes it is less expensive. The meters commonly used measure the amperes, volts, and time automatically, and register the electric power supplied in watt hours.



INDEX

Absorption, of heat by lampblack, 143-144. of gases by charcoal, 57. of light waves, 135-138.

Accommodation of the eye, 123.

Acetanilid, 259.

Acetylene, as illuminant, 152-153. manufacture of, 152-153. properties of, 220.

Acid, boric, 253. carbolic, 152, 251, 252. hydrochloric, 55, 80, 227, 238, 241. lactic, 230. oxalic, 247, 248. salicylic, 253. sulphuric, 55, 80, 240, 241, 307. sulphurous, 242.

Acids, action on litmus, 220.

Adenoids, 51.

Adulterants, detection of, 16.

Air, characteristics of, 81-83, 86, 189. compressibility of, 91. expansion of, 10-11. humidity, 38, 39. pumps, 201-205. transmits sound, 269. weight of, 86. See Atmosphere.

Alcohol, 234. in patent medicines, 260.

Alizarin, 248.

Alkali, 222.

Alternating current, 351.

Alum, 247. in baking powder, 230.

Ammeter, 341, 343.

Ammonia, 152. a base, 221-222. in bath, 226. in manufacture of ice, 98. neutralizing chlorine, 240.

Ampere, 342.

Anemia, 259.

Angle, of incidence, 110. of reflection, 110. of refraction, 114.

Aniline, 152, 245.

Animal charcoal, 58.

Animal transportation, 132.

Antichlor, 240.

Antipyrin, 259.

Armature, 319, 320. dynamo, 350. motor, 335.

Artificial lighting, 148-153.

Atmosphere, 81. carbon dioxide in, 54-55. height of, 81. nitrogen and oxygen in, 262. pressure of, 82-86. water vapor in, 36-38. weight, 86. See Air.

Atmospheric pressure, 82-86.

Atomizer, 92.

Atoms, 102.

Automobiles, gas engines, 185.

Axis of a lens, 119.

Bacteria, 133. as nitrogen makers, 263. destroyed by sunlight, etc., 133, 250, 251. diseases caused by, 133. in butter and cheese, 133.

Baking powder, 229-230.

Baking soda, 227-229.

Barograph, 87.

Barometer, aneroid, 84-85. mercury, 84. use in weather predictions, 86-87.

Bases, action on litmus, 221-222. properties, 220-222.

Battery, electric, 311.

Beans, as food, 66. roots take in nitrogen, 263.

Bell, electric, 319-321.

Benzine, 150. as a cleaning agent, 227.

Benzoate of soda, 253.

Bicarbonate of soda, in fire extinguisher, 55, 56. in Rochelle salt, 227. in soda mints, 231. in seidlitz powder, 231.

Bicycle pumps, 202.

Blasting, by electricity, 314.

Bleaching, 237-243. by chlorine, 238-240.

Bleaching powder, 239-240.

Body, human, 63-64. a conductor of electricity, 292.

Boiling, 31. amount of heat absorbed, 31-32. of milk, 32. of water, 77. point, 15.

Bomb calorimeter, 61.

Borax, as meat preservative, 253. as washing powder, 226.

Boric acid, as meat preservative, 253.

Boyle's law, 95-96.

Bread, 232-233. unleavened, 233.

Bread making, 232-235.

Breathing, hygienic habits of, 50. by mouth, 50-51.

Burns, treatment of, 52-53.

Butter, adulteration test, 16. bacteria in, 133.

Buttermilk, 230.

Caisson, 203-204.

Calcium carbide, 152-153. in making nitrogenous fertilizer, 264.

Calico printing, 249.

Calorie, 27-28, 61-62.

Calorimeter, 61.

Camera, 128-129. films, 129. lens, 129. plates, 129.

Camping, water supply, 195-197.

Candle, 148-149. as standard for light-measure, 104-105.

Candle-power, 105-107.

Carbide, calcium, 152-153, 264.

Carbohydrates, 64-65, 149.

Carbolic acid, 152. as disinfectant, 251.

Carbon, 56, 66. in voltaic cells, 308.

Carbon dioxide, 53. as fire extinguisher, 55-56. commercial use, 55-56. in baking soda, 228. in fermentation, 234. in health, 54. in plants, 55. preparation of, 55. source of, 53. test for, 228.

Catarrh, 259.

Caustic lime, 222..

Caustic potash, 222.

Caustic soda, 218, 222. to make a salt, 227.

Caves and caverns, 71.

Cell, dry, 310. gravity, 309-310. voltaic, 306-308, 310.

Cells of human body, 63, 64, 66.

Centigrade thermometer, 15.

Central heating plant, 19.

Chalk, in making carbon dioxide, 55.

Charcoal as a filter, 57. commercially, 57. preparation, 57-58.

Chemical action, and electricity, 307, 315-317. and light, 126, 127.

Chemistry, in daily life, 218, 219.

Chills, 38.

Chloride of lime, in bleaching, 240. disinfectant, 251.

Chlorine, and hydrogen, 239. effect upon human body, 239. in bleaching, 238-240. influence of light upon, 126. presence in salt, 227.

Circuit, electric, 321. local, in telegraph, 325-326.

City water supply, 206-212.

Clarinet, 297.

Cleaning of material, 226, 243.

Climate, influenced by presence of water, 29, 40.

Clover, nitrogen producers, 263.

Coal, 30.

Coal gas, 150, 151. by-products, 152.

Coal oil, 149, 150.

Coal tar dyes, 152, 218, 245.

Cogwheels, 170.

Coil, current-bearing, 320. magnetic field about, 331-333.

Coke, 152.

Cold storage, 97.

Color, 134-141. and heat, 142, 143. influenced by light, 137. of opaque bodies, 136, 137. of transparent bodies, 135, 136.

Color blindness, 140, 141. designs in cloth, 248, 249.

Colors, compound, 138, 139. essential, 139-140. primary, 135. simple, 138. spectrum, 134-135. variety in dyeing, 247, 248.

Combustion, heat of, 45. spontaneous, 52.

Commutator, 335.

Compass, 328.

Compound colors, 138, 139.

Compound machine, 171.

Compound substances, 103.

Compression of air, 91, 92. cause of heat, 96.

Compression pumps, 201, 205.

Concave lens, 118.

Condensation, 33. heat set free, 40.

Conduction of heat, 25.

Conductivity metals, 321.

Conductors, electric, 321, 322.

Conservation, of energy, 58, 59. of matter, 58, 59.

Convection, 24, 25.

Convex lens, 118.

Cooling, by evaporation, 35-36. by expansion, 97.

Copper, in electric cell, 307.

Core, iron, 319.

Corn, bleached with sulphurous acid, 242.

Cotton, mercerized, 218. bleaching, 241. dyeing, 245-247.

Cough sirup, 258.

Crane, compound machine, 172.

Cream of tartar, 229.

Creosote oil, 254.

Crude petroleum, 149, 150.

Current, electric, 306, 312. alternating, 349. induced, 346-347. measurement of, 340. resistance, 312, 343, 345. strength, 339, 340, 344.

Dams, 214-216.

Decay, 49.

Decomposition of soil by water, 70-74.

Degrees Fahrenheit and Centigrade, 15.

Density, 11.

Designs in cloth, printed, 248, 249. woven, 249.

Developer in photography, 128.

Dew, 36, 37.

Dew point, 38.

Diarrhea, 251.

Diet, 62, 66. economy on table, 66-69.

Discord, reason for, 271.

Disease, and surface water, 76. relation of light to, 131-132.

Disease disinfectants, 250, 251, 252.

Distillation, 34-35. in commerce, 35. of petroleum, 149-150. of soft coal, 150. of water, 34, 35, 77.

Diving suits, 204.

Door bells, 319-321.

Drainage, of land, 194, 195. sewage, 196, 198, 199, 201.

Drilled well, 199.

Drinking water, 75-77. in camping, 195-196. and rural supplies, 198, 201.

Driven well, 196-197.

Drought, 217.

Drugs, 255, 260.

Dry cell, 312.

Dyeing, 244-249. color designs, 248.

Dyeing, direct, 245. home, 247. indirect, 247. variety of color, 247.

Dyes, 218, 244, 245.

Dynamo, 346. alternating current, 349. source of energy, 346-347.

Ear, in man, 301-303. care of, 303.

Earth, conductor of electricity, 326.

Echo, 277.

Economy in buying food, 66-69.

Effort, muscular, 155, 160.

Electric, battery, 311. bell, 319-321. bread toasters, 314. conductors and non-conductors, 321-322. cost of, energy, 352. current, 306, 312. flatiron, 313. heating pad, 314. lights, 314. street cars, 337.

Electricity, heat, 312-315, 339. as a magnet, 319, 331-333. practical uses of, 312-317.

Electrodes, of cell, 308.

Electrolytic metals, 317.

Electromagnets, 319.

Electromotive force, 308. unit of, 344.

Electroplating, 315.

Electrotyping, 317.

Elements, 102-103.

Emulsion, 224.

Energy, conservation of, 58, 59. transformations of, 58, 59.

Engine, steam, 183-185. gas, 185-186. horse power, 173.

Erosion, 73-74.

Essential colors, 139-140.

Evaporation, 35-39. cooling effect, 35-36. effect of temperature on, 35, 36. effect of air on, 38. freezing by, 98. heat absorbed, 36. of perspiration, 38.

Expansion, of air, 10, 11. cooling effect of, 97. disadvantage and advantage of, 11-13. of liquids, 9-11. of solids, 10, 11. of water, 9, 10, 11, 12. Eye, 122-125. headache, 124, 125. how focused, 122, 123. nearsighted and farsighted, 123. strain, 125.

Fahrenheit thermometer, 15.

Fats, 65. in soap making, 223.

Fermentation, 232-236. by yeast, 234-236.

Ferric compounds, 248.

Fertilizers, 262-265. nitrogen, 262. phosphorus, 263, 264. potash, 263-265.

Field magnet, 336.

Filings, iron, 329.

Film, photographic, 129.

Filter, charcoal, 57.

Filtering water, 77.

Fire, 9. and oxygen, 45, 47. and tinder box, 47. making of, 51. primitive production of, 47. produced by friction, 47. spontaneous combustion, 52. sores and burns, 52-53. extinguisher, 55, 56.

Fireless cooker, 25, 26.

Fireplaces, 17, 18.

Fixing, in photography, 128.

Flame, hydrogen, 80.

Flood, Johnstown, 214, 215. relation to forests, 217.

Flour, self-raising, 231.

Flume, 177.

Flute, 297.

Focal length, 118.

Focus, of lens, 118.

Fog, 37.

Food, 60-69. carbohydrates, 64, 65. economy in buying, 66-69. fats, 65. fuel value of, 60-62. need of, 63, 64. preservatives, 252. proteids, 66. value, 67. waste, 60. water in, 75.

Foot pound, 172.

Force and motion, 156, 157. and work, 156, 157. magnetic lines of, 329-331, 334. muscular, 155, 160.

Force pumps, 192, 193.

Forests and water supply, 216-217.

Forging of iron, 40, 41.

Formaldehyde, 253.

Freezing, effect of salt, 44. effect on ground and rocks, 42. expansion of water on, 41. ice cream freezer, 44.

Frequency in music, 273, 275.

Fresh air, 22-24, 49. amount consumed by gas burner, 22. and health, 49, 50. in underground work, 202. in work under water, 203-205.

Friction, 173, 174. losses by, 174, 210. source of heat and fire, 47.

Frost, 36, 37.

Fruit, canned, bleached with sulphurous acid, 242. colored with coal tar dyes, 253.

Fuel value of foods, 60-62. table of fuel values, 67.

Fulcrum, 159, 160.

Fumigation, 251.

Fundamental tone, 290, 291, 292.

Furnace, hot air, 19.

Fuse, 340.

Fusion, heat of, 40.

Galvanometer, 341.

Gas, acetylene, 152, 153. and unburned carbon, 151. coal, 151, 152. effect of heat on volume, 96, 97. effect of pressure on volume, 95-96. engine, 185-186. for cooking, 151, 152. illuminating, 92, 93, 150, 151. liquefaction, 97, 98. meter, 93, 94. natural, 152.

Gasolene, 149, 150. as cleaning agent, 227, 243. in gas engine, 185, 186.

Gauge, pressure, 92-94.

Gelatin, plate and film, 129.

Glass, kinds of, 119. molding of, 40. non-conductor, 321.

Grape juice, fermented with millet, 233.

Gravity cell, 309, 310.

Grease, and lye, 221. and soap making, 223.

Gulf Stream, 24.

Hard water, and soap, 225.

Harp, 295.

Headache, 124, 125. powders, 259.

Health, effect of diet, 62, 64.

Heat, 9. absorbed in boiling, 31-32. and disease germs, 250. and food, 252. and friction, 47. and light, 142, 147. and oxidation, 45, 48, 49. and wave motion, 145-147. conduction, 25. convection, 24, 25. from burning hydrogen, 80. from electricity, 312-315, 339. needed to melt substances, 39. of fusion, 40. of vaporization, 32. produced by compression, 96. relation of water to weather, 29, 40. set free by freezing water, 40. sources of, 29-30. specific, 28-29. temperature, 27. unit of, 27, 28.

Heating effect of electric current, 312-315.

Heating of buildings: central heating plant, 19. fireplaces, 17-18.

Heating, furnaces, 19. hot water, 19-22.

Helix, 318.

Horse power, 173, 351.

Hot water heating, 19-22.

Hues, primary, 135.

Humidity, 38. proper percentage for health and comfort, 38, 39.

Humus, 216, 217.

Hydrocarbons, 149.

Hydrochloric acid, composition, 227. in bleaching, 241. to make a salt, 227. to make carbon dioxide, 55. to make chlorine, 238. to make hydrogen, 80.

Hydrogen, 65, 66. and chlorine, 239. and water, 79. chemical conduct, 126-127. flame, 80. in voltaic cell, 307. peroxide, 53, 252. preparation, 80. to liquefy, 97.

Ice, lighter than water, 42. manufacture of, 98, 99.

Ice cream freezers, 44.

Illuminating gas, manufacture of, 150, 151. measurement of quantity consumed, 93, 94. test of pressure, 92, 93.

Illumination, intensity of, 105, 106.

Image, in mirror, 108, 111.

Incandescent lighting, 107, 314.

Incidence, angle of, 110.

Inclined plane, 162-166. screw, 166. wedge, 166.

Indigo, 218.

Induced current, 346-347.

Ink spots, removal of, 243.

Insoluble substances, 71.

Insulators, electric, 324.

Intensity, of light, 105-107. of sound, 270-271.

Interval, in musical scale, 283.

Iron, forging, 41. filings, 329. galvanizing, 49. oxidation of, 48.

Irrigation, 193-194.

Isobaric lines, 88, 91.

Isothermal lines, 89, 91.

Johnstown flood, 214, 215.

Kerosene, 149, 150.

Kilowatt, 351.

Lactic acid, 230.

Leaves, 132, 262.

Lens, 117-121. concave, 118. converging, 118. crystalline, of eye, 122. focal length, 118. material, 119. refractive power, 119.

Lever, 158-162. examples, 160-162. fulcrum, 159, 160.

Life, and carbon dioxide, 54. and nitrogen, 261. and oxygen, 49, 54.

Lifting pumps, 189-192.

Light, absorption, 135-138. and heat, 142-147. a wave motion, 145-147. bent rays, 113, 114. chemical action, 126-127. disease, 131-132. essential to life, 131, 132. fading illumination, 105, 106. influence on color, 134. reflection of, 109-112. refraction of, 113-125. travels in a straight line, 108. white, composed of colors, 134.

Lighting, artificial, 148-153.

Lime, chloride of, 240, 251.

Limewater, 220. and carbon dioxide, 228.

Linen, bleaching, 241. dyeing, 245-247.

Lines, of force, 329-331, 334. isobaric, 88, 91. isothermal, 89, 91.

Liquefaction of gases, 97, 98.

Liquid air, 98.

Liquid soap, 223, 224.

Litmus, action of acids, 220. action of bases, 221, 222. action of neutral substance, 222.

Logwood dyes, 245, 247, 248.

Los Angeles aqueduct, 211.

Lye, 221, 222.

Machines, compound, 171. inclined plane, 162-166. lever, 158-162. pulley, 166-169. wheel and axle, 169-171.

Madder, for dyes, 245.

Magnet, 328. electro-, 319. field of, 329-331. lines of force about, 329-331. poles of, 330-332. properties of electricity, 318.

Magnetic, needle, 328. poles, 329-331.

Magnifying power, of a lens, 115. of a microscope, 115. of a telescope, 115.

Mammoth Cave of Kentucky, 71.

Manganese dioxide, 46. chlorine made from, 238. oxygen made from, 46.

Marble, for carbon dioxide, 55.

Matches, 47. safety, 47-48.

Matching colors, 137.

Matter, conservation of, 58, 59.

Meat, 66. preservation of, 253.

Mechanical devices, 154, 155.

Melting, 39, 40.

Melting point, 40.

Melting substances without a definite melting point, 40.

Mercerized cotton, 218.

Mercury, barometer, 84. thermometer, 14-17.

Metals, electroplating, 317. preservation by paint, 253-254. veins deposited by precipitation, 72, 73. welding, 315.

Meter, gas, 93, 94.

Microoerganisms, 132, 133.

Microscope, 115.

Milk, boiling point, 32. Pasteurized, 250.

Minerals, in foods, 62, 63. in water, 70, 71.

Mirrors, 108-112. distance of image behind mirror, 111. distance of object in front of mirror, 111. image a duplicate of object. 111.

Molding of glass, 40.

Molecule, 100-103.

Mordants, 247, 248, 249.

Morphine, 257.

Morse, telegraphic code, 324.

Motion, in sound, 266, 278, 280. in work, 156.

Motor, electric, 336. principle of, 333. street car, 337.

Mouth breathing, 50. cause of, 51.

Movable pulley, 167, 168.

Music, 278.

Musical instruments, percussion, 299. stringed, 284-295. wind, 295, 299.

Musical scale, 282.

Naphtha in gas engines, 185.

Naphthalene, 152.

Narcotics, 255.

Natural gas, 152.

Needle, magnetic, 328.

Negative, electrode, 308. photographic, 130.

Neutral substance, 222. and litmus, 222.

Neutralization, 222.

Niagara Falls, 176.

Nitrogen, 66. and bacteria, 263. and plant life, 261. in atmosphere, 261. in fertilizer, 262-265. in food, 66. preparation of, 261. properties of, 261.

Noise in music, 280.

Non-conductors, of electricity, 321-322. of heat, 25.

Nutcracker, as a lever, 162.

Oboe, 297.

Octave, 284.

Odors, 101.

Ohm, unit of resistance, 345.

Oil, gasoline, 149, 150. kerosene, 149, 150. lubricating, 174. olive, 16.

Orchestra grouping, 299.

Ore, 72.

Organ pipes, 297.

Overtones, 290-293.

Oxalic acid, 247, 248.

Oxidation, 45-59. and decay, 49. heat the result of, 49-52. in human body, 49, 53. of iron, 48.

Oxygen, 66. and bleaching, 239. and combustion, 45. and food, 66. and plants, 55. and the human body, 50. and water, 79, 80. in the atmosphere, 45. preparation of, 46.

Paint, as wood and metal preservatives, 253, 254. removal of stains, 243.

Paper making, 219.

Paraffin, 150, 321.

Pasteurized milk, 250.

Patent medicines, 257-260.

Peas, sources of nitrogen, 263.

Pelton wheel, 177.

Percussion instruments, 299.

Period of a body, 273.

Peroxide of hydrogen, 53, 252

Petrolatum, 150.

Petroleum, 149, 150.

Phonograph, 303-305.

Phosphorus, in fertilizer, 263, 264. in making nitrogen, 261. in matches, 47, 48. poisoning by, 47.

Photography, 127-131.

Photometer, 107.

Pianos, 284-292.

Pin wheel, 181.

Pitch of sound, 280, 281. cause of, 282. in wind instruments, 296-299.

Plane, inclined, 162-166.

Plants, and atmosphere, 55. and light, 131-132. and nitrogen, 261.

Plate developing, photographic, 128.

Pneumatic dispatch tube, 205.

Poles, magnetic, 330-332. of cell, 308.

Positive electrode, 308.

Potash, in fertilizer, 263-265.

Potassium chlorate and oxygen, 46. permanganate, 100. tartrate and Rochelle salt, 227.

Power, candle, 105-107. electric, 351. horse, 173, 351. sources of, 174, 175, 185. transmission by belts, 171. water, 176-180.

Precipitation, 72, 73.

Preservatives, food, 252. wood and metal, 253-254.

Pressure, atmospheric, 82-86. calculation of atmospheric, 83, 84. calculation of gas, 92, 93. calculation of water, 94. gauge, 92-94. of illuminating gas, 93. relation of pressure of gas to volume, 95, 96. water pressure, 208-211, 214-216. within the body, 86.

Primary colors, 135.

Print, photographic, 131.

Printing, color designs in cloth, 248, 249. electrotype, 317.

Prisms, 135. refraction through, 117.

Proteids, 66.

Pulleys, 166-169. applications of, 169.

Pump, 187-205. air, 201-205. force, 192, 193. lifting, 189-192.

Pupil of the eye, 122.

Pure food laws, bleaching, 242. preservatives, 252.

Purification of water, 77, 196.

Push button, 321.

Radiator, 19-21.

Railroads, grading of, 165-166.

Rain, 36, 37.

Rainbow, 134.

Rain water, 225.

Reflection, angle of, 110. of light, 109-112. of sound, 278, 279.

Refraction, angle of, 114. by atmosphere, 114. of light, 113. uses of, 115-116.

Relay, telegraph, 325.

Reservoir, 214. artificial, 211. construction of, 214-216. natural, 211.

Resistance, electrical, 312. internal, of cell, 343. unit of, 345.

Resonance, 276.

River, volume and value of, 180.

Roads, application of inclined plane to, 165-166.

Rochelle salt, 227, 231.

Rocks, effect of freezing water on, 42-43. water as a solvent, 71.

Rosin, obtained by distillation, 35.

Safety matches, 47-48.

Salicylic acid, 253.

Salt, 227-228.

Salts, 227. general properties, 227. in ocean, 227. smelling, 222.

Saturation of air, 37.

Scale, musical, 282.

Screw, and inclined plane, 166.

Seaweed, 265.

Seidlitz powder, 231.

Self-raising flour, 231.

Sewage, disposition of, 198-199. of camps, 196. source of revenue, 201.

Sewer gas, 57.

Silk, bleaching, 241. dyeing, 245-247.

Silver chloride, 127-131.

Simple colors, 138.

Simple substances, 103.

Siren, 280.

Smelling salts, 222.

Snow, 36-37.

Soap, 222-224. and hard water, 225. liquid, 223-224. preparation, 223.

Soda, baking, 227, 228-229. benzoate, 253. caustic, 218, 222, 223, 227. washing, 225, 226, 229.

Soda mints, 231.

Sodium, bicarbonate, 56, 227, 228, 230-231. carbonate, 228. chloride, 228.

Soil, deposited by streams, 73.

Solenoid, 318.

Solution, 70.

Soothing sirup, 258.

Sound, and motion, 266, 278. musical, 278. nature of, 266. reflection, 277. speed of, 271-272. transmission of, 267-271. velocity of, 271-272. waves, 272-274.

Sounder, telegraph, 324.

Sounding board, 277.

Sour milk in cooking, 230.

Specific heat, 28-29.

Spectrum, 134-135.

Speed, of sound, 271, 272.

Spontaneous combustion, 52.

Stains, removal of, 226, 243.

Standpipes, 212.

Starch, 65.

Steam, and work, 183-184. engine, 183-185. heat of vaporization, 32. heating by, 33. turbine, 183-184.

Steel, forging and annealing, 16.

Stoves, 18-19.

Streams, carriers of mud, 73. volume of, 179-180.

Street cars, electric, 337.

Stringed instruments, 284-295.

Strings, vibrating, 286-290.

Sugar, 16, 65. fermented by yeast, 234.

Sulphur, 66. as disinfectant, 251. in making sulphurous acid, 242.

Sulphuric acid, in bleaching, 240,241. in fire extinguisher, 55. in making of hydrogen, 80. in voltaic cell, 307.

Sulphurous acid, in bleaching, 242. preparation, 242.

Sun, energy derived from, 143-144. source of heat, 29-30.

Sunlight, 135. and bacteria, 133. and chemical action, 126-127.

Sympathetic vibrations, 274-277.

Tallow, 105, 148.

Tartar, cream of, 229.

Telegraph, 322. long distance, 327. relay, 325. sounder, 324.

Telephone, 350-351.

Temperature, 13-14. as measurement of heat present, 27. in detecting adulterants, 17. in forging steel, 16. in making sirups, 16. measurement of, 14-15.

Thermometer, 14-17. Centigrade, 15. Fahrenheit, 15.

Tinder box, 47.

Transmission, of light, 145-147. of sound, 267-271.

Tuning fork, 266, 273, 278, 290.

Turbine, steam, 183. water, 178.

Turpentine, and grease, 226. by distillation, 35.

Unleavened bread, 233.

Vacuum, sound in, 268.

Vapor, in atmosphere, 36-38.

Vaporization, heat of, 32.

Varnish, on candies, 253.

Vegetable matter, and coal, 30. and gas, 30. and oil, 30.

Veins, formation in rock, 72-73.

Velocity, of sound, 271-272.

Ventilation, 21-24, 54. need of, 38.

Vibration, of strings, 286-290. sympathetic, 274-277.

Viola, 295.

Violin, 295.

Violoncello, 295.

Vocal cords, 300.

Voice, 300.

Volt, 344.

Voltage, 345.

Voltaic cell, 306-308, 310.

Voltmeter, 344.

Volume, of a stream, 179-180. relation of pressure of a gas, 95-96.

Washing powders, 224-226. soda, 229.

Water, action in nature, 70-74. amount used daily per person, 181. and hydrogen, 79. and oxygen, 79, 80. as solvent, 70-71. boiling, 77. boiling point, 15. composition, 79-80. condensation, 33. dams and reservoirs, 214-216. density, 11. distilled, 34, 77. drinking, 75-77, 195-201. electrolysis, 79-80. evaporation, 33-34. expansion, 9-10, 41-42. filtration, 77. freezing, 40-41. hard, 225. heat of fusion, 40. impurities, 76-77. in atmosphere, 36-38. in food, 75. in human body, 75. in vegetables, 75. influence on climate, 29, 40. irrigation, 193-194. minerals in, 70-71. ocean, 265. power, 176-180. precipitates, 72, 73. pressure, 208-211, 214-216. purification, 77. rain, 225. running, value of, 178-180. source of, 78. steam, 32. waves, 145-147. weight, 208-209, 215. wells, 195-201. wheels, 176-180. work under, 203-205.

Water supply, and forests, 216-217. cost, 212-214. of city, 206-212, 217.

Watt, 351.

Waves, heat, 145-147. light, 145-147. sound, 268, 272-274. water, 145-147.

Weather, bureau, 87-91. forecasts, 38-39, 86-88. relation of water to, 29, 40.

Weather maps, 89-91.

Wedge, and inclined plane, 166.

Weight, of air, 86. of water, 208-209, 215.

Welding, by electricity, 315.

Wells, 195-201. drilled, 199. driven, 196-197.

Wheel and axle, 169-171. cogwheels, 170. windlass, 169.

Wheelbarrow as lever, 160-161.

White light, nature of, 135.

Wind instruments, 297-301.

Windlass, 169.

Windmill, 174-175, 180-182.

Winds, 24.

Wine, 232, 234.

Wood, as source of charcoal, 58. ashes in soap making, 223. in paper making, 219. preservation, 253-254.

Wool, bleaching, 241. dyeing, 245-247.

Work, 156-186. and steam, 183-184. and water, 176-180. conservation, 174-175. formula, 157. machines, 157-175. unit of, 172-173. waste, 173.

Woven designs in cloth, 249.

Yeast, 234-236. wild, 235-236.

Zinc, in galvanizing iron, 49. in making hydrogen, 80. in voltaic cell, 307-308.



PLANT LIFE AND PLANT USES

By JOHN GAYLORD COULTER, Ph. D.

$1.20

An elementary textbook providing a foundation for the study of agriculture, domestic science, or college botany. But it is more than a textbook on botany—it is a book about the fundamentals of plant life and about the relations between plants and man. It presents as fully as is desirable for required courses in high schools those large facts about plants which form the present basis of the science of botany. Yet the treatment has in view preparation for life in general, and not preparation for any particular kind of calling.

The subject is dealt with from the viewpoint of the pupil rather than from that of the teacher or the scientist. The style is simple, clear, and conversational, yet the method is distinctly scientific, and the book has a cultural as well as a practical object.