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If the Leyden jars of a Wimshurst machine are connected up and the discharging balls placed at a suitable distance apart, the electricity produced by rotating the plates is discharged in the form of a brilliant zigzag spark between the balls, accompanied by a sharp crack. The resemblance between this spark and forked lightning is at once evident, and in fact it is lightning in miniature. The discharging balls are charged, as we have seen, with opposite kinds of electricity, and these charges are constantly trying to reach one another across the intervening air, which, being an insulator, vigorously opposes their passage. There is thus a kind of struggle going on between the air and the two charges of electricity, and this keeps the air in a state of constant strain. But the resisting power of the air is limited, and when the charges reach a certain strength the electricity violently forces its way across, literally rupturing or splitting the air. The particles of air along the path of the discharge are rendered incandescent by the heat produced by the passage of the electricity, and so the brilliant flash is produced. Just as a river winds about seeking the easiest course, so the electricity takes the path of least resistance, which probably is determined by the particles of dust in the air, and also by the density of the air, which becomes compressed in front, leaving less dense air and therefore an easier path on each side.

The connexion between lightning and the sparks from electrified bodies and electrical machines was suspected by many early observers, but it remained for Benjamin Franklin to prove that lightning was simply a tremendous electric discharge, by actually obtaining electricity from a thunder-cloud. Franklin was an American, born at Boston in 1706. He was a remarkable man in every way, and quite apart from his investigations in electricity, will always be remembered for the great public services he rendered to his country in general and to Philadelphia in particular. He founded the Philadelphia Library, the American Philosophical Society, and the University of Pennsylvania.

Franklin noticed many similarities between electricity and lightning. For instance, both produced zigzag sparks, both were conducted by metals, both set fire to inflammable materials, and both were capable of killing animals. These resemblances appeared to him so striking that he was convinced that the two were the same, and he resolved to put the matter to the test. For this purpose he hit upon the idea of using a kite, to the top of which was fixed a pointed wire. At the lower end of the flying string was tied a key, insulated by a piece of silk ribbon. In June 1752, Franklin flew his kite, and after waiting a while he was rewarded by finding that when he brought his knuckle near to the key a little spark made its appearance. This spark was exactly like the sparks obtained from electrified bodies, but to make things quite certain a Leyden jar was charged from the key. Various experiments were then performed with the jar, and it was proved beyond all doubt that lightning and electricity were one and the same.

Lightning is then an enormous electric spark between a cloud and the Earth, or between two clouds, produced when opposite charges become so strong that they are able to break down the intervening non-conducting layer of air. The surface of the Earth is negatively electrified, the electrification varying at different times and places; while the electricity of the air is usually positive, but frequently changes to negative in rainy weather and on other occasions. As the clouds float about they collect the electricity from the air, and thus they may be either positively or negatively electrified, so that a discharge may take place between one cloud and another, as well as between a cloud and the Earth.

Lightning flashes take different forms, the commonest being forked or zigzag lightning, and sheet lightning. The zigzag form is due to the discharge taking the easiest path, as in the case of the spark from a Wimshurst machine. Sheet lightning is probably the reflection of a flash taking place at a distance. It may be unaccompanied by thunder, as in the so-called "summer lightning," seen on the horizon at night, which is the reflection of a storm too far off for the thunder to be heard. A much rarer form is globular or ball lightning, in which the discharge takes the shape of a ball of light, which moves slowly along and finally disappears with a sudden explosion. The cause of this form of lightning is not yet understood, but it is possible that the ball of light consists of intensely heated and extremely minute fragments of ordinary matter, torn off by the violence of the lightning discharge. Another uncommon form is multiple lightning, which consists of a number of separate parallel discharges having the appearance of a ribbon.

A lightning flash probably lasts from about 1/100,000 to 1/1,000,000 of a second, and in the majority of cases the discharge is oscillatory; that is to say, it passes several times backwards and forwards between two clouds or between a cloud and the Earth. At times it appears as though we could see the lightning start downwards from the cloud or upwards from the Earth, but this is an optical illusion, and it is really quite impossible to tell at which end the flash starts.

Death by lightning is instantaneous, and therefore quite painless. We are apt to think that pain is felt at the moment when a wound is inflicted. This is not the case however, for no pain is felt until the impression reaches the brain by way of the nerves, and this takes an appreciable time. The nerves transmit sensations at a speed of only about one hundred feet per second, so that in the case of a man killed by a bullet through the brain, no pain would be felt, because the brain would be deprived of sensibility before the sensation could reach it. Lightning is infinitely swifter than any bullet, so life would be destroyed by it before any pain could be felt.

On one occasion Professor Tyndall, the famous physicist, received accidentally a very severe shock from a large battery of Leyden jars while giving a public lecture. His account of his sensations is very interesting. "Life was absolutely blotted out for a very sensible interval, without a trace of pain. In a second or so consciousness returned; I saw myself in the presence of the audience and apparatus, and, by the help of these external appearances, immediately concluded that I had received the battery discharge. The intellectual consciousness of my position was restored with exceeding rapidity, but not so the optical consciousness. To prevent the audience from being alarmed, I observed that it had often been my desire to receive accidentally such a shock, and that my wish had at length been fulfilled. But, while making this remark, the appearance which my body presented to myself was that of a number of separate pieces. The arms, for example, were detached from the trunk, and seemed suspended in the air. In fact, memory and the power of reasoning appeared to be complete long before the optic nerve was restored to healthy action. But what I wish chiefly to dwell upon here is, the absolute painlessness of the shock; and there cannot be a doubt that, to a person struck dead by lightning, the passage from life to death occurs without consciousness being in the least degree implicated. It is an abrupt stoppage of sensation, unaccompanied by a pang."

Occasionally branched markings are found on the bodies of those struck by lightning, and these are often taken to be photographic impressions of trees under which the persons may have been standing at the time of the flash. The markings however are nothing of the kind, but are merely physiological effects due to the passage of the discharge.

During a thunderstorm it is safer to be in the house than out in the open. It is probable that draughts are a source of some danger, and the windows and doors of the room ought to be shut. Animals are more liable to be struck by lightning than men, and a shed containing horses or cows is a dangerous place in which to take shelter; in fact it is better to remain in the open. If one is caught in a storm while out of reach of a house or other building free from draughts and containing no animals, the safest plan is to lie down, not minding the rain. Umbrellas are distinctly dangerous, and never should be used during a storm. Wire fences, hedges, and still or running water should be given a wide berth, and it is safer to be alone than in company with a crowd of people. It is extremely foolish to take shelter under an isolated tree, for such trees are very liable to be struck. Isolated beech trees appear to have considerable immunity from lightning, but any tree standing alone should be avoided, the oak being particularly dangerous. On the other hand, a fairly thick wood is comparatively safe, and failing a house, should be chosen before all other places of refuge. Horses are liable to be struck, and if a storm comes on while one is out driving it is safer to keep quite clear of the animals.

Thunder probably is caused by the heating and sudden expansion of the air in the path of the discharge, which creates a partial vacuum into which the surrounding air rushes violently. Light travels at the rate of 186,000 miles per second, and therefore the flash reaches us practically instantaneously; but sound travels at the rate of only about 1115 feet per second, so that the thunder takes an appreciable time to reach us, and the farther away the discharge the greater the interval between the flash and the thunder. Thus by multiplying the number of seconds which elapse between the flash and the thunder by 1115, we may calculate roughly the distance in feet of the discharge. A lightning flash may be several miles in length, the greatest recorded length being about ten miles. The sounds produced at different points along its path reach us at different times, producing the familiar sharp rattle, and the following rolling and rumbling is produced by the echoes from other clouds. The noise of a thunder-clap is so tremendous that it seems as though the sound would be heard far and wide, but the greatest distance at which thunder has been heard is about fifteen miles. In this respect it is interesting to compare the loudest thunder-clap we ever heard with the noise of the famous eruption of Krakatoa, in 1883, which was heard at the enormous distance of nearly three thousand miles.

When Franklin had demonstrated the nature of lightning, he began to consider the possibility of protecting buildings from the disastrous effects of the lightning stroke. At that time the amount of damage caused by lightning was very great. Cathedrals, churches, public buildings, and in fact all tall edifices were in danger every time a severe thunderstorm took place in their neighbourhood, for there was absolutely nothing to prevent their destruction if the lightning chanced to strike them. Ships at sea, too, were damaged very frequently by lightning, and often some of the crew were killed or disabled. To-day, thanks to the lightning conductor, it is an unusual occurrence for ships or large buildings to be damaged by lightning. The lightning strikes them as before, but in the great majority of cases it is led away harmlessly to earth.

Franklin was the first to suggest the possibility of protecting buildings by means of a rod of some conducting material terminating in a point at the highest part of the building, and leading down, outside the building, into the earth. Lightning conductors at the present day are similar to Franklin's rod, but many improvements have been made from time to time as our knowledge of the nature and action of the lightning discharge has increased. A modern lightning conductor generally consists of one or more pointed rods fixed to the highest parts of the building, and connected to a cable running directly to earth. This cable is kept as straight as possible, because turns and bends offer a very high resistance to the rapidly oscillating discharge; and it is connected to large copper plates buried in permanently moist ground or in water, or to water or gas mains. Copper is generally used for the cable, but iron also may be employed. In any case, the cable must be of sufficient thickness to prevent the possibility of its being deflagrated by the discharge. In ships the arrangements are similar, except that the cable is connected to the copper sheathing of the bottom.

The fixing of lightning conductors must be carried out with great care, for an improperly fixed conductor is not only useless, but may be a source of actual danger. Lightning flashes vary greatly in character, and while a carefully erected lightning conductor is capable of dealing with most of them, there are unfortunately certain kinds of discharge with which it now and then is unable to deal. The only absolutely certain way of protecting a building is to surround it completely by a sort of cage of metal, but except for buildings in which explosives are stored this plan is usually impracticable.

The electricity of the atmosphere manifests itself in other forms beside the lightning. The most remarkable of these manifestations is the beautiful phenomenon known in the Northern Hemisphere as the Aurora Borealis, and in the Southern Hemisphere as the Aurora Australis. Aurora means the morning hour or dawn, and the phenomenon is so called from its resemblance to the dawn of day. The aurora is seen in its full glory only in high latitudes, and it is quite unknown at the equator. It assumes various forms, sometimes appearing as an arch of light with rapidly moving streamers of different colours, and sometimes taking the form of a luminous curtain extending across the sky. The light of the aurora is never very strong, and as a rule stars can be seen through it. Auroras are sometimes accompanied by rustling or crackling sounds, but the sounds are always extremely faint. Some authorities assert that these sounds do not exist, and that they are the result of imagination, but other equally reliable observers have heard the sounds quite plainly on several occasions. Probably the explanation of this confliction of evidence is that the great majority of auroras are silent, so that an observer might witness many of them without hearing any sounds. The height at which auroras occur is a disputed point, and one which it is difficult to determine accurately; but most observers agree that it is generally from 60 to 125 miles above the Earth's surface.

There is little doubt that the aurora is caused by the passage of electric discharges through the higher regions of the atmosphere, where the air is so rarefied as to act as a partial conductor; and its effects can be imitated in some degree by passing powerful discharges through tubes from which the air has been exhausted to a partial vacuum. Auroral displays are usually accompanied by magnetic disturbances, which sometimes completely upset telegraphic communication. Auroras and magnetic storms appear to be connected in some way with solar disturbances, for they are frequently simultaneous with an unusual number of sunspots, and all three run in cycles of about eleven and a half years.

THE ELECTRIC CURRENT

Somewhere about the year 1780 an Italian anatomist, Luigi Galvani, was studying the effects of electricity upon animal organisms, using for the purpose the legs of freshly killed frogs. In the course of his experiments he happened to hang against an iron window rail a bundle of frogs' legs fastened together with a piece of copper wire, and he noticed that the legs began to twitch in a peculiar manner. He knew that a frog's leg would twitch when electricity was applied to it, and he concluded that the twitchings in this case were caused in the same way. So far he was quite right, but then came the problem of how any electricity could be produced in these circumstances, and here he went astray. It never occurred to him that the source of the electricity might be found in something quite apart from the legs, and so he came to the conclusion that the phenomenon was due to electricity produced in some mysterious way in the tissues of the animal itself. He therefore announced that he had discovered the existence of a kind of animal electricity, and it was left for his fellow-countryman, Alessandro Volta, to prove that the twitchings were due to electricity produced by the contact of the two metals, the iron of the window rail and the copper wire.

Volta found that when two different metals were placed in contact in air, one became positively charged, and the other negatively. These charges however were extremely feeble, and in his endeavours to obtain stronger results he hit upon the idea of using a number of pairs of metals, and he constructed the apparatus known as the Voltaic pile, Fig. 6. This consists of a number of pairs of zinc and copper discs, each pair being separated from the next pair by a disc of cloth moistened with salt water. These are piled up and placed in a frame, as shown in the figure. One end of the pile thus terminates in a zinc disc, and the other in a copper disc, and as soon as the two are connected by a wire or other conductor a continuous current of electricity is produced. The cause of the electricity produced by the voltaic pile was the subject of a long and heated controversy. There were two main theories; that of Volta himself, which attributed the electricity to the mere contact of unlike metals, and the chemical theory, which ascribed it to chemical action. The chemical theory is now generally accepted, but certain points, into which we need not enter, are still in dispute.

There is a curious experiment which some of my readers may like to try. Place a copper coin on a sheet of zinc, and set an ordinary garden snail to crawl across the zinc towards the coin. As soon as the snail comes in contact with the copper it shrinks back, and shows every sign of having received a shock. One can well imagine that an enthusiastic gardener pestered with snails would watch this experiment with great glee.

Volta soon found that it was not necessary to have his pairs of metals in actual metallic contact, and that better results were got by placing them in a vessel filled with dilute acid. Fig. 7 is a diagram of a simple voltaic cell of this kind, and it shows the direction of the current when the zinc and the copper are connected by the wire. In order to get some idea of the reason why a current flows we must understand the meaning of electric potential. If water is poured into a vessel, a certain water pressure is produced. The amount of this pressure depends upon the level of the water, and this in turn depends upon the quantity of water and the capacity of the vessel, for a given quantity of water will reach a higher level in a small vessel than in a larger one. In the same way, if electricity is imparted to a conductor an electric pressure is produced, its amount depending upon the quantity of electricity and the electric capacity of the conductor, for conductors vary in capacity just as water vessels do.

This electric pressure is called "potential," and electricity tends to flow from a conductor of higher to one of lower potential. When we say that a place is so many feet above or below sea-level we are using the level of the sea as a zero level, and in estimating electric potential we take the potential of the earth's surface as zero; and we regard a positively electrified body as one at a positive or relatively high potential, and a negatively electrified body as one at a negative or relatively low potential. This may be clearer if we think of temperature and the thermometer. Temperatures above zero are positive and represented by the sign +, and those below zero are negative and represented by the sign -. Thus we assume that an electric current flows from a positive to a negative conductor.

The current from a simple voltaic cell does not remain at a constant strength, but after a short time it begins to weaken rapidly. The cell is then said to be polarized, and this polarization is caused by bubbles of hydrogen gas which accumulate on the surface of the copper plate during the chemical action. These bubbles of gas weaken the current partly by resisting its flow, for they are bad conductors, and still more by trying to set up another current in the opposite direction. For this reason the simple voltaic cell is unsuitable for long spells of work, and many cells have been devised to avoid the polarization trouble. One of the most successful of these is the Daniell cell. It consists of an outer vessel of copper, which serves as the copper plate, and an inner porous pot containing a zinc rod. Dilute sulphuric acid is put into the porous pot and a strong solution of copper sulphate into the outer jar. When the circuit is closed, the hydrogen liberated by the action of the zinc on the acid passes through the porous pot, and splits up the copper sulphate into copper and sulphuric acid. In this way pure copper, instead of hydrogen, is deposited on the copper plate, no polarization takes place, and the current is constant.

Other cells have different combinations of metals, such as silver-zinc, or platinum-zinc, and carbon is also largely used in place of one metal, as in the familiar carbon-zinc Leclanch? cell, used for ringing electric bells. This cell consists of an inner porous pot containing a carbon plate packed round with a mixture of crushed carbon and manganese dioxide, and an outer glass jar containing a zinc rod and a solution of sal-ammoniac. Polarization is checked by the oxygen in the manganese dioxide, which seizes the hydrogen on its way to the carbon plate, and combines with it. If the cell is used continuously however this action cannot keep pace with the rate at which the hydrogen is produced, and so the cell becomes polarized; but it soon recovers after a short rest.

The so-called "dry" cells so much used at the present time are not really dry at all; if they were they would give no current. They are in fact Leclanch? cells, in which the containing vessel is made of zinc to take the place of a zinc rod; and they are dry only in the sense that the liquid is taken up by an absorbent material, so as to form a moist paste. Dry cells are placed inside closely fitting cardboard tubes, and are sealed up at the top. Their chief advantage lies in their portability, for as there is no free liquid to spill they can be carried about and placed in any position.

We have seen that the continuance of the current from a voltaic cell depends upon the keeping up of a difference of potential between the plates. The force which serves to maintain this difference is called the electro-motive force, and it is measured in volts. The actual flow of electricity is measured in amperes. Probably all my readers are familiar with the terms volt and ampere, but perhaps some may not be quite clear about the distinction between the two. When water flows along a pipe we know that it is being forced to do so by pressure resulting from a difference of level. That is to say, a difference of level produces a water-moving or water-motive force; and in a similar way a difference of potential produces an electricity-moving or electro-motive force, which is measured in volts. If we wish to describe the rate of flow of water we state it in gallons per second, and the rate of flow of electricity is stated in amperes. Volts thus represent the pressure at which a current is supplied, while the current itself is measured in amperes.

We may take this opportunity of speaking of electric resistance. A current of water flowing through a pipe is resisted by friction against the inner surface of the pipe; and a current of electricity flowing through a circuit also meets with a resistance, though this is not due to friction. In a good conductor this resistance is small, but in a bad conductor or non-conductor it is very great. The resistance also depends upon length and area of cross-section; so that a long wire offers more resistance than a short one, and a thin wire more than a thick one. Before any current can flow in a circuit the electro-motive force must overcome the resistance, and we might say that the volts drive the amperes through the resistance. The unit of resistance is the ohm, and the definition of a volt is that electro-motive force which will cause a current of one ampere to flow through a conductor having a resistance of one ohm. These units of measurement are named after three famous scientists, Volta, Amp?re, and Ohm.

A number of cells coupled together form a battery, and different methods of coupling are used to get different results. In addition to the resistance of the circuit outside the cell, the cell itself offers an internal resistance, and part of the electro-motive force is used up in overcoming this resistance. If we can decrease this internal resistance we shall have a larger current at our disposal, and one way of doing this is to increase the size of the plates. This of course means making the cell larger, and very large cells take up a lot of room and are troublesome to move about. We can get the same effect however by coupling. If we connect together all the positive terminals and all the negative terminals of several cells, that is, copper to copper and zinc to zinc in Daniell cells, we get the same result as if we had one very large cell. The current is much larger, but the electro-motive force remains the same as if only one cell were used, or in other words we have more amperes but no more volts. This is called connecting in "parallel," and the method is shown in Fig. 8. On the other hand, if, as is usually the case, we want a larger electro-motive force, we connect the positive terminal of one cell to the negative terminal of the next, or copper to zinc all through. In this way we add together the electro-motive forces of all the cells, but the amount of current remains that of a single cell; that is, we get more volts but no more amperes. This is called connecting in "series," and the arrangement is shown in Fig. 9. We can also increase both volts and amperes by combining the two methods.

A voltaic cell gives us a considerable quantity of electricity at low pressure, the electro-motive force of a Leclanch? cell being about 1 1/2 volts, and that of a Daniell cell about 1 volt. We may perhaps get some idea of the electrical conditions existing during a thunderstorm from the fact that to produce a spark one mile long through air at ordinary pressure we should require a battery of more than a thousand million Daniell cells. Cells such as we have described in this chapter are called primary cells, as distinguished from accumulators, which are called secondary cells. Some of the practical applications of primary cells will be described in later chapters.

Besides the voltaic cell, in which the current is produced by chemical action, there is the thermo-electric battery, or thermopile, which produces current directly from heat energy. About 1822 Seebeck was experimenting with voltaic pairs of metals, and he found that a current could be produced in a complete metallic circuit consisting of different metals joined together, by keeping these joinings at different temperatures. Fig. 10 shows a simple arrangement for demonstrating this effect, which is known as the "Seebeck effect." A slab of bismuth, BB, has placed upon it a bent strip of copper, C. If one of the junctions of the two metals is heated as shown, a current flows; and the same effect is produced by cooling one of the junctions. This current continues to flow as long as the two junctions are kept at different temperatures. In 1834 another scientist, Peltier, discovered that if a current was passed across a junction of two different metals, this junction was either heated or cooled, according to the direction in which the current flowed. In Fig. 10 the current across the heated junction tends to cool the junction, while the Bunsen burner opposes this cooling, and keeps up the temperature. A certain amount of the heat energy is thus transformed into electrical energy. At the other junction the current produces a heating effect, so that some of the electrical energy is retransformed into heat.

THE ACCUMULATOR

If we had two large water tanks, one of which could be emptied only by allowing the bottom to fall completely out, and the other by means of a narrow pipe, it is easy to see which would be the more useful to us as a source of water supply. If both tanks were filled, then from the first we could get only a sudden uncontrollable rush of water, but from the other we could get a steady stream extending over a long period, and easily controlled. The Leyden jar stores electricity, but in yielding up its store it acts like the first tank, giving a sudden discharge in the form of a bright spark. We cannot control the discharge, and therefore we cannot make it do useful work for us. For practical purposes we require a storing arrangement that will act like the second tank, giving us a steady current of electricity for a long period, and this we have in the accumulator or storage cell.

It will be remembered that one of the troubles with a simple voltaic cell was polarization, caused by the accumulation of hydrogen; and that this weakened the current by setting up an opposing electro-motive force tending to produce another current in the opposite direction. In the present case a similar opposing or back electro-motive force is produced, and as soon as the battery current is stopped and the electrodes are connected, we get a current in the reverse direction, and this current continues to flow until the two gases have recombined, and the electrodes have regained their original condition. Consequently we can see that in order to electrolyze water, our battery must have an electro-motive force greater than that set up in opposition to it, and at least two Daniell cells are required.

This apparatus thus may be made to serve to some extent as an accumulator or storage cell, and it also serves to show that an accumulator does not store up or accumulate electricity. In a voltaic cell we have chemical energy converted into electrical energy, and here we have first electrical energy converted into chemical energy, and then the chemical energy converted back again into electrical energy. This is a rough-and-ready way of putting the matter, but it is good enough for practical purposes, and at any rate it makes it quite clear that what an accumulator really stores up is not electricity, but energy, which is given out in the form of electricity.

In 1881, Faure hit upon the idea of coating the plates with a paste of red-lead, and this greatly shortened the time of forming. At first it was found difficult to make the paste stick to the plates, but this trouble was got rid of by making the plates in the form of grids, and pressing the paste into the perforations. Many further improvements have been made from time to time, but instead of tracing these we will go on at once to the description of a present-day accumulator. There are now many excellent accumulators made, but we have not space to consider more than one, and we will select that known as the "Chloride" accumulator.

The positive plate of this accumulator is of the Plant? type, but it is not simply a casting of pure lead, but is made by a building-up process which allows of the use of a lead-antimony mixture for the grids. This gives greater strength, and the grids themselves are unaffected by the chemical changes which take place during the charging and discharging of the cell. The active material, that is the material which undergoes chemical change, is pure lead tape coiled up into rosettes, which are so designed that the acid can circulate through the plates. These rosettes are driven into the perforations of the grid by a hydraulic press, and during the process of forming they expand and thus become very firmly fixed. The negative plate has a frame made in two parts, which are riveted together after the insertion of the active material, which is thus contained in a number of small cages. The plate is covered outside with a finely perforated sheet of lead, which prevents the active material from falling out. It is of the utmost importance that the positive and negative plates should be kept apart when in the cell, and in the Chloride accumulator this is ensured by the use of a patent separator made of a thin sheet of wood the size of the plates. Before being used the wood undergoes a special treatment to remove all substances which might be harmful, and it then remains unchanged either in appearance or composition. Other insulating substances, such as glass rods or ebonite forks, can be used as separators, but it is claimed that the wood separator is not only more satisfactory, but that in some unexplained way it actually helps to keep up the capacity of the cell. The plates are placed in glass, or lead-lined wood or metal boxes, and are suspended from above the dilute sulphuric acid with which the cells are filled. A space is left below the plates for the sediment which accumulates during the working of the cell.

Accumulators are usually charged from a dynamo or from the public mains, and the electro-motive force of the charging current must be not less than 2 1/2 volts for each cell, in order to overcome the back electro-motive force of the cells themselves. It is possible to charge accumulators from primary cells, but except on a very small scale the process is comparatively expensive. Non-polarizing cells, such as the Daniell, must be used for this purpose.

The practical applications of accumulators are almost innumerable, and year by year they increase. As the most important of these are connected with the use of electricity for power and light, it will be more convenient to speak of them in the chapters dealing with this subject. Minor uses of accumulators will be referred to briefly from time to time in other chapters.

MAGNETS AND MAGNETISM

"Let there be two needles provided of an equal Length and Bigness, being both of them touched by the same lodestone; let the Letters of the Alphabet be placed on the Circles on which they are moved, as the Points of the Compass under the needle of the Mariner's Chart. Let the Friend that is to travel take one of these with him, first agreeing upon the Days and Hours wherein they should confer together; at which times, if one of them move the Needle, the other Needle, by Sympathy, will move unto the same letter in the other instantly, though they are never so far distant; and thus, by several Motions of the Needle to the Letters, they may easily make up any Words or Sense which they have a mind to express." This is wireless telegraphy in good earnest!

The lodestone is a natural magnet. If we rub a piece of steel with a lodestone we find that it acquires the same properties as the latter, and in this way we are able to make any number of magnets, for the lodestone does not lose any of its own magnetism in the process. Such magnets are called artificial magnets. Iron is easier to magnetize than steel, but it soon loses its magnetism, whereas steel retains it; and the harder the steel the better it keeps its magnetism. Artificial magnets, therefore, are made of specially hardened steel. In this chapter we shall refer only to steel magnets, as they are much more convenient to use than the lodestone, but it should be remembered that both act in exactly the same way. We will suppose that we have a pair of bar magnets, and a horse-shoe magnet, as shown in Fig. 13.

If we roll a bar magnet amongst iron filings we find that the filings remain clinging to it in two tufts, one at each end, and that few or none adhere to the middle. These two points towards which the filings are attracted are called the poles of the magnet. Each pole attracts filings or ordinary needles, and one or two experiments will show that the attraction becomes evident while the magnet is still some little distance away. If, however, we test our magnet with other substances, such as wood, glass, paper, brass, etc., we see that there is no attraction whatever.

The simplest method of magnetizing a piece of steel by means of one of our bar magnets is the following: Lay the steel on the table, and draw one pole of the magnet along it from end to end; lift the magnet clear of the steel, and repeat the process several times, always starting at the same end and treating each surface of the steel in turn. A thin, flat bar of steel is the best for the purpose, but steel knitting needles may be made in this way into useful experimental magnets.

We have seen that a magnet has two poles or points where the magnetism is strongest. It might be thought that by breaking a bar magnet in the middle we should get two small bars each with a single pole, but this is not the case, for the two poles are inseparable. However many pieces we break a magnet into, each piece is a perfect magnet having a north and south pole. Thus while we can isolate a positive or a negative charge of electricity, we cannot isolate north or south magnetism.

Any one who experiments with magnets must be struck with the distance at which one magnet can influence filings or another magnet. If a layer of iron filings is spread on a sheet of paper, and a magnet brought gradually nearer from above, the filings soon begin to move about restlessly, and when the magnet comes close enough they fly up to it as if pulled by invisible strings. A still more striking experiment consists in spreading filings thinly over a sheet of cardboard and moving a magnet to and fro underneath the sheet. The result is most amusing. The filings seem to stand up on their hind legs, and they march about like regiments of soldiers. Here again invisible strings are suggested, and we might wonder whether there really is anything of the kind. Yes, there is. To put the matter in the simplest way, the magnet acts by means of strings or lines of force, which emerge from it in definite directions, and in a most interesting way we can see some of these lines of force actually at work.

Place a magnet, or any arrangement of magnets, underneath a sheet of glass, and sprinkle iron filings from a muslin bag thinly and evenly all over the glass. Then tap the glass gently with a pencil, and the filings at once arrange themselves in a most remarkable manner. All the filings become magnetized by induction, and when the tap sets them free for an instant from the friction of the glass they take up definite positions under the influence of the force acting upon them. In this way we get a map of the general direction of the magnetic lines of force, which are our invisible strings.

Magnetic dip also is seen to be a natural result of the Earth's magnetic influence. Here in England, for instance, the north magnetic pole is much nearer than the south magnetic pole, and consequently its influence is the stronger. Therefore a magnetized needle, if free to do so, dips downwards towards the north. At any place where the south magnetic pole is the nearer the direction of the dip of course is reversed. If placed immediately over either magnetic pole the needle would take up a vertical position, and at the magnetic equator it would not dip at all, for the influence of the two magnetic poles would be equal. A little study of Fig. 14, which represents a dipping needle at different parts of the earth, will make this matter clearer. N and S represent the Earth's north and south magnetic poles, and the arrow heads are the north poles of the needles.

Since the Earth is a magnet, we should expect it to be able to induce magnetism in a bar of iron, just as our artificial magnets do, and we can show that this is actually the case. If a steel poker is held pointing to and dipping down towards the north, and struck sharply with a piece of wood while in this position, it acquires magnetic properties which can be tested by means of a small compass needle. It is an interesting fact that iron pillars and railings which have been standing for a long time in one position are found to be magnetized. In the northern hemisphere the bases of upright iron pillars are north poles, and their upper ends south poles, and in the southern hemisphere the polarity is reversed.

The most valuable application of the magnetic needle is in the compass. An ordinary pocket compass for inland use consists simply of a single magnetized needle pivoted so as to swing freely over a card on which are marked the thirty-two points of the compass. Ships' compasses are much more elaborate. As a rule a compound needle is used, consisting of eight slender strips of steel, magnetized separately, and suspended side by side. A compound needle of this kind is very much more reliable than a single needle. The material of which the card is made depends upon whether the illumination for night work is to come from above or below. If the latter, the card must be transparent, and it is often made of thin sheet mica; but if the light comes from above, the card is made of some opaque material, such as very stout paper. The needle and card are contained in a sort of bowl made of copper. In order to keep this bowl in a horizontal position, however the ship may be pitching and rolling, it is supported on gimbals, which are two concentric rings attached to horizontal pivots, and moving in axes at right angles to one another. Further stability may be obtained by weighting the bottom of the bowl with lead. There are also liquid compasses, in which the card is floated on the surface of dilute alcohol, and many modern ships' compasses have their movements regulated by a gyrostat.

The large amount of iron and steel used in the construction of modern vessels has a considerable effect upon the compass needle, and unless the compass is protected from this influence its readings are liable to serious errors. The most satisfactory way of giving this protection is by placing on each side of the compass a large globe of soft iron, twelve or more inches in diameter.

It is an interesting fact that the Earth's magnetism is subject to variation. The declination and the dip slowly change through long periods of years, and there are also slight annual and even daily variations.

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