POSITIVE Bis.m.u.th Cobalt Pota.s.sium Nickel Sodium Lead Tin Copper Platinum Silver Zinc Cadmium a.r.s.enic Iron Red phosphorus Antimony Tellurium Selenium NEGATIVE

Other things being equal, the hotter the joint in comparison with the free ends of the bars the stronger the current of electricity.

Within certain limits the current is, in fact, proportional to this difference of temperature. It always flows in the same direction if the joint is not overheated, or, in other words, raised above a certain temperature.

The electromotive force and current of a thermo-electric couple is very much smaller than that given by an ordinary voltaic cell. We can, however, multiply the effect by connecting a number of pairs together, and so forming a pile or battery. Thus figure 23 shows three couples joined "in series," the positive pole of one being connected to the negative pole of the next. Now, if all the junctions on the left are hot and those on the right are cool, we will get the united effect of the whole, and the total current will flow through the wire W, joining the extreme bars or positive and negative poles of the battery. It must be borne in mind that although the bis.m.u.th and antimony of this thermo-electric battery, like the zinc and copper of the voltaic or chemico-electric battery, are respectively positive and negative to each other, the poles or wires attached to these metals are, on the contrary, negative and positive. This peculiarity arises from the current starting between the bis.m.u.th and antimony at the heated junction.

The internal resistance of a "thermo-electric pile" is, of course, very slight, the metals being good conductors, and this fact gives it a certain advantage over the voltaic battery. Moreover, it is cleaner and less troublesome than the chemical battery, for it is only necessary to keep at the required difference of temperature between the hot and cold junctions in order to get a steady current. No solutions or salts are required, and there appears to be little or no waste of the metals. It is important, however, to avoid sudden heating and cooling of the joints, as this tends to destroy them.

Clammond, Gulcher, and others have constructed useful thermo-piles for practical purposes. Figure 24 ill.u.s.trates a Clammond thermo- pile of 75 couples or elements. The metals forming these pairs are an alloy of bis.m.u.th and antimony for one and iron for the other.

Prisms of the alloy are cast on strips of iron to form the junctions. They are bent in rings, the junctions in a series making a zig-zag round the circle. The rings are built one over the other in a cylinder of couples, and the inner junctions are heated by a Bunsen gas-burner in the hollow core of the battery. A gas- pipe seen in front leads to the burner, and the wires WW connected to the extreme bars or poles are the electrodes of the pile.

Thermo-piles are interesting from a scientific point of view as a direct means of transforming heat into electricity. A sensitive pile is also a delicate detector of heat by virtue of the current set up, which can be measured with a galvanometer or current meter. Piles of antimony and bis.m.u.th are made which can indicate the heat of a lighted match at a distance of several yards, and even the radiation from certain of the stars.

Thermo-batteries have been used in France for working telegraphs, and they are capable of supplying small installations of the electric light or electric motors for domestic purposes.

The action of the thermo-pile, like that of a voltaic cell, can be reversed. By sending a current through the couple from the antimony to the bis.m.u.th we shall find the junction cooled. This "Peltier effect," as it is termed, after its discoverer, has been known to freeze water, but no practical application has been made of it.

A very feeble thermo-electric effect can be produced by heating the junction of two different pieces of the same substance, or even by making one part of the same conductor hotter than another.

Thus a sensitive galvanometer will show a weak current if a copper wire connected in circuit with it be warmed at one point.

Moreover, it has been found by Lord Kelvin that if an iron wire is heated at any point, and an electric current be pa.s.sed through it, the hot point will shift along the wire in a direction contrary to that of the current.

CHAPTER IV.

THE ELECTRICITY OF MAGNETISM.

We have already seen how electricity was first produced by the simple method of rubbing one body on another, then by the less obvious means of chemical union, and next by the finer agency of heat. In all these, it will be observed, a substantial contact is necessary. We have now to consider a still more subtle process of generation, not requiring actual contact, which, as might be expected, was discovered later, that, mainly through the medium of magnetism.

The curious mineral which has the property of attracting iron was known to the Chinese several thousand years ago, and certainly to the Greeks in the times of Thales, who, as in the case of the rubbed amber, ascribed the property to its possession of a soul.

Lodestone, a magnetic oxide of iron (FE3O4), is found in various parts of China, especially at T'szchou in Southern Chihli, which was formerly known as the "City of the Magnet." It was called by the Chinese the love-stone or thsu-chy, and the stone that s.n.a.t.c.hes iron or ny-thy-chy, and perchance its property of pointing out the north and south direction was discovered by dropping a light piece of the stone, if not a sewing needle made of it, on the surface of still water. At all events, we read in Pere Du Halde's Description de la Chine, that sometime in or about the year 2635 B.C. the great Emperor Hoang-ti, having lost his way in a fog whilst pursuing the rebellious Prince Tchiyeou on the plains of Tchou-lou, constructed a chariot which showed the cardinal points, thus enabling him to overtake and put the prince to death.

A magnetic car preceded the Emperors of China in ceremonies of state during the fourth century of our era. It contained a genius in a feather dress who pointed to the south, and was doubtless moved by a magnet floating in water or turning on a pivot. This rude appliance was afterwards refined into the needle compa.s.s for guiding mariners on the sea, and a.s.sisting the professors of feng- shui or geomancy in their magic rites.

Magnet.i.te was also found at Heraclea in Lydia, and at Magnesium on the Meander or Magnesium at Sipylos, all in Asia Minor. It was called the "Heraclean Stone" by the people, but came at length to bear the name of "Magnet" after the city of Magnesia or the mythical shepherd Magnes, who was said to have discovered it by the attraction of his iron crook.

The ancients knew that it had the power of communicating its attractive property to iron, for we read in Plato's "Ion" that a number of iron rings can be supported in a chain by the Heraclean Stone. Lucretius also describes an experiment in which iron filings are made to rise up and "rave" in a bra.s.s basin by a magnet held underneath. We are told by other writers that images of the G.o.ds and G.o.ddesses were suspended in the air by lodestone in the ceilings of the temples of Diana of Ephesus, of Serapis at Alexandria, and others. It is surprising, however, that neither the Greeks nor Romans, with all their philosophy, would seem to have discovered its directive property.

During the dark ages pieces of Lodestone mounted as magnets were employed in the "black arts." A small natural magnet of this kind is shown in figure 25, where L is the stone shod with two iron "pole-pieces," which are joined by a "keeper" A or separable bridge of iron carrying a hook for supporting weights.

Apparently it was not until the twelfth century that the compa.s.s found its way into Europe from the East. In the Landnammabok of Ari Frode, the Norse historian, we read that Flocke Vildergersen, a renowned viking, sailed from Norway to discover Iceland in the year 868, and took with him two ravens as guides, for in those days the "seamen had no lodestone (that is, no lidar stein, or leading stone) in the northern countries." The Bible, a poem of Guiot de Provins, minstrel at the court of Barbarossa, which was written in or about the year 890, contains the first mention of the magnet in the West. Guiot relates how mariners have an "art which cannot deceive" of finding the position of the polestar, that does not move. After touching a needle with the magnet, "an ugly brown stone which draws iron to itself," he says they put the needle on a straw and float it on water so that its point turns to the hidden star, and enables them to keep their course. Arab traders had probably borrowed the floating needle from the Chinese, for Bailak Kibdjaki, author of the Merchant's Treasure, written in the thirteenth century, speaks of its use in the Syrian sea. The first Crusaders were probably instrumental in bringing it to France, at all events Jacobus de Vitry (1204-15) and Vincent de Beauvais (1250) mention its use, De Beauvais calling the poles of the needle by the Arab words aphron and zohran.

Ere long the needle was mounted on a pivot and provided with a moving card showing the princ.i.p.al directions. The variation of the needle from the true north and south was certainly known in China during the twelfth, and in Europe during the thirteenth century.

Columbus also found that the variation changed its value as he sailed towards America on his memorable voyage of 1492. Moreover, in 1576, Norman, a compa.s.s maker in London, showed that the north- seeking end of the needle dipped below the horizontal.

In these early days it was supposed that lodestone in the pole- star, that is to say, the "lodestar" of the poets or in mountains of the far north, attracted the trembling needle; but in the year 1600, Dr. Gilbert, the founder of electric science, demonstrated beyond a doubt that the whole earth was a great magnet. A magnet, as is well known, has, like an electric battery, always two poles or centres of attraction, which are situated near its extremities.

Sometimes, indeed, when the magnet is imperfect, there are "consequent poles" of weaker force between them. One of the poles is called the "north," and the other the "south," because if the magnet were freely pivotted like a compa.s.s needle, the former would turn to the north and the latter to the south.

Either pole will attract iron, but soft or annealed iron does not retain the magnetism nearly so well as steel. Hence a boy's test for the steel of his knife is only efficacious when the blade itself becomes magnetic after being touched with the magnet. A piece of steel is readily magnetised by stroking it from end to end in one direction with the pole of a magnet, and in this way compa.s.s needles and powerful bar magnets can be made.

The poles attract iron at a distance by "induction," just as a charge of electricity, be it positive or negative, will attract a neutral pith ball; and Dr. Gilbert showed that a north pole always repels another north pole and attracts a south pole, while, on the other hand, a south pole always repels a south pole and attracts a north pole. This can be proved by suspending a magnetic needle like a pithball, and approaching another towards it, as ill.u.s.trated in figure 26, where the north pole N attracts the south S. Obviously there are two opposite kinds of magnetic poles, as of electricity, which always appear together, and like magnetic poles repel, unlike magnetic poles attract each other.

It follows that the magnetic pole of the compa.s.s needle which turns to the north must be unlike the north and like the south magnetic pole of the earth. Instead of calling it the "north," it would be less confusing to call it the "north-seeking" pole of the needle.

Gilbert made a "terella," or miniature of the earth, as a magnet, and not only demonstrated how the compa.s.s needle sets along the lines joining the north and south magnetic poles, but explained the variation and the dip. He imagined that the magnetic poles coincided with the geographical poles, but, as a matter of fact, they do not, and, moreover, they are slowly moving round the geographical poles, hence the declination of the needle, that is to say its angle of divergence from the true meridian or north and south line, is gradually changing. The north magnetic pole of the earth was actually discovered by Sir John Ross north of British America, on the coast of Boothia (lat.i.tude 70 degrees 5' N, longitude 96 degrees 46' W), where, as foreseen, the needle entirely lost its directive property and stood upright, or, so to speak, on its head. The south magnetic pole lies in the Prince Albert range of Victona Land, and was almost reached by Sir James Clark Ross.

The magnetism of the earth is such as might be produced by a powerful magnet inside, but its origin is unknown, although there is reason to believe that ma.s.ses of lodestone or magnetic iron exist in the crust. Coulomb found that not only iron, but all substances are more or less magnetic, and Faraday showed in 1845 that while some are attracted by a magnet others are repelled. The former he called paramagnetic and the latter diamagnetic bodies.

The following is a list of these.--

Paramagnetic Diamagnetic Iron Bis.m.u.th Nickel Phosphorus Cobalt Antimony Aluminium Zinc Manganese Mercury Chromium Lead Cerium Silver t.i.tanium Copper Platinum Water Many ores and Alcohol salts of the Tellurium above metals Selenium Oxygen Sulphur Thallium Hydrogen Air

We have theories of magnetism that reduce it to a phenomenon of electricity, though we are ignorant of the real nature of both. If we take a thin bar magnet and break it in two, we find that we have now two shorter magnets, each with its "north" and "south"

poles, that is to say, poles of the same kind as the south and north--magnetic poles of the earth. If we break each of these again, we get four smaller magnets, and we can repeat the process as often as we like. It is supposed, therefore, that every atom of the bar is a little magnet in itself having its two opposite poles, and that in magnetising the bar we have merely partially turned all these atoms in one direction, that is to say, with their north poles pointing one way and their south poles the other way, as shown in figure 27. The polarity of the bar only shows itself at the ends, where the molecular poles are, so to speak, free.

There are many experiments which support this view. For example, if we heat a magnet red hot it loses its magnetism, perhaps because the heat has disarranged the particles and set the molecular poles in all directions. Again, if we magnetise a piece of soft iron we can destroy its magnetism by striking it so as to agitate its atoms and throw them out of line. In steel, which is iron with a small admixture of carbon, the atoms are not so free as in soft iron, and hence, while iron easily loses its magnetism, steel retains it, even under a shock, but not under a cherry red- heat. Nevertheless, if we put the atoms of soft iron under a strain by bending it, we shall find it retain its magnetism more like a bit of steel.

It has been found, too, that the atoms show an indisposition to be moved by the magnetising force which is known as HYSTERESIS. They have a certain inertia, which can be overcome by a slight shock, as though they had a difficulty of turning in the ranks to take up their new positions. Even if this molecular theory is true, however, it does not help us to explain why a molecule of matter is a tiny magnet. We have only pushed the mystery back to the atom. Something more is wanted, and electricians look for it in the const.i.tution of the atom, and in the luminiferous ether which is believed to surround the atoms of matter, and to propagate not merely the waves of light, but induction from one electrified body to another.

We know in proof of this ethereal action that the s.p.a.ce around a magnet is magnetic. Thus, if we lay a horse-shoe magnet on a table and sprinkle iron filings round it, they will arrange themselves in curving lines between the poles, as shown in figure 28. Each filing has become a little magnet, and these set themselves end to end as the molecules in the metal are supposed to do. The "field"

about the magnet is replete with these lines, which follow certain curves depending on the arrangement of the poles. In the horse- shoe magnet, as seen, they chiefly issue from one pole and sweep round to the other. They are never broken, and apparently they are lines of stress in the circ.u.mambient ether. A pivoted magnet tends to range itself along these lines, and thus the compa.s.s guides the sailor on the ocean by keeping itself in the line between the north and south magnetic poles of the earth. Faraday called them lines of magnetic force, and said that the stronger the magnet the more of these lines pa.s.s through a given s.p.a.ce. Along them "magnetic induction" is supposed to be propagated, and a magnet is thus enabled to attract iron or any other magnetic substance. The pole induces an opposite pole to itself in the nearest part of the induced body and a like pole in the remote part. Consequently, as unlike poles attract and like repel, the soft iron is attracted by the inducing pole much as a pithball is attracted by an electric charge.

The resemblances of electricity and magnetism did not escape attention, and the derangement of the compa.s.s needle by the lightning flash, formerly so disastrous at sea, pointed to an intimate connection between them, which was ultimately disclosed by Professor Oersted, of Copenhagen, in the year 1820. Oersted was on the outlook for the required clue, and a happy chance is said to have rewarded him. His experiment is shown in figure 29, where a wire conveying a current of electricity flowing in the direction of the arrow is held over a pivoted magnetic needle so that the current flows from south to north. The needle will tend to set itself at right angles to the wire, its north or north-seeking pole moving towards the west. If the direction of the current is reversed, the needle is deflected in the opposite direction, its north pole moving towards the east. Further, if the wire is held below the needle, in the first place, the north pole will turn towards the east, and if the current be reversed it will move towards the west.

The direction of a current can thus be told with the aid of a compa.s.s needle. When the wire is wound many times round the needle on a bobbin, the whole forms what is called a galvanoscope, as shown in figure 30, where N is the needle and B the bobbin. When a proper scale is added to the needle by which its deflections can be accurately read, the instrument becomes a current measurer or galvanometer, for within certain limits the deflection of the needle is proportional to the strength of the current in the wire.

A rule commonly given for remembering the movement of the needle is as follows:--Imagine yourself laid along the wire so that the current flows from your feet to your head; then if you face the needle you will see its north pole go to the left and its south pole to the right. I find it simpler to recollect that if the current flows from your head to your feet a north pole will move round you from left to right in front. Or, again, if a current flows from north to south, a north pole will move round it like the sun round the earth.

The influence of the current on the needle implies a magnetic action, and if we dust iron filings around the wire we shall find they cling to it in concentric layers, showing that circular lines of magnetic force enclose it like the water waves caused by a stone dropped into a pond. Figure 31 represents the section of a wire carrying a current, with the iron filings arranged in circles round it. Since a magnetic pole tends to move in the direction of the lines of force, we now see why a north or south pole tends to move ROUND a current, and why a compa.s.s needle tries to set itself at right angles to a current, as in the original experiment of Oersted. The needle, having two opposite poles, is pulled in opposite directions by the lines, and being pivoted, sets itself tangentically to them. Were it free and flexible, it would curve itself along one of the lines. Did it consist of a single pole, it would revolve round the wire.

Action and re-action are equal and opposite, hence if the needle is fixed and the wire free the current will move round the magnet; and if both are free they will circle round each other. Applying the above rule we shall find that when the north pole moves from left to right the current moves from right to left. Ampere of Paris, following Oersted, promptly showed that two parallel wires carrying currents attracted each other when the currents flowed in the same direction, and repelled each other when they flowed in opposite directions. Thus, in figure 32, if A and B are the two parallel wires, and A is mounted on pivots and free to move in liquid "contacts" of mercury, it will be attracted or repelled by B according as the two currents flow in the same or in opposite directions. If the wires cross each other at right angles there is no attraction or repulsion. If they cross at an acute angle, they will tend to become parallel like two compa.s.s needles, when the currents are in one direction, and to open to a right angle and close up the other way when the currents are in opposite directions, always tending to arrange themselves parallel and flowing in the same direction. These effects arise from the circular lines of force around the wire. When the currents are similar the lines act as unlike magnetic poles and attract, but when the currents are dissimilar the lines act as like magnetic poles and repel each other.

Another important discovery of Ampere is that a circular current behaves like a magnet; and it has been suggested by him that the atoms are magnets because each has a circular current flowing round it. A series of circular currents, such as the spiral S in figure 33 gives, when connected to a battery C Z, is in fact a skeleton ELECTRO-MAGNET having its north and south poles at the extremities. If a rod or core of soft iron I be suspended by fibres from a support, it will be sucked towards the middle of the coil as into a vortex, by the circular magnetic lines of every spire or turn of the coil. Such a combination is sometimes called a solenoid, and is useful in practice.

When the core gains the interior of the coil it becomes a veritable electromagnet, as found by Arago, having a north pole at one end and a south pole at the other. Figure 34 ill.u.s.trates a common poker magnetised in the same way, and supporting nails at both ends. The poker has become the core of the electromagnet. On reversing the direction of the current through the spiral we reverse the poles of the core, for the poker being of soft or wrought iron, does not retain its magnetism like steel. If we stop the current altogether it ceases to be a magnet, and the nails will drop away from it.

Ampere's experiment in figure 32 has shown us that two currents, more or less parallel, influence each other; but in 1831 Professor Faraday of the Royal Inst.i.tution, London, also found that when a current is started and stopped in a wire, it induces a momentary and opposite current in a parallel wire. Thus, if a current is STARTED in the wire B (fig. 32) in direction of the arrow, it will induce or give rise to a momentary current in the wire A, flowing in a contrary direction to itself. Again, if the current in B be STOPEED, a momentary current is set up in the wire A in a direction the same as that of the exciting current in B. While the current in B is quietly flowing there is no induced current in A; and it is only at the start or the stoppage of the inducing or PRIMARY current that the induced or SECONDARY current is set up.

Here again we have the influence of the magnetic field around the wire conveying a current.

This is the principle of the "induction coil" so much employed in medical electricity, and of the "transformer" or "converter" used in electric illumination. It consists essentially, as shown in figure 35, of two coils of wire, one enclosing the other, and both parallel or concentric. The inner or primary coil P C is of short thick wire of low resistance, and is traversed by the inducing current of a battery B. To increase its inductive effect a core of soft iron I C occupies its middle. The outer or secondary coil S C is of long thin wire terminating in two discharging points D1 D2.

An interrupter or hammer "key" interrupts or "makes and breaks"

the circuit of the primary coil very rapidly, so as to excite a great many induced currents in the secondary coil per second, and produce energetic sparks between the terminals D1 D2. The interrupter is actuated automatically by the magnetism of the iron core I C, for the hammer H has a soft iron head which is attracted by the core when the latter is magnetised, and being thus drawn away from the contact screw C S the circuit of the primary is broken, and the current is stopped. The iron core then ceases to be a magnet, the hammer H springs back to the contact screw, and the current again flows in the primary circuit only to be interrupted again as before. In this way the current in the primary coil is rapidly started and stopped many times a second, and this, as we know, induces corresponding currents in the secondary which appear as sparks at the discharging points. The effect of the apparatus is enhanced by interpolating a "condenser"

C C in the primary circuit. A condenser is a form of Leyden jar, suitable for current electricity, and consists of layers of tinfoil separated from each other by sheets of paraffin paper, mica, or some other convenient insulator, and alternate foils are connected together. The wires joining each set of plates are the poles of the condenser, and when these are connected in the circuit of a current the condenser is charged. It can be discharged by joining its two poles with a wire, and letting the two opposite electricities on its plates rush together. Now, the sudden discharge of the condenser C C through the primary coil P C enhances the inductive effect of the current. The battery B, here shown by the conventional symbol [Electrical Symbol] where the thick dash is the negative and the thin dash the positive pole, is connected between the terminals T1 T2, and a COMMUTATOR or pole- changer R, turned with a handle, permits the direction of the current to be reversed at will.