Part 2 (1/2)
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 s.h.i.+ft 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.