a contrivance similar to that used by Mr. M'Gauley* will be found very useful: this consists of a beam of brass supported by a horizontal axis at к, having at one end a ball of soft iron, L, suspended, and at the other a fork of thick copper wire, so arranged that by its own weight it will fall into two cups of mercury fixed at м, and thus connect them with each other. One of these cups is connected by a wire with the screw D, whilst the other is, by a wire z, connected with one end of the battery, the screw E being in communication with the other. As soon as these connexions are completed, the bar s, becoming magnetic, attracts the ball L, which, falling, raises the fork м from the cups; thus breaking contact with the battery, and producing a vivid spark attended with a loud snap, and combustion of the mercury. The bars losing their magnetism, the fork F falls by its own weight, and reestablishes connexion with the battery; L is again attracted, and so on, the beam rapidly vibrating amid a complete shower of sparks from the mercury, producing a most brilliant spectacle in a dark room. 401. A rapid succession of powerful currents being at each rupture of contact sent through the long coil, the shock felt at the screws GH, or at the cylinder connected with them, becomes intensely painful, completely paralysing the arms of the persons grasping the conductors; with these currents evolved at GH, chemical decomposition may be performed and other effects produced, as with a voltaic battery. If a piece of charcoal (336) be placed on 6, and a platina wire connected with H be placed against it, whilst the machine is in action, a series of minute sparks from the induced currents will be observed. 402. As electric currents are induced by other currents passing near the conductors, in which they are excited, the theory of Ampere (383) receives considerable support from the facts enumerated in this chapter. Granting with him • Rep. British Association, vol. vi., p. 24. THEORY OF AMPERE. 277 that a magnet is full of perpetually moving currents of electricity, it induces magnetism in a bar of iron, by exciting similar currents, as in the case already mentioned (386), and then the remarkable fact of magnets exciting electric currents in wires moved near them, will be resolved into the same case of currents exciting currents. In fact, it permits us to generalise the phenomena of magnetism and electro-dynamics, in a very important and satisfactory manner. The apparently mysterious phenomena produced by revolving plates of different metals under magnetic needles, in causing them to move, may be referred to a similar explanation; the currents in the needles exciting similar currents in the revolving plate, which by their reaction on the needles cause it to oscillate and revolve. CHAPTER XVIII. THERMO-ELECTRICITY. Excitaton of Thermo-electric Currents by two Metals, 403—by one Metal unequally heated, 405— Rotations produced by, 406—Chemical Decompositions, and Sparks from Currents induced by, 407. Currents evolved by Metals plunged into Fused Salts, 408. 403. WHEN two different metals, as copper and bismuth, are soldered together and connected by wires to a multiplier (362), a powerful electric current becomes developed on heating the point of juncture of the two metals with a spirit lamp. If the multiplier be sufficiently delicate, the deviation of the needles will occur when the point of connexion of both metals is grasped in the hand; a very slight elevation of temperature being sufficient to produce this effect. In general, the most powerful currents are evolved by heating the more crystalline metals, as bismuth and antimony; and they increase within certain limits with the increase of temperature. The following list contains the names of several metals, any two of which being employed as a source of electricity, by heating them at their point of junction, currents are developed in such a manner that each metal becomes positive to all below, and negative to all above it, in the list: +Bismuth Lead Tin Gold Silver Copper Zinc Iron - Antimony. CHEMICAL DECOMPOSITIONS. 279 404. This mode of developing electricity was discovered in 1821, by Prof. Seebeck, of Berlin, and has been studied. with success by Prof. Cumming, of Cambridge, Mr. Sturgeon, and many other philosophers. In examining these currents, as they are of too low intensity to force their way through very long conducting wires, the multiplier should be constructed in the manner already explained (362), but the wire coil should be short, and composed of thick and soft copper wire, so as to offer as little opposition as possible to the passage of the currents. 405. It is by no means necessary to employ two metals in these experiments, for if two pieces of copper wire be twisted. together, and connected with the multiplier, a current of electricity takes place on holding a spirit lamp on one side of the juncture. Even platina and gold wires will evolve these currents; so that they are to be regarded as arising from a series of decompositions and recombinations of electricity produced by the action of heat, and not resulting, at least necessarily, from oxydation or other chemical action. When an uniform bar of metal is heated at one end, the cold portions assume negative and the hot ones positive electricity.* * 406. The phenomena of electro-magnetic rotation may be readily produced by means of thermo-electric currents; for this purpose twist round each end of a bar of bismuth an inch in length, a thick copper wire, and having amalgamated (366) their other ends, immerse them in the circular troughs AB of the apparatus for the rotation of a conducting wire round the pole of a magnet (367). Apply a spirit lamp to one end of the bar of bismuth, and as soon as the latter becomes warm, a current of electricity will pass through the apparatus from the copper to the bismuth, and the conducting wires suspended on the poles will begin to revolve with rapidity. 407. The intensity of the evolved electricity increases by Becquerel, Traité, tom. i., 234, ii. 35. combining a series of alternations of two metals, as copper and platina, or bismuth and antimony, as in the ordinary electric pile (330). And by the current excited by a large number of alternations of platina and iron, M. Botto*, of Turin, succeeded in decomposing water and various saline solutions. In 1836, Chev. Antinorit of Florence, by connecting a thermoelectric battery with an helix of insulated copper wire, about 500 feet in length, obtained on breaking contact a vivid spark from the induced or secondary current produced by the passage of the primary thermo-electric current (390). Shortly afterwards, Prof. Wheatstonet repeated this experiment with success, using a battery of thirty-three pairs of bismuth and antimony, forming a cylindrical bundle, 1.2 inches long, and 0.75 inch in diameter, with a coil of insulated copper ribbon 50 feet long, and 1.5 inch broad Mr. Watkins has since obtained the same results, by using a single pair of plates of bismuth and antimony, each being 0.5 inch long, 0.12 inch thick, and weighing but five grains. The same gentleman, by using a thermo-electric battery of thirty pairs, each plate being 1.5 inch square, and 0.33 inch thick, and heating one end of the arrangement with a hot iron, whilst the other was kept cool with ice, succeeded in exciting an electro-magnet to such an extent as to support a weight of ninety-eight pounds. 408. Dr. Andrews,|| of Belfast, has discovered that platina wires connected with a multiplier, and plunged into fused salts, are traversed by an electric current. This may be shown by connecting a piece of platina wire with one screw of the multiplier (362), and bending its free end into a loop. On fusing a little borax in the loop, by means of the blowpipe, and quickly inserting the previously heated end of a second platina wire also connected with the multiplier, into • Bibliothéq. Univers., tom. xxxiii. p. 259. + Indicatore Sanese, Dec. 13, 1836. § Phil. Mag., vol. xi. pp. 304, 399. Phil. Mag., x. p. 414. Ibid, vol. x. p. 433. |