gives a feeble current, but by combining a number of pairs in the fashion of B, Fig. 9, where the shaded bars represent pieces of antimony and the plain bars bismuth, it is found that when all the joints on one side are heated, a powerful current equal to the sum of the elementary ones will circulate in the wire w connecting the terminal pieces P P, which correspond to the poles of the voltaic pile.

Generators of electricity have been constructed on this plan, by Becquerel, Noë, Clammond and others; but, owing to the deterioration of the joints, they have not come into general use. The latest and most efficient is that of Clammond, which we illustrate in Fig. 10. It is heated by a fire of coke; but the same inventor has also designed a thermo-electric battery which is kept in action by gas. The latter is now used for electro-plating in the note-printing department of the Bank of France. The former is also applicable to this purpose or to electric lighting.

The metals used by M. Clammond are an alloy of bismuth and antimony for one, and iron for the other. Prisms of the alloy are cast on strips of sheet iron, which connect the right face of one prism to the left face of the next. In this way a set of 50 elements in a ring is produced at one casting. These rings are piled up one above another, as shewn at F in the figure, as many as 60 rings being superposed in one battery sometimes. The rings are connected "in series" just as the elements are, and thus the current obtained at the poles of the pile is the sum total of all the elementary

currents.

The column of elements F is placed round a cast iron tube which is heated by the air rising from the

burning coke in the furnace c, and circulating in the chambers C D E, before escaping by the funnel A. The iron tube in contact with the inner joints of the pile

heats these without allowing the hot air and fumes to attack them. On the other hand, the outer joints are kept cool by radial plates of copper в, which expose a

considerable cooling surface to the air. By this means the mean temperature of the heated face of the pile is kept at 360° Centigrade and that of the cooled face at 80° C. The power of a battery of 60 rings or 3000 elements when heated to this degree is about equal to a Bunsen battery of 60 large cells. Strictly speaking, the electro-motive force is 109 volts, and the internal resistance is 15.5 ohms. Such a battery consumes from 10 to 12 lbs. of coke per hour. Two of them connected in series will feed a pair of Serrin electric lamps, each giving a light of 400 to 700 candles.

Another mode of generating electricity by heat has recently been discovered by Dr. Brard of La Rochelle; but though the plan is interesting the results are as yet very insignificant. It consists in forming an electro-generative fire-brick, which when put into a furnace is consumed, and in consuming gives birth to an electric current which can be led away for useful purposes. The brick is composed of two slabs or plates laid over each other. The lower slab is of coal dust made into a paste with treacle or coal tar, and moulded under pressure so as to pit its upper surface and perforate it through and through. Strips of brass are also embedded in the mass and connected to a projecting end. The upper slab is prepared from nitrate of potash mixed with ashes, melted, and poured on the pitted surface of the lower slab. Strips of brass are also embedded in it, as in the case of the lower slab. The projecting strips are the positive and negative poles of the electro-generative element.

Such a brick weighing a few pounds will yield a current strong enough to ring an electric bell for an hour or two. When several are connected together like

the cells of a voltaic battery, the united current is equal to the sum of the currents from each brick. Dr. Brard has also constructed an electro-generative torch or candle by making a brick of coal dust and treacle, and dipping it in melted nitrate. The power of yielding heat as well, is, of course, the peculiar merit of this electro-generative fuel.

The electric current produces many curious effects, nearly all of which have been usefully applied; but we shall only enumerate the most important of these. It decomposes chemicals, and hence has given rise to the electro-plating industry and many other processes, such as the storage of electricity, the rectification of alcohol, the manufacture of aniline colours, and the reduction of watery tumours in the art of medicine. In overcoming the resistance of a wire, or, better still, some semi-conductor, such as carbon, it produces light and heat, hence we have the electric light and the redhot platinum wire used in the actual cautery. When it is caused to flow through a wire coiled round a pivoted magnetic needle it causes the needle to move, and if a bar of soft iron is put within the coil in lieu of the needle, that bar will become a magnet. From the first of these two important effects we have the needle telegraph of Cooke and Wheatstone and the reflecting galvanometer for measuring currents, while from the second we have an infinite variety of appliances, such as the printing telegraph, the electric bell, the telephone, and the magneto-electric motor.

CHAPTER III.

INDUCTION.

WE come now to the principle known as electrical induction, a principle which is of the highest importance, and on which all mechanical generators of electricity are based. The name is derived from the fact that an electrified body induces a charge of electricity in a nonelectrified body brought within its influence. For example, let è, Fig. 11, be a body charged with positive electricity, and let it be brought near a nonelectrified body n p which, for the sake of demonstration, is usually a brass cylinder mounted on an insulating stem of glass, and having a pair of pith balls hung from its two ends. As the cylinder approaches p the pith balls will be seen to diverge, thereby showing that the ends of the cylinder have become electrified; and it will be found that the balls at n indicate a negative charge at that end, and the balls at p indicate a positive charge at the other. In short, the positive electricity on P has apparently separated the two fluids, which before were neutral in the cylinder, and attracted the negative or opposite fluid to the nearest point, n, while it has

FIG. 11.