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two liquids. Grove's is like Bunsen's, except that a platinum plate is substituted for the carbon, and Smee's consists of a zinc plate and a silver plate roughly coated with platinum immersed side by side in a vessel of sulphuric acid and water. The object in roughening the silver with platinum is to allow the hydrogen to discharge itself from the fine points of platinum, and not remain upon the silver plate to "polarise" it, or, in other words, reduce its "electro-motive" power. The last three kinds of battery, and many others, are, however, rather adapted for experimental purposes than common use, and hence it is unnecessary to dwell upon them longer.

We have spoken of the voltaic "circuit" of resistance," ," "current," and "electro-motive" power, and it will be well to have a clear idea of the meaning of these terms. The "circuit" is the complete course, or round, traversed by the current of electricity in setting out from the spot where it is generated and returning there. It may consist of a straight wire, such as a telegraph line, or the bobbin of wire in an electro-magnet, or a sparking coil; it may be in part made up of an electrotyper's bath, an electric lamp, an electric motor, or even of the earth itself; but still the entire round is called the circuit. Whatever it is composed of, it exercises a certain "resistance" to the passage of the current, much in the same way as friction in a pipe resists the flow of water through it; and the force which impels the current through the circuit is the electro-motive force. In a voltaic battery, as we have before stated, the electro-motive force mainly depends on the two kinds of substance which form the plates and the number of elements which go to make

C

up the battery. In the analogy of flowing water which we have employed it corresponds to the "head" or pressure of the water in the pipes. After this explanation it will be easy to understand that the electric current is the quantity of electricity driven through the circuit, and that its strength, or the quantity flowing per second, will be directly as the electro-motive force driving it, and inversely as the resistance it encounters. This relation is a fundamental one in the science of electricity, and is known to electricians as Ohm's Law.

Besides the voltaic "pile," or battery, there is another very useful combination of two different metals for generating currents of electricity. This is termed the thermo-pile, because the heat takes the place of chemical oxidation in keeping up the supply of electricity. It is based on the remarkable discovery of Professor Seebeck, that when two different metals are soldered together so as to form a closed ring or circuit,

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as at A in Fig. 9, and one of the junctions is heated more than the other, an electric current is set up in the circuit. With one pair of junctions a very feeble current is produced, just as a single voltaic element

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

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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

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