into the form of two blocks placed on each side of the

plate of zinc z. These three plates are kept apart by the wooden cover, which has a circular hole in it to allow a wire or electrode from the zinc plate to pass through. In the particular form shown this wire is bent into a hook h, so as to hang the plates from a windlass axle and raise them out of the jar at will by turning the windlass. A wire from the hook forms the negative (-) pole of the cell, and a terminal t in connection with the two carbon plates, forms the positive (+) pole of the cell as shown. These plates are immersed in a solution of bichromate (or yellow prussiate) of potash, sulphuric acid (vitriol) and water. In the action of the battery gases collect on the carbon plates, and reduce its power, but these can be liberated by raising the plates from the solution after the battery is out of use. Stirring the battery has a like effect; and sometimes a continual circulation of the liquid from one cell to the next is resorted to for the same purpose. When this is done, z the plates may be kept in the solution, and the battery constantly at work. The electromotive force of the bichromate battery is higher than other batteries, and but for this tendency to "polarise with gas it would be a very useful combination for electric lighting.

S

A convenient portable cell invented by Mr. Warren de la Rue, is shown in Fig. 8. This consists of a stick of zinc z and a strip of silver s surrounded by a coating of chloride of silver. These two "plates are immersed in a solution of ammonium chloride in water. To keep the two plates from touching within

FIG. 8.

the cell, the rod of chloride is put in a sleeve of parchment paper. A cork or paraffin plug fills the mouth of the glass tube forming the jar, and the electrodes from the plates pass through it. It is also provided with a little vent hole v to allow gases to escape, when the cell is in action. For medical purposes, or for making electric tests of submarine cables and instruments, this compact little battery is very convenient, and as it does not readily "polarise "it is also very reliable and constant.

A recent addition to the catalogue of batteries is the iron cell of Mr. Alfred Bennet, which has the merit of simplicity and cheapness. It can be made by taking an empty meat-tin, and filling it part full with a solution of caustic potash, or carbonate of soda in water, then inserting a sheet or plate of zinc. The iron tin serves the double purpose of jar and negative metal, while the zinc is the positive metal, and the solution is the exciting liquid. When the two metals are joined outside the cell, a current flows from the iron to the zinc. Polarisation by accumulation of gases on the iron is further prevented by surrounding the interior of the tin with iron filings, which are found to discharge the gases much in the same way as the broken manganese in the Leclanché cell does. Since, however, the iron and zinc plates must not touch within the cell, it is necessary to separate the zinc from the filings by a partition or wrapper of a porous nature, such as clay, cloth, or parchment paper. Bennet's cell is also useful for electric bells.

Bunsen's battery is composed of a zinc plate immersed in sulphuric acid, and a carbon plate in nitric acid, with a porous partition between to separate the

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,

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