a mechanical method; viz., to use as a negative plate some plate to which the hydrogen could not stick in the form of a film. The other method was to give the hydrogen, as it appeared in its nascent condition on the surface of the negative plate (see § 7), some chemical reducing work to do. We will now describe several forms of so-called 'constant' battery-cells under the heads of mechanical and chemical respectively.

Note.-Electromotive force.-The poles of the open cell exhibit a certain AV; the greater this is, the greater (cæteris paribus) will be the current when the circuit is closed. When the current is running, that which urges it is called electromotive force; it can be measured in terms of the ▲ V that would appear were the circuit cut. We shall hear more of electromotive force (usually called E. M. F.) when we come to Ohm's Law in Chapter XIII.

In Chapter XV. § 13 we shall see that when the hydrogen is got rid of by chemical means we not only obviate polarisation, but we gain a positive advantage in a greater E.M.F. (see preceding note).

I. Mechanical.-The earliest form of cell in ordinary use that comes under this head is the Smee's. In this the copper plate is replaced by a sheet of thick silver foil that is covered with very finely divided platinum (or 'platinum-black'). Off these points the hydrogen rises to the top, and does not remain as a film covering the negative plate. The substitution of silver for copper gives, moreover, a slightly greater E. M.F.

II. Chemical. Single-fluid cells.-The most important singlefluid cell in which the hydrogen is used up in doing chemical reduction is the-

(i.) Bichromate cell.-Here the negative plate is of carbon (a special kind) instead of copper; and the liquid is composed of a solution of potassium-bichromate in water, mixed with sulphuric acid. The nascent hydrogen is employed in reducing the chromic acid; so that chromic sulphate is formed. This battery has a high E.M.F., yields no fumes, is simple in its arrangements, and therefore is adapted for general laboratory use. The zinc plates must

be removed from the liquid when not in use.

(ii.) Latimer Clark's standard cell.-For purposes of comparison rather than for practical use, Latimer Clark proposed the following cell.

As a negative plate, pure mercury. Over this a paste obtained

N

by boiling sulphate of mercury with a saturated solution of zinc sulphate.

As a positive plate, zinc; this resting on the sulphate paste. Insulated wires are connected with both plates.

This cell can be easily prepared. Its value consists in the fact that, when left with the circuit open, its E.M.F. is very constant, and can be used as a standard of comparison. When, however, the circuit is closed, and a current flows, the E.M.F. does not remain constant.

III. Chemical.

Two fluid cells.-There are many elements in which the negative plate is separated from the positive plate by a porous pot or partition. The positive plate is surrounded by a liquid that acts upon it when the circuit is closed, and the negative plate by a liquid containing something that the nascent hydrogen can reduce.

(i.) Bunsen's cell.-In this battery the + plate is zinc, surrounded by dilute sulphuric acid. The plate is gas-carbon; it

stands in a porous pot, and is surrounded by nitric acid, the porous vessel thus preventing the two acids from mixing, while yet allowing the chain of chemical changes to pass with the current through its pores.

In the figure, P represents the entire cell; while F Z VC represent the outer vessel, zinc, porous pot, and carbon respectively.

The zinc dissolves in the dilute acid, forming zinc sulphate; while the corresponding hydrogen, set free against the carbon, reduces the nitric acid to lower oxides of nitrogen.

(ii.) Grove's cell.-This is an earlier form than the preceding. It differs from it only in having platinum instead of carbon. The shape of the porous pot is flat, to suit the flat plates of platinumfoil.

(iii.) Daniell's cell-In this the only essential difference from the above is that we have copper in a saturated solution of copper sulphate, instead of platinum (or carbon) in nitric acid. But it is usually constructed having the zinc and acid in the porous pot, while very often the copper itself forms the outside vessel. The zinc may be surrounded by a semi-saturated solution of zinc sulphate, or of common salt, instead of by dilute sulphuric acid.

Here the hydrogen reduces the copper sulphate; sulphuric acid is formed, while copper is deposited on the copper plate. It is therefore necessary to keep up the strength of the copper sulphate solution by a supply of crystals of that salt.

In one very portable form we have but a single vessel; at the bottom is a plate of copper on which is a layer of crystals of sulphate of copper; over this is a layer of sawdust; and, resting on this, a plate of zinc immersed in dilute sulphuric acid or in a solution of sulphate of zinc. Insulated wires form connections with the plates.

6

In gravity batteries' the liquids are kept apart simply by the fact that the less dense liquid forms a layer above the more dense.

(iv.) Leclanche's cell.-In this the plate is a zinc rod immersed in a strong solution of ammonium chloride. In the porous pot is a carbon rod, round which is tightly packed a mixture of manganese dioxide and of powdered carbon. The porous pot

FIG. ii.

gases.

The ammonium chloride solution soon soaks through and moistens the powder in the porous pot, and then the cell is ready for use.

Zinc chloride is formed in the outer vessel, the zinc displacing ammonium. In the inner vessel is set free ammonia, while the remaining hydrogen of the ammonium reduces the manganese dioxide.

§ 12. Remarks on Cells and on Batteries.-We have now mentioned the chief cells that are of interest to the general student. A few remarks will be made in conclusion.

Chemical action in the cell.—For a discussion of the manner in which the chemical action takes place, in what way the hydrogen displaced by the zinc 'travels with the current' and appears at the negative plate, and what relation the amount of chemical action bears to the strength of the current, the student is referred to Chapter XII.

Efficiency of different cells.-We have stated that different cells give a different ▲ V at their terminals when the circuit is open, and a different E.M.F. (which, when the other conditions are the same, will cause different currents) when the circuit is completed. For a further discussion of this matter we refer the reader to the sections on Electromotive force' in Chapters XIII. and XV.

This matter

Again, cells differ from one another in offering more or less resistance to the passage of the current through them. will be discussed in Chapter XIII.

Coupling cells together.-If we connect n cells as we connected the elements in the Couronne des tasses we get a AV in the open circuit, or an E.M.F. in the closed circuit, which is n times that of a single cell.

Again, if we connect all the positive plates together and all the negative plates together respectively, we have what amounts to one large cell whose plates are ʼn times the size of those of a single cell. Such an arrangement has the E.M.F. of only one cell.

The advantages of the above two methods of coupling in different circumstances respectively will be discussed in Chapter XIII., and the proper terms will be there given.

Amalgamating zinc.-If the zinc be wetted with dilute acid it is readily amalgamated with mercury.

Or, again, if a little sodium be added to the mercury it enables one to amalgamate zinc and other metals with greater ease.

Use of carbons. In order to obviate the tendency of the carbons to 'soak up' the liquids in which they are immersed-a result very unpleasant and very injurious to the binding screws attached to the carbons-it is usual to soak the upper part of the carbons in melted paraffin wax. The surface must then be scraped at the places where the binding screws make contact.