Constructed under these conditions, in which the depolarising material rests on a plate, or, better still, at the bottom of a metal jar, is the most handy form for numerous purposes. The use of agglomerate plates should be reserved for special cases.
In these cells the zinc is placed in the upper part, and when in work the very dense solution of zincate of potash which is formed falls to the bottom of the jar, where it lies in a syrupy layer, so that to the very end the zinc is in contact with the dilute alkaline liquid which should attack it. This it does under these conditions with perfect regularity, whilst plates of zinc immersed as far as the bottom of batteries of the same kind are eaten away with increasing rapidity from bottom to top. A plate of zinc 3 mm. thick, placed horizontally, can be eaten away so regularly as to have at last a uniform thickness of only •2-. 3 mm. In batteries at rest the separation of the solution of potash and the saturated solution of zincate which falls to the bottom may be clear enough to permit of renewing the exciting liquid by syphoning off the saturated parts to replace them. Soda gives apparently the same results as potash, both as regards E.M.F. and internal resistance. Potash avoids the formation of creeping salts. This inconvenience can, however, be avoided in large cells by simply covering the surface with a liquid not attackable by soda. It is useful to employ sufficiently concentrated solutions, up to 30-40 per cent., for instance.
A solution containing 5-6 per cent, of potash dissolves but very little oxide of zinc; and a solution of alkaline zincate precipitates oxide of zinc when diluted with water. There is, therefore, a far greater relative proportion of unused alkali the more dilute the solution is. When the alkaline solution in the battery is saturated, though the circuit be closed, the zinc continues nevertheless to be attacked; but then oxide of zinc is deposited as much at the bottom of the jar as on the zinc, which it covers with a hard crust. The appearance of these deposits indicates the exhaustion of the battery. The carbonic acid of the air is absorbed by the potash of the batteries, especially if the surface is not protected by a cover or layer of liquid. This absorption, which otherwise is very slow, is only inconvenient so far as it renders useless the quantity of potash converted into the carbonate state. When the alkaline solution is saturated with oxide of zinc and remains exposed long enough to the action of the air, it causes insoluble crystals to be deposited in water (which appear to be oxide or hydrate of zinc). One serious inconvenience is found in nearly all batteries intended to remain untouched for a long period; that is that the zinc, even with an open circuit, is rapidly attacked at the level of the liquid and to a small distance below the surface.
It may in this way be com* pletely cut in two, even if it be a thick plate, and the working of the cell be stopped. The use of insulating Tarnish or protecting bands of rubber gets over this difficulty. If the zinc be suspended by a wire or rod of any other soluble metal, a local couple is formed which rapidly destroys the electrode at the point of attachment. A rod or plate of bras* or of amalgamated copper may be used as a support for the zinc. Amalgamated zinc will not form a couple with amalgamated brass, even after a long period of immersion. This method is adopted for all batteries required for any length of time. The zinc it always completely immersed and suspended by a conductor of amalgamated brass connected with a terminal.
The oxide of copper battery can be made in many different forms, according to the service it is intended for. Models with an external iron jar offer the advantage of being hermetically closed, readily portable, and of great solidity - very important qualities for cells enclosing a caustic liquid. In one of then forms (Fig. 86) the outer jar, of .09m. diameter, looks like a shell. It couititutes the positive pole; a terminal A projects as shown. The outside of the jar is paraffined when cold, to as to prevent rust and short circuiting. The zinc D is formed of a cylinder '02m. diameter, soldered to a rod of amalgamated brass K, fitted in a rnbber tube G, and carries the terminal F. The tube is also traversed by a metal lobe terminated by a ping H, formed of split rubber tubing. These cells ate usually delivered filled with the potash only necessary to throw in the proper quantity of oiide of copper, which spreads over the bottom at B, and to close the cell by means of the robber stopper carrying the zinc This at-rangement is particularly handy for ose in rooms for telephones or electric bells. This type will give a duty amounting to nearly 2 amperes. A smaller type of •05m. diameter is fully capable of several years' electria bell work.
Fig. 87 shows another type of hertactically -closed cell which hat more recently been brought into use. It hat a large surface, '22m. diameter, and gives a duty of 8 to 10 ampfcres, which allows of its being used for the same purposes as the Bunsen and bichromate batteries, viz.: - Charging accumulators, lighting houses, electro-plating, induction coils, etc. The arrangement of this cell is very similar to that of the foregoing. The oxide of copper B is sitni-trly spread over the bottom of the jar; the zinc D, consisting of a long plate rolled in spiral form to oner a large surface. Is suspended from an ebonite cover G, filled on to the month of the jar by means of screw 3, as shown, and having a rubber washer to serure the joint. These large elements hate the same charge as large trough cells, via-, 2 kilo, potash and .9 kilo, oxide of copper, and can be used instead of them for all purposes. They are capable of a considerable amount of work. For instance, a trough battery has kept an Edison 5-candle lamp alight for more than 300 hours. By using 6 elements one can carry on nickel-plating work during nearly 2 months at the rate of 7 hours per day, which requires 3 Hun sen elements.