Part 3 (1/2)

Gum lac resin 5 ”

Pota.s.sium bisulphate 3 ”

These are to be thoroughly incorporated, forced into steel moulds (containing the central carbon core) at a temperature of 100 C. (212 Fahr.), under a pressure of 300 atmospheres, say 4,500 lbs. to the square inch.

No. 3.

_Barbier and Leclanche's Patent._

Manganese dioxide 49 parts.

Graphite 44 ”

Pitch (”brai gras”) 9 ”

Sulphur 3/5 ”

Water 2/5 ”

The materials having been reduced to fine powder, and the proportion of water stated having been added, are intimately mixed together by hand or mechanically. The moist mixture is moulded at the ordinary temperature, either by a simple compressing press, or by a press in which two pistons moving towards each other compress the block on two opposite faces; or the mixture may be compressed by drawing, as in the manufacture of electric light carbon. After compression, the products are sufficiently solid to be manipulated. They are then put in a stove, or oven, the temperature of which is gradually raised to about 350 C. (about 662 Fahr.); a temperature which is insufficient to decompose the depolarising substance (manganese dioxide), but sufficient to drive out first the volatile parts of the agglomerating material, and then to transform its fixed parts in a body unattackable by the ammonia of the cell. During the gradual heating, or baking, which lasts about two hours, what remains of the water in the agglomerate is driven off; then come the more volatile oils contained in the pitch, and finally the sulphur. The sulphur is added to the mixture, not as an agglomerative, but as a chemical re-agent (and this is a characteristic feature in the invention), acting on what remains of the pitch, as it acts on all carbo-hydrides at a high temperature, transforming it partially into volatile sulphuretted compounds, which are expelled by the heat, and partially into a fixed and unattackable body, somewhat similar to vulcanite. The action of the sulphur on the pitch can very well be likened to its action on caoutchouc (which is likewise a hydro-carbon) during the process of vulcanisation.

These agglomerate blocks, however prepared, are placed in gla.s.s or porcelain containing vessels, as shown in Fig. 9, with a rod of zinc, separated from actual contact with the carbon by means of a couple of crossed indiarubber bands, which serve at the same time to hold the zinc rods upright. The exciting solution, as in the case of the ordinary Leclanche consists in a solution of ammonium chloride.

[Ill.u.s.tration: Fig. 9.]

Among the various advantages claimed for the agglomerate form of Leclanche over the ordinary type, may be mentioned the following:--

1st.--The depolarising power of the manganese oxide is used to the best advantage, and that, owing to this, the electro-motive force of the battery is kept at the same point.

2nd.--That, owing to the absence of the porous cell, there is less internal resistance in the battery and therefore more available current.

3rd.--That the resistance of the battery remains pretty constant, whatever work be put upon it.

4th.--That, owing to the fact that the liquid comes into contact with both elements immediately, the battery is ready for use directly on being charged.

5th.--That the renewal or recharging is exceedingly easy, since the elements can be removed together, fresh solution added, or new depolarising blocks subst.i.tuted.

But when this battery came to be put to the test of practical work, it was found the block form could not be credited with all these advantages, and that their chief superiority over the old cell consisted rather in their lower internal resistance than in anything else. Even this is not an advantage in the case of bell work, except when several bells are arranged _in parallel_, so that a large current is required.

The blocks certainly polarise more quickly than the old form, and it does not appear that they depolarise any more rapidly. Probably the enormous pressure to which the blocks are subjected, in the first two processes, renders the composition almost impermeable to the pa.s.sage of the fluid, so that depolarisation cannot take place very rapidly.

Another and serious objection to these blocks is that, after a little work, pieces break away from the blocks and settle on the zinc. This sets up a ”short circuit,” and the zincs are consumed whether the battery is in action or not.

The author has had no opportunity for making any practical tests with the blocks prepared by process No. 3, but he is under the impression that the blocks would be even more friable than those prepared under greater pressure.

-- 30. A third form of Leclanche, and one which has given considerable satisfaction, is the one known as ”Judson's Patent.” This consists, as shown at Fig. 10, in a cylinder of corrugated carbon encased in an outer coating of an insulating composition. Inside the cell are two or more thin carbon sheets, cemented to the sides of the cell by Prout's elastic glue, or some similar compound, so as to leave s.p.a.ces, which are filled in with granular carbon and manganese. The surface of the plates is perforated, so as to allow ready access to the exciting fluid. The zinc rod, which is affixed to the cover, stands in the centre of the cell, touching it at no part. Owing to the very large surface presented by the corrugations in the carbon, and by the perforated carbon plates, the internal resistance of this form of battery is very low; hence the current, if employed against a small outer resistance, is large. But this, except in the case of bells arranged in parallel, is of no great advantage.

[Ill.u.s.tration: Fig. 10.]

-- 31. The ordinary form of Leclanche is found in market in three sizes, viz., No. 1, No. 2, and No. 3. Unfortunately, all makers do not use these numbers in the same manner, so that while some call the smallest, or _pint_ size, No. 1, others give this name to the largest, or _three-pint_, size. No. 2 is always quart size, and this is the one commonly employed. When several cells are employed to work a number of bells, it is well, in order that they may not receive injury, that they be enclosed in a wooden box. As it is necessary that the batteries should be inspected from time to time, boxes are specially made with doubled hinged top and side, so that when the catch is released these fall flat; thus admitting of easy inspection or removal of any individual cell. This form of battery box is shown at Fig. 11.

[Ill.u.s.tration: Fig. 11. BATTERY IN BOX.]

-- 32. There are certain ills to which the Leclanche cells are liable that require notice here. The first is _creeping_. By creeping is meant the gradual crystallisation of the sal ammonium up the inside and round the outside of the gla.s.s containing jar. There are two modes of preventing this. The first consists in filling in the neck with melted pitch, two small funnel-like tubes being previously inserted to admit of the addition of fresh sal ammoniac solution, and for the escape of gas.