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.
This mode cannot be recommended, as it is almost impossible to remove the pitch (in case it be required to renew the zinc, etc.) without breaking the gla.s.s vessel. The best way to remove the pitch is to place the cell in a large saucepan of cold water, and set it on a fire until the water boils. The pitch is, by this treatment, so far softened that the elements can be removed and the pitch sc.r.a.ped away with a knife.
[Ill.u.s.tration: Fig. 12.]
By far the better mode is to rub round the inside and outside of the neck of the jar with tallow, or melted paraffin wax, to the depth of an inch or thereabouts. This effectually prevents creeping and the consequent loss of current. Messrs. Gent, of Leicester, have introduced a very neat modification of the Leclanche cell, with a view to obviate altogether the evils deriving from creeping. This cell is ill.u.s.trated at Fig. 12, and the following is the description supplied by the patentees:--"All who have had experience of batteries in which a solution of salts is used are aware of the difficulty experienced in preventing it creeping over the outside of the jar, causing local loss, and oftentimes emptying the jar of its solution. Many devices have been tried to prevent this, but the only effectual one is our patent insulated jar, in which a recess surrounds the top of the jar, this recess being filled with a material to which the salts will not adhere, thus keeping the outside of the jar perfectly clean. It is specially adapted for use in hot climates, and is the only cell in which jars may touch each other and yet retain their insulations. We confidently recommend a trial of this cell. Its price is but little in excess of the ordinary Leclanche." The battery should be set up in as cool a place as possible, as heat is very conducive to creeping. It is also important that the battery should be placed as near as convenient to the bell.
Sometimes the zincs are seen to become coated with a black substance, or covered with crystals, rapidly wasting away at the same time, although doing little or no work; a strong smell of ammonia being given off at the same time. When this occurs, it points to an electrical leakage, or short circuit, and this, of course, rapidly exhausts the battery. It is of the utmost importance to the effective working of any battery that not the slightest leakage or _local action_ should be allowed to take place. However slight such loss be, it will eventually ruin the battery.
This leakage may be taking place in the battery, as a porous cell may be broken, and carbon may be touching the zinc; or out of the battery, along the conducting wires, by one touching the other, or through partial conductivity of a damp wall, a metallic staple, etc., or by creeping. If loss or local action has taken place, it is best, after discovering and repairing the faults (see also _testing wires_), to replace the old zincs by new ones, which are not costly.
-- 33. There is yet a modification of the Leclanche which is sometimes used to ring the large bells in hotels, etc., known as the Leclanche reversed, since the zinc is placed in the porous pot, this latter being stood in the centre of the stoneware jar, the s.p.a.ce between the two being packed with broken carbon and manganese dioxide. By this means a very much larger negative surface is obtained. In the Grenet cell, the porous cell is replaced by a canvas bag, which is packed full of lumps of graphite and carbon dioxide, a central rod of carbon being used as the electrode. This may be used in out-of-the-way places where porous cells are not readily obtainable, but I cannot recommend them for durability.
-- 34. The only other type of battery which it will be needful to notice in connection with bell work is one in which the depolariser is either chromic acid or a compound of chromic acid with potash or lime. Chromic acid consists of hydrogen united to the metal chromium and oxygen.
Pota.s.sic dichromate (bichromate of potash: bichrome) contains pota.s.sium, chromium, and oxygen. If we represent pota.s.sium by K, chromium by Cr, and oxygen by O, we can get a fair idea of its const.i.tution by expressing it as K_{2}Cr_{2}O_{7}, by which it is shown that one molecule of this body contains two atoms of pota.s.sium united to two atoms of chromium and seven atoms of oxygen. Bichromate of potash readily parts with its oxygen; and it is upon this, and upon the relatively large amount of oxygen it contains, that its efficiency as a depolariser depends. Unfortunately, bichromate of potash is not very soluble in water; one pint of water will not take up much more than three ounces of this salt. Hence, though the solution of pota.s.sium bichromate is an excellent depolariser as long as it contains any of the salt, it soon becomes exhausted. When bichromate of potash is used in a cell along with sulphuric acid and water, sulphate of potash and chromic acid are formed, thus:--
K_{2}Cr_{2}O_{7} + H_{2}SO_{4} + H_{2}O = K_{2}SO_{4} + 2H_{2}CrO_{4} ---------------- ----------- ------ ----------- ------------- 1 molecule of & 1 molecule & 1 give 1 molecule & 2 molecules bichrome. of molecule of of sulphuric of sulphate chromic acid. water. of potash. acid.
From this we learn that before the pota.s.sium bichromate enters into action in the battery, it is resolved into chromic acid. Chromic acid is now prepared cheaply on a large scale, so that pota.s.sium bichromate may always be advantageously replaced by chromic acid in these batteries; the more so as chromic acid is extremely soluble in water. In the presence of the hydrogen evolved during the action of the battery (-- 18) chromic acid parts with a portion of its oxygen, forming water and sesquioxide of chromium, Cr_{2}O_{3}, and this, finding itself in contact with the sulphuric acid, always used to increase the conductivity of the liquid, forms sulphate of chromium. The action of the hydrogen upon the chromic acid is shown in the following equation:--
2H_{2}CrO_{4} + 3H_{2} = 5H_{2}O + Cr_{2}O_{3} ------------- ------ ------- ----------- 2 molecules of 3 molecules 5 molecules 1 molecule chromic & of give of water. & of acid. hydrogen. chromium sesquioxide.
[Ill.u.s.tration: Fig. 13.]
-- 35. The "bottle" form of the bichromate or chromic acid battery (as ill.u.s.trated at Fig. 13) is much employed where powerful currents of short duration are required. It consists of a globular bottle with a rather long wide neck, in which are placed two long narrow graphite plates, electrically connected to each other and to one of the binding screws on the top. Between these two plates is a sliding rod, carrying at its lower extremity the plate of zinc. This sliding rod can be lowered and raised, or retained in any position, by means of a set screw. The zinc is in metallic connection with the other binding screw.
This battery (which, owing to the facility with which the zinc can be removed from the fluid, is extremely convenient and economical for short experiments) may be charged with either of the following fluids:--
FIRST RECIPE.
_Bichromate Solution._
Bichromate of potash (finely powdered) 3 oz.
Boiling water 1 pint.
Stir with a gla.s.s rod, allow to cool, then add, in a fine stream, with constant stirring,
Strong sulphuric acid (oil of vitriol) 3 fluid oz.
The mixture should be made in a glazed earthern vessel, and allowed to cool before using.
SECOND RECIPE.
_Chromic Acid Solution._
Chromic acid (chromic trioxide) 3 oz.
Water 1 pint.
Stir together till dissolved, then add gradually, with stirring,