CHAPTER II.

ON THE CHOICE OF BATTERIES FOR ELECTRIC BELL WORK.

[S] 17. If we immerse a strip of ordinary commercial sheet zinc in dilute acid (say sulphuric acid 1 part by measure, water 16 parts by measure[4]), we shall find that the zinc is immediately acted on by the acid, being rapidly corroded and dissolved, while at the same time a quantity of bubbles of gas are seen to collect around, and finally to be evolved at the surface of the fluid in contact with the plate. Accompanying this chemical action, and varying in a degree proportionate to the intensity of the action of the acid on the zinc, we find a marked development of _heat_ and _electricity_. If, while the bubbling due to the extrication of gas be still proceeding, we immerse in the same vessel a strip of silver, or copper, or a rod of graphite, taking care that contact _does not_ take place between the two elements, no perceptible change takes place in the condition of things; but if we cause the two strips to touch, either by inclining the upper extremities so as to bring them in contact out of the fluid like a letter [Lambda], or by connecting the upper extremities together by means of a piece of wire (or other conductor of electricity), or by causing their lower extremities in the fluid to touch, we notice a very peculiar change. The extrication of bubbles around the zinc strip ceases entirely or almost entirely, while the other strip (silver, copper, or graphite) becomes immediately the seat of the evolution of the gaseous bubbles. Had these experiments been performed with chemically pure metallic zinc, instead of the ordinary impure commercial metal, we should have found some noteworthy differences in behaviour. In the first place, the zinc would have been absolutely unattacked by the acid before the immersion of the other strip; and, secondly, all evolution of gas would entirely cease when contact between the two strips was broken.

[Footnote 4: In mixing sulphuric acid with water, the acid should be added in a fine stream, with constant stirring, to the water, and not the water to the acid, lest the great heat evolved should cause the acid to be scattered about.]

As the property which zinc possesses of causing the extrication of gas (under the above circumstances) has a considerable influence on the efficiency of a battery, it is well to understand thoroughly what chemical action takes place which gives rise to this evolution of gas.

[S] 18. All acids may be conveniently regarded as being built up of two essential portions, viz.: firstly, a strongly electro-negative portion, which may either be a single body, such as _chlorine_, _iodine_, _bromine_, etc., or a compound radical, such as _cyanogen_; secondly, the strongly electro-positive body _hydrogen_.

Representing, for brevity's sake, hydrogen by the letter H., and chlorine, bromine, iodine, etc., respectively by Cl., Br., and I., the constitution of the acids derived from these bodies may be conveniently represented by:--

H Cl H Br H I ---- ---- --- Hydrochloric[5] Hydrobromic Hydriodic Acid. Acid. Acid.

[Footnote 5: Spirits of salt.]

and the more complex acids, in which the electro-negative component is a compound, such as sulphuric acid (built up of 1 atom of sulphur and 4 atoms of oxygen, united to 2 atoms of hydrogen) or nitric acid (consisting of 1 nitrogen atom, 6 oxygen atoms, and 1 hydrogen atom), may advantageously be retained in memory by the aid of the abbreviations:--

H_{2}SO_{4} HNO_{6} ----------- ------- Sulphuric and Nitric Acid.[6] Acid.[7]

[Footnote 6: Oil of vitriol.]

[Footnote 7: Aquafortis.]

When zinc _does_ act on an acid, it displaces the hydrogen contained in it, and takes its place; the acid losing at the same time its characteristic sourness and corrosiveness, becoming, as chemists say, _neutralized_. _One_ atom of zinc can replace _two_ atoms of hydrogen, so that one atom of zinc can replace the hydrogen in two equivalents of such acids as contain only one atom of hydrogen.

This power of displacement and replacement possessed by zinc is not peculiar to this metal, but is possessed also by many other bodies, and is of very common occurrence in chemistry; and may be roughly likened to the substitution of a new brick for an old one in a building, or one girder for another in an arch.

It will be well, therefore, to remember that in all batteries in which acids are used to excite electricity by their behaviour along with zinc, the following chemical action will also take place, according to which acid is employed:--

Hydrochloric Acid and Zinc, equal Zinc Chloride and Hydrogen Gas.

2HCl + Zn = ZnCl_{2} + H_{2}

or:--

Sulphuric Acid and Zinc, equal Zinc Sulphate and Hydrogen Gas.

H_{2}SO_{4} + Zn = ZnSO_{4} + H_{2}

Or we may put this statement into a general form, covering all cases in which zinc is acted on by a compound body containing hydrogen, representing the other or electro-negative portion of the compound by X:--

Zn + H_{2}X = ZnX + H_{2}

the final result being in every case the corrosion and solution of the zinc, and the extrication of the hydrogen gas displaced.

[S] 19. We learn from the preceding statements that no electricity can be manifested in a battery or cell (as such a combination of zinc acid and metal is called) without consumption of zinc. On the contrary, we may safely say that the more rapidly the _useful_ consumption of zinc takes place, the greater will be the electrical effects produced. But here it must be borne in mind that if the zinc is being consumed when we are _not_ using the cell or battery, that consumption is sheer waste, quite as much as if we were compelled to burn fuel in an engine whether the latter were doing work or not. For this reason the use of commercial zinc, in its ordinary condition, is not advisable in batteries in which acids are employed, since the zinc is consumed in such, whether the battery is called upon to do electrical work (by placing its plates in connection through some conducting circuit) or not. This serious objection to the employment of commercial zinc could be overcome by the employment of chemically purified zinc, were it not that the price of this latter is so elevated as practically to preclude its use for this purpose. Fortunately, it is possible to confer, on the ordinary crude zinc of commerce, the power of resisting the attacks of the acid (so long as the plates are not metallically connected; or, in other words, so long as the "circuit is broken"), by causing it to absorb superficially a certain amount of mercury (quicksilver). The modes of doing this, which is technically known as _amalgamating the zinc_, are various, and, as it is an operation which every one who has the care of batteries is frequently called upon to perform, the following working details will be found useful:--

[S] 20. To amalgamate zinc, it should first be washed with a strong solution of common washing soda, to remove grease, then rinsed in running water; the zinc plates, or rods, should then be dipped into a vessel containing acidulated water ([S] 17), and as soon as bubbles of hydrogen gas begin to be evolved, transferred to a large flat dish containing water. While here, a few drops of mercury are poured on each plate, and caused to spread quickly over the surface of the zinc by rubbing briskly with an old nail-brush or tooth-brush. Some operators use a kind of mop, made of pieces of rag tied on the end of a stick, and there is no objection to this; others recommend the use of the fingers for rubbing in the mercury. This latter plan, especially if many plates have to be done, is very objectionable: firstly, on the ground of health, since the mercury is slowly but surely absorbed by the system, giving rise to salivation, etc.; and, secondly, because any jewellery, etc., worn by the wearer will be whitened and rendered brittle. When the entire surface of the zinc becomes resplendent like a looking-glass, the rubbing may cease, and the zinc plate be reared up on edge, to allow the superfluous mercury to drain off. This should be collected for future operations. It is important that the mercury used for this purpose should be pure. Much commercial mercury contains lead and tin. These metals can be removed by allowing the mercury to stand for some time in a vessel containing dilute nitric acid, occasional agitation being resorted to, in order to bring the acid into general contact with the mercury. All waste mercury, drainings, brushings from old plates, etc., should be thus treated with nitric acid, and finally kept covered with water. Sprague, in his admirable work on electricity, says:--"Whenever the zinc shows a grey granular surface (or rather before this), brush it well and re-amalgamate, remembering that a saving of mercury is no economy, and a free use of it no waste; for it may all be recovered with a little care. Keep a convenient sized jar, or vessel, solely for washing zinc in, and brush into this the dirty grey powder which forms, and is an amalgam of mercury with zinc, lead, tin, etc., and forms roughnesses which reduce the protection of the amalgamation. Rolled sheet zinc should always be used in preference to cast. This latter is very hard to amalgamate, and has less electro-motive power[8]; but for rods for use in porous jars, and particularly with saline solutions, cast-zinc is very commonly used. In this case great care should be taken to use good zinc cuttings, removing any parts with solder on them, and using a little nitre as a flux, which will remove a portion of the foreign metals."

[Footnote 8: Power to set up a current of electricity.]

[S] 21. Another and very convenient mode of amalgamating zinc, specially useful where solid rods or masses of zinc are to be used, consists in weighing up the zinc and setting aside four parts of mercury (by weight) for every hundred of the zinc thus weighed up. The zinc should then be melted in a ladle, with a little tallow or resin over the top as a flux. As soon as melted, the mercury should be added in and the mixture stirred with a stick. It should then be poured into moulds of the desired shape. This is, perhaps, the best mode of amalgamating cast zincs.

[S] 22. Some operators recommend the use of mercurial salts (such as mercury nitrate, etc.) as advantageous for amalgamating; but, apart from the fact that these salts are generally sold at a higher rate than the mercury itself, the amalgamation resulting, unless a very considerable time be allowed for the mercuric salts to act, is neither so deep nor so satisfactory as in the case of mercury alone. It may here be noted, that although the effect of mercury in protecting the zinc is very marked in those batteries in which acids are used as the exciting fluids, yet this action is not so observable in the cases in which solutions of _salts_ are used as exciters; and in a few, such as the Daniell cell and its congeners, the use of amalgamated zinc is positively a disadvantage.

[S] 23. If, having thus amalgamated the zinc plate of the little battery described and figured at [S] 9, we repeat the experiment therein illustrated, namely, of joining the wires proceeding from the two plates over a suspended magnetic needle, and leave them so united, we shall find that the magnetic needle, which was originally very much deflected out of the line of the magnetic meridian (north and south), will very quickly return near to its old and normal position; and this will be found to take place long before the zinc has been all consumed, or the acid all neutralised. Of course, this points to a rapid falling off in the transmission of the electric disturbance along the united wires; for had _that_ continued of the same intensity, the deflection of the needle would evidently have remained the same likewise. What, then, can have caused this rapid loss of power? On examining (without removing from the fluid) the surface of the copper plate, we shall find that it is literally covered with a coating of small bubbles of hydrogen gas, and, if we agitate the liquid or the plates, many of them will rise to the surface, while the magnetic needle will at the same time give a larger deflection. If we entirely remove the plates from the acid fluid, and brush over the surface of the copper plate with a feather or small pledget of cotton wool fastened to a stick, we shall find, on again immersing the plates in the acid, that the effect on the needle is almost, if not quite, as great as at first; thus proving that the sudden loss of electrical energy was greatly due to the adhesion of the free hydrogen gas to the copper plate. This peculiar phenomenon, which is generally spoken of as the _polarisation of the negative plate_, acts in a twofold manner towards checking the electrical energy of the battery. In the first place, the layer of hydrogen (being a bad conductor of electricity) presents a great resistance to the transmission of electrical energy from the zinc plate where it is set up to the copper (or other) plate whence it is transmitted to the wires, or _electrodes_. Again, the _copper_ or other receiving plate, in order that the electric energy should be duly received and transmitted, should be more electro-negative than the zinc plate; but the hydrogen gas which is evolved, and which thus adheres to the negative plate, is actually very highly electro-positive, and thus renders the copper plate incapable of receiving or transmitting the electric disturbance. This state of things may be roughly likened to that of two exactly equal and level tanks, Z and C, connected by a straight piece of tubing. If Z be full and C have an outlet, it is very evident that Z can and will discharge itself into C until exhausted; but if C be allowed to fill up to the same level as Z, then no farther flow can take place between the two.

It is, therefore, very evident that to ensure anything like constancy in the working of a battery, at least until all the zinc be consumed or all the acid exhausted, some device for removing the liberated hydrogen must be put into practice. The following are some of the means that have been adopted by practical men:--

[S] 24. _Roughening the surface of the negative plate_, which renders the escape of the hydrogen gas easier. This mode was adopted by Smee in the battery which bears his name. It consists of a sheet of silver, placed between two plates of zinc, standing in a cell containing dilute sulphuric acid, as shown at Fig. 5.

The silver sheet, before being placed in position, is _platinised_; that is to say, its surface is covered (by electro-deposition) with a coating of platinum, in the form of a fine black powder. This presents innumerable points of escape for the hydrogen gas; and for this reason this battery falls off much less rapidly than the plain zinc and smooth copper form. A modification of Smee's battery which, owing to the large negative surface presented, is very advantageous, is Walker's graphite cell. In this we have a plate of zinc between two plates of gas-carbon ("scurf"), or graphite. The surface of this body is naturally much rougher than metal sheets; and this roughness of surface is further assisted by coating the surface with platinum, as in the case of the Smee. The chief objection to the use of graphite is its porosity, which causes it to suck up the acid fluid in which the plates stand, and this, of course, corrodes the brass connections, or binding screws.

Other _mechanical_ means of removing the hydrogen have been suggested, such as brushing the surface of the plate, keeping the liquid in a state of agitation by boiling or siphoning; but the only really efficient practical means with which we are at present acquainted are _chemical_ means. Thus, if we can have present at the negative plate some substance which is greedy of hydrogen, and which shall absorb it or combine with it, we shall evidently have solved the problem. This was first effected by Professor Daniell; and the battery known by his name still retains its position as one of the simplest and best of the "constant" forms of battery. The term "constant," as applied to batteries, does not mean that the battery is a constancy, and will run for ever, but simply that so long as there is in the battery any fuel (zinc, acid, etc.), the electrical output of that battery will be constant. The Daniell cell consists essentially in a rod or plate of zinc immersed in dilute sulphuric acid, and separated from the copper or collecting plate by a porous earthen pot or cell. Around the porous cell, and in contact with the copper plate, is placed a solution of sulphate of copper, which is maintained saturate by keeping crystals of sulphate of copper (blue stone, blue vitriol) in the solution. Sulphate of copper is a compound built up of copper Cu, and of sulphur oxide SO_{4}. When the dilute sulphuric acid acts on the zinc plate or rod ([S] 18), sulphate of zinc is formed, which dissolves in the water, and hydrogen is given off:--

Zn + H_{2} SO_{4} = Zn SO_{4} + H_{2}.

Zinc and sulphuric acid produce zinc sulphate and free hydrogen.

Now this free hydrogen, by a series of molecular interchanges, is carried along until it passes through the porous cell, and finds itself in contact with the solution of copper sulphate. Here, as the hydrogen has a greater affinity for, or is more greedy of, the sulphur oxide, SO_{4}, than the copper is, it turns the latter out, takes its place, setting the copper free, and forming, with the sulphur oxide, sulphuric acid. The liberated copper goes, and adheres to the copper plate, and, far from detracting from its efficacy, as the liberated hydrogen would have done, actually increases its efficiency, as it is deposited in a roughened form, which presents a large surface for the collection of the electricity. The interchange which takes place when the free hydrogen meets the sulphate of copper (outside the porous cells) is shown in the following equation:--

H_{2} + Cu SO_{4} = H_{2} SO_{4} + Cu.

Free hydrogen and copper sulphate produce sulphuric acid and free copper.

[S] 25. The original form given to this, the Daniell cell, is shown at Fig. 6, in which Z is the zinc rod standing in the porous pot P, in which is placed the dilute sulphuric acid. A containing vessel, V, of glazed earthenware, provided with a perforated shelf, S, on which are placed the crystals of sulphate of copper, serves to hold the copper sheet, C, and the solution of sulphate of copper. T and T' are the terminals from which the electricity is led where desired.

In another form, the copper sheet itself takes the form and replaces the containing vessel V; and since the copper is not corroded, but actually increases in thickness during action, this is a decided advantage. A modification, in which the porous cell is replaced by _sand_ or by _sawdust_, is also constructed, and known as "Minotto's" cell: this, owing to the greater thickness of the porous layer, offers more resistance, and gives, consequently, less current. By taking advantage of the greater specific gravity (_weight, bulk for bulk_) of the solution of sulphate of copper over that of water or dilute sulphuric acid, it is possible to construct a battery which shall act in a manner precisely similar to a Daniell, without the employment of any porous partition whatsoever. Fig. 7 illustrates the construction of one of these, known as "Gravity Daniells."

In this we have a plate, disc, or spiral of copper, C, connected by an insulated copper wire to the terminal T'. Over this is placed a layer of crystals of copper sulphate; the jar is then filled nearly to the top with dilute sulphuric acid, or with a strong solution of sulphate of zinc (which is more lasting in its effects, but not so energetic as the dilute sulphuric acid), and on the surface of this, connected to the other terminal, T, is allowed to rest a thick disc of zinc, Z. Speaking of these cells, Professor Ayrton, in his invaluable "Practical Electricity," says:--"All gravity cells have the disadvantage that they cannot be moved about; otherwise the liquids mix, and the copper sulphate solution, coming into contact with the zinc plate, deposits copper on it. This impairs the action, by causing the zinc to act electrically, like a copper one. Indeed, without any shaking, the liquids mix by diffusion, even when a porous pot is employed; hence a Daniell's cell is found to keep in better order if it be always allowed to send a weak current when not in use, since the current uses up the copper sulphate solution, instead of allowing it to diffuse." The use of a solution of zinc sulphate to act on the zinc rod, or plate, is always to be preferred in the Daniell cell, when long duration is of more consequence than energetic action.

[S] 26. There are many other bodies which can be used in batteries to absorb the hydrogen set free. Of several of these we need only take a passing notice, as the batteries furnished by their use are unfit for electric bell work. Of these we may mention nitric acid, which readily parts with a portion of the oxygen ([S] 18) and reconverts the free hydrogen into water. This acid is used as the "depolarizer"[9] in the "Grove" and in the "Bunsen" cell. Another very energetic "depolariser" is chromic acid, either in solution, in dilute sulphuric acid, or in the form of potassic dichromate (bichromate of potash: bichrome). As one form of chromic cell has found favour with some bell-fitters, we shall study its peculiarities farther on.

[Footnote 9: Depolarizer is the technical name given to any body which, by absorbing the free hydrogen, removes the false polarity of the negative plate.]

Another class of bodies which readily part with their oxygen, and thus act as depolarisers, are the oxides of lead and manganese. This latter oxide forms the basis of one of the most useful cells for electric bell work, namely: the one known as the "Leclanch['e]." As the battery has been, and will probably remain, long a favourite, the next paragraph will be devoted to its consideration.

[S] 27. The Leclanch['e] cell, in its original form, consists in a rod or block of gas carbon (retort scurf: graphite) standing in an upright porous pot. Around this, so as to reach nearly to the top of the porous cell, is tightly packed a mixture of little lumps of graphite and black oxide of manganese (manganic dioxide: black wad), the porous cell itself being placed in an outer containing vessel, which usually takes the form of a square glass bottle. A zinc rod stands in one corner of the bottle, and is prevented from coming into actual contact with the porous cell by having an indiarubber ring slipped over its upper and lower extremities. The glass containing vessel is then filled to about two-thirds of its height with a solution of ammonium chloride (sal ammoniac) in water, of the strength of about 2 oz. of the salt to each pint of water. This soon permeates the porous cell and reaches the mixture inside. The general appearance of the Leclanch['e] cell is well shown at Fig. 8.

In order to ensure a large surface of contact for the terminal of the carbon rod or plate, it is customary to cast a leaden cap on the top thereof; and, as the porosity of the graphite, or carbon, is very apt to allow the fluid in the battery to creep up to and corrode the terminal, and thus oppose resistance to the passage of electricity, the upper end of the carbon, before the lead cap is cast on, is soaked for some time in melted paraffin wax, at a temperature of 110 deg. Centigrade: that is somewhat hotter than boiling water heat. This, if left on the outside, would prevent the passage of electricity almost entirely; so lateral holes are drilled into the carbon before the cap is finally cast on. The action that takes place in the Leclanch['e] cell may be summarised as follows:--

When the zinc, Zn, is acted on by the ammonium chloride, 2NH_{4}Cl, the zinc seizes the chlorine and forms with it zinc chloride, ZnCl_{2}, while the ammonium, 2NH_{4}, is liberated. But this ammonium, 2NH_{4}, does not escape. Being electro-positive, it is impelled towards the negative plate, and in its passage thereto meets with another molecule of ammonium chloride, from which it displaces the ammonium, in this wise: 2NH_{4} + 2NH_{4}Cl = 2NH_{4}Cl + 2NH_{4}; in other words, this electro-positive ammonium is able, by virtue of its electrical charge, to displace the ammonium from the combined chloride. In so doing, it sets the liberated ammonium in an electro-positive condition, as it was itself, losing at the same time its electrical charge. This interchange of molecules goes on (as we saw in the case of the Daniell's cell, [S] 24) until the surface of the carbon is reached. Here, as there is no more ammonium chloride to decompose, the ammonium 2NH_{4} immediately splits up into ammonia 2NH_{3} and free hydrogen H_{2}. The ammonia escapes, and may be detected by its smell; while the hydrogen H_{2}, finding itself in contact with the oxide of manganese, 2MnO_{2}, seizes one atom of its oxygen, O, becoming thereby converted into water H_{2}O; while the manganese dioxide, 2MnO_{2}, by losing one atom of oxygen, is reduced to the form of a lower oxide of manganese, known as manganese sesquioxide, Mn_{2}O_{3}. Expressed in symbols, this action may be formulated as below:--

In the zinc compartment--

Zn + 2NH_{4}Cl = ZnCl_{2} + 2NH_{3} + H_{2}

In the peroxide of manganese compartment--

H_{2} + 2MnO_{2} = Mn_{2}O_{3} + H_{2}O.

Ammonia gas therefore slowly escapes while this battery is in action, and this corrodes all the brass work with which it comes into contact, producing a bluish green verdigris. If there be not sufficient ammonium chloride in solution, the water alone acts on the zinc: zinc oxide is produced, which renders the solution milky. Should this be the case, more sal ammoniac must be added. It is found that for every 50 grains of zinc consumed in this battery, about 82 grains of sal ammoniac and 124 grains of manganese dioxide are needed to neutralize the hydrogen set free. It is essential for the efficient working of this battery that both the manganese dioxide and the carbon should be free from powder, otherwise it will cake together, prevent the passage of the liquid, and present a much smaller surface to the electricity, than if in a granular form. For this reason, that manganese dioxide should be preferred which is known as the "needle" form, and both this and the carbon should be sifted to remove dust.

[S] 28. In the admirable series of papers on electric bell fitting which was published in the _English Mechanic_, Mr. F. C. Allsop, speaking of the Leclanch['e] cell, says:--"A severe and prolonged test, extending over many years, has proved that for general electric bell work the Leclanch['e] has no equal; though, in large hotels, etc., where the work is likely to be very heavy, it may, perhaps, be preferable to employ a form of the Fuller bichromate battery. It is very important that the battery employed should be a thoroughly reliable one and set up in a proper manner, as a failure in the battery causes a breakdown in the communication throughout the whole building, whilst the failure of a push or wire only affects that portion of the building in which the push or wire is fixed. A common fault is that of putting in (with a view to economy) only just enough cells (when first set up) to do the necessary work. This is false economy, as when the cells are but slightly exhausted the battery power becomes insufficient; whereas, if another cell or two had been added, the battery would have run a much longer time without renewal, owing to the fact that each cell could have been reduced to a lower state of exhaustion, yet still the battery would have furnished the necessary power; and the writer has always found that the extra expense of the surplus cells is fully repaid by the increased length of time the battery runs without renewal."

[S] 29. Another form of Leclanch['e], from which great things were expected at its introduction, is the one known as the "Agglomerate block," from the fact that, instead of simply placing the carbon and manganese together loosely in a porous cell, solid blocks are formed by compressing these materials, under a pressure of several tons, around a central carbon core, to which the terminal is attached in the usual manner. The following are some of the compositions used in the manufacture of agglomerate blocks:--

No. 1.

Manganese dioxide 40 parts. Powdered gas carbon 55 " Gum lac resin 5 "

No. 2.

Manganese dioxide (pyrolusite) 40 parts. Gas carbon (powdered) 52 " Gum lac resin 5 " Potassium bisulphate 3 "

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

No. 3.

_Barbier and Leclanch['e]'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 deg. C. (about 662 deg. 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 glass 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 Leclanch['e] consists in a solution of ammonium chloride.

Among the various advantages claimed for the agglomerate form of Leclanch['e] 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 substituted.

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 passage 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.

[S] 30. A third form of Leclanch['e], 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 spaces, 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.

[S] 31. The ordinary form of Leclanch['e] 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.

[S] 32. There are certain ills to which the Leclanch['e] 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 glass 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 glass 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 scraped away with a knife.

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 Leclanch['e] cell, with a view to obviate altogether the evils deriving from creeping. This cell is illustrated 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 Leclanch['e]." 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.

[S] 33. There is yet a modification of the Leclanch['e] which is sometimes used to ring the large bells in hotels, etc., known as the Leclanch['e] reversed, since the zinc is placed in the porous pot, this latter being stood in the centre of the stoneware jar, the space 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.

[S] 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. Potassic dichromate (bichromate of potash: bichrome) contains potassium, chromium, and oxygen. If we represent potassium by K, chromium by Cr, and oxygen by O, we can get a fair idea of its constitution 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 potassium 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 potassium 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 potassium 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 potassium 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 ([S] 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.

[S] 35. The "bottle" form of the bichromate or chromic acid battery (as illustrated 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 glass 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,

Sulphuric acid 3 oz.

This also must not be used till cold.

In either case the bottle must not be more than three parts filled with the exciting fluid, to allow plenty of room for the zinc to be drawn right out of the liquid when not in use.

[S] 36. The effects given by the above battery, though very powerful, are too transient to be of any service in continuous bell work. The following modification, known as the "Fuller" cell, is, however, useful where powerful currents are required, and, when carefully set up, may be made to do good service for five or six months at a stretch. The "Fuller" cell consists in an outer glass or glazed earthern vessel, in which stands a porous pot. In the porous pot is placed a large block of amalgamated zinc, that is cast around a stout copper rod, which carries the binding screw. This rod must be carefully protected from the action of the fluid, by being cased in an indiarubber tube. The amalgamation of the zinc must be kept up by putting a small quantity of mercury in the porous cell. The porous cells must be paraffined to within about half an inch of the bottom, to prevent too rapid diffusion of the liquids, and the cells themselves should be chosen rather thick and close in texture, as otherwise the zinc will be rapidly corroded. Water alone is used as the exciting fluid in the porous cell along with the zinc. Speaking of this form of cell, Mr. Perren-Maycock says:--"The base of the zinc is more acted on (when bichromate crystals are used), because the porous cells rest on the crystals; therefore let it be well paraffined, as also the top edge. Instead of paraffining the pot in strips all round (as many operators do) paraffin the pot all round, except at one strip about half an inch wide, and let this face the carbon plate. If this be done, the difference in internal resistance between the cell with paraffined pot and the same cell with pot unparaffined will be little; but if the portion that is unparaffined be turned away from the carbon, it will make very nearly an additional 1 ohm resistance. It is necessary to have an ounce or so of mercury in each porous cell, covering the foot of the zinc; or the zincs may be cast short, but of large diameter, hollowed out at the top to hold mercury, and suspended in the porous pot. The zinc is less acted on then, for when the bichromate solution diffuses into the porous pot, it obviously does so more at the bottom than at the top."

Fig. 14 illustrates the form usually given to the modification of the Fuller cell as used for bell and signalling work.

[S] 37. Before leaving the subject of batteries, there are certain points in connection therewith that it is absolutely essential that the practical man should understand, in order to be able to execute any work satisfactorily. In the first place, it must be borne in mind that a cell or battery, when at work, is continually setting up electric undulations, somewhat in the same way that an organ pipe, when actuated by a pressure of air, sets up a continuous sound wave. Whatever sets up the electric disturbance, whether it be the action of sulphuric acid on zinc, or caustic potash on iron, etc., is called _electromotive force_, generally abbreviated E.M.F. Just in the same manner that the organ pipe could give no sound if the pressure of air were alike inside and out, so the cell, or battery, cannot possibly give _current_, or evidence of electric flow, unless there is some means provided to allow the _tension_, or increased atomic motion set up by the electromotive force, to distribute itself along some line of conductor or conductors not subjected to the same pressure or E.M.F. In other words, the "current" of electricity will always tend to flow from that body which has the highest tension, towards the body where the strain or tension is less. In a cell in which zinc and carbon, zinc and copper, or zinc and silver are the two elements, with an acid as an excitant, the zinc during the action of the acid becomes of higher "potential" than the other element, and consequently the undulations take place towards the negative plate (be it carbon, copper, or silver). But by this very action the negative plate immediately reaches a point of equal tension, so that no current is possible. If, however, we now connect the two plates together by means of any conductor, say a copper wire, then the strain to which the carbon plate is subjected finds its exit along the wire and the zinc plate, which is continually losing its strain under the influence of the acid, being thus at a lower potential (electrical level, strain) than the carbon, can and does actually take in and pass on the electric vibrations. It is therefore evident that no true "current" can pass unless the two elements of a battery are connected up by a conductor. When this connection is made, the circuit is called a "_closed circuit_." If, on the contrary, there is no electrical connection between the negative and positive plates of a cell or battery, the circuit is said to be open, or _broken_. It may be that the circuit is closed by some means that is not desirable, that is to say, along some line or at some time when and where the flow is not wanted; as, for instance, the outside of a cell may be _wet_, and one of the wires resting against it, when of course "leakage" will take place as the circuit will be closed, though no useful work will be done. On the other hand, we may actually take advantage of the practically unlimited amount of the earth's surface, and of its cheapness as a conductor to make it act as a portion of the conducting line. It is perfectly true that the earth is a very poor conductor as compared with metals. Let us say, for the sake of example, that damp earth conducts 100,000 times worse than copper. It will be evident that if a copper wire 1/20 of an inch in section could convey a given electric current, the same length of earth having a section of 5,000 inches would carry the same current equally well, and cost virtually nothing, beyond the cost of a metal plate, or sack of coke, presenting a square surface of a little over 70 inches in the side at each end of the line. This mode of completing the circuit is known as "the earth plate."

[S] 38. The next point to be remembered in connection with batteries is, that the electromotive force (E.M.F.) depends on the _nature_ of the elements (zinc and silver, zinc and carbon, etc.) and the excitants used in the cell, and has absolutely nothing whatever to do with their _size_. This may be likened to difference of temperature in bodies. Thus, whether we have a block of ice as large as an iceberg or an inch square, the temperature will never exceed 32 deg.F. as long as it remains ice; and whether we cause a pint or a thousand gallons of water to boil (under ordinary conditions), its temperature will not exceed 212 deg.F. The only means we have of increasing the E.M.F., or "tension," or "potential," of any given battery, is by connecting up its constituent cells in _series_; that is to say, connecting the carbon or copper plate of the one cell to the zinc of the next, and so on. By this means we increase the E.M.F. just in the same degree as we add on cells. The accepted standard for the measure of electromotive force is called a VOLT, and 1 volt is practically a trifle less than the E.M.F. set up by a single Daniell's cell; the exact amount being 1.079 volt, or 1-1/12 volt very nearly. The E.M.F. of the Leclanch['e] is very nearly 1.6 volt, or nearly 1 volt and 2/3. Thus in Fig. 15, which illustrates 3 Leclanch['e] cells set up in series, we should get

1.6 volt 1.6 " 1.6 " --------- 4.8 volts

as the total electromotive force of the combination.

[S] 39. The _current_, or amplitude of the continuous vibrations kept up in the circuit, depends upon two things: 1st, the electromotive force; 2nd, the resistance in the circuit. There is a certain amount of resemblance between the flow of water under pressure and electricity in this respect. Let us suppose we have a constant "head" of water at our disposal, and allow it to flow through a tube presenting 1 inch aperture. We get a certain definite flow of water, let us say 100 gallons of water per hour. More we do not get, owing to the resistance opposed by the narrowness of the tube to a greater flow. If now we double the capacity of the exit tube, leaving the pressure or "head" of water the same, we shall double the flow of water. Or we may arrive at the same result by doubling the "head" or pressure of water, which will then cause a double quantity of water to flow out against the same resistance in the tube, or conductor. Just in the same way, if we have a given pressure of electric strain, or E.M.F., we can get a greater or lesser flow or "current" by having less or more resistance in the circuit. The standard of flowing current is called an AMP[E']RE; and 1 amp[e']re is that current which, in passing through a solution of sulphate of copper, will deposit 18.35 grains of copper per hour. The unit of resistance is known as an OHM. The resistance known as 1 ohm is very nearly that of a column of mercury 1 square millim[e']tre (1/25 of an inch) in section, and 41-1/4 inches in height; or 1 foot of No. 41 gauge pure copper wire, 33/10000 of an inch in diameter, at a temperature of 32 deg. Fahr., or 0 deg. Centigrade.

[S] 40. Professor Ohm, who made a special study of the relative effects of the resistance inserted in the circuit, the electromotive force, and the current produced, enunciated the following law, which, after him, has been called "OHM'S LAW." It is that if we divide the number of electromotive force units (volts) employed by the number of resistance units (ohms) in the entire circuit, we get the number of current units (amp[e']res) flowing through the circuit. This, expressed as an equation is shown below:

E/R = C or Electromotive force/Resistance = Current.

Or if we like to use the initials of volts, amp[e']res, and ohms, instead of the general terms, E, R, and C, we may write V/R = A, or Volts/Ohms = Amp[e']res.

From this it appears that 1 volt will send a current of 1 amp[e']re through a total resistance of 1 ohm, since 1 divided by 1 equals 1. So also 1 volt can send a current of 4 amp[e']res through a resistance of 1/4 of an ohm, since 1 divided by 1/4 is equal to 4. We can therefore always double the current by halving the resistance; or we may obtain the same result by doubling the E.M.F., allowing the resistance to remain the same. In performing this with batteries we must bear in mind that the metals, carbon, and liquids in a battery do themselves set up resistance. This resistance is known as "_internal resistance_," and must always be reckoned in these calculations. We can _halve_ the internal resistance by _doubling_ the size of the negative plate, or what amounts to the same thing by connecting two similar cells "_in parallel_;" that is to say, with both their zincs together, to form a positive plate of double size, and both carbons or coppers together to form a single negative of twice the dimensions of that in one cell. Any number of cells thus coupled together "_in parallel_" have their resistances reduced just in proportion as their number is increased; hence 8 cells, each having a resistance of 1 ohm if coupled together _in parallel_ would have a joint resistance of 1/8 ohm only. The E.M.F. would remain the same, since this does not depend on the size of the plate (see [S] 38). The arrangement of cells in parallel is shown at Fig. 16, where three Leclanch['e] cells are illustrated thus coupled. The following little table gives an idea of the E.M.F. in volts, and the internal resistance in ohms, of the cells mostly used in electric bell work.

TABLE SHOWING E.M.F. AND R. OF BATTERIES.

----------------+-------------------+-----------------+--------------- Name of Cell. | Capacity of Cell. | Electromotive | Resistance in | | force in Volts. | Ohms. ----------------+-------------------+-----------------+--------------- Daniell | 2 quarts | 1.079 | 1 " Gravity | 2 quarts | 1.079 | 10 Leclanch['e] | 1 pint | 1.60 | 1.13 " | 2 pints | 1.60 | 1.10 " | 3 pints | 1.60 | 0.87 Agglomerate | 1 pint | 1.55 | 0.70 " | 2 pints | 1.55 | 0.60 " | 3 pints | 1.55 | 0.50 Fuller | 1 quart | 1.80 | 0.50 ----------------+-------------------+-----------------+---------------

From this it is evident that if we joined up the two plates of a Fuller cell with a short wire presenting no appreciable resistance, we should get a current of (1.80 divided by 0.50) 3.6 amp[e']res along the wire; whereas if a gravity Daniell were employed the current flowing in the same wire would only be a little over 1/10 of an amp[e']re, since 1.079/10 = 0.1079. But every wire, no matter how short or how thick, presents _some_ resistance; so we must always take into account both the internal resistance (that of the battery itself) and the external resistance (that of the wires, etc., leading to the bells or indicators) in reckoning for any given current from any cell or cells.