Fig. 1.
Fig. 1.
Fig. 2.
Fig. 2.
§ 9. A very simple experiment will make this quite clear. Two strips, one of copper and the other of zinc, 1" wide by 4" long, have a 12" length of copper wire soldered to one extremity of each. A small flat piece of cork, about 1" long by 1" square section, is placed between the two plates, at the end where the wires have been soldered, this portion being then lashed together by a few turns of waxed string. (The plates should not touch each other at any point.) If this combination (which constitutes a very primitive galvanic couple) beimmersed in a tumbler three-parts filled with water, rendered just sour by the addition of a few drops of sulphuric or hydrochloric acid, we shall get a manifestation of electrical effects. If a delicately poised magnetic needle be allowed to take up its natural position of north and south, and then the wires proceeding from the two metal strips twisted in contact, so as to be parallel to and over the needle, as shown inFig. 1, the needle will be impelled out of its normal position, and be deflected more or less out of the line ofthe wire. If the needle be again allowed to come to restN.andS.(the battery or couple having been removed), and then the tumbler be held close over the needle, as inFig. 2, so that the needle points from the copper to the zinc strip, the needle will be again impelled or deflected out of its natural position, but in this case in the opposite direction.
§ 10. It is a well-known fact that if a wire, or any other conductor, along which the electric undulation (or, as is usually said, the electric current) is passing, be brought over and parallel to a suspended magnetic needle, pointing north and south, the needle is immediately deflected from this north and south position, and assumes a new direction, more or less east and west, according to the amplitude of the current and the nearness of the conductor to the needle. Moreover, the direction in which the north pole of the needle is impelled is found to be dependent upon the direction in which the electric waves (or current) enter the conducting body or wire. The law which regulates the direction of these deflections, and which is known, from the name of its originator, as Ampère's law, is briefly as follows:—
§ 11. "If a current be caused to flowoverand parallel to a freely suspended magnetic needle, previously pointing north and south, the north pole will be impelled to theLEFTof theenteringcurrent. If, on the contrary, the wire, or conductor, be placedbelowthe needle, the deflection will, under similar circumstances, be in the opposite direction, viz.: thenorth pole will be impelled to theRIGHTof theenteringcurrent." In both these cases the observer is supposed to be looking along the needle, with itsN.seeking pole pointing at him.
§ 12. From a consideration of the above law, in connection with the experiments performed at§ 9, it will be evident that inside the tumbler the zinc ispositiveto the copper strip; while, viewed from the outside conductor, the copper is positive to the zinc strip.[3]
§ 13. A property of current electricity, which is the fundamental basis of electric bell-ringing, is that of conferring upon iron and steel the power of attracting iron and similar bodies, or, as it is usually said, of rendering iron magnetic. If a soft iron rod, say about 4" long by ½" diameter, be wound evenly from end to end with three or four layers of cotton-covered copper wire, say No. 20 gauge, and placed in proximity to a few iron nails, etc., no attractive power will be evinced; but let the two free ends of the wire be placed in metallic contact with the wires leading from the simple battery described at§ 9, and it will be found that the iron has become powerfully magnetic, capable of sustaining several ounces weight of iron and steel, so long as the wires from the battery are in contact with the wire encircling the iron; or, in other words, "the soft iron is a magnet, so long as an electric current flows round it." If contact between the battery wires and the coiled wiresbe broken, the iron loses all magnetic power, and the nails, etc., drop off immediately. A piece of soft iron thus coiled with covered or "insulated" wire, no matter what its shape may be, is termed an "electro-magnet." Their chief peculiarities, as compared with the ordinary permanent steel magnets or lodestones, are, first, their great attractive and sustaining power; secondly, the rapidity, nay, instantaneity, with which they lose all attractive force on the cessation of the electric flow around them. It is on these two properties that their usefulness in bell-ringing depends.
§ 14. If, instead of using asoftiron bar in the above experiment, we had substituted one ofhardiron, or steel, we should have found two remarkable differences in the results. In the first place, the bar would have been found to retain its magnetism instead of losing it immediately on contact with the battery being broken; and, in the second place, the attractive power elicited would have been much less than in the case of soft iron. It is therefore of the highest importance, in all cases where rapid and powerful magnetisation is desired, that thecoresof the electro-magnets should be of the very softest iron. Long annealing and gradual cooling conduce greatly to the softness of iron.
Fig. 3.Magnets, showing Lines of Force.
Fig. 3.Magnets, showing Lines of Force.
§ 15. There is yet another source of electricity which must be noticed here, as it has already found application in some forms of electric bells and signalling, and which promises to enter into more extended use. If we sprinkle some iron filings over a bar magnet, or a horse-shoe magnet, we shall find that the filings arrangethemselves in a definite position along the lines of greatest attractive force; or, as scientists usually say, the iron filings arrange themselves in the direction of the lines of force. The entire space acted on by the magnet is usually known as its "field."Fig. 3gives an idea of the distribution of the iron filings, and also of the general direction of the lines of force. It is found that if a body be moved before the poles of a magnet in such a direction as to cut the lines of force, electricity is excited in that body, and also around the magnet. The ordinary magneto-electric machines of the shops are illustrations of the application of this property of magnets. They consist essentially in a horse-shoemagnet, in front of which is caused to rotate, by means of appropriate gearing, or wheel and band, an iron bobbin, or pair of bobbins, coiled with wire. The ends of the wire on the bobbins are brought out and fastened to insulated portions of the spindle, and revolve with it. Two springs press against the spindle, and pick up the current generated by the motion of the iron bobbins before the poles of the magnet. It is quite indifferent whether we use permanent steel magnets or electro-magnets to produce this effect. If we use the latter, and more especially if we cause a portion of the current set up to circulate round the electro-magnet to maintain its power, we designate the apparatus by the name ofDynamo.
Fig. 4.Typical Dynamo, showing essential portions.
Fig. 4.Typical Dynamo, showing essential portions.
§ 16. Our space will not permit of a very extended description of the dynamo, but the following brief outline of its constructive details will be found useful to the student. A mass of soft iron (shape immaterial) is wound with many turns of insulated copper wire, in such a manner that, were an electrical current sent along the wire, the mass of iron would become strongly north at one extremity, and south at the other. As prolongations of the electro-magnet thus produced are affixed two masses of iron facing one another, and so fashioned or bored out as to allow a ring, or cylinder of soft iron, to rotate between them. This cylinder, or ring of iron, is also wound with insulated wire, two or more ends of which are brought out in a line with the spindle on which it rotates, and fastened down to as many insulated sections of brass cylinder placed around thecircumference of the spindle. Two metallic springs, connected to binding screws which form the "terminals" of the machine, serve to collect the electrical wave set up by the rotation of the coiled cylinder (or "armature") before the poles of the electro-magnet. The annexed cut (Fig. 4) will assist the student in getting a clear idea of the essential portions in a dynamo:—Eis the mass of wrought iron wound with insulated wire, and known as thefield-magnet.NandSare cast-iron prolongations of the same, and are usually bolted to the field-magnet. When current is passing these become powerfully magnetic.Ais the rotating iron ring, or cylinder, known as thearmature, which is also wound with insulated wire,B, the ends of which are brought out and connected to the insulated brass segmentsknown as thecommutator,C. Upon this commutator press the two springsDandD', known as thebrushes, which serve to collect the electricity set up by the rotation of the armature. Thesebrushesare in electrical connection with the two terminals of the machineF F', whence the electric current is transmitted where required; the latter being also connected with the wire encircling the field-magnet,E.
When the iron mass stands in the direction of the earth's magnetic meridian, even if it have not previously acquired a little magnetism from the hammering, etc., to which it was subjected during fitting, it becomes weakly magnetic. On causing the armature to rotate by connecting up the pulley at the back of the shaft (not shown in cut) with any source of power, a very small current is set up in the wires of the armature, due to the weak magnetism of the iron mass of the field-magnet. As this current (or a portion of it) is caused to circulate around this iron mass, through the coils of wire surrounding the field-magnet, this latter becomes more powerfully magnetic (§ 13), and, being more magnetically active, sets up a more powerful electrical disturbance in the armature.
This increased electrical activity in the armature increases the magnetism of this field-magnet as before, and this again reacts on the armature; and these cumulative effects rapidly increase, until a limit is reached, dependent partly on the speed of rotation, partly on the magnetic saturation of the iron of which the dynamo is built up, and partly on the amount of resistance in the circuit.
[2]This refers, of course, to those portions of the metals which are out of the acid. For reasons which will be explained farther on, the condition of the metals in the acid is just the opposite to this.
[2]This refers, of course, to those portions of the metals which are out of the acid. For reasons which will be explained farther on, the condition of the metals in the acid is just the opposite to this.
[3]From some recent investigations, it would appear that what we usually term the negative is really the point at which the undulation takes its rise.
[3]From some recent investigations, it would appear that what we usually term the negative is really the point at which the undulation takes its rise.
§ 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 ofheatandelectricity. 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 contactdoes nottake place between the two elements, no perceptible change takes place in thecondition 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 Λ, 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.
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.
§ 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 aschlorine,iodine,bromine, etc., or a compound radical, such ascyanogen; secondly, the strongly electro-positive bodyhydrogen.
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 ClH BrH I┗━━┛┗━━┛┗━━┛HydrochloricAcid[5].HydrobromicAcid.HydriodicAcid.
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:—
H2SO4HNO6┗━━━┛┗━━┛SulphuricAcid[6].andNitricAcid[7].
When zincdoesact 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.Oneatom of zinc can replacetwoatoms 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 ispossessed 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 AcidandZinc,equalZinc ChlorideandHydrogen Gas.2HCl+Zn=ZnCl2+H2
or:—
Sulphuric AcidandZinc,equalZinc SulphateandHydrogen Gas.H2SO4+Zn=ZnSO4+H2
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 + H2X = ZnX + H2
the final result being in every case the corrosion and solution of the zinc, and the extrication of the hydrogen gas displaced.
§ 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 theusefulconsumption of zinc takes place, the greater will be the electrical effects produced. But here it must be borne in mindthat if the zinc is being consumed when we arenotusing 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 asamalgamating 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:—
§ 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 (§ 17), and as soon asbubbles 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 itwell 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."
§ 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.
§ 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 ofsaltsare used as exciters; and in a few, such as the Daniell cell and its congeners, the use of amalgamated zinc is positively a disadvantage.
§ 23. If, having thus amalgamated the zinc plate of the little battery described and figured at§ 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 hadthatcontinued 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 thefluid) 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 thepolarisation 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, orelectrodes. Again, thecopperor 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,ZandC, connected by a straight piece of tubing. IfZbe full andChave an outlet, it is very evident thatZcan and will discharge itself intoCuntil exhausted; but ifCbe allowed to fill up to the same level asZ, 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:—
§ 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 atFig. 5.
Fig. 5.
Fig. 5.
The silver sheet, before being placed in position, isplatinised; 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 bycoating 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.
Othermechanicalmeans 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 arechemicalmeans. 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 batteryknown 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 SO4. When the dilute sulphuric acid acts on the zinc plate or rod (§ 18), sulphate of zinc is formed, which dissolves in the water, and hydrogen is given off:—
Zn+H2SO4=ZnSO4+H2.Zincandsulphuric acidproducezinc sulphateandfree 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, SO4, 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 fromits 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:—
H2+CuSO4=H2SO4+Cu.Free hydrogenandcopper sulphateproducesulphuric acidandfree copper.
Fig. 6. Daniell Cell.Fig. 6. Daniell Cell.
Fig. 6. Daniell Cell.
§ 25. The original form given to this, the Daniell cell, is shown atFig. 6, in whichZis the zinc rod standing in the porous potP, 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, servesto hold the copper sheet,C, and the solution of sulphate of copper.TandT'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 vesselV; 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 bysandor bysawdust, 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. 7illustrates the construction of one of these, known as "Gravity Daniells."
Fig. 7. Gravity Cell.Fig. 7. Gravity Cell.
Fig. 7. Gravity Cell.
In this we have a plate, disc, or spiral of copper,C, connected by an insulated copper wire to the terminalT'. 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 thatthey 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.
§ 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 thebatteries 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 (§ 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.
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 "Leclanché." As the battery has been, and will probably remain, long a favourite, the next paragraph will be devoted to its consideration.
§ 27. The Leclanché 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 Leclanché cell is well shown atFig. 8.
Fig. 8.Fig. 8.
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 porosityof 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° 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 Leclanché cell may be summarised as follows:—
When the zinc, Zn, is acted on by the ammonium chloride, 2NH4Cl, the zinc seizes the chlorine and forms with it zinc chloride, ZnCl2, while the ammonium, 2NH4, is liberated. But this ammonium, 2NH4, 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: 2NH4+ 2NH4Cl = 2NH4Cl + 2NH4; 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,§ 24) until the surface of the carbon is reached. Here, as there is no more ammonium chloride to decompose, the ammonium 2NH4immediately splits up into ammonia2NH3and free hydrogen H2. The ammonia escapes, and may be detected by its smell; while the hydrogen H2, finding itself in contact with the oxide of manganese, 2MnO2, seizes one atom of its oxygen, O, becoming thereby converted into water H2O; while the manganese dioxide, 2MnO2, by losing one atom of oxygen, is reduced to the form of a lower oxide of manganese, known as manganese sesquioxide, Mn2O3. Expressed in symbols, this action may be formulated as below:—
In the zinc compartment—
Zn + 2NH4Cl = ZnCl2+ 2NH3+ H2
In the peroxide of manganese compartment—
H2+ 2MnO2= Mn2O3+ H2O.
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.
§ 28. In the admirable series of papers on electric bell fitting which was published in theEnglish Mechanic, Mr. F. C. Allsop, speaking of the Leclanché cell, says:—"A severe and prolonged test, extending over many years, has proved that for general electric bell work the Leclanché 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."
§ 29. Another form of Leclanché, from which greatthings 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.