No. 2.

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

No. 3.

Barbier and Leclanché's Patent.

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

These agglomerate blocks, however prepared, are placed in 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 Leclanché consists in a solution of ammonium chloride.

Among the various advantages claimed for the agglomerate form of Leclanché 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.

§ 30. A third form of Leclanché, 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.

§ 31. The ordinary form of Leclanché 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.

§ 32. There are certain ills to which the Leclanché 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 Leclanché 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 Leclanché." The battery should be set up in as cool a place as possible, as heat is very conducive to creeping. It is also important that the battery should be placed as near as convenient to the bell.

Sometimes the zincs are seen to become coated with a black substance, or covered with crystals, rapidly wasting away at the same time, although doing little or no work; a strong smell of ammonia being given off at the same time. When this occurs, it points to an electrical leakage, or short circuit, and this, of course, rapidly exhausts the battery. It is of the utmost importance to the effective working of any battery that not the slightest leakage or local action should be allowed to take place. However slight such loss be, it will eventually ruin the battery. This leakage may be taking place in the battery, as a porous cell may be broken, and carbon may be touching the zinc; or out of the battery, along the conducting wires, by one touching the other, or through partial conductivity of a damp wall, a metallic staple, etc., or by creeping. If loss or local action has taken place, it is best, after discovering and repairing the faults (see also testing wires), to replace the old zincs by new ones, which are not costly.

§ 33. There is yet a modification of the Leclanché which is sometimes used to ring the large bells in hotels, etc., known as the Leclanché 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.

§ 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 K₂Cr₂O₇, 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:—

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 (§ 18) chromic acid parts with a portion of its oxygen, forming water and sesquioxide of chromium, Cr₂O₃, 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:—

§ 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.

§ 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.

§ 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."

§ 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°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°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 Leclanché is very nearly 1·6 volt, or nearly 1 volt and 2/3. Thus in Fig. 15, which illustrates 3 Leclanché cells set up in series, we should get

as the total electromotive force of the combination.

§ 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 Ampère; and 1 ampère 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 millimètre (1/25 of an inch) in section, and 41¼ 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° Fahr., or 0° Centigrade.

§ 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 (ampères) 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, ampères, and ohms, instead of the general terms, E, R, and C, we may write V/R = A, or Volts/Ohms = Ampères.

From this it appears that 1 volt will send a current of 1 ampère through a total resistance of 1 ohm, since 1 divided by 1 equals 1. So also 1 volt can send a current of 4 ampères through a resistance of ¼ of an ohm, since 1 divided by ¼ 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 ⅛ ohm only. The E.M.F. would remain the same, since this does not depend on the size of the plate (see § 38). The arrangement of cells in parallel is shown at Fig. 16, where three Leclanché 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.

CHAPTER III.
ON ELECTRIC BELLS AND OTHER SIGNALLING APPLIANCES.

§ 41. An electric bell is an arrangement of a cylindrical soft iron core, or cores, surrounded by coils of insulated copper wire. On causing a current of electricity to flow round these coils, the iron becomes, for the time being, powerfully magnetic (see § 13). A piece of soft iron (known as the armature), supported by a spring, faces the magnet thus produced. This armature carries at its free extremity a rod with a bob, clapper or hammer, which strikes a bell, or gong, when the armature, under the influence of the pull of the magnet, is drawn towards it. In connection with the armature and clapper is a device whereby the flow of the current can be rapidly interrupted, so that on the cessation of the current the iron may lose its magnetism, and allow the spring to withdraw the clapper from against the bell. This device is known as the "contact breaker" and varies somewhat in design, according to whether the bell belongs to the trembling, the single stroke, or the continuous ringing class.

§ 42. In order that the electric bell-fitter may have an intelligent conception of his work, he should make a small electric bell himself. By so doing, he will gain more practical knowledge of what are the requisites of a good bell, and where defects may be expected in any he may be called upon to purchase or examine, than he can obtain from pages of written description. For this reason I reproduce here (with some trifling additions and modifications) Mr. G. Edwinson's directions for making an electric bell:—[10]

How to make a bell.—The old method of doing this was to take a piece of round iron, bend it into the form of a horse-shoe, anneal it (by leaving it for several hours in a bright fire, and allowing it to cool gradually as the fire goes out), wind on the wire, and fix it as a magnet on a stout board of beech or mahogany; a bell was then screwed to another part of the board, a piece of brass holding the hammer and spring being fastened to another part. Many bells made upon this plan are still offered for sale and exchange, but their performance is always liable to variation and obstruction, from the following causes:—To insure a steady, uniform vibratory stroke on the bell, its hammer must be nicely adjusted to move within a strictly defined and limited space; the least fractional departure from this adjustment results in an unsatisfactory performance of the hammer, and often a total failure of the magnet to move it. In bells constructed on the old plan, the wooden base is liable to expansion and contraction, varying with the change of weather and the humidity, temperature, etc., of the room in which the bells are placed. Thus a damp, foggy night may cause the wood to swell and place the hammer out of range of the bell, while a dry, hot day may alter the adjustment in the opposite direction. Such failures as these, from the above causes alone, have often brought electric bells into disrepute. Best made bells are, therefore, now made with metallic (practically inexpansible) bases, and it is this kind I recommend to my readers.

The Base, to which all the other parts are fastened, is made of ¾ in. mahogany or teak, 6 in. by 4 in., shaped as shown at Fig. 17, with a smooth surface and French polished. To this is attached the metallic base-plate, which may be cut out of sheet-iron, or sheet-brass (this latter is better, as iron disturbs the action of the magnet somewhat), and shaped as shown in Fig. 18; or it may be made of cast-iron, or cast in brass; or a substitute for it may be made in wrought-iron, or brass, as shown in Fig. 19. I present these various forms to suit the varied handicrafts of my readers; for instance, a worker in sheet metal may find it more convenient to manufacture his bell out of the parts sketched in Figs. 17, 18, 20^A, 21, 23, 24^A, and 25; but, on the other hand, a smith or engineer might prefer the improved form shown at Fig. 31, and select the parts shown at Figs. 20^A, 22, 19, choosing either to forge the horse-shoe magnet, Fig. 20, or to turn up the two cores, as shown at Fig. 21 (A), to screw into the metal base, Fig. 21 B, or to be fastened by nuts, as shown at Fig. 19. The result will be the same in the end, if good workmanship is employed, and the proper care taken in fixing and adjusting the parts. A tin-plate worker may even cut his base-plate out of stout block tin, and get as good results as if the bell were made by an engineer. In some makes, the base-plate is cut or stamped out of thick sheet-iron, in the form shown by the dotted lines on Fig. 18, and when thus made, the part A is turned up at right angles to form a bracket for the magnet cores, the opposite projection is cut off, and a turned brass pillar is inserted at B to hold the contact screw, or contact breaker (§ 41).

The Magnet may be formed as shown at Fig. 20, or at Fig. 20^A. Its essential parts are: 1st. Two soft iron cores (in some forms a single core is now employed); 2nd. An iron base, or yoke, to hold the cores together; 3rd. Two bobbins wound with wire. The old form of magnet is shown at Fig. 20. In this form the cores and yoke are made out of one piece of metal. A length of round Swedish iron is bent round in the shape of a horseshoe; this is rendered thoroughly soft by annealing, as explained further on. It is absolutely essential that the iron be very soft and well annealed, otherwise the iron cores retain a considerable amount of magnetism when the current is not passing, which makes the bell sluggish in action, and necessitates a higher battery power to make it work (see § 14). Two bobbins of insulated wire are fitted on the cores, and the magnet is held in its place by a transverse strip of brass or iron secured by a wood screw passing between the two bobbins. The size of the iron, the wire, the bobbins, and the method of winding is the same as in the form next described, the only difference being that the length of the iron core, before bending to the horse-shoe form, must be such as to allow of the two straight portions of the legs to be 2 in. in length, and stand 1⅜ apart when bent. We may now consider the construction of a magnet of the form shown at Fig. 20^A. To make the cores of such a magnet, to ring a 2½ in. bell, get two 2 inch lengths of 5/16 in. best Swedish round iron, straighten them, smooth them in a lathe, and reduce ¼ in. of one end of each to 4/16 of an in., leaving a sharp shoulder, as shown at Fig. 21 A. Next, get a 2-in. length of angle iron, drill in it two holes 1⅜ apart, of the exact diameter of the turned ends of the cores, and rivet these securely in their places; this may be done by fastening the cores or legs in a vice whilst they are being rivetted. Two holes should be also bored in the other flange to receive the two screws, which are to hold the magnet to the base, as shown at Fig. 21 B. The magnet is now quite equal to the horse-shoe form, and must be made quite soft by annealing. This is done by heating it in a clear coal fire to a bright red heat, then burying it in hot ashes, and allowing it to cool gradually for a period of from 12 to 24 hours; or perhaps a better guide to the process will be to say, bury the iron in the hot ashes and leave it there until both it and they are quite cold. The iron must be brought to a bright cherry red heat before allowing it to cool, to soften it properly, and on no account must the cooling be hurried, or the metal will be hard. Iron is rendered hard by hammering, by being rapidly cooled, either in cold air or water, and hard iron retains magnetism for a longer time than soft iron. As we wish to have a magnet that will only act as such when a current of electricity is passing around it, and shall return to the state of a simple piece of unmagnetised iron when the current is broken, we take the precaution of having it of soft iron. Many bells have failed to act properly, because this precaution has been neglected, the "residual" (or remaining) magnetism holding down the armature after contact has been broken. When the magnet has been annealed, its legs should be polished with a piece of emery cloth, and the ends filed up level and smooth. If it is intended to fasten the cores into the base-plate, this also should be annealed, unless it be made of brass, in which case a thin strip of soft iron should connect the back ends of the two legs before they are attached to the brass base (an iron yoke is preferable, as it certainly is conducive to better effects to have a massive iron yoke, than to have a mere strip as the connecting piece). It will also be readily understood and conceded that the cores should be cut longer when they are to be fastened by nuts, to allow a sufficient length for screwing the ends to receive the nuts. The length and size of the legs given above are suitable for a 2½ in. bell only; for larger bells the size increases 1/16 of an inch, and the length ¼ of an inch, for every ½ in. increase in the diameter of the bell.