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 gla.s.s 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 quant.i.ty 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-Mayc.o.c.k 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."
[Ill.u.s.tration: Fig. 14.]
Fig. 14 ill.u.s.trates 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 pa.s.s on the electric vibrations. It is therefore evident that no true "current" can pa.s.s 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 c.o.ke, 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 32F. 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 212F. The only means we have of increasing the E.M.F., or "tension," or "potential," of any given battery, is by connecting up its const.i.tuent 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 1079 volt, or 1-1/12 volt very nearly. The E.M.F. of the Leclanche is very nearly 16 volt, or nearly 1 volt and 2/3. Thus in Fig. 15, which ill.u.s.trates 3 Leclanche cells set up in series, we should get
16 volt 16 "
16 "
--------- 48 volts
as the total electromotive force of the combination.
[Ill.u.s.tration: Fig. 15.]
-- 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 quant.i.ty 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 AMPeRE; and 1 ampere is that current which, in pa.s.sing through a solution of sulphate of copper, will deposit 1835 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 millimetre (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 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 (amperes) 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, amperes, and ohms, instead of the general terms, E, R, and C, we may write V/R = A, or Volts/Ohms = Amperes.
From this it appears that 1 volt will send a current of 1 ampere through a total resistance of 1 ohm, since 1 divided by 1 equals 1. So also 1 volt can send a current of 4 amperes 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 -- 38). The arrangement of cells in parallel is shown at Fig.
16, where three Leclanche cells are ill.u.s.trated 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.
[Ill.u.s.tration: Fig. 16.]
TABLE SHOWING E.M.F. AND R. OF BATTERIES.
----------------+-------------------+-----------------+--------------- Name of Cell.Capacity of Cell.ElectromotiveResistance inforce in Volts.Ohms.
----------------+-------------------+-----------------+--------------- Daniell2 quarts10791 " Gravity2 quarts107910 Leclanche1 pint160113 "2 pints160110 "3 pints160087 Agglomerate1 pint155070 "2 pints155060 "3 pints155050 Fuller1 quart180050 ----------------+-------------------+-----------------+---------------
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 (180 divided by 050) 36 amperes 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 ampere, since 1079/10 = 01079. 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.
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_ cla.s.s.
-- 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 bra.s.s 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 s.p.a.ce; 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.
[Footnote 10: "Amateur Work."]
[Ill.u.s.tration: Fig. 17.]
[Ill.u.s.tration: Fig. 18.]
_The Base_, to which all the other parts are fastened, is made of 3/4 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-bra.s.s (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 bra.s.s; or a subst.i.tute for it may be made in wrought-iron, or bra.s.s, 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 bra.s.s pillar is inserted at B to hold the contact screw, or contact breaker (-- 41).
[Ill.u.s.tration: Fig. 19.]
[Ill.u.s.tration: Fig. 20.]
[Ill.u.s.tration: Fig. 20 A.]
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 pa.s.sing, 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 bra.s.s or iron secured by a wood screw pa.s.sing 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 1-3/8 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 2-1/2 in. bell, get two 2 inch lengths of 5/16 in. best Swedish round iron, straighten them, smooth them in a lathe, and reduce 1/4 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 1-3/8 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 f.l.a.n.g.e 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 pa.s.sing 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 bra.s.s, in which case a thin strip of soft iron should connect the back ends of the two legs before they are attached to the bra.s.s base (an iron yoke is preferable, as it certainly is conducive to better effects to have a ma.s.sive 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 s.c.r.e.w.i.n.g the ends to receive the nuts. The length and size of the legs given above are suitable for a 2-1/2 in. bell only; for larger bells the size increases 1/16 of an inch, and the length 1/4 of an inch, for every 1/2 in. increase in the diameter of the bell.
[Ill.u.s.tration: Fig. 21.]
The _Bobbins_, on which the wire that serves to carry the magnetising current is to be wound, next demand our attention. They may be turned out of boxwood, ebony, or ebonite, or out of any hard wood strong enough and dense enough to allow of being turned down thin in the body, a very necessary requirement to bring the convolutions of wire as near the coil as possible without touching it. Some amateurs use the turned ends of cotton reels or spools, and glue them on to a tube of paper formed on the cores themselves. If this tube be afterwards well covered with melted paraffin wax, the plan answers admirably, but of course the bobbins become fixtures on the magnets. There are some persons who are clever enough to make firm bobbins out of brown paper (like rocket cases), with reel ends, that can be slipped off and on the magnet cores.
To these I would say, "by all means at your command, do so if you can."
The size of the bobbins for a 2-1/2 in. bell should be: length 1-3/4 in., diameter of heads 3/4 of an in., the length increasing 1/4 of an in. and the diameter 1/8 of an in. for every additional 1/2 in. in the diameter of the bell. The holes throughout the bobbins should be of a size to fit the iron cores exactly, and the cores should project 1/8 of an inch above the end of the bobbins when these are fitted on. The wire to be wound on the bobbins is sold by all dealers in electrical apparatus. It is copper wire, covered with cotton or with silk, to ensure insulation. Mention has already been made of what is meant by insulation at -- 3, but, in order to refresh the reader"s memory, Mr. G.
Edwinson"s words are quoted here. "To insulate, as understood by electricians, means to protect from leakage of the electric current, by interposing a bad conductor of electricity between two good conductors, thus insulating[11] or detaching them from electric contact."
[Footnote 11: _Insula_ in Latin means an island, hence an electrified body is said to be insulated when surrounded by non-conductors, as an island by the sea.]
The following list will enable my readers to see at a glance the value of the substances mentioned here as conductors or insulators, the best conductors being arranged from the top downwards, and the bad conductors or insulators opposed to them in similar order, viz., the worst conductors or best insulators being at the top:--