Electric Bells and All About Them Part 6

-- 47. We can now glance at several modifications in the shape and mode of action of electric bells and their congeners. Taking Figs. 33 A and B as our typical forms of trembling bell, the first notable modification is one by means of which the bell is made to give a single stroke only, for each contact with the battery. This form, which is known as the "single stroke bell," lends itself to those cases in which it may be required to transmit preconcerted signals; as also where it is desired to place many bells in one circuit. Fig. 34 ill.u.s.trates the construction of the single stroke bell. It differs from the trembling bell in the mode in which the electro-magnet is connected up to the binding screws.

In the trembling bell, Fig. 33, the circuit is completed through the platinum screw pillar, to the binding screw marked Z, hence the circuit is rapidly made and broken as long as by any means contact is made with the battery, and the binding screws L and Z. But in the single stroke bell, Fig. 34, the wires from the electro-magnet are connected directly to the two binding screws L and Z, so that when contact is made with the battery, the armature is drawn to the poles of the electro-magnet, and kept there so long as the battery current pa.s.ses. By this means, only one stroke or blow is given to the bell for each contact of the battery.

Of course, directly the connection with the battery is broken, the spring which carries the armature and clapper flies back ready to be again attracted, should connection again be made with the battery. To regulate the distance of the armature from the poles of the electro-magnets, a set screw Q takes the place of the platinum screw in the ordinary form, while to prevent the hammer remaining in contact with the bell (which would produce a dull thud and stop the clear ring of the bell), a stop (_g_) is set near the end of the armature, or two studs are fixed on the tips of the poles of the electro-magnets. The mode of adjusting this kind of bell, so as to obtain the best effect, differs a little from that employed in the case of the trembling bell. The armature must be pressed towards the poles of the electro-magnets, until it rests against the stop or studs. A piece of wood or cork may be placed between the armature and the set screw Q, to retain the armature in this position, while the rod carrying the hammer or clapper is being bent (if required) until the hammer just clears the bell. If it touches the bell, a thud instead of a ring is the result; if it is set off too far, the ring will be too weak. The armature can now be released, by removing the wood or cork, and the set screw Q driven forwards or backwards until the best effect is produced when tested with the battery. The tension of the armature spring must be carefully looked to in these single stroke bells. If it is too strong, the blow will be weak; if too weak, the hammer trembles, so that a clear single stroke is not obtainable, as the spring _chatters_.

-- 48. _The continuous ringing bell_ is the modification which next demands our attention. In this, the ringing action, when once started by the push,[12] or other contact maker, having been touched, continues either until the battery is exhausted, or until it is stopped by the person in charge. The great use of this arrangement is self-evident in cases of burglar alarms, watchman's alarms, etc., as the continuous ringing gives notice that the "call" has not received attention. The continuous ringing bell differs but little from the ordinary trembling bell. The chief difference lies in the addition of an automatic device whereby contact is kept up with the battery, even after the "push"

contact has ceased. As it is desirable for the person in charge to be able to stop the ringing at will, without proceeding to the place where the "push" stands, so it is not usual to make the continuous ringing arrangement dependent on the "push," though, of course, this could be done, by causing it to engage in a catch, which would keep up the contact, when once made. Continuous ringing bells may be conveniently divided into two cla.s.ses; viz., 1st, those in which a device is attached to the framework of the bell; which device, when once upset by the first stroke of the bell, places the bell in direct communication with the battery independent of the "push" or usual contact; and 2ndly, those in which a separate device is used, for the same purpose. This latter arrangement admits of the use of an ordinary trembling bell.

[Footnote 12: A "push," of which several forms will hereafter be described and figured, consists essentially in a spring carrying a stud, standing directly over, but not touching, another stud, fixed to a base.

The lower stud is connected to one terminal of battery, the spring is connected to the bell. When the spring is pressed down, the two studs come into contact, the current flows, and the bell rings.]

[Ill.u.s.tration: Fig. 35.]

[Ill.u.s.tration: Fig. 36.]

Fig. 35 ill.u.s.trates the action of bells of the first cla.s.s. In the first place it will be noticed that there are three binding screws instead of two, as in the ordinary pattern, one marked C connected as usual with the carbon element of the battery; another marked L, which connects with line wire, and a third, Z, connected by means of a branch wire (shunt wire), proceeding from the zinc of the battery. It will be seen, that if the battery current is by means of the push caused to flow through the coils of the electro-magnets, the armature is attracted as usual by them, and in moving towards them, releases and lets fall the lever contact, which, resting on the contact screw, completes the circuit between Z and C, so that the bell is in direct communication with its battery, independently of the push. Hence the bell continues ringing, until the lever is replaced. This can be done, either by pulling a check string (like a bell-pull) attached to an eye in the lever, or by means of a press-b.u.t.ton and counter-spring; as shown in Fig. 36, A and B.

[Ill.u.s.tration: Fig. 37.]

[Ill.u.s.tration: Fig. 38.]

[Ill.u.s.tration: Fig. 39.]

In continuous ringing bells of the second cla.s.s, a detent similar to that shown at Fig. 35 D is used, but this, instead of being actuated by the electro-magnet belonging to the bell itself, is controlled by a separate and entirely independent electro-magnet, which, as it may be wound with many coils of fine wire, and have a specially light spring for the armature, can be made very sensitive. This second electro-magnet, which serves only to make contact with a battery, is known as a _Relay_, and is extensively employed in many cases where it is desired to put one or more batteries into, or out of circuit, from a distance. The relay may be looked upon as an automatic hand, which can be made to repeat at a distant point contacts made or broken by hand at a nearer one. Fig. 37 shows this arrangement, attached to the same base board as the bell itself. On contact being made with the push, the current enters at C, circulates round the cores of the relay, thus converting it into a magnet. The armature _a_ is thereby pulled to the magnet, and in so doing releases the detent lever, which falls on the contact screw, thus at one and the same time breaking the circuit through the relay, and making the circuit through the bell magnets B B', back to the battery by Z. A second modification of this mode of causing an ordinary bell to ring continuously is shown at Fig. 38, the peculiar form of relay used therewith being ill.u.s.trated at Fig. 39. Here, the relay is placed on a separate base board of its own, and could, if necessary, be thrown out of circuit altogether, by means of a _switch_,[13] so that the bell can be used as an ordinary bell or continuous action at will. It will be noticed that the relay has in this sketch only one core. But the delicacy of the action is not impaired thereby, as the armature, by means of the steel spring _s_, is made to form part and parcel of the magnet, so that it becomes magnetised as well as the core, and is attracted with more force than it would be, if it were magnetically insulated. The battery current enters by the wires C and W, pa.s.ses round the coils of the electro-magnet, and returns by Z.

In so doing it energises the electro-magnet E, which immediately attracts its armature A. The forward movement of the armature A, releases the pivoted arm L, to which is attached a platinum-tipped contact p.r.o.ng P. This, it will be noticed, is in metallic connection with the pillar P', and with the base, and, therefore, through the wire W, with the battery. When the arm L falls, the contact p.r.o.ng completes the circuit to the bell, through the insulated pillar X. The relay is thus thrown out of the circuit at the same time that the bell is thrown in. A device similar to those ill.u.s.trated at Fig. 36 can be employed to reset the arm L.

[Footnote 13: Described at -- 61.]

[Ill.u.s.tration: Fig. 40.]

A rather more complicated arrangement for continuous bell ringing is shown at Fig. 40. It is known as Callow's, and is peculiarly adapted to ringing several bells from one attachment, etc. Owing to the relay in this form being wound with two sets of wires, it takes a little more battery power; but this disadvantage is compensated by its many good points. The following description, taken from F. C. Allsop's papers in the _English Mechanic_, will render the working of Callow's attachment perfectly clear. "When the b.u.t.ton of the push P is pressed, the current in the main circuit flows from the positive pole C of the battery D through the relay coil _a_, and thence by the wire _d_ and push P, to the zinc of the battery. This attracts the armature A of the relay R, closing the local bell circuit, the current flowing from C of the battery to armature A of the relay R, through contact post _p_, terminal L of the bell, through bell to terminal Z, and thence by the wire _g_ to the zinc of the battery. Part of the current also flows along the wire from the bell terminal L through the relay coil _b_ and switch W, to terminal Z of the bell, thus keeping the armature of the relay down, after the main circuit (through the push) has been broken; the bell continuing to ring until the shunt circuit is broken by moving the arm of the switch W over to the opposite (or non-contact) side. The bell can also be stopped by short circuiting the relay, which can be effected by an ordinary push. It will be seen that more than one bell can be rung from the same attachment, and the bell can, by moving the arm of the switch W, be made continuous ringing or not, at will. If the arm of the switch is moved over to the opposite side to which it is shown in the figure, the shunt circuit of the bell through the relay is broken, and the bell will ring only so long as the b.u.t.ton of the push is kept in.

This continuous arrangement is very convenient for front doors, etc., where trouble is experienced in securing immediate attention to the summons. Instead of being taken to the switch, as in Fig. 40, the two wires are taken to a contact piece fixed on the side of the door frame, and so arranged that when the door is opened, it either short circuits or breaks the shunt circuit: thus when the push is pressed, the bell rings until the door is opened, the continual ringing of the bell insuring prompt attention."

Mr. H. Thorpe, of 59, Theobald's Road, London, has devised a very ingenious arrangement for the continuous ringing of one or more bells for a stated period of time. This is shown at Fig. 40 A. It is set in action by pulling the ring outside the bottom of the core. The bell or bells then start ringing, as contact is established and kept up. The novelty lies in the fact that the duration of the contact, and consequently of the ringing, can be accurately timed from 5 seconds to 30 seconds, by merely inserting a pin at different holes in the rod, as shown. After the bells have rung the required time the instrument automatically resets itself.

[Ill.u.s.tration: Fig. 40 A.]

-- 49. The modifications we are now about to consider, differ from the ordinary bell, either in the shape or material of the bell itself, the relative disposition of the parts, or some structural detail; but not upon the introduction of any new principle. The most striking is certainly the Jensen bell, which is shown in section at Fig. 41.

[Ill.u.s.tration: Fig. 41.]

According to Mr. Jensen's system of electric bells, the bell may take any desired form, that of the ordinary church bell being preferred, and the electro-magnetic apparatus is placed entirely inside the bell itself. To attain this end the electro-magnetic apparatus must be compact in form. A single electro-magnet has pole pieces at each end opposite to which an armature is suspended from a pivot and balanced by the hammer of the bell. At the back of the armature there may be a make and break arrangement, whereby a continuous succession of strokes is effected, or this may be omitted, in which case a single stroke is given when the contact with the battery is made, or both may be effected by separate wires, make contact with one wire, and a single stroke is struck; make it with the other and the current pa.s.ses through the make and break and a succession of strokes is heard. When the contact-breaker is used, it is so arranged that a slight rub is caused at every stroke, so keeping the contact clean. The flexible break, with the ingenious wiping contact, is a great improvement over the ordinary screw, which often becomes disarranged.

The form of the magnet is such that a considerable degree of magnetic force is caused by a comparatively small battery power. The electro-magnetic apparatus being within the bell the latter forms a very effective and handsome s.h.i.+eld for the former. Not only can the bell s.h.i.+eld the electro-magnet from wet but the whole of the conducting wires as well.

The bell may be screwed to a tube through which pa.s.ses the conducting wire, which makes contact with an insulated metallic piece in the centre of the top of the bell. Both the wire and the contact piece are as completely s.h.i.+elded from the weather as if within the bell itself.

[Ill.u.s.tration: Fig. 42.]

The great point of departure is the discarding of the unsightly magnet box, and the hemispherical bell (_see_ Fig. 32), and subst.i.tuting a bell of the Church type (see Fig. 42), and placing inside it an electro-magnet specially arranged. The inventors use a single solenoidal magnet of a peculiar construction, by which the armature is attracted by both poles simultaneously. By this means less than half the usual quant.i.ty of wire is required, thus reducing the external resistance of the circuit one half. Moreover the armature, besides being magnetised by induction, as acted on in the ordinary method of making electric bells, is by Messrs. Jensen's plan directly polarised by being in actual magnetic contact by the connection of the gimbal (which is one piece with the armature) with the core iron of their magnet. It is thus induced to perform the largest amount of work with the smallest electro-motive force. Instead of the armature and clapper being in a straight line attached to a rigid spring, which necessitates a considerable attractive power to primarily give it momentum, in the Jensen Bell the armature and hammer are in the form of an inverted [U], and being perfectly balanced from the point of suspension, the lines of force from a comparatively small magnetic field suffice to set this improved form of armature into instant regular vibration. By using a flexible break and make arrangement instead of the usual armature spring and set screw (at best of most uncertain action), it is found that a much better result is attained, and by this device the armature can be set much nearer the poles of the magnet with sufficient traverse of the hammer. This is in strict accordance with the law of inverse squares, which holds that the force exerted between two magnetic poles is inversely proportionate to the square of the distance between them, or, in other words, that magnets increase proportionately in their power of attraction as they decrease in the square of the distance. It will now be seen why these bells require so little battery power to ring them: firstly, the armature and hammer are so perfectly balanced as to offer but little resistance; secondly, the external resistance to the current is reduced; and thirdly, the best possible use is made of the electro-magnetic force at disposal.

-- 50. The next modification which demands attention is the so-called "Circular bell." This differs from the ordinary form only in having the action entirely covered by the dome. Except, perhaps, in point of appearance, this presents no advantages to that. The bells known as "Mining bells" resemble somewhat in outward appearance the circular bell; but in these mining bells the action is all enclosed in strong, square teak cases, to protect the movement, as far as possible, from the effects of the damp. All the parts are, for the same reason, made very large and strong; the armature is pivoted instead of being supported on a spring, the hammer shank being long, and furnished with a heavy bob.

The domes or bells are from 6 inches to 12 inches in diameter, and are generally fitted with _single stroke_ movement, so as to enable them to be used for signalling. The hammer shank, with its bob, and the dome, which stands in the centre of the case, are the only parts left uncovered, as may be seen on reference to Figs. 43 A and B, where the exterior and interior of such a bell are shown.

[Ill.u.s.tration: Fig. 43 A.]

[Ill.u.s.tration: Fig. 43 B.]

[Ill.u.s.tration: Fig 44.]

-- 51. In the "Electric Trumpet," introduced by Messrs. Binsw.a.n.ger, of the General Electric Company, we have a very novel and effective arrangement of the parts of an electric bell and telephone together.

This instrument, along with its battery, line and push, is ill.u.s.trated at Fig. 44, where A is a hollow bra.s.s cylinder, in which is placed an ordinary electro-magnet similar to Figs. 20 or 20 A. At the front end, near B, is affixed by its edges a thin disc of sheet iron, precisely as in the Bell telephone,[14] and over against it, at B, is an insulated contact screw, as in the ordinary trembling bell. On the disc of sheet iron, at the spot where the screw touches, is soldered a speck of platinum. The wires from the electro-magnet are connected, one to the upper binding screw, the other to the bra.s.s case of the instrument itself, which is in metallic communication with the sheet iron disc.

The return wire from the contact screw is shown attached to the insulated piece, and is fastened to another binding screw (not visible) on the base board. When contact is made with the battery, through the press or push, the magnet becomes energised, and pulls the iron disc or diaphragm towards it, causing it to buckle inwards. In doing this, contact is broken with the screw B; consequently the diaphragm again straightens out, as the magnet no longer pulls it. Again contact is made; when of course the same round of performances is continuously repeated. As the plate or diaphragm vibrates many hundreds of times per second, it sets up a distinctly musical and loud sound wave, not unlike the note of a cornet-a-piston, or a loud harmonium reed. With a number of these "trumpets," each diaphragm being duly tuned to its proper pitch, it would be possible to construct a novel musical instrument, working solely by electricity. The "pushes" need only take the form of pianoforte keys to render the instrument within the grasp of any pianoforte or organ player.

[Footnote 14: See "Electrical Instrument Making for Amateurs." Whittaker & Co. Second edition.]

-- 52. Sometimes the gong or "dome" of the ordinary bell is replaced by a coil spring, as in the American clocks; sometimes quaint forms are given to the parts covering the "movement," so as to imitate the head of an owl, etc. But bells with these changes in outward form will not present any difficulty, either in fixing or in management, to those who have mastered the structural and working details given in this chapter.

CHAPTER IV.

ON CONTACTS, PUSHES, SWITCHES, KEYS, ALARMS, AND RELAYS.

-- 53. All the appliances which have hitherto been described, would be utterly useless for the purposes intended, had we not at hand some means of easily, certainly and rapidly completing and breaking the circuit between the bell or bells, on the one hand, and the battery on the other. This necessary piece of apparatus, which is simply a contact maker, receives different names, dependent on its application. When it is intended to be actuated directly by hand, it is known as a "push," a "pressel," or "pull," according to the mode in which the contact is made. At Fig. 45, A, B, C, D, and E, show the outward forms of various "pushes," in wood and china, as sent out by the leading makers. (The ones figured are from Messrs. Binsw.a.n.ger & Co.) At F is a sectional view of one of these pushes, and G shows the interior when the cover has been removed. From these two latter ill.u.s.trations it will be easily understood that the "push" consists essentially in two pieces of metal one or both of which are springs, and one of which is connected with one of the wires from the battery, while the other is attached to the wire proceeding to the bell. When the b.u.t.ton is pressed the upper spring comes into contact with the lower metal spring or plate. The circuit is now complete; hence the bell rings. But as soon as the finger is removed from the stud or b.u.t.ton of the "push," the spring returns to its old place, contact being thereby broken when the bell ceases to ring, unless it be fitted with a continuous ringing arrangement (see -- 48). In fastening the leading wires to these pushes, care must be taken that the ends of the wires be sc.r.a.ped, and sand papered quite clean and bright, bent into a loop which must be inserted under the head of the screw that holds the wire to the spring pieces; the screws being then tightened up carefully to ensure a good grip and contact with the wires.

[Ill.u.s.tration: Fig. 45.]

[Ill.u.s.tration: Fig. 46.]

-- 54. A "pressel" (Fig. 46) is simply a push which instead of being made a fixture by being fastened in the wall or door, is attached to a metallic wired line, so that it is generally made to resemble somewhat in outward appearance the k.n.o.b or ta.s.sel of the bell-pull of the last generation, the interior arrangement is precisely similar to that of the push; that is to say, the pressel consists in a pear-shaped or acorn-shaped hollow wooden box, with a projecting k.n.o.b or b.u.t.ton below.

This b.u.t.ton is attached to a spring, the tension of which keeps the k.n.o.b protruding from the end of the box, and at the same time prevents contacts with the second spring at the bottom of the box. Two insulated wires, one from the battery, the other from the bell, are connected to separate screws at the top of the pressel. One of these screws connects with the lower spring, the other with the upper.

[Ill.u.s.tration: Fig. 47.]

-- 55. The "pull" (Fig. 47), as its name implies, makes contact and rings the bell on being pulled. The k.n.o.b has a rather long shank bar, around which is coiled a pretty stiff spring. At the farther extremity is an ebonite or boxwood collar ending in a rather wider metal ring. The wires from the bell and battery are connected respectively to two flat springs, _a a'_, by the screws _b b'_. When the k.n.o.b is pulled, the metal collar touches both springs, and the circuit is completed. Closely allied to the "pull" is a form of bedroom contact, which combines pear-push or pressel and pull in one device. This will be readily understood on reference to Fig. 48. Another form of bedroom pull, with ordinary rope and ta.s.sel, consists in a box containing a jointed metal lever, standing over a stud, from which it is kept out of contact by a counter spring. To the projecting end of the lever is attached the bell rope. When this is pulled the lever touches the stud, contact is made, and the bell rings. This is clearly shown in Fig. 49 A. In all these contacts, except the door pull (Fig. 47) where the friction of the action of pulling keeps the surfaces bright, the points of contact should be tipped with platinum. Another form of contact to be let in the floor of the dining-room, within easy reach of the foot of the carver, or other persons at the head of the table, is shown at Fig. 49 B.

[Ill.u.s.tration: Fig. 48.]

[Ill.u.s.tration: Fig. 49 A.]

[Ill.u.s.tration: Fig. 49 B.]