§ 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 classes; 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.

Fig. 35 illustrates the action of bells of the first class. 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-button and counter-spring; as shown in Fig. 36, A and B.

In continuous ringing bells of the second class, 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 illustrated 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, passes 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 prong 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 prong 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 illustrated at Fig. 36 can be employed to reset the arm L.

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 button 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 button 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.

§ 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.

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 passes 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 shield for the former. Not only can the bell shield the electro-magnet from wet but the whole of the conducting wires as well.

The bell may be screwed to a tube through which passes 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 shielded from the weather as if within the bell itself.

The great point of departure is the discarding of the unsightly magnet box, and the hemispherical bell (see Fig. 32), and substituting 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 quantity 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.

§ 51. In the "Electric Trumpet," introduced by Messrs. Binswanger, 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 illustrated at Fig. 44, where A is a hollow brass 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 brass 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.

§ 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. Binswanger & 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 illustrations 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 button 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 button 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 scraped, 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.

§ 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 knob or tassel 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 knob or button below. This button is attached to a spring, the tension of which keeps the knob 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.

§ 55. The "pull" (Fig. 47), as its name implies, makes contact and rings the bell on being pulled. The knob 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 knob 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 tassel, 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.

Mr. Mackenzie has introduced a very ingenious contrivance whereby the ringer may know whether the bell at the distant end has rung. This is effected by inclosing in the push a device similar to that shown at Fig. 43 A. That is to say, an electro-magnet wound with wire, and surmounted by a thin iron disc, is placed in circuit with the line wires. The ringing of the bell rapidly magnetises and demagnetises the electro-magnet, and causes a humming sound, which clearly indicates whether the bell is ringing or not. As this device can be made very small, compact, and not liable to derangement, it is of easy application.

§ 56. The next form of contact to which our attention must be directed, is that known as the burglar alarm, with its variant of door-contacts, sash-contacts, till-contacts, etc.

The "burglar's pest" (as the contrivance we illustrate is called) is one of the most useful applications of electricity for the protection of property against thieves. It consists usually, first, of a brass plate (Fig. 50), upon which a platinum contact piece is fixed, and second, of a spring made of hardened brass or steel insulated from the plate; or of a cylindrical box with a spiral spring inside (see Fig. 51). It is so arranged that as long as the stud is kept pressed in, the platinum points of contact are kept apart; this is the position when fixed in the rebate of a closed door or window; but as soon as opened, the stud passes outward through the hole, and the points of contact come together and complete the circuit of the wires in connection with the bell. The bell is best to be a continuous ringing one. It may be fixed in the master's bedroom, or outside the premises in the street.

A form of floor contact, which may be placed under a light mat or carpet, illustrated at Fig. 52, serves to give notice if anyone be waiting at the door, or stepping into places which are desired to be kept private. All these arrangements, to be serviceable, should be connected with continuous ringing bells (see § 48). Wherever it is likely that these arrangements may stand a long time without being called into play, it is better to employ some form of contact in which a rubbing action (which tends to clean the surfaces and then make a good contact) is brought into play, rather than a merely dotting action. For this reason, spring contacts in which the springs connected with the wires are kept apart by an insulating wedge (shown at Fig. 53) as long as the door or window are kept closed, are preferred. In the case of windows, strips of brass let into the frame on each side of the sash, are thrown into contact by the springs a and a' in the sash itself, as shown at Fig. 54. For shop doors and others, where a short contact only is required, and this only when the door is opened, a contact such as shown at Fig. 55 is well adapted. It consists, as will be seen, in a peculiarly shaped pivoted trigger a, which is lifted forwards when the door is opened, so that it makes contact with the spring b. Owing to the curved shape of the arm of the trigger, the contact is not repeated when the door is closed.

§ 57. In all forms of burglar or thief alarms, the ordinary system of having the circuit broken, until contact is made by the intruder involuntarily making contact at some point, presents one great disadvantage; and that is, that if "notre ami l'ennemi," viz., the thief or burglar, be anything of an electrician (and alas! to what base uses may not even science be perverted) he will begin by cutting all suspicious-looking wires before he attempts to set about any serious work. This disadvantage may be entirely overcome by the adoption of a simple modification, known as the "closed circuit system" of bell ringing. For this the bells, etc., are continuously in contact with the batteries, but owing to the peculiar connections, do not ring unless the circuit is broken. To render the working of such a system clear to my readers, I quote the description given in the English Mechanic, by one of our leading electricians:—

Writing on the subject of Closed Circuit Bell-ringing, Mr. Perren Maycock says:—"This is principally adopted for alarm purposes. Its superiority over the open circuit system lies in the fact that notice is given on opening (breaking) the circuit, which is the reverse to the usual practice. In the ordinary method it becomes necessary to have a contact maker, differing in form for various purposes and situations, which, along with the leading wires, must be artfully concealed. All this entails great expense; besides which one can never be sure that the contacts and wires are in proper order without actually trying each one. On the other hand, with the "closed circuit" system, one has merely to place the wire in any convenient position, it being better seen than hidden. The very fact that alarm is given on breaking the contact renders the method applicable in circumstances and under conditions which would render the "open" method difficult and expensive, if not impossible. One can always be certain that everything is in order. The modern burglar, electrically educated as regards common practise in such matters, would naturally make a point of cutting all wires that fall in his path. From these and other obvious considerations, it is evident how simple and yet how perfect a means of protection such a system provides. I will now proceed to explain the manner of application. The bell used differs from the ordinary, only in the arrangement of its external connections.

Fig. 56 A represents a single-alarm circuit. When contact is broken externally, there is a closed circuit in which are the battery and bell magnet coils. Consequently the armature is drawn away from the contact stud, close up to the electro-magnet, and is held so. When a break occurs, the armature flies back, completes the local circuit, and rings so long as the external circuit remains broken. There is a switch for use when the alarm is not required.

Fig. 56 B represents a case in which notice is given at two places. By insulating a key as shown, reply signalling can be carried on between the points at which the bells are placed. A special gravity Daniell modification (§ 25) is used for this class of work (Fig. 57): a narrow lead cylinder, about 2" in diameter, watertight except at the bottom, where it opens out into an inverted cone, the surface of which is pierced with holes. This stands immersed in dilute sulphuric acid. A saturated solution of copper sulphate is next carefully introduced, so as to displace the acid upwards. Crystals of sulphate of copper are introduced into the open end at the top of cylinder, to fill the perforated portion at the bottom. From the wooden cover of cell a thick flat ring of amalgamated zinc hangs suspended in the dilute acid. Care should be taken not to introduce the zinc till the two solutions have become well separated. During action this becomes coppered, while in contact with the sulphate of copper, but it is not attacked by the acid. It is, however, preferable to paint that portion of the lead, which is surrounded by the acid. The height of the cell is about 14."

It will be readily understood that if this latter system be employed, special contacts, which break contact when the pressure is removed, must be employed for the door or window contacts. A simple form is shown at Fig. 58.

Contacts similar to Figs. 50, 53 and 54, may be fitted on tills or drawers.

§ 58. Another useful application of "contact" is for the notification of any rise or fall of temperature beyond certain fixed limits. The devices used for this purpose are known as "fire alarms," "frost alarms," and "thermometer alarms." The thermometer alarm is at once the most effective and trustworthy of the forms known, as, besides its delicacy, it has the advantage of being able to give notice of low, as well as of abnormally high temperature. The form usually given to the electric alarm thermometer, is well shown at Fig. 59. It consists in an ordinary thermometer with a wire projecting into the tube to a certain point, say 100 degrees. The mercury in the bulb being also connected with another wire. When the temperature is within the usual climatic range, the mercury does not reach the upper wire. If by reason of fire or any other abnormal heat, the temperature rises beyond that to which the instrument is set, the mercury rises and touches the upper wire, contact is thus established, and the bell rings.

By giving the thermometer the shape of a letter U, it is possible to notify also a fall below a certain degree, as well as a rise beyond a certain fixed point. These thermometers are specially used by nurserymen and others, to warn them of the too great lowering of temperature, or vice versâ, in the houses under their charge.

Other forms of fire alarms are shown at Fig. 60 and 61. If a strip be built up of two thin layers of dissimilar metals riveted together, as the two metals do not expand at the same rate, the strip will bend to the right if heated, and to the left if cooled. In the instrument shown at Fig. 60, the application of heat causes the flexible strip carrying the contact screw, to bend over till it touches the lower stop, when, of course, the bell rings. If two stops are employed instead of the lower one only, the bell will ring when a low temperature is reached, which causes the strip to bend in the opposite direction.