CHAPTER IV.

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

[S] 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 [S] 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.

[S] 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.

[S] 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.

[S] 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.

Legge's Window Blind contact is an arrangement by which the blind is secured at the bottom by attaching it to a hook or button. A slight pressure against the blind (caused by anyone trying to enter after having broken a window) sets the electric bell in motion unknown to the intruder.

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 [S] 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.

[S] 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 ([S] 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.

[S] 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 vers[^a]_, 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.

At Fig. 61 is illustrated a novel form, in which the expansion of air causes contact to be made. It consists in an air chamber hermetically closed by a corrugated metal plate I, similar to that used in the aneroid barometers. When the temperature rises to a certain point, the expansion of the air in the chamber brings the metallic plate into contact with the screw, as shown below. This closes the circuit and rings the bell in the usual manner. In all these fire or thermometer alarms, the exact degree of heat at which the bell shall ring, can be pretty accurately adjusted by means of the contact screws.

[S] 59. Closely allied to these forms of contacts are the devices whereby an ordinary clock or watch can be made to arouse the over-drowsy sleeper by the ringing of an electric bell, which in this case should be of the continuous type. All these depend in their action upon some arrangement whereby when the hour hand of the clock or watch arrives at a certain given point in its travel, it makes contact between the battery and bell. In general the contact piece is attached bodily to the clock, but in the very ingenious arrangement illustrated at Fig. 62 (devised by Messrs. Binswanger) the contacts are attached to an outer case, and as the case of the watch itself forms one point of contact, any watch that will slip in the case, may be set to ring the bell.

Messrs. Gent, of Leicester, have also perfected an electric watchman's clock, which records the number of places the watchman in charge has visited or missed on his rounds. This we illustrate at Fig. 63. We quote Messrs. Gent's own words, in the following description:--

"It consists of an eight-day clock, to which is attached a disc or table revolving upon a vertical axis and driven by the mechanism of the clock. The disc is covered with a sheet of paper, attached to it by a binding screw so that it can be removed when used and a clean sheet substituted for it. Each sheet of paper is divided longitudinally into hours and, if necessary, parts of hours, and crosswise into as many divisions as there are places to be visited by the watchman--any number from one to twenty. Each division has a corresponding marker, which indicates, by the impression it makes upon the paper, the time the watchman visits the place connected with that marker. Wires are carried from the terminals of the clock, one to the battery, and one to each press-button fixed at the points intended to be visited by the watchman; another wire is carried from each press-button to the other end of the battery. The action is very simple: when the button is pressed in the current passes through a coil carrying an armature and contact breaker with a point at the end of a long arm; a hammer-like motion is given to the pointer, and a distinct perforation made in the card. It is usual to have the press-button in a box locked up, of which the watchman only has the key.

"The clock may be in the office or bedroom of the manager or head of the establishment, who can thus, from time to time, satisfy himself of the watchman's vigilance. The record should be examined in the morning, and replaced by a clean sheet of card.

"This clock received the special mention of Her Majesty's Commissioners in Lunacy, and has been adopted by some of the largest asylums in the country.

"We have recently made an important improvement by adding a relay for every marker, thus enabling a local battery of greater power to be used for actuating the markers. This has made no alteration in the appearance of the clock, as the relays are contained within the cornice at the top of the clock case."

[S] 60. By means of a float, it is possible to give notice of the height of water in a tank, a reservoir, or even of the state of the tide. In these cases all that is needed is a float with an arm, having a suitable contact attached, so that when the water rises to the level of the float and lifts it, it causes the contact piece to complete the circuit through a set screw. Or the float may be attached to an arm having a certain play in both directions, _i.e._, up and down, within which no contact is made, as the arm has a contact piece on either side, which can touch either an upper or a lower contact screw, according to whether the tide is low or high, or whether the lock or tank is nearly empty or too full.

[S] 61. Sometimes it is convenient to be able to ring an ordinary trembling bell continuously, as when a master wishes to wake a member of his family or a servant; or again, to cut a given bell or bells out of circuit altogether. The arrangements by which this can be effected, are known as "switches." Of switches there are two kinds, namely, _plugswitches_ or _interruptors_, and _lever switches_. The former consists essentially in two stout plates of brass affixed to a base board of any insulating material. These brass plates are set parallel to each other, a short distance apart, and the centre of the facing edge is hollowed out to take a brass taper plug. A binding or other screw is fixed to each brass plate, to connect up to the leading wires. When the plug is in its socket, the circuit between the two plates (and consequently between the battery and bell, etc.) is complete; when the plug is out, the contact is broken. This form of switch is subject to work out of order, owing to the fact that the taper plug gradually widens the hole, so that the contact becomes uncertain or defective altogether. By far the better form of switch is the lever switch, as shown at Fig. 64. This consists in a movable metal lever or arm, which is held by a strong spring in contact with the upper binding screw. It can be made to slide over to the right or left of the centre, at its lower or free end, as far as the binding screws or studs shown, which act at once as stops and point of connection to wires. When the arm or lever is in the centre no contact is made but if it be pushed over to the right, it slides on a brass strip let into and lying flush with the base. Contact is thus made between the upper binding screw and the left-hand screw. If there is another brass strip on the left-hand side (as shown in the figure), contact may be made with another bell, etc., by sliding the arm to the left; or again, if no metal strip be placed on the left side the contact may be broken by pushing the arm towards the left-hand stud.

[S] 62. A _key_ is another form of contact, by means of which a long or short completion of circuit can be made by simply tapping on the knob. It is particularly useful when it is desired to transmit signals, either by ringing or otherwise. It consists, as may be seen at Fig. 65, of a lever or arm of brass, pivoted at its centre, furnished with a spring which keeps the portion under the knob out of contact with the stud in the front of the base-board. As both the stud and the lever are connected to binding screws communicating with the battery and bell, etc., it is evident that on depressing the key the circuit with the bell will be completed for a longer or shorter period, varying with the duration of the depression. Hence, either by using preconcerted signals of short and long rings to signify certain common words, such as a long ring for _No_, and a short one for _Yes_, or by an adaptation of the ordinary Morse code, intelligible conversation can be kept up between house and stable, etc., etc., by means of a key and a bell. As Mr. Edwinson has given much time to the elucidation of this system of bell signalling, I cannot do better than quote his instructions, as given in _Amateur Work_:--

"For this purpose preconcerted signals have been agreed upon or invented as required, and these have been found to be irksome and difficult to remember, because constructed without any reference to a definite plan. We may, however, reduce bell signals to a definite system, and use this system or code as a means to carry on conversation at a distance as intelligently as it can be done by a pair of telegraph instruments. In fact, the Morse telegraph code can be easily adopted for use with electric bells of the vibrating or trembling type, and its alphabet, as appended below, easily learnt. The letters of the alphabet are represented by long strokes and short strokes on the bell, as here shown.--

A .- B -... C -.-. D -.. E . F ..-. G --. H .... I .. J .--- K -.- L .-.. M -- N -. O --- P .--. Q --... R .-. S ... T - U ..- V ...- W .-- X -..- Y -.-- Z --.. Ch ---- [:A] ([ae]) .-.- [:O] ([oe]) ---. [:U] (ue) ..-- 1 .---- 2 ..--- 3 ...-- 4 ....- 5 ..... 6 -.... 7 --... 8 ---.. 9 ----. 0 -----

"It will be noticed that the strokes to represent a letter do not in any case exceed four, and that all the figures are represented by five strokes of varying length to each figure. Stops, and other marks of punctuation, are represented by six strokes, which are in their combination representations of two or three letters respectively, as shown below:--

Comma (,) by A A A or .-.-.- Full stop (.) " I I I " ...... Interrogation (?) " U D " ..--.. Hyphen (-) " B A " -....- Apostrophe (') " W G " .----. Inverted commas (") " A F " .-..-. Parenthesis () " K K " -.--.- Semi-colon (;) " K Ch " .----- Surprise (!) " N Ch " -.---- Colon (:) " I Ch " ..----

"In sending signals to indicate stops, no regard must be had to the letters which they represent; these are only given as aids to memory, and are not to be represented separately on the bell. Bell signals must be given with a certain amount of regularity as to time; indeed, to carry on a conversation in this way it is necessary to be as punctilious in time as when playing a piece of music on a piano, if the signals are to be understood. The dots of the signal should therefore be represented in time by _one_, and the dashes by _two_, whilst the spaces between words and figures where a stop does not intervene should be represented by a pause equal to that taken by a person counting _three_, the space between a word and a stop being of the same duration. To make this more clear I give an example. The mistress signals to her coachman:--

G | E | T | | T | H | E | --. | . | - | | - | .... | . 221 | 1 | 2 |3| 2 | 1111 | 1 | 3

C | A | R | R | I | A | G | E | -.-. | .- | .-. | .-. | .. | .- | --. | . | 2121 | 12 | 121 | 121 | 11 | 12 | 221 | 1 | 3

R | E | A | D | Y .-. | . | .- | -.. | -.-- 121 | 1 | 12 | 211 | 2122

"The coachman replies:--

R | E | A | D | Y .-. | . | .- | -.. | -.-- 121 | 1 | 12 | 211 | 2122

"When the mistress is ready she signals:--

B | R | I | N | G | | T | H | E | -... | .-. | .. | -. | --. | | - | .... | . | 2111 | 121 | 11 | 21 | 221 | 3| 2 | 1111 | 1 | 3

C | A | R | R | I | A | G | E -.-. | .- | .-. | .-. | .. | .- | --. | . 2121 | 12 | 121 | 121 | 11 | 12 | 221 | 1

"And the coachman replies with a single long ring to signify that he understands. It will be found convenient to have an answering signal from the receiving end of the line to each word separately. This must be sent in the pause after each word, and consists of the short signal E . when the word is understood, or the double short signal I .. when the word is not understood. A negative reply to a question may be given by the signal for N -., and an affirmative by the signal for [AE] .-.-; other abbreviations may be devised and used where desired. The code having been committed to memory, it will be quite easy to transpose the words and send messages in cypher when we wish to make a confidential communication; or the bells may be muffled under a thick cloak, and thus, whilst the measured beats are heard by the person for whom the signal is intended, others outside the room will not be annoyed by them."

[S] 63. At [S] 48, we noticed that a device known as a _Relay_ is a convenient, if not an essential mode of working continuous ringing bells. Here we will direct our attention to its structural arrangement, and to its adaptations. Let us suppose that we had to ring a bell at a considerable distance, so far indeed that a single battery would not energise the electro-magnets of an ordinary bell, sufficiently to produce a distinct ring. It is evident that if we could signal, ever so feebly, to an attendant at the other end of the line to make contact with another battery at the distant end of the line to _his_ bell, by means, say, of a key similar to that shown at Fig. 65, we should get a clear ring, since this second battery, being close to the bell, would send plenty of current to energise the bell's magnets. But this would require a person constantly in attendance. Now the _relay_ does this automatically; it _relays_ another battery in the circuit. The manner in which it effects this will be rendered clear, on examination of Fig. 66. Here we have an armature A attached to a light spring, which can play between an insulated stop C, and a contact screw B. The play of this armature can be regulated to a nicety by turning the screws B or C. These two screws are both borne by a double bent arm (of metal) affixed to the pillar D. This pillar is separated from the rest of the frame by an insulating collar or washer of ebonite, so that no current can pass from E to D, unless the armature be pulled down so as to make contact with the contact screw B. Just under the armature, stands the electro-magnet G, which when energised can and does pull down the armature A. It will be readily understood that if we connect the wires from the electro-magnet G, to the wires proceeding from the battery and push (or other form of contact) at the distant station, the electro-magnet, being wound with a large quantity of fine wire, will become sufficiently magnetized to pull the armature down through the small space intervening between C and B; so that if the screws D and E are connected respectively to the free terminals of a battery and bell coupled together at the nearer station, this second battery will be thrown into circuit with the bell, and cause it to ring as well and as exactly as if the most skilful and most trustworthy assistant were in communication with the distant signaller. Every tap, every release of the contact, (be it push, key, or switch) made at the distant end, will be faithfully reproduced at the nearer end, by the motion of the armature A. For this reason we may use a comparatively weak battery to work the relay, which in its turn brings a more powerful and _local_ battery into play, for doing whatever work is required. In cases where a number of calls are required to be made simultaneously from one centre, as in the case of calling assistance from several fire engine stations at once, a relay is fixed at each station, each connected with its own local battery and bell. The current from the sending station passes direct through all the relays, connecting all the local batteries and bells at the same time. This is perhaps the best way of ringing any number of bells from one push or contact, at a distant point. Ordinary trembling bells, unless fitted with an appropriate contrivance, cannot well be rung if connected up in _series_. This is owing to the fact that the clappers of the bells do not all break or make contact at the same time, so that intermittent ringing and interruptions take place. With single stroke bells, this is not the case, as the pulling down of the armature does not break the contact.

[S] 64. We now have to consider those contrivances by means of which it is possible for an attendant to know when a single bell is actuated by a number of pushes in different rooms, etc., from whence the signal emanates. These contrivances are known as _indicators_. Indicators may be conveniently divided into 3 classes, viz.:--1st, indicators with _mechanical_ replacements; 2nd, those with electrical replacements; and 3rdly, those which are self replacing. Of the former class we may mention two typical forms, namely, the ordinary "fall back" indicator, and the drop indicator. All indicators depend in their action on the sudden magnetisation of an electro-magnet by the same current that works the electric bell at the time the call is sent. To understand the way in which this may be effected, let the reader turn to the illustration of the Relay (Fig. 66), and let him suppose that the pillar D, with its accompanying rectangle B C, were removed, leaving only the electro-magnet G, with its frame and armature A. If this armature holds up a light tablet or card, on which is marked the number of the room, it is evident that any downward motion of the armature, such as would occur if the electro-magnet were energised by a current passing around it, would let the tablet fall, so as to become visible through a hole cut in the frame containing this contrivance. It is also equally evident that the card or tablet would require replacing by hand, after having once fallen, to render it capable of again notifying a call. Fig. 67 shows the working parts of one of these "drop" indicators, as sent out by Messrs. Binswanger. In another modification, known as Thorpe's "Semaphore Indicator," we have a most ingenious application of the same principle in a very compact form. In this (Fig. 68), the electro-magnet is placed directly behind a disc-shaped iron armature, on which is painted or marked the number of the room etc. (in this case 4); this armature is attached by a springy shank to the drop bar, shown to the left of the electro-magnet. In front of the armature is a light metal disc, also pivoted on the drop bar. This engages in a catch above, when pushed up so as to cover the number. When pushed up, the spring of the armature retains it in its place so that the number is hidden. When the current passes around the electro-magnet, the armature is pulled toward it, and thus frees the covering disc, which therefore falls, and displays the number. The ordinary form of "fall back" indicator (a misnomer, by the way, since the indicator falls forwards) is well illustrated at Fig 69. Here we have an ordinary electro-magnet A, with its wires _w_ _w'_ standing over an armature B attached to a spring C, which bears on its lower extremity, a toothed projection which serves to hold up the short arm of the bent lever D, which supports the number plate E. When the electro-magnet A is energised by the current, it pulls up the armature B, which releases the detent D from the tooth C; the number plate therefore falls forwards, as shown by the dotted lines, and shows itself at the aperture E', which is in front of the indicator frame. To replace the number out of sight, the attendant pushes back the plate E, till it again engages the bent lever D in the tooth C. This replacement of the number plate, which the attendant in charge is obliged to perform, gives rise to confusion, if through carelessness it is not effected at once, as two or more numbers may be left showing at one time. For this reason, indicators which require no extraneous assistance to replace them, are preferred by many. Indicators with electrical replacements meet in part the necessities of the case. This form of indicator consists usually of a permanent bar magnet pivoted near its centre, so that it can hang vertically between the two poles of an electro-magnet placed at its lower extremity. The upper extremity carries the number plate, which shows through the aperture in the frame. This bar magnet is made a trifle heavier at the upper end, so that it must rest against either the one or other pole of the electro-magnet below. If the _north_ pole of the bar magnet rests against the _right_ hand pole of the electro-magnet when the number does not show, we can cause the bar magnet to cross over to the other pole, and display the number by sending a current through the electro-magnet in such a direction as to make its right hand pole a north pole, and its left hand a south pole. This is because the two north poles will repel each other, while the south will attract the north. On being once tilted over, the bar magnet cannot return to its former position, until the person who used the bell sends a current in the opposite direction (which he can do by means of a reversing switch), when the poles of the electro-magnet being reversed, the bar magnet will be pulled back into its original position. Indicators of this class, owing to the fact that their replacement depends on the _polarity_ of the bar magnet, are also known as "polarised indicators."

[S] 65. For general efficiency and trustworthiness, the _pendulum indicator_; as shown at Fig. 70, is unsurpassed. It consists of an electro-magnet with prolongation at the free end on which is delicately pivoted a soft iron armature. From the centre of this armature hangs, pendulum fashion, a light brass rod carrying a vane of fluted silver glass, or a card with a number on it, as may be found most convenient. This vane or card hangs just before the aperture in the indicator frame. Stops are usually placed on each side of the pendulum rod to limit the swing. When the electro-magnet is magnetised by the passage of the current, the armature is pulled suddenly on one side, and then the pendulum swings backwards and forwards in front of the aperture for some minutes before it comes to rest. When fitted with silver fluted glass, the motion of the vane is clearly visible even in badly lighted places. As the pendulum, after performing several oscillations, comes to rest by itself in front of the aperture, this indicator requires no setting. Messrs. Binswanger fit these indicators with double core magnets, and have a patented adjustment for regulating the duration of the swings of the pendulum, which may be made to swing for two or three minutes when the circuit is completed by pressing the push; it then returns to its normal position, thus saving the servant the trouble of replacing the "drop."

Messrs. Gent, of Leicester, have also patented a device in connection with this form of indicator, which we give in the patentee's own words:--"The objection so frequently urged against the use of Electric Bells, that the servants cannot be depended upon to perform the operation of replacing the signals, cannot any longer apply, for the pendulum signals require no attention whatever. It consists of an electro-magnet having forks standing up in which [V] openings are made. An armature of soft iron, with a piece of thin steel projecting at each end lies suspended at the bottom of the [V] opening, a brass stem carrying the signal card is screwed into the armature, the action being, that when a current is allowed to pass through the electro-magnet the armature with the pendulum is drawn towards it and held there until the current ceases to pass, when it instantly looses its hold of the armature, which swings away and continues to oscillate for two or three minutes, so that if the servant happens to be out of the way, it may be seen on her return which pendulum has been set in motion. The Pendulum Indicator we have recently patented is entirely self-contained. The magnet has its projecting poles riveted into the brass base which carries the flag. The flag is constructed as Fig. 70, but swings in closed bearings, which prevents its jerking out of its place, and enables us to send it out in position ready for use. It will be seen this _patented_ improvement makes all screws and plates as formerly used for securing the parts unnecessary. It will be seen at once that this is simplicity itself, and has nothing about it which may by any possibility be put out of order, either by warping or shrinking of the case or carelessness of attendants."

There is only one point that needs further notice with regard to these pendulum indicators, and that is, that since the rapid break and make contact of the ringing bell interferes somewhat with the proper action of the indicator magnet, it is always advisable to work the indicator by means of a relay (fixed in the same frame) and a _local_ battery. This is shown in Fig. 71, where a second pair of wires attached to C and C, to the extreme right of the indicator frame, are brought from the same battery to work the indicator and contained relay. It is not advisable, however, with the pendulum indicator, to use the same battery for the indicator; the relay should throw a local battery into the indicator circuit. In Fig. 71 six pushes are shown to the left of the indicator frame. These, of course, are supposed to be in as many different rooms.

We close this chapter with an engraving of a very compact and neat form of drop indicator devised by Messrs. Gent, and called by them a "Tripolar Indicator." It consists, as the name implies, of a single magnet, having one end of the iron core as one pole, the other end extending on each side like a [V], forming, as it were, three poles. Though but one bobbin is used, the effect is very powerful. There are no springs or other complications, so that the arrangement is adapted for ship use, as are also those represented at Figs. 67 and 68. Pendulum and fall-back indicators, as well as polarised indicators, owing to the delicacy of the adjustments, are unfitted for use on board ship, or in the cabs of lifts, where the sudden jolts and jerks are sure to move the indicators, and falsify the indications. The tripolar indicator is illustrated at Fig. 72.