Electric Bells and All About Them Part 7

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 ill.u.s.trate is called) is one of the most useful applications of electricity for the protection of property against thieves. It consists usually, first, of a bra.s.s plate (Fig. 50), upon which a platinum contact piece is fixed, and second, of a spring made of hardened bra.s.s 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 pa.s.ses 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.

[Ill.u.s.tration: Fig. 50.]

[Ill.u.s.tration: Fig. 51.]

Legge's Window Blind contact is an arrangement by which the blind is secured at the bottom by attaching it to a hook or b.u.t.ton. 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.

[Ill.u.s.tration: Fig. 52.]

[Ill.u.s.tration: Fig. 53.]

A form of floor contact, which may be placed under a light mat or carpet, ill.u.s.trated 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 bra.s.s 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.

[Ill.u.s.tration: Fig. 54.]

[Ill.u.s.tration: Fig. 55.]

-- 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 Mayc.o.c.k says:--"This is princ.i.p.ally 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 circ.u.mstances 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.

[Ill.u.s.tration: Fig. 56 A.]

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.

[Ill.u.s.tration: Fig. 56 B.]

[Ill.u.s.tration: Fig. 57.]

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 cla.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 59.]

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 versa_, 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.

[Ill.u.s.tration: Fig. 60.]

[Ill.u.s.tration: Fig. 61.]

At Fig. 61 is ill.u.s.trated 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.

-- 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 ill.u.s.trated at Fig. 62 (devised by Messrs. Binsw.a.n.ger) 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.

[Ill.u.s.tration: Fig. 62.]

[Ill.u.s.tration: Fig. 63.]

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 ill.u.s.trate 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 subst.i.tuted 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-b.u.t.ton fixed at the points intended to be visited by the watchman; another wire is carried from each press-b.u.t.ton to the other end of the battery. The action is very simple: when the b.u.t.ton is pressed in the current pa.s.ses 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-b.u.t.ton 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."

-- 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.

[Ill.u.s.tration: Fig. 64.]

-- 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 bra.s.s affixed to a base board of any insulating material. These bra.s.s plates are set parallel to each other, a short distance apart, and the centre of the facing edge is hollowed out to take a bra.s.s taper plug. A binding or other screw is fixed to each bra.s.s 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 bra.s.s 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 bra.s.s 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 pus.h.i.+ng the arm towards the left-hand stud.

-- 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 k.n.o.b.

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 bra.s.s, pivoted at its centre, furnished with a spring which keeps the portion under the k.n.o.b 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 -----