Popular Science Monthly/Volume 18/November 1880/The Electric Burglar-Alarm

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ELABORATE as are the ordinary agencies for the protection of property, they afford but a partial security. Well-lighted streets, careful watchmen, numerous policemen, and strong and ingeniously arranged bolts and bars, are certainly obstacles not easily overcome. But, in his quest of other men's riches, the accomplished burglar has not found them insurmountable. However extensive and vigilant a police force, it can not have all points under its surveillance at once, and this gives the burglar the opportunity which he rarely fails to improve. Bolts and bars are, doubtless, good things in their way; but the experienced cracksman has a cunning beyond them. In the contest between him and the locksmith, the victory has not always been with the latter, though he has produced that marvel of skill and workmanship—the modern safe-lock. The burglar's tools are not such as are thwarted by nice mechanical combinations. Explosives and the simple mechanical powers in his hands have a wonderful range of utility, and are able to frequently set at naught the most elaborate contrivances. The protection afforded by these combined agencies is, however, only realizable to its full extent in the business centers of large cities. In resident districts, and in suburban and country situations where policemen are often few and far between, reliance has chiefly to be placed upon fastenings; and these often prove insufficient. Yet it is especially important to the owner of property that his protection be good, for recovery is very difficult. The advantages are so largely with the thieves, that they can frequently make the search a long and costly, and often a fruitless one. The cost is, in fact, the main bar to recovery. Only when stolen property is of large value does it pay to regain it. Small amounts, such as are usually taken from private houses, are practically irrecoverable.

No practicable extension of the ordinary agencies can greatly increase present security. Bars and bolts have now approached very closely to their limit of strength and ingenuity, and police surveillance is as extensive and perhaps as effective as circumstances will permit. Greater protection must be sought in some further agency—one that will reproduce as nearly as possible the condition of watchfulness present in the daytime. This the electric burglar-alarm is designed to do, and does with a good degree of success. In its earlier forms there were many defects, but in a development of twenty years these have been mostly corrected. It has now attained to a simplicity of construction and certainty of action that make it one of the most useful and trustworthy of man's servitors. Though widely known and appreciated both in this country and abroad, there are probably many not acquainted with it, to whom a brief description will not be without value.

However the details of construction differ, the essential elements of every system are, a bell to give the alarm, an annunciator to indicate the point from which it proceeds, wires from all the openings of a building, and a battery to furnish the current. These elements are combined in various ways, depending upon the special circumstances of the particular case, but the manner of use is practically the same.

The main piece of apparatus, remarkable alike for the simplicity of its construction and the range of its performance, is the annunciator. In the earlier forms of the alarm, the indications were made by means of a simple switch-board provided with buttons bearing the names of the apartments protected. When an alarm sounded, the depression of each of these buttons in turn, until the bell ceased ringing, was necessary to determine its locality. This is still quite largely used, as it is cheaper than the more perfect annunciator, which tells at a glance where the disturbance in the circuit is. In shape and size this latter instrument resembles an ordinary mantel-clock. The indications are given by devices on the face, which vary with different makers. In one form they are made by arrows, which lie horizontal when in normal position, and point to the names of the apartments printed above them when indicating. In another form, cards drop down in front ofPSM V18 D068 Burglar alarm area indicator.jpgFig. 1. apertures arranged in rows on the face, and in still another the name and number of a room are uncovered by a falling piece when an alarm is sounded. The needle-instrument is shown in Fig, 1, Once made, the indications remain until the parts are restored by some one, A small switch at one side completes or opens the circuit through the instrument, and one on the other side controls the connection with the bell, A row of studs at the base of the apparatus allows any opening to be disconnected that may be desired. Aside from its giving an alarm when an attempt is made to enter a building, the annunciator has an important use in showing whether a place is properly closed. If any window or door has been forgotten, it will infallibly point it out. In large business houses where there are many openings, this feature is of the greatest value. By disconnecting the bell, this test can be made a silent one.

The mechanism operating the indicators is of the simplest description. In the needle-instrument, an arm on the pivot of the needle is held in position by the hooked end of a lever, the other end of which forms the armature of an electro-magnet. The connection between the lever and the supported arm is very slight, so that a small movement of the former allows the latter to fall. When the circuit is closed this takes place. The armature in moving toward the magnet raises the hooked end of the lever, releasing the arm which drops and turns its needle. In the instrument using the card, the card is carried on the end of an arm held up in a similar manner by a hook on the armature of the magnet. The depression of the armature allows the arm to drop by its weight. The restoring of the arms to position is done by a sliding frame raised by a handle or button on the base of the instrument. Delicate as the movements of the apparatus are, it is not easily put out of order. The points of contact of the hook and arm are so made as to reduce the wear to a minimum. The mechanism is all inclosed, and the exposed parts, such as the needles, switch-handles, etc, finished in polished metal. The annunciator and bell are usually combined into one piece of apparatus, but they may be put up separate when desired.

The bell used is that common with different forms of electrical instruments. It consists of a gong and a clapper vibrated by the combined action of an electro-magnet and a spring. The magnet, when the current passes, draws the clapper to itself and in doing so opens the circuit; this destroys its magnetism and allows the spring to carry the clapper back. This "make" and "break" action, rapidly repeated as long as the current is passing, produces a continuous ringing of the bell. Reference to Fig. 2[1] will make this movement clear.

Fig. 2.
PSM V18 D069 Alarm bell mechanism.jpg

One end of the wire of the coils of the magnet M M is secured to the binding-post B, and the other to the post C. The arm of the clapper k is a rather stiff spring, which in its normal position holds the armature e carried by it from the poles of the magnet. It then presses against the spring r, attached to the post D. The posts A and E holding the wires from the battery are respectively connected with B and D by metallic strips. The current enters at A, traverses the coils of the magnet M M, passes through the armature e, and out by way of spring r and posts D and E. In doing so, the soft-iron cores of the magnet are magnetized and attract the armature e. This in moving breaks its contact with the spring r, and interrupts the current. The clapper then springs back into position. In the bell now generally used the ringing continues not only while the door or window is open, but until the indicating parts of the annunciator are restored to position.

This secures the proper resetting of the apparatus in readiness for a new alarm. The result is obtained very simply by making the clapper turn a switch, which cuts from the circuit the opened window or door, and allows the current to pass directly from the battery to the bell.

The door and window attachments for closing the circuit by the movements of these parts are of various forms. Those used on doors are simply little push-pins placed in the casing. The pin slides in an insulated case provided with metallic strips. When it is pressed in, the contact between it and the strips is broken and the circuit opened. When the pressure is released, the pin springs out, closing the circuit. The slightest movement of a door allows this motion of the pin to take place. In one form the pin and a metallic casing are so arranged that the attempt to keep the pin pushed in, when the door is opened, by inserting a knife-blade, establishes the circuit and gives the alarm. These push-buttons may be constructed to close the circuit, either by pushing in or springing out, and in both forms have a great variety of uses. They may be placed under the carpet, in the hall, on the stairs, in front of a window, or wherever any one entering would be liable to step. A sufficient number properly disposed could make intrusion without giving an alarm simply impossible. The window attachments are usually simple springs placed in the casing so that the movement of the sash presses them together. One form consists of a roller on the the end of a spring arm, which keeps it pressed out from contact with a metal strip, through which the circuit is completed. Placed in the casing, the roller stands out and is received in a pocket in the edge of the sash, so that the motion of the sash brings the roller arm and metal strip into contact. For the purpose of ventilation, the pocket in the upper sash is usually elongated to give a free movement through any desired distance. When the lower sash is left open, security can be gained by covering a push-pin in the window-sill with a flower-pot or other obstruction, the removal of which is necessary to gain entrance. The wires forming the circuit are of insulated copper, carefully put up so as to be completely hidden from view. They are run in grooves in the wood-work, carried beneath a floor, or on its face, according to circumstances. Once in place, they remain unchanged for any period, causing neither trouble nor expense.

The Le Clanché battery, shown in Fig. 3, is the one universally employed with this apparatus. It is very simple in construction, exhales no noxious gases when in operation, does not waste the material when no current is passing, and needs but very little attention. The positive pole is a piece of gas carbon placed in a porous cell filled with coarse-grained peroxide of manganese and carbon. The cell is sealed at the top with pitch, and a lead cap on the carbon receives the wire. The negative pole is formed of a rod of amalgamated zinc. Both poles are immersed in a solution of sal-ammoniac contained in a glass jar. Four of these elements put up in a wooden case constitute the battery usually furnished.

These appliances provided, the most common way of using the system is to make it complete in each building, the alarm apparatus being placed in a sleeping-apartment in a private house, and in the

Fig. 3.
PSM V18 D071 Alarm system battery.jpg

watchman's room in a place of business. So arranged, the condition of the circuit is this: In the daytime, when the doors and windows are open, the circuit is continuous at all points except at the alarm apparatus. At night this is reversed, the circuit being closed at the instrument, and broken at all the points protected, A movement at any of these points which closes the circuit gives the alarm and turns the proper needle on the annunciator. The connection with the alarm is made at night by an attendant, and broken at any desired time in the morning. In private houses fitted with electric bells, a clock is often provided that disconnects the alarm in the morning and turns the current on to a bell placed in the servants' room. The movement by which this is done is something similar to that of the ordinary alarm-clock.

The protection afforded with such an apparatus in good working order is probably as perfect as it can be made. It is generally impossible to cut the wires from the outside of the building, and unless this is done intrusion will start the alarm. Even if the wires be cut, buttons under the carpet or circuit-closers in interior doors will reveal the burglar's presence in perhaps every case.

Valuable as is the protection in any particular case of attempted robbery, the general immunity from such attempts that the presence of the apparatus secures is of still greater moment. Burglars will not generally take such risks as those imposed by an efficient alarm system, and will therefore give a house so protected a wide berth. The only case in which there is room for failure of the system is when the battery power is not sufficient to operate the alarm. But it is a very simple matter to provide against this. Tests once every month or two, and the experience soon gained in using the battery, will enable one to know at any time the state of the system. None of the other parts need ever cause any solicitude.

While in the great majority of cases the plan of giving the alarm to some one in the building broken into affords perfect security, in some it does not. In business centers, determined and cunning burglars, accustomed to take large chances, might frequently overpower the watchman and stop the alarm before it excited outside attention. To meet this difficulty the plan is sometimes adopted of making the alarm sound in a central office of the company furnishing the apparatus. One company doing this has adopted a system that seems to be beyond circumvention. Each building protected is connected on a closed circuit with the central office, at which place delicate galvanometers are used as indicators. The circuit of each building is independent of all others. Any change in the resistance of any circuit is instantly shown by the deflection of the proper needle, and an alarm started. The opening of a protected door or window breaks the circuit, as does the cutting of the line, and of course gives an alarm. If the burglar could carry the wire to the ground and insert just the proper resistance, no signal would be given at the company's office, but this is impossible, as the resistance is not only that of the wire but of the apparatus in circuit. The only way to get around it is to tunnel under the building, but even then circuit-breakers judiciously disposed would generally lead to detection. Nothing is gained, so far as the safe is concerned, in this case, as it is independently protected. It is placed in a light wooden cabinet lined with a metallic casing, consisting of two sheets of tin-foil insulated from each other by a thin sheet of non-conducting material. The wires from a battery are connected each with one of the sheets of foil. So delicate is the insulation that the sticking of a pin in the cabinet closes the circuit and deflects the needle, and sounds the alarm in the central office. This system, though not yet in extensive use, is gaining in favor among merchants having valuable stores of goods. A similar plan of protecting private houses whose occupants are away is practiced to some extent. The apparatus used in this case is much less delicate, and the protection therefore not so good.

The cost of applying the burglar-alarm to any house will vary in each case. It depends upon the size of the annunciator required and the number of openings to be protected. The prices charged by the different American manufacturers differ very little. Annunciators range in price from thirty dollars with four indications to one hundred dollars with twenty. The annunciator used should have as many indications as there are rooms protected. The cost of circuit-closers, including the placing in position and laying the wire, is three dollars a window when both sashes are connected. The same devices for doors vary from one and a half to two and a half dollars. In ordinary city houses it is only necessary to connect the windows and doors, front and back, of the first two stories and the opening in the roof. The entire cost will not generally exceed one hundred dollars. In the country the cost would of course be somewhat greater, in the average house probably between a hundred and fifty and two hundred dollars. The apparatus once in, the only expense is the maintenance of the battery. This will generally be very small, probably not more than a dollar a year. Considering the security gained, the outlay required is not excessive, and builders find that it is fully made up to them in increased rents. It is not improbable that the apparatus will eventually be considered as necessary to the complete equipment of a house as now are water-and gas-pipes.

Intimately connected with the burglar-alarm system, though having a different object, is an automatic fire-alarm, somewhat extensively introduced during recent years. The system consists in placing in the ceiling of a room a number of mercury bulbs, which close an electric circuit when the temperature rises above a certain point and set off an alarm. One form of the bulb or thermostat is shown in Fig. 4. The PSM V18 D073 Fire alarm thermostat.jpgFig.—4. Thermostat. wire from the lower end is in permanent contact with the mercury, but that in the upper end comes in contact with it only when a given temperature is reached. The bulbs are usually manufactured to make this contact at a temperature of 120° Fahr. The thermostat is set in a bell-shaped shield of sheet-metal, only the rim of which, when in the ceiling, is exposed. They are placed about twenty feet apart in large rooms, a couple being sufficient for those of ordinary size. This alarm, like that for burglars, may be complete in a building, or may give its signal at a central office. In some of the larger cities this latter plan has been carried into practice. Each building is provided with an annunciator placed on the front, where it can be readily inspected. The signal given at the office indicates the building, and the annunciator on the face of the building gives the room in which the fire is located.

Together these alarms form an excellent protection against the two most common dangers to which buildings and their contents are exposed. The addition of the call-bell system, now so common in hotels, business houses, and the better class of residences, completes an electrical equipment that leaves little to be desired in the way of security and convenience.


  1. Figs. 2 and 3 are reproduced, through the kindness of Mr. George B. Prescott, from his works on "The Electric Light," etc., and "Electricity," etc.