Burglar Alarm Signals.— For simple do mestic purposes the arrangement employed for burglar alarm signals is also closely akin to the systems just described, the main difference being that the finger push button is displaced by a circuit closing contact in the frame of a window or the jamb of a door. To bring about the result desired the circuit closer is placed on the frame of the window in such a way that the movable portion of the circuit closer projects beyond the surface of the window frame. The contacts of the circuit closer are held apart normally by a simple spring. In order that when the window is closed this projection may not be depressed, a piece is cut out of the window sash at a point directly opposite the projecting circuit closer. In an analogous manner the circuit closer is attached to a door jamb. The wires leading to the circuit closer are concealed behind the woodwork. When a window is raised or a door is opened by an intruder the contact points of the circuit closer are brought together, whereupon the annunciator bell is rung, giving the alarm, and at the same time the room where the door or window has been opened is indi cated by the dropping of the corresponding shutter in the annunciator. The annunciator is located in any desired part of the house, usually in a bedroom. The method described is termed an open circuit method. In such sys tems what is known as open circuit batteries, such as the ordinary dry batteries, may be used. In some cases, to guard against a momentary opening of a door or window, such as would only occasion a short ring of the alarm bell, not sufficient perhaps to arouse a sleeper, an auxiliary arrangement is provided at the an nunciator consisting of a drop arranged to close the bell circuit continuously until the drop is reset. A small switch is provided near the annunciator by means of which the alarm cir cuit may be opened during the daytime so that needless alarm may not be given by the ordi nary opening of windows. Other switches are also used for the purpose of testing the vari ous circuits to see that they are in proper working condition. See ELECTRIC ANNUNCIATOR.
Central Office Burglar Alarm System.— This system, as the name implies, relates to one in which the ringing of an electromagnetic alarm in a central office will announce the presence of intruders in the building or build ings in which the protecting apparatus is in stalled. The central office may be any con venient distance from the protected buildings. These •systems are frequently termed electric protective systems. The plan usually adopted is to run a net-work of wires through parti lions, across floors, skylights, etc. These wires are part of a circuit extending to the central office, and the said wires are so connected with the circuit that any interference with them, after they have been "set," will cut out a high resistance, consisting of a coil of wire, suitably placed in the circuit of the protected building. The cutting out or short-circuiting of this re sistance will so increase the strength of the current in the circuit as to operate a "double balanced" instrument in the central office. If on the other hand the resistance is not "cut out? but instead the wires of the circuit be cut or broken, by accident or design on the part of intruders, the consequent absence of current, or even a slight decrease of current, will likewise cause an alarm in the central station. The doubled-balanced instrument at the central office is usually a relay, the armature of which carries a needle that normally stands at a zero point, from which point it may be deflected in either direction. An increase of current on the circuit deflects it in one direction— a decrease of current allows a spring or weight to deflect it in the other direction. In either case, the alarm is given when the needle moves and an attendant is despatched to the premises from which the alarm has emanated. Each relay is of
course suitably numbered or otherwise desig•, nated in the central office so that the building with which it is connected is laiown.
Telegraph Message Service or Emergency This service relates to the supplying or calling of messengers, policemen, firemen, etc., at the call of or by a °subscriber? in whose house or office a °call box" has been placed. This call box is electrically.eonnected by means of a wire with a central office at which the messengers are located, and from which office communication with, police and fire headquarters can quickly be made. Each call box is numbered and is supplied with what are termed "make and break" attachments which are set in operation by the turning of a crank on the cover of the box. These attachments, thus operated, transmit automatically to the central office the number of the box, which at once indicates to that office the location of the signaling box. The construction and opera tion of the call box are as follows: A crank is mounted rigidly with a recoil spring on a shaft. On this shaft is also mounted, but loosely, a cog-wheel. A "break-wheel," having a number of slots in its periphery, is geared with the cog wheel in such a manner that it receives a tend ency to turn in a given direction, but it is normally prevented from turning by the engage ment of a pin on its side with a curved cam which is attached to a prolongation of the crank arm. When, however, the crank lever is turned to, say the right, preparatory to send ing in a call, the cam is automatically moved out of the path of the said pin and the break-wheel is then free to move. By a suitable pawl and ratchet, the cog-wheel is prevented from mov ing when the crank is turned to the right. The effect of turning the crank is to wind the recoil spring. When the crank is let go the spring unwinds and turns with it the break-wheel which completes a revolution; at the end of which it is again held by the pin as before. The break-wheel is made part of the circuit leading from the box to the central office. A flat metal spring which is also part of the said circuit rests on the periphery of the break-wheel in such a manner that when the wheel is making its revolution the spring falls into the slots on the said periphery and opens the circuit as many times as there are slots provided. If, for instance, the number of a given box is 24, there will be cut on the periphery two slots in close succession, then an interval of unbroken metal surface followed by four slots in succession. The circuit in such a case, as the wheel rotates, will be opened twice in succession, closed for an interval, and then opened four times in succes sion, with the result that a bell at the central office will give out, first two strokes, and then four strokes, on its gong. A large number of such boxes can be placed on one circuit without causing confusion. The call box just described suffices to send in a swift call for a messenger. When it is desired to make it possible to call a policeman, doctor or fireman by the same type of box, it is provided with a "stop" that projects through the cover in such a way that the crank cannot be moved beyond a certain point. The ordinary position of the "stop" would be at the messenger call, in which case the number of the box only would be sent in when the crank is turned. If, however, a doctor should be desired, the stop is moved to a point marked "doctor," on the cover of the box, and in turn ing the crank it is moved up to that point. This farther movement. of the crank has the effect of bringing into operation one or more, additionel slots on theperiphery of the wheel in consequence of which the box number is preceded by one or more single strokes on the bell, which indicate to the central office attendant that a doctor, fireman or policeman, as the case may be, is desired.