NETISM.
The Electric Door-Bell.— This is perhaps the best known and one of the simplest methods of electric signaling in its operation it em ploys a primary battery, a push button at the door, a vibrating bell within the house, and a wire connecting the push button and the bell. The bell and its armature, the connecting wire, the push button and the battery, comprise the electric circuit. The push button is merely a key consisting of two strips of metal which when pressed together allowed the current to flow. The electric bell consists of a coil of wire wound around a soft iron core. Its armature carries at its free end a small hammer which, when the armature is attracted, strikes against a small gong, ringing it. The vibration of the hammer is brought about by a very simple device. Normally the armature rests against a contact point which is a part of the circuit ; the armature itself also being a part of the circuit. The circuit is normally open at the push button. When this button is pressed the circuit is closed and the armature of the bell is forthwith attracted, its hammer striking the gong at that instant. In the act of moving forward, however, the armature leaves the con tact point against which it had been resting, with the result that the circuit is opened at that point. This has the effect of permitting the magnet of the bell to lose its magnetism, where upon the armature falls upon its contact point, again closing the circuit with the result that the armature is again attracted, which action is continuously repeated so long as the push button is pressed in. A small spiral or tension spring suffices to keep the push button normally open. See ELECTRIC BELL.
Annunciator Signals.— The ((annunciator" or call-bell systems so generally used in hotels and offices utilize the principle of the door bell. Annunciator systems in fact might be termed multiple electric door-bell systems. Usually a wire runs from each room to a central point or office where the annunciator is placed. This annunciator contains within its case a small electro-magnet for each room. One battery is
caused to supply the current for all the circuits by simply connecting the wire from each room to the terminals of the battery. In a similar way one bell at the annunciator is caused to respond to all the calls that come. The arma ture of each electro-magnet within the case carries a small shutter, on which is placed the number or letter of the room with which it is connected. This shutter is held out of sight by means of a small catch hook attached to the armature of the magnet until the push button in a given room is pressed, where upon the armature is attracted, releasing the shutter, which instantly drops, showing the number. At the same time the annunciator bell rings, directing the attention of the clerk to the call. In other forms of annunciators the fall ing of the armature is caused to deflect a small arrow on the face of the case, beneath where is marked the number, name or letter of the room. There may he almost any number of rooms for 1 up to 100 or more connected with one annunciator. In practice a single wire, called the ((common return" wire, is usually run from the battery and annunciator to all rooms. From each room a separate, individual wire is then run, from the ((common return" in the room, through the push button and then down to the electro-magnet in the annunciator, there by supplying a separate circuit for each room. In order to restore the shutters to their normal position after the call has been received, a rod is provided with a knob at its lower end which projects below the under side of. the case con taining the annunciator magnets. By pushing up the rod a cross-bar within the case is raised and this cross-bar in turn lifts up and resets the shutter and armature for the next call. In the later form of annunciators which are now frequently operated by alternating current of low voltage, the shutters of the annunciator are electromagnetically reset by merely push ing a button, thereby closing the circuit of an electromagnet.