Fig. 239.—Example of an electric doorbell installation.
Electric bells vary in price according to design and workmanship. A bell outfit may be purchased complete for $1 but it is advisable to install a bell of better construction, as few pieces of household mechanism repay their cost in service so often as a well-made bell. The bell should be rigid, well-constructed, and the contact piece F should be adjustable. This part F, being the most important of the moving parts of the bell, is shown separately in Fig. 240. Only the ends of the magnet coils with their cores are shown in the figure. The contact is made at A, by the pressure of the spring against the end of the adjustable screw D. When the screw is properly adjusted it is locked securely in place by the nut G. The screw D is held with a screw-driver and the nut G forced into position to prevent any movement. If the screw is moved, so that contact is lost at A, the bell will not ring. In the better class of bells the point of the screw and its contact at A are made of platinum to insure long life. With each movement of the armature a spark forms at the contact which wears away the point, so that to insure good service these points must be made of refractory material.
Fig. 240.—Diagram of the vibrating mechanism used in buzzers and doorbells.
Buzzers.
—Electric bells are often objectionable as signal calls because of their clamor, but with the removal of the bell the vibrating armature serves equally well as a signal but without the undesirable noise. With the bell and tapper removed the operating mechanism of such a device works with a sound that has given to them the name of buzzers. Fig. 241 illustrates the form of an iron-cased buzzer for ordinary duty. The working parts are enclosed by a stamped steel cover that may be easily removed. The mechanism is quite similar to that already described in the doorbell and Fig. 240 shows in detail the working parts. The noise, from which the device takes its name, is produced by the armature and spring in making and breaking contact.
Burglar Alarms.
—A burglar alarm is any device that will give notice of the attempted entrance of an intruder. It is usually in the form of a bell or buzzer placed in circuit with a battery, as a doorbell system, in which the contact piece is placed to detect the opening of a door or window. The contact is arranged to start the alarm whenever the window or door is opened beyond a certain point. The attachment shown in Fig. 242 is intended to form the contact for a window. It is set in the window frame so that the lug C will be depressed and close the alarm circuit in case the sash is raised sufficiently to admit a man. Each window may be furnished with a similar device and the doors provided with suitable contacts which together form a system to operate in a single alarm. During the time when the alarm is not needed it is disconnected by a switch. The windows and doors are sometimes connected with an annunciator which will indicate the place from which an alarm is given. An annunciator used for this purpose designates the exact point at which the contact is made and removes the necessity of searching for the place of attempted entrance.
Fig. 241.—The electric buzzer.
Fig. 242.—Contact for a window burglar alarm.
Fig. 243.—Trip contact which announces the opening of a door.
Fig. 244.—Contact for a door alarm.
Fig. 245.—Doorway or hall matting with contacts for electric alarm.