[Illustration: 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. [Illustration: 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.] In Fig. 243 is illustrated one form of door trip which may be used on a door to announce its opening. This trip makes electric connection in the alarm circuit when the opening door comes into contact with the swinging piece _T_, but no contact is made as the door closes. The trip is fastened with screws at _D_ to the frame above the door. The opening door comes into contact with _T_ and moves it forward until the electric circuit is formed at _C_; after the door has passed, a spring returns it to place. As the door is closed, the part _T_ is moved aside without making electric contact.