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.
Fig. 244 is another form of door alarm that makes contact when the door is opened and remains in contact until the door is closed. The part P is set into the door frame of the door in such position that the contact at C is held open when the door is closed. When the door is opened a spring in C closes the contact and causes the alarm to sound. It continues to sound until the door is closed and the contact is broken. When the use of the alarm is not required, the contact-maker is turned to one side and the contact is held open by a catch. It is put out of use by pressing the plunger to one side.
The matting shown in Fig. 245 is provided with spring contacts so placed that no part may be stepped upon without sounding the alarm. When placed in a doorway and properly connected with a signal, no person can enter without starting an alarm. The matting is attached to the alarm by the wires C and contacts are set at close intervals so that a footstep on the mat must close at least one contact.
Annunciators.
—It is often convenient for a bell or buzzer to serve two or more push buttons placed in different parts of the house. In order that there may be means of designating the push button used—when the bell is rung—an annunciator is provided. This is a box arranged with an electric bell and the required number of pointers and fingers corresponding to the push buttons. In Fig. 246 is shown an annunciator with which two push buttons are served by the single bell. The annunciator is placed at the most convenient place of observation, usually in the kitchen. When the bell rings the pointer indicates the push button that has last been used. In hotels or apartment houses an annunciator with a single bell may thus serve any number of push buttons. In a burglar-alarm system the annunciator numbers are arranged to indicate the windows and other openings at which entrance might be made. When the alarm sounds the annunciator indicates the place from which the alarm is made.
Table Pushes.
—Call bells to be rung from the dining-room table are connected with an annunciator or to a separate bell. The table pushes may be temporarily clamped on the edge of the table and connected by a cord to an attachment set in the floor or the connection may be made by a foot plate set on the floor. In Fig. 247 is shown a form of push P which is intended to be clamped to the edge of the table under the cloth. The plate F forms the floor connection. It is set permanently with the upper edge flush with the surface of the floor. The part S, in which the connecting cord terminates, when inserted in the floor plate, makes contact at the points C to form an electric circuit with the battery. The foot plate shown in Fig. 248 is only an enlarged push button which is set under the table in convenient positions to be pressed with the foot. Its connection might be made as indicated or with the same floor connection as that of the preceding figure. Fig. 249 is a simpler form of floor push in which a metallic plug is inserted in the floor plate. When the plug R is pressed, contact is made at the points C to form the circuit with the battery and bell.
Fig. 246.—A kitchen annunciator.
Fig. 247.—Plug attachment and table push for a dining table.
Fig. 248.—Foot plate and contact for table bell.
Fig. 249.—Call bell attachment with detachable contact piece.