Fig. 52.
Legge's Window Blind contact is an arrangement by which the blind is secured at the bottom by attaching it to a hook or button. A slight pressure against the blind (caused by anyone trying to enter after having broken a window) sets the electric bell in motion unknown to the intruder.
A form of floor contact, which may be placed under a light mat or carpet, illustrated at [Fig. 52], serves to give notice if anyone be waiting at the door, or stepping into places which are desired to be kept private. All these arrangements, to be serviceable, should be connected with continuous ringing bells (see [§ 48]). Wherever it is likely that these arrangements may stand a long time
without being called into play, it is better to employ some form of contact in which a rubbing action (which tends to clean the surfaces and then make a good contact) is brought into play, rather than a merely dotting action. For this reason, spring contacts in which the springs connected with the wires are kept apart by an insulating wedge (shown at [Fig. 53]) as long as the door or window are kept closed, are preferred. In the case of windows, strips of brass let into the frame on each side of the sash, are thrown into contact by the springs a and a' in the sash itself, as shown at [Fig. 54]. For shop doors and others, where a short contact only is required, and this only when the door is opened, a contact such as shown at [Fig. 55] is well adapted. It consists, as will be seen, in a peculiarly shaped pivoted trigger a, which is lifted forwards when the door is opened, so that it makes contact with the spring b. Owing to the curved shape of the arm of the
trigger, the contact is not repeated when the door is closed.
Fig. 53.