| 摘要 |
A burglar alarm system mounted in a reinforced doorway and comprising a control circuit having a normally open switch point consisting of a flange projecting from the backer plate towards the striker plate of the door lock, the flange being out of contact with, but close to, a long latch bolt projecting into the latch bolt hole for locking the door, and a contact having an electrical polarity opposite to that of the backer plate. The contact extends towards, but is normally slightly spaced from, the flange whereby the switch point is open and the control circuit is not energized. Displacement of the latch bolt towards the flange by a burglar tool and resultant displacement of the flange towards the contact causes the flange to touch the contact and energizes the control circuit to set off the alarm.
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