Patent GB-962249-A (GB)
Title: Improvements in or relating to magnetic triggers
Inventor: JURY ROBERT JOHN
IPC: E05G1/14
Filed: 1961-11-29
Abstract: 962,249. Preventing theft. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Jan. 31, 1963 [Nov. 29, 1961], No. 42755/61. Headings E2A and E2X. A magnetic trigger, suitable for actuating a device for marking the contents of a safe or strong box in the event of an attempted theft, comprises two magnetic bodies 9, 15, at least one of which is permanently magnetised, and a rigid pendulum 12, 13 mechanically linked to one of the bodies, the bodies having complementary surfaces which are normally in contact and the arrangement being such that displacement of the pendulum from its equilibrium position causes the body to which it is linked to pivot relative to the other body and then to separate from it against the magnetic attraction between the bodies. In the embodiment described the pendulum is suspended in a gas-tight vessel 1 and the rod 12 carries a cup 14 rigidly secured to an annular magnet 15 normally in contact with a similar magnet 9 fixed to the cover 2 of the vessel. A spring 41 controlled by an adjusting screw 11 bears upon the cup 14 and regulates the force required to separate the magnets. The device is located inside a safe, and if the safe is tilted or subjected to vibration the resultant swing of the pendulum causes the magnet 15 to pivot about the edge of the magnet 9 so that the attraction between them falls off rapidly until it will no longer support the weight of the pendulum. The cup 14, in falling, breaks the tubular closure 38 of a capsule 37 containing compressed air, and the release of the air into the vessel 1 causes the discharge of an indelible dye contained in the vessel through an outlet 44. The outlet is normally closed by a bursting diaphragm 45. Means are provided for locking the pendulum against movement during the legitimate use of the safe, such means comprising a bellows tube 16, 17 divided by the lower cover plate 3 of the vessel into two parts communicating through an aperture 18 controlled by a valve 19. The tube contains sufficient hydraulic fluid to extend either of the bellows fully when the other is collapsed, and the lower bellows 17 is urged towards its extended position by a spring 25 and bears by means of its end 34 against a lever 31 pivoted at 30. One arm 33 of the lever is urged upwardly by a spring 35 and the other arm 32 is adapted to abut the inside of the safe door. When the door is closed the arm 33 is in its lowest position and the bellows 17 is fully extended so that the pendulum is free. If the door is opened in legitimate circumstances the spring 35 collapses the bellows 17 and the fluid therein flows freely through the valve 19 to extend the bellows 16 to the position shown in the drawing in which the pendulum is secured. When the door is slammed shut the arm 33 drops at once to its lowest position leaving the bellows 17 free to extend under the action of the spring 25, but the rate of extension is controlled by the downward flow of fluid through the valve 19. This is such that the pendulum is not released until the vibrations caused by the slamming of the door have died away.