(57) The alarm apparatus for indicating the passage of an intruder across the boundery
of a predetermined region comprises at least one pair of yieldable fluid-filled tubes
(2, 4) for burying side by side along the boundary of the region, a diaphragm-type,
two-directional differential pressure transducer (8) connected to one end of each
tube and arranged to output an electrical signal as a result of the deformation of
the diaphragm means (16a. 16b) caused by a difference between the pressures in the
two tubes, valve means (22) mounted in at least one bypass duct (18) of the transducer
and arranged to put the two tubes suddenlv into fluid communication when the difference
between the pressures in the two tubes exceeds a predetermined threshold value, and
capillary means (24) which put the two tubes into fluid communication and are interposed
in the bypass duct hydraulically in parallel with the valve means.
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