| 摘要 |
A housing (10, 12, 14) has first and second flexible membranes (32, 42) dividing it into two sealed chambers plus a third chamber from which the movable member (38) projects. Each membrane co-operates with a respective cup (40, 50) and return springs (54, 56) tend to urge both cups away from an end wall (26). The two cups are interconnected by a single piece member (58) which is inextensible but which can readily be bent in two to enable the cups to come into contact with one another. Each sealed chamber can individually be selectively connected to a source of reduced pressure via a respective orifice or pipe fitting (30, 28). The position of the movable member (38) can thus be changed.
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