| 摘要 |
This control device consists of a slider disposed within the portion of the hollow head which rotates with the rules; this slide is provided with a guide T-shaped and member is pivotally interconnected to a thumb-piece accessible from outside the head. The pivotal connection between the slider and the thumb-piece is constantly maintained through a coil compression spring prestressed between the inner wall of the head and a stud rigid with the slider opposite the pivotal connection. The thumb-piece comprises on its bottom a projection engageable in a cavity formed in the head base. The relative positions of the pivotal connecting means and the projection are so calculated that the spring generates a self-locking torque preventing any uncontrolled released of the mechanism.
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