The invention concerns a portable displacement measuring instrument, notably a micrometer, that comprises a body (11), an anvil (2), a spindle (3) moveable with respect to the body (11), an encoder (7) detecting an axial displacement of the spindle, a rotationally-mounted thimble (4) being actionable by a user for bringing the spindle (3) into contact with a workpiece and a transmission mechanism being provided between the thimble (4) and the spindle (3) for converting a rotation of the thimble in a translation of the spindle towards the anvil.;The transmission mechanism comprises a constant force mechanism configured for disengaging the transmission mechanism when a movement of the spindle towards the anvil is opposed by a load being equal or surpassing a threshold load. The constant force mechanism (5) comprises an adjusting mechanism (51, 511, 52, 55) arranged for setting the threshold load within a predefined range.
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