Reference mark selection arrangement for measuring instrument

Claim 1

In a measuring instrument of the type comprising a measuring scale which extends along a measuring direction and defines a measuring direction and plurality of reference marks, a scanning unit which includes means for detecting the reference marks and means for detecting at least one selector element positioned adjacent to the scale, and means for generating a reference pulse only when one of the reference marks and the selector element are simultaneously detected, such that only those reference marks which are associated with one of the selector elements are brought into action, the improvement comprising: means for defining a guide which extends along the measuring direction adjacent the scale; a column of elements coupled to the guide, said column of elements comprising the at least one selector element and a plurality of filler elements, positioned end to end, said guide, selector element, and filler elements shaped such that the column of elements is slideable along the guide yet is prevented from moving laterally out of contact with the guide; and holding elements positioned at two spaced locations of the guide, said holding elements movable between a first position, in which the column of elements is slideable along the guide, and a second position, in which the end elements in the column of elements are prevented from moving longitudinally past the holding elements; said column of elements being of a length selected completely to fill the length of the guide between the holding elements in order to hold the at least one selector element in position, aligned with a selected one of the reference marks.