The magnitude of vector O͞A is set up by turning pinion 1, and its direction by turning gear 2' which belongs to link 2. The rotation of pinion 1, meshing with the annular gear rack of link 3, is transmitted by spur rack 3 to pinion 4 which is rigidly attached to screw 6. Nut 5 moves along screw 6 and thereby changes the distance O͞A. When link 2 is turned, link 3 turns through the same angle without rotating gear 1 and screw 6. This, therefore, does not change the magnitude of vector O͞A.
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