Rack 2 slides in fixed guide A and meshes with gear 6 which, in turn, meshes with rack 3, sliding in fixed guide C. Gear 6 is connected by turning pair E to rack 7 which slides in fixed guide B and meshes with pinion 5. Pinion 5 rotates about fixed axis D and is rigidly attached to gear 8 which meshes with two-sided rack 9. Rack 9 slides in fixed guide K and meshes with pinion 10 which is connected by turning pair F to link 1. Link 1 slides in fixed guide M. Pinion 10 meshes with rack 4 which slides in fixed guide N. The three addends are proportional to the linear displacements s₂, s₃ and s₄ of racks 2, 3 and 4. The linear displacement s₁ of link 1 is proportional to the half-sum of addends s₂, s₃ and s₄, which are entered by shifting racks 2, 3 and 4. Thus s₁(s₂+s₃+s₄)/2 since the numbers of teeth of gears 5 and 8 are related by the equation z₈=2z₅. The required sum is determined by reading on the displacement of link 1 on a special scale (not shown).
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