slide-rule is mechanical device formerly used by engineers and scientists for rapid and approximate multiplication, division, extraction of roots, raising to powers, and other simple computations. slide-rule has been almost totally superseded by small hand-held electronic calculator. It consists of ruler engraved with logarithmic scales, and sliding central strip. principle of slide-rule is translation of all computations to equivalent additions or subtractions that can be carried out on set of scales sliding over each other.