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In practical mechanics, portable hand-operated device for raising heavy weights through short distances, exerting great pressures, or holding assembled work firmly in position. The ratio of the load to the amount of force applied to the handle can be made quite high by using a gear or screw to regulate the upward extension. A ratchet allows a heavy weight to be raised in short successive stages. Though limited by the requirements of portability and ease of manual operation, jacks may lift, or exert a force of, several tons. A familiar example is the automobile jack, used to raise one end of a car to change a tire.
This entry comes from Encyclopædia Britannica Concise. For the full entry on jack, visit Britannica.com.