jack, in practical mechanics, portable hand-operated device for raising heavy weights through short distances, exerting great pressures, or holding assembled work firmly in position, as in jacking up a building to prevent settling or keeping it in position while replacing a foundation.
The jack’s effectiveness stems from the ratio of the load, or weight of the object raised, to the amount of force applied to the handle of the jack. This ratio can be made quite high by the use of a gear or screw to regulate the upward extension of the jack. A ratchet allows a heavy weight to be raised in short successive stages, with the jack locking whenever force is not being applied to its handle. Though limited in capacity by the requirements of portability and ease of manual operation, a jack may be built to lift, or exert a force of, several tons. A familiar example of this device is the automobile jack, which is used to raise one end of a car to permit the changing of a tire.