jack, mechanical device used to multiply a relatively small applied force so that it can lift and support heavy loads, or sometimes, move massive objects into a desired position. The lever jack, often used in lifting automobiles, has a lever combined with a ratchet; the lever is used to lift the load a small distance and the ratchet prevents the load from falling back while the lever is reset so that the process can be repeated. In the screw jack the load is moved or lifted by the turning of a screw; the pitch of the screw threads is arranged so that friction is sufficient to hold the load in place when the torque applied to the screw is released. In yet another form of jack a hydraulic device is used. See hydraulic machine.