The aneroid manometer consists of a metal bellows, which is connected to the compression cuff. Variations of pressure within the system cause the bellows to expand and collapse. The movement of the bellows rotates a gear that turns a needle, pivoted on bearings, across a calibrated dial. Since the BP recorded with the aneroid manometer depends upon the elasticity of the metal bellows, it is subject to errors inherent in the elastic properties of the metal. For this reason the aneroid manometer must be calibrated against a mercury manometer at intervals specified by the manufacturer.