A grounded metallic conductor with its upper extremity extending above the structure that is to be protected from damage due to lightning. The upper extremity, called the air terminal, should be raised well above the top of the structure, to yield an adequate radius of protection. The path to ground must consist of a conductor of low total resistance and must contain no points of high local resistance. Lower ends should be buried deeply enough in the earth that they will always be in good contact with soil moisture. connexion to water pipes usually affords a good ground. Compare lightning arrester.