Lightning rod improvement studies

Although lightning rods have long been used to limit damage from lightning, there are currently no American standards for the shape and form of these devices. Following tradition, however, sharp-tipped Franklin rods are widel y installed despite evidence that, on occasion, lightning strikes objects i n their vicinity. In recent tests of various tip configurations to determin e which were preferentially struck by lightning, several hemispherically ti pped, blunt rods were struck but none of the nearby, sharper rods were "hit " by lightning. Measurements of the currents from the tips of lightning rods exposed to str ong electric fields under negatively charged cloud bases show that the emis sions consist of periodic ion charge bursts that act to reduce the strength of the local fields. After a burst of charge, no further. emissions occur until that charge has moved away from the tip. Laboratory measurements of t he emissions fi om a wide range of electrodes exposed to strong, normal-pol arity thunderstorm electric fields show that positive ions are formed and m ove more readily over sharp-tipped electrodes than over blunter ones. From these findings, it appeals that the electric held rates of intensification over sharp rods must be much greater than those over similarly exposed blun t rods for the initiation of upward-going leaders. Calculations of the relative strengths of the electric fields above similar ly exposed sharp and blunt rods show that although the fields, prior to any emissions, are much stronger at the tip of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the held is greater than that o ver an otherwise similar, sharper I od at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy form ation of ions in the surrounding ail; the field strengths over blunt rods c an be much stronger than those at distances greater than 1 cm over sharper ones. The results of this study suggest that moderately blunt metal rods (with ti p height-to-tip radius of curvature ratios of about 680:1) are better light ning strike receptors than are sharper rods or very blunt ones.