From Academic Kids
Construction and uses
A lightning rod is connected via a low-resistance cable to the earth or water below, where the charge may be safely dissipated. Lightning rods sometimes possess a short circuit to the ground that is interrupted by a thin non-conductor over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist corrosion well, it may be covered in salt to improve its electrical connection with the ground.
In telegraphy and telephony a lightning rod is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Similarly, high-tension power lines carry a lighter conductor wire over the main power conductors. This conductor is grounded at various points along the link. Electrical substations usually have a web of the lighter conductor wires covering the whole plant.
Considerable material is used in the construction of lightning arrestors, so it is prudent to work out where a new arrestor will have the greatest effect. Historical understanding of lightning assumed that each rod protected a cone of 45 degrees. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building.
A better technique to determine the effect of a new arrestor is called the rolling sphere technique and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 60m near the ground.
Electricity travels along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a sphere that travels from the cloud to the ground.
For lightning protection it suffices to consider all possible spheres as they touch potential strike points. To determine which strike points consider a sphere rolling over the terrain. At each point we are simulating a potential leader position and where the sphere touches the ground the lightning is most likely to strike. Points which the sphere cannot roll across and touch are safest from lightning. Lightning rods should be placed where they will prevent the sphere from touching a structure.
It is commonly believed, erroneously, that a rod ending in a sharp point at the peak is the best means to conduct the current of a lightning strike to the ground. According to field research, a rod with a rounded or spherical end is better. "Lightning Rod Improvement Studies" by Moore et al say:
Calculations of the relative strengths of the electric fields above similarly 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 field is greater than that over an otherwise similar, sharper rod at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can 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 tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.
Lightning damage has been with humanity since we started building structures. Early structures made of wood and stone tended to be short and in valleys and as a result lightning hit rarely. As buildings became taller lightning became a significant threat. Lightning can damage structures made of most materials (masonry, wood, concrete and even steel) as the huge currents involved can heat materials, and especially water to high temperatures causing fire, loss of strength and explosions from superheated steam and air.
The lighting rod seen in the picture above is sourced from Nikola Tesla's American patent US#1,266,175. The patent was granted due to a fault in Franklin's original theory of operation; the pointed lightning rod actually ionizes the air around itself, rendering the air conductive, which in turn raises the probability of a strike. Many years after receiving his patent, in 1919 Dr. Tesla wrote an article for The Electrical Experimenter entitled "Famous Scientific Illusions", in which he explains the logic of Franklin's pointed lightning rod and discloses his improved method and apparatus. Despite the publicity, the pointed lightning rod is still misapplied today.
In the United States, the pointed lightning rod, and more accurately the "lightning attractor", was invented by Benjamin Franklin as part of his groundbreaking explorations of electricity. Franklin speculated that with an iron rod sharpened to a point at the end "the electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike...." Franklin had speculated about lightning rods for several years before his reported kite experiment. In fact, the experiment took place because he was tired of waiting for Christ Church in Philadelphia to be completed so he could place a lighting rod on top of it.
There was some resistance from churches who felt that it was defying divine will to install these rods. Franklin countered that there is no religious objection to roofs on buildings to resist precipitation, so lightning, which he proved to be simply a giant electrical spark, should be no different.
In the 19th century the lightning rod became a symbol of American ingenuity and a decorative motif. Lightning rods were often embellished with ornamental glass balls (now prized by collectors) that also served to provide visual sign of a lightning strike (when the rod is struck the glass ball shatters and falls off, indicating to the owner which rod got struck and that they should check it and the grounding wire for damage). The ornamental appeal of these glass balls has also been incorporated into weather vanes.