![]() ![]() ![]() Today, most authorities agree that lightning rods define and control the points where lightning will strike the structure and then guide the current safely into ground. “Considerable cavities” had been made in the earth near the rod, sunk about three feet underground, and the lightning had produced several furrows in the ground “some yards in length.” Franklin was pleased by these reports, and replied to Kinnersley that “a conductor formed of nail rods, not much above a quarter of an inch thick, served well to convey the lightning” but “when too small, may be destroyed in executing its office.” Franklin sent the reports from South Carolina to Kinnersley with a recommendation to use larger, more substantial conductors and a deeper, more extensive grounding system to protect the foundation of the house against the effects of surface arcs and explosions in the soil. Nearly all the staples that held the conductor to the outside of the house had also been loosened. Moreover, several sections of the iron down conductor, each about a half-inch in diameter and hooked together, had become unhooked by the discharge (see figure 4). In the other, three brass points, each about seven inches long and mounted on top of an iron rod, had evaporated. In one case, the points and a length of the brass down conductor had melted. Vessels also, having a sharp pointed Rod fix’d on the Top of their Masts, with a Wire from the Foot of the Rod reaching down, round one of the Shrouds, to the Water, will not be hurt by Lightning.īefore Kinnersley’s letter, Franklin had received reports of two similar strikes to protected houses in South Carolina. A House thus furnished will not be damaged by Lightning, it being attracted by the Points, and passing thro the Metal into the Ground without hurting any Thing. If the House or Barn be long, there may be a Rod and Point at each End, and a middling Wire along the Ridge from one to the other. To the upper End of the Rod fasten about a Foot of Brass Wire, the Size of a common Knitting-needle, sharpened to a fine Point the Rod may be secured to the House by a few small Staples. The Method is this: Provide a small Iron Rod (it may be made of the Rod-iron used by the Nailers) but of such a Length, that one End being three or four Feet in the moist Ground, the other may be six or eight Feet above the highest Part of the Building. It has pleased God in his Goodness to Mankind, at length to discover to them the Means of securing their Habitations and other Buildings from Mischief by Thunder and Lightning. His laboratory analogy was that “a wet Rat can not be kill’d by the exploding electrical Bottle, when a dry Rat may.” Franklin also noted that out in the open during a thunderstorm, clothing tends to become wet, thereby providing a conducting path outside the body. That supposition then led to some practical advice against taking shelter under a single, isolated tree during a thunderstorm crouching in an open field is less dangerous. In the fourth letter, 5 he applied his knowledge of electricity to lightning by introducing the concept of the sparking or striking distance: If two electrified gun barrels “will strike at two Inches Distance, and make a loud Snap to what great a Distance may 10 000 Acres of Electrified Cloud strike and give its Fire, and how loud must be that Crack!” Based on his previous experiments with sharp points, Franklin then postulated that when an electrified cloud passes over a region, it might draw electricity from, or discharge electricity to, high hills and trees, lofty towers, spires, masts of ships, and chimneys.
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