Tuesday, 11 February 2003: 3:30 PM
How has the Original Franklin Rod Evolved into Today's Lightning Protection System?
The Philadelphia experiments and observations on static electricity, as led and communicated by Benjamin Franklin, were important because some were new and novel and because their interpretations helped to stimulate the development of electricity as a science and the beginnings of modern physics. This work also led to the famous sentry box and kite experiments that proved once and for all that thunderclouds are electrified and that lightning is an electrical discharge. The latter discoveries, in turn, validated the key assumptions that lay behind Franklin's supposition that tall, grounded rods might protect structures from lightning damage. Here, we will trace how Franklin's ideas about "the wonderful effects of pointed bodies" evolved into the design of the first protective rods, and then we will describe some important improvements that Franklin made to this design in the years from 1752 to 1762, after experience was gained through practice. Today, most authorities agree that the main functions of a lightning rod and the associated conductors are to define and control the points where lightning will attach to a structure and then to provide safe paths for the current to flow to ground. In a letter written in 1762, Franklin noted that "Indeed, in the construction of an instrument so new, and of which we could have so little experience, it is rather lucky that we should at first be so near the truth as we seem to be, and commit so few errors." Lucky indeed - today virtually every lightning protection code in the world still recommends grounded metallic rods for protecting ordinary structures, and the basic elements of their design and installation are, in essence, the same as Franklin's specifications of 1762.