The fallacy of Franklin’s pointed lightning rod
Nikola Tesla next addresses himself to the lightning rod, and comments as follows:
The display of atmospheric electricity has since ages been one of the most marvelous spectacles afforded to the sight of man. Its grandeur and power filled him with fear and superstition. For centuries, he attributed lightning to agents godlike and supernatural and its purpose in the scheme of this universe remained unknown to him.
Now we have learned that the waters of the ocean are raised by the sun and maintained in the atmosphere delicately suspended and that they are wafted to distant regions of the globe, where electric forces assert themselves in upsetting the sensitive balance and causing precipitation, thus sustaining all organic life. There is every reason to hope that man will soon be able to control this life-giving flow of water, and thereby solve many pressing problems of his existence.
Atmospheric electricity became of special scientific interest in Franklin’s time, Faraday had not yet announced his epochal discoveries in magnetic induction but static frictional machines were already generally used in physical laboratories. Franklin’s powerful mind at once leaped to the conclusion that frictional and atmospheric electricity were identical. To our present view, this inference appears obvious, but in his time the mere thought of it was little short of blasphemy.
He investigated the phenomena and argued that if they were of the same nature, then the clouds could be drained of their charge exactly as the ball of a static machine, and in 1749 he indicated in a published memoir how this could be done by the use of pointed metal rods.
The earliest trials were made by Dalbrand in France, but Franklin himself was the first to obtain a spark by using a kite, in June, 1752. When these atmospheric discharges manifest themselves today in our wireless station we feel annoyed and wish that they would stop, but to the man who discovered them, they brought tears of joy.
The lightning conductor in its classical form was invented by Benjamin Franklin in 1755, and immediately upon its adoption proved a success to a degree. As usual, however, its virtues were often exaggerated. So, for instance, it was seriously claimed that in the city of Pietermaritzburg (capital of Natal, South Africa) no lightning strokes occurred after the pointed rods were installed, although the storms were as frequent as before.
Experience has shown that just the opposite is true. A modern city like New York, presenting innumerable sharp points and projections in good contact with the earth, is struck much more often than an equivalent area of land. Statistical records, carefully compiled and published from time to time, demonstrate that the danger from lightning to property and life has been reduced to a small percentage by Franklin’s invention, but the damage by fire amounts, nevertheless, to several million dollars annually. It is astonishing that this device, which has been in universal use for more than one century and a half, should be found to involve a gross fallacy in design and construction which impairs its usefulness and may even render its employment hazardous under certain conditions.
All points or projections on the surface of a conductor of such vast dimensions as the earth would be quite ineffective were it not for other influences. These will be elucidated with reference to Figure 4, in which our artist of the Impressionist school has emphasized Franklin’s notion that his rod was drawing electricity from the clouds. If the earth were not surrounded by an atmosphere which is generally oppositely charged it would behave, despite all its irregularities of surface, like a polished sphere.
But owing to the electrified masses of air and cloud the distribution is greatly modified. Thus in Figure 4, the positive charge of the cloud induces in the earth an equivalent opposite charge, the density at the surface of the latter diminishing with the cube of the distance from the static center of the cloud. A brush discharge is then formed at the point of the rod and the action Franklin anticipated takes place.
In addition, the surrounding air is ionized and rendered conducting and, eventually, a bolt may hit the building or some other object in the vicinity. The virtue of the pointed end to dissipate the charge, which was uppermost in Franklin’s mind, is, however, infinitesimal. Careful measurements show that it would take many years before the electricity stored in a single cloud of moderate size would be drawn off or neutralized through such a lightning conductor.
The grounded rod has the quality of rendering harmless most of the strokes it receives, though occasionally the charge is diverted with damaging results. But, what is very important to note, it invites danger and hazard on account of the fallacy involved in its design. The sharp point which was though advantageous and indispensable to its operation is really a defect detracting considerably from the practical value of the device.
I have produced a much improved form of lightning protector characterized by the employment of a terminal of considerable area and large radius of curvature which makes impossible undue density of the charge and ionization of the air. These protectors act as quasi-repellents, and so far have never been struck though exposed a long time. Their safety is experimentally demonstrated to greatly exceed that invented by Franklin. By their use, property worth millions of dollars which is now annually lost can be saved.
Photo 2 (piece of metal): The top portion of a lightning rod, designed by Benjamin Franklin — from The Frankliniana Collection, The Franklin Institute, Inc., Philadelphia, Photo Credit: Peter Harholdt