The Fallacy of Franklin’s Pointed Lightning-Rod
Famous Scientific Illusions · II — The Electrical Experimenter · February 1919 · pages 694, 728 and 730
This page reproduces the second part of Famous Scientific Illusions, the three-part essay Nikola Tesla wrote for The Electrical Experimenter in 1919 — the part in which he takes the most widely trusted instrument in lightning protection, Franklin’s pointed rod, and shows what its point actually does. The text is transcribed in full, the figures are the ones printed with it, and the page is presented in the reading mode of this archive: paper by default, night on demand.
A Point That Invites the Strike
Franklin’s rod was meant to do two things: carry a stroke safely to earth, and drain a charged cloud before it could strike at all. The first it does reasonably well. The second, Tesla shows, it barely does at all — and in the attempt it does something worse: the point ionises the air at the tip, and the ionised air is precisely what a discharge travels through.
The reasoning in the article is that of a man who spent his life on high-frequency currents. A point of length h on a sphere of radius r concentrates the density of the charge; the advantage of using the point instead of a large smooth body shrinks in the ratio (r+h)/(nr+h) as the body grows — so on a conductor as vast as the earth, points and projections are effectively useless. What remains is the brush discharge, and a conductor that invites what it was built to prevent. Tesla’s own answer, patented the year before, is a terminal of considerable area and large radius of curvature.
Facsimile of page 694, where the second part opens in the lower half of the page. The article then jumps to pages 728 and 730 — the magazine filled the intervening pages with advertising.
THE ELECTRICAL EXPERIMENTER
FEBRUARY, 1919 · PAGES 694, 728 AND 730
Nikola Tesla, writing specially for The Electrical Experimenter — and the second of his three “predominant scientific fallacies”.
In this original and revolutionizing discussion, Nikola Tesla gives us something really new to think about. First — Does the moon rotate on its axis? Second — Is the Franklin pointed lightning rod correct in theory and operation? Third — Do wireless signals fly thru space by means of so-called Hertzian waves in the ether, or are they propagated thru the earth at prodigious velocity by means of earth-bound oscillations? World-famous conundrums these questions which have been answered in many ways by some of the greatest scientists. Dr. Tesla explains these three predominant scientific fallacies in a masterly way, so that everyone can understand them.
II. The Fallacy of Franklin’s Pointed Lightning-Rod
Full text of the part, reproduced as printed — the wording and the spelling of 1919 are preserved.
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 god-like 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, 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 publisht memoir how this could be done by the use of pointed metal rods.
The earliest trials were made by Dalibrand 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 Piatermaritzburg (capital of Natal, South Africa) no lightning strokes occurred after the pointed rods were installed, altho 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 equivalent area of land.
Statistical records, carefully compiled and publisht 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.
For explanation of this curious fact I may first refer to Fig. 3, in which s is a metallic sphere of radius r, such as the capacity terminal of a static machine, provided with a sharply pointed pin of length h, as indicated. It is well known that the latter has the property of quickly dissipating the accumulated charge into the air.
To examine this action in the light of present knowledge we may liken electric potential to temperature. Imagine that sphere s is heated to T degrees and that the pin or metal bar is a perfect conductor of heat so that its extreme end is at the same temperature T.
Then if another sphere of larger radius, v1, is drawn about the first and the temperature along this boundary is T1, it is evident that there will be between the end of the bar and its surrounding a difference of temperature T – T1 which will determine the outflow of heat.
Obviously, if the adjacent medium was not affected by the hot sphere this temperature difference would be greater and more heat would be given off. Exactly so in the electric system.
The diagram printed with the article, with its original caption: “Diagram Used to Explain the Fallacy of the Franklin Pointed Lightning Rod, and an Analogy Whereby the Author Shows in a Clear Manner How the Charged Sphere May for Illustration be Considered as Heated to a High Degree, and the Heat Allowed to Escape at a Known Rate.”
Let q be the quantity of the charge, then the sphere — and owing to its great conductivity also the pin — will be at the potential \(q/r\). The medium around the point of the pin will be at the potential \(q/r_1 = q/(r+h)\) and, consequently, the difference
\[\frac{q}{r} – \frac{q}{r+h} = \frac{qh}{r(r+h)}\]
Suppose now that a sphere S of much larger radius R = nr is employed containing a charge Q this difference of potential will be, analogously \(Qh/R(R+h)\).
According to elementary principles of electro-statics the potentials of the two spheres s and S will be equal if \(Q = nq\) in which case
\[\frac{Qh}{R(R+h)} = \frac{nqh}{nr(nr+h)} = \frac{qh}{r(nr+h)}\]
Thus the difference of potential between the point of the pin and the medium around the same will be smaller in the ratio \(\dfrac{r+h}{nr+h}\) when the large sphere is used.
In many scientific tests and experiments this important observation has been disregarded with the result of causing serious errors.
Its significance is that the behavior of the pointed rod entirely depends on the linear dimensions of the electrified body. Its quality to give off the charge may be entirely lost if the latter is very large.
For this reason, 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 Fig. 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 Fig. 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.
The plate printed with the article, with its original caption: “Tesla Explains the Fallacy of the Franklin Pointed Lightning Rod… The Density of the Dots Indicates the Intensity of the Charges.”
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 thru such a lightning conductor.
The grounded rod has the quality of rendering harmless most of the strokes it receives, tho 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 thought advantageous and indispensable to its operation, is really a defect detracting considerably from the practical value of the device. I have produced a much more 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 tho 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.
*Refer to the October, 1918, issue of this journal wherein Dr. Tesla’s new form of non-pointed lightning rod was fully described and illustrated.
Nikola Tesla