Popular Science Monthly/Volume 7/August 1875/The Form of Lightning-Rods
|THE FORM OF LIGHTNING-RODS.|
THE season when the attention of the public will be directed to protection from lightning is now approaching, and it is of the utmost importance that correct views in regard to the construction and erection of lightning-rods should prevail. We have in this country a class of men who have devoted themselves to the business of making money out of the fears which thunder and lightning inspire, and it unfortunately happens that the majority of these men care more for the money which they obtain than for the actual protection which they afford to their customers. To them, complicated arrangements, that can be defended with any show of reason, are a most important matter, for, on the ground of greater cost and efficiency, a more liberal harvest is obtained. In this connection there has been no more fertile source of imposition than the fallacy that lightning travels only on the surface of metallic conductors, for it has led to the construction of lightning-rods of which the cross-sections are stars, tubes, and all sorts of complicated devices. A recent note in The Popular Science Monthly, translated from the Comptes Rendus, indorses this view, and, as no editorial protest has been added, it may have a tendency to mislead many. Let us, therefore, consider the facts in the case.
In looking up the history of this subject, the first mistake that we meet is the confounding of static with dynamic electricity, or rather an utter ignorance of what static electricity is. The author of the note to which we have referred evidently supposes that all electricity produced by the ordinary frictional machine is static—which most assuredly is not the case. In making this mistake, however, he is not by any means alone. Dozens of writers have committed the same error, and it is not long since a medical man wrote a book on the curative powers of static, as distinguished from dynamic, electricity, while any physicist would have told him that in the entire volume there was not a single case described in which static electricity was used! Whenever electricity is in motion, that is to say, when it is flowing along a conductor, it is dynamic, no matter from what source it may be obtained. When at rest—that is, when it is in equilibrium—it is static. Dynamic electricity may be produced by the ordinary plate or cylinder machine; static electricity may have its origin in a voltaic battery.
Knowing that electricity at rest always tends to diffuse itself on the surface, in fact, that it always confines itself to the surface, it became, at an early period, a question whether electricity in motion did not follow the same law. Pouillet determined the question in a very ingenious manner. He took a cylindrical wire of a certain size and measured the resistance which it offered to a current of electricity. He then rolled the wire out flat and measured the resistance again; it was found to be the same, although it is evident that the extent of the surface of the conductor was by this means greatly increased. Other experimenters have determined the question by different methods, but always with the same result. The committee of the French Academy, which included Becquerel, De la Rive, Pouillet, and others, adopted a solid square bar as the best form for lightning-rods; and Sir William Snow Harris, though often quoted as favoring rods which present a large surface, says: "Provided the quantity of metal be present, the form under which we place it is evidently of no consequence to its conducting powers, since it would be absurd to suppose that a mass of metal, under any form, did not conduct electricity in all its particles; indeed, we know that it does so."
In attempting to determine this question, Pouillet and others seem always to have used electricity produced by a voltaic battery; and although, to the mind of every scientific physicist, such experiments are conclusive, the objection has been raised that they do not fairly determine the case for electricity of such high tension as lightning. To meet such objections, the writer of this article, many years ago, instituted the following experiments:
Take a strip of gold-leaf half an inch wide, and two or three inches long; pass through it a moderate charge from a six-jar electrical battery, and it will be entirely burned up. The circumference of the gold in this case is one inch, and this, of course, is the measure of the surface. Now, take a gold wire one-sixteenth of an inch in diameter, and pass through it the most powerful charge that can be obtained from the same battery; the wire will remain unaffected, although it presents but one-fifth the surface.
The difference between the action of static electricity and electricity in motion is very well shown by the following simple experiment: Take a large Leyden jar, one of say two gallons measurement, having the usual knob and other arrangements, as shown in the figure. In the wooden cover insert a glass tube, carrying at its upper extremity
a wire lying horizontally across it, this wire having a good-sized ball at each end, so that the discharge may take the form of a spark or an explosion, and not pass off silently. Between the horizontal wire and the knob of the jar stretch a strip of gold-leaf (B), and charge the jar in the usual manner. So long as the electricity does not flow through the gold-leaf, the latter will remain uninjured, although it is evidently charged as intensely as the machine can charge it. But, if we discharge the jar by laying one ball of the discharger on the outer coating of the jar, and the other on the knob (A), the gold-leaf will be destroyed. If, for the strip of gold-leaf, a wire the one-thirtieth of an inch in diameter be substituted, the charge will be carried off without its doing any damage. Here we see that, while the electricity was at rest (static), the gold-leaf was quite, capable of receiving as heavy a charge as the most powerful machine could impart; but, the moment the electricity began to flow (became dynamic), the gold-leaf was destroyed, notwithstanding its great surface, while a wire of far less surface afforded a perfect way for the charge to pass off.
Experiments in this direction might be multiplied ad infinitum, and, when properly conducted, they all lead to the same conclusion, which is, that, when made of the same metal, the efficiency of any rod is in direct proportion to its weight per foot. It may be round, square, tubular, ribbon-like, or in the form of a rope consisting of several strands; it makes no difference. For ourselves, we give the preference to a simple flat ribbon as being most easily applied and less obtrusive, but wires and wire ropes are very convenient, more easily procured, and quite as good.
That M. Nouel has neither experimented upon the subject nor given deep thought to it, is evident from the fact that he advises us to substitute hollow pipes for the present solid rods. As the interior surface of a pipe is incapable of receiving a charge of static electricity, it is evident that, if this law applies to lightning-conductors, the capacity of a pipe or tube would be just doubled by slitting it and spreading it out flat.
- We give to the terms static and dynamic the old meanings, as evidently does the writer under review. According to the new definitions suggested and advocated by Profs. Thomson and Tait, dynamics includes statics. The point is one which does not affect the main question, however.