Publication: Electrical Review
New York, NY, United States
The Chambers Lightning-Rod.
BY E. W. CLAYPOLE.
In the second part of the third volume of the proceedings of the Davenport Academy, is a short paper by W. H. Pratt, entitled "The Chambers Rod and the Phoenix Mill Fire." In that paper it seems to me that Prof. Pratt has satisfactorily proved that the fire in the mill was caused by lightning. The fact that the mill was clean, and had not been running for some weeks, is sufficient to meet the allegation that the fire may have been caused by spontaneous combustion, aided by explosive dust. It might have been added, that there was no explosion. Moreover, one man bore distinct testimony to having seen the lightning strike the mill.
The mill, it appears, was fitted with the "Chambers" rod, the peculiarity of which is that it has no ground connection, and professes to dissipate the electric charge without conveying it to the earth. Much discussion has been aroused in regard to the protective power of this mode of constructing and setting up lightning-rods. Experience, which should be the final, court of appeal, is not, in this case, so conclusive as it might be. Lightning rods of all kinds are so carelessly put up, and, what is more, so carelessly kept up, that accidents from lightning are not infrequent even to "protected" buildings. That well-made and well-kept lightning-rods are, however, not only efficacious, but thoroughly effective, in preventing damage, is proved by the fact that no loss has been sustained by the British Navy from this cause since the adoption of Sir W. Snow Harris's lightning conductors. It should be borne in mind that a rod does not avert the electric discharge, but only the danger and damage that would be caused by an electric explosion. Ships of the British Navy are probably struck as often now as formerly, but no explosion ensues, and no mischief is done. Yet, in spite of all the "protection" afforded by lightning-rods in this country, in some parts of which every house and barn is fitted with them, I cannot learn that the fire insurance companies make much or any diminution in their rates of premium on buildings so fitted. These remarks apply to all kinds of lightning-rods. Whatever their construction, unless well made and well kept, they are a source of danger rather than of safety.
But there is an element of danger in the Chambers rod, arising from the principle on which it is constructed, that does not exist in the case of the usual "earth-fast" rods. These latter act on the principle that the air and the earth are always in opposite states of electric tension. Our knowledge does not allow us to say if this is absolutely and necessarily true, but experience shows that it is perfectly safe in practice to assume that it is so. Consequently, the action of the rod depends on its power of communicating by a continuous and infusible conductor between the one and the other, by which means the electric tension is equalized, either by a slow and silent discharge, as commonly occurs, or by a sudden and sharp flash.
The Chambers rod, on the other hand, is constructed on the principle that different masses of air are also in different conditions of electric tension, and that these different masses of air are very near each other. The first of these two principles is true beyond question. This is proved by the constant passage of sparks or flashes of lightning between cloud and cloud, or, more accurately speaking, between one body of air and another, without "striking", the earth at all. Probably this is by far the commonest mode of restoring the disturbed equilibrium. Not one flash, apparently, among hundreds that occur, strikes the earth, or anything upon it. Were it otherwise, so many storms could not happen without any mischievous consequences. Comparatively seldom do we hear that anything or anybody has been struck by one of the hundreds of electric flashes occurring during every summer. They discharge themselves in the air. An overcharged mass relieves itself by "flashing" into one with less tension at no great distance. So far, the principles on which the Chambers rod is constructed are well founded.
But the second principle is open to grave suspicion, and this suspicion, to say the least, is a serious objection to the general adoption of this system of "protection." In assuming that these masses of air of unequal electric tension are in close proximity, the advocates of this system claim more than can be granted. Without raising any question of the meaning of the word "proximity," it is evident that it. must mean "striking distance." Now, that two masses of air of unequal tension are usually within striking distance is amply proved by the fact above mentioned — that almost all the electric discharges occur between cloud and cloud. But that this is not always true is proved with equal certainty by the other fact that sometimes the discharge takes place between the cloud and the earth. Against the former we need no protection, because discharges between masses of air of unequal tension are harmless to the earth and things on it. It is the latter kind of discharge solely against which lightning-rods are intended to afford protection. The assumption that a mass of air overcharged is always near enough to one that is undercharged to flash into it being baseless, renders any mode of protection founded upon it to a great extent untrustworthy. I say to a great extent, because a mass of overcharged air, though naturally out of reach of a mass of undercharged air, may be brought within reach of the same by artificial means — and this is what is attempted by the Chambers lightning-rod. A cloud or over-excited mass of air at one place may be out of striking distance of a mass of under-excited air, but if a long copper rod be laid from one to the other, or from near the one to near the other, the spark may pass. Consequently, if a large or long building be fitted with the Chambers rod, with no earth connection, it is more than probable that some of the most distant points will be beyond the influence of a thunder-cloud that is able to discharge into a nearer one, and may, consequently, take the charge and pass it off into the less electrically intense air that overhangs them. In this way, a building so protected may be struck and not injured. It is, however, obvious that this protective power must rapidly diminish with the size, and especially with the length, of the structure; and when this is reduced to small dimensions, it is more than possible that all the points are within striking distance of the same cloud. Consequently, there is, then, no discharge, and the danger to the building is vastly increased by the presence of the rod.
Now, the Phoenix Mill was a structure of this kind. It measured only fifty by thirty feet, and all its points were probably within the influence of the mass of excited air from which the flash proceeded. Not having any path provided for its escape, it took the conductor and then the building in its passage to the earth. It is needless to add that on this view, an ordinary conductor, with good earth-plate, would have efficiently protected the building.
It may be replied that the conductor was insulated from the roof, but the reply would be futile. It matters little or nothing whether this conductor, or any other, be insulated or not. A flash that can leap from the cloud to the rod can certainly leap from the rod to the roof.
The necessary inferences from the views above put forward are :
1. That the Chambers rod possesses no virtue which ordinary rods do not possess, except a slightly lower cost.
2. That the Chambers rod can, and doubtless does, in many cases, protect buildings when they are extensive and the points at considerable distances from each other.
3. That the Chambers rod loses its virtue when applied to small buildings.
4. That the Chambers rod has least protective power when the danger is greatest — that is when the mass of charged air is very large.
5. That while these objections diminish the protective power of the Chambers rod, a rod with good earth connection of sufficient size, and in perfect order, with a sufficient number of points, affords protection, if not absolute, yet so nearly absolute, that the danger from lightning to a building thus protected is infinitesimally small.