Page:Popular Science Monthly Volume 2.djvu/132

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THE POPULAR SCIENCE MONTHLY.

A Classified Catalogue of the Lepidoptera of Canada. By Alexander Milton Ross. Toronto, 1872.

Report submitted to the Trustees of Cornell University in behalf of a Majority of the Committee on Mr. Sage's Proposal to endow a College for Women. By Andrew D. White. Ithaca, 1872.

Report on the Climatology and Epidemics of Minnesota. By Charles N. Hewit, M. D. Philadelphia. 1872.

Short-hand and Reporting. A Lecture. By Charles A. Sumner, with Appendix. San Francisco, 1872.

 


MISCELLANY.

The Ground Connection of Lightning-Rods.—It is asserted, by all the later authorities on the subject of lightning-rods, that a proper ground termination of the rod is of the very first importance to its efficiency as a protection against accidents by lightning. The electricity of the cloud will select the easiest path into the earth, or, as it is technically stated, follow the line of least resistance; and it is to furnish a path less resisting than the building itself that the lightning-rod is erected. But it is not enough that the rod have a sufficient conducting capacity. The current must be able to leave it, at the place where it terminates in the ground, as fast as it passes along the rod, else there is an accumulation, or damming up as it were, in the rod, which, when it has attained a certain volume or intensity, will relieve itself with explosive violence; and thus the appliance becomes an actual source of danger to the building, rather than a means of protection. Mr. David Brooks, in an able paper on "Lightning and Lightning-rods," published in the August number of the "Journal of the Franklin Institute," says on this point: "I do not say that a greater proportion of buildings having lightning-rods are destroyed or injured than of those not having them, although those making careful observations do give that as a result of their statistics. I shall undertake to show that this difficulty consists in the defective connection of these plates with the earth, and also that with a proper connection with the earth they are almost, if not an absolute, means of protection." Says Prof. John Phin, in his admirable brochure on "Lightning-Rods and how to construct Them:" "Upon the perfection of the ground termination mainly depends the value of the lightning-rod. If this be defective, no other good features can possibly make up for it. And yet, so little is it understood, that a careful examination of a very large number of rods leads us to believe that fully one-half the lightning-rods in existence are defective in this respect, and consequently furnish but an insufficient protection."

All objects may be said to conduct electricity, but they vary greatly in their conducting capacity. Copper conducts six times as well as iron, and iron thousands of times better than water, and water again thousands of times better than dry earth. That is to say, a rod of iron, to have the same conducting capacity as a rod of copper, would require to be of six times the sectional area, while, if a rod or column of water were employed, it would require to be many thousands of times greater in sectional area than the iron, and dry earth again many thousands of times larger than the column of water. In connecting a rod with the ground, allowance has to be made for this difference in conducting capacity, sufficient earth-surface being joined to the rod to give a conducting capacity approaching to or equalling that of the rod. Otherwise the lightning discharge, unable to find a free passage into the ground, accumulates until the tension becomes so great that it bursts from the rod with explosive violence, taking the track which affords the readiest means of escape, and often doing serious damage in its progress.

Accidents of this character are by no means rare. Mr. Henry Wilde, in a communication to the Mechanic s Magazine, gives two cases of fire, resulting from the ignition of the gas by lightning in buildings where it left the conductor and took to the gas-pipes. In one instance, the discharge passed down a wire rope suspended by the side of a tall chimney, and, leaving the lower end of the rope, which was some ten feet from the ground, darted across a space of sixteen feet to a gas-meter in the cellar of an adjoining cotton-warehouse, where it