Page:Popular Science Monthly Volume 43.djvu/474

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feet of the air, he mentioned the fact that in Norway there were two kinds of thunderstorms: one occurred in the summer, he believed, when the lightning clouds were high, and very little damage was done; on the contrary, in winter time the clouds were very low, and the churches were frequently struck.

Lord Rayleigh, Sir William Thomson, and Prof. Rowland discussed the questions whether or not the experiments actually represented the actual conditions. M. de Fonveille called attention to the most extraordinary lightning conductor in existence, the Eiffel Tower, and the fact that Paris was practically free from calamities produced by lightning, because a sufficient number of lightning rods had been erected according to the principles advocated by the many official boards, substantially the same as the conference. Prof. George Forbes thought a copper alternative path better than an iron one. Sir James Douglas, speaking from an experience of forty years with a large number of conductors, was of the opinion that the rods, when properly constructed, were entirely adequate. In the matter of copper versus iron he pointed out the practical consideration that iron corroded rapidly compared with copper. Mr. Walker further pointed out that this corrosion of iron was not a question of weather alone, but, as in the case of the top of the chimney of a factory, there would be some chemical action. Mr. Symons brought out with respect to iron conductors that galvanizing would not entirely overcome the difficulty.

We now come to Lodge's book upon Lightning Conductors and Lightning Guards, and shall get from it a more satisfactory understanding of his experiments and deductions. He believes that the current ideas on the character of the lightning discharge were not altogether correct, because the momentum of an electric current and the energy of an electrostatic charge had both been more or less overlooked. The application of the known fact of electrokinetic momentum revolutionized the treatment of certain phenomena. The old drain-pipe idea of conveying electricity gently from cloud to earth was thus proved fallacious, and the problem of protection became at the same time more complex and more interesting. His position, therefore, is not that lightning rods are useless, but that few or none of the present types are absolute and complete safeguards; and he believes it possible to so modify existing protective systems as to afford more certain protection. The problem is one, lie very clearly shows, far removed from the old idea of conduction.

To-day we know from the experiments of Hertz, Lodge, and others that when an electric current flows steadily in one direction in a conductor, its intensity is the same in all parts of the wire; but if it be of an oscillatory character—i. e., a current revers-