Newsletter of the Mercurians, in the Society for the History of Technology
Volume 10 No. 1, November 1997
In Joseph Henry's Own Words
The next series of experiments . . . was on the induction from thunder clouds. For this purpose the tin covering of the roof of the house in which I resided was used as an inductive plate. A wire was soldered to the edge of the roof near the gutter, was passed into my study and out again through holes in the window-sash, and terminated in connection with a plate of metal in a deep well immediately in front of the house. By breaking the continuity of that part of the wire which was in the study, and introducing into the opening a magnetizing spiral, needles placed in this could be magnetized by a flash of lightning so distant that the thunder could scarcely be heard. The electrical disturbance produced in this case was also found to be of an oscillatory character, a discharge first passing through the wire from the roof to the well, then another in the opposite direction, and soon until equilibrium was restored. This result was arrived at in this case, as well as in that of the Leyden jar, before mentioned, by placing the same, or a similar needle, in succession, in spirals of greater and greater number of turns; for example, in a spiral of a single turn the needle would be magnetized plus, or in the direction due to the first and more powerful wave. By increasing the number of coils, the action of the second wave became dominant, so that it would more than neutralize the magnetism produced by the first wave, and leave the needle minus. By further increasing the number of turns, the third wave would be so exalted as to neutralize the effects of the preceding two, and so on. In the case of induction by lightning, the same result was obtained by placing a number of magnetizing spirals, of different magnetizing intensities, in the opening of the primacy conductor, the result of which was to produce the magnetization of an equal number of needles, plus and minus, indicating alternative currents in opposite directions.
Excerpt from "Memorial of Joseph Henry,"
Smithsonian Miscellaneous Collections,
Volume 21 (Washington, D.C., 1881), pp. 152-153.