This figure illustrated Joseph Henry's 1859 account of an experiment he had performed years earlier while at Princeton. In describing this component of his study of induction from clouds, Henry explained that "the inductive action of the electrical discharge in the heavens was exerted on the natural electricity of a surface of about 1,600 square feet [the tin roof], and a considerable portion of this passed down through the wire" that ran through his room to a ground. There, the "intensity of magnetism and the direction of the current were ascertained by presenting the end of the needle [hanging from a cork, d, near the coiled wire, a ] to a small compass represented by c." Henry noted that the "inductive action" magnetized the needle "whenever a flash of lightning was perceived, though it might be a distance of several miles." Moreover, this magnetizing could occur at such great distances that the needle could be "removed, its magnetic condition observed, and another needle put in its place, before the noise of the thunder reached the ear."
To argue his case further, Henry pointed out that "the effect here described was not produced by the actual transfer of any electricity from the cloud, but was simply the result of induction at a distance."
From Report of the Commissioner of Patents for 1859, Agricultural Section, in a lengthy paper on Meteorology, part V in a series begun in the 1855 Report (pp. 477-478)