William Henly: Note on the Electrical Phenomena of a Thunderstorm8
AD: American Philosophical Society
Wednesday Sepr. 30. 1772
At 10 Minutes past 2: p.m. I heard what I thought (but was not certain) was a Clap of Thunder. However on putting out my Rod, I observed the balls to diverge 1½ or 2 Inches.9 Presently I heard a very distinct and loud clap, when the Electricity which before was positive, instantly changed to negative and the balls (having first closed,) diverged to 3 Inches distance. The approach of excited wax would now nearly double the[ir distance?] of divergence, which I observed near a dozen times; and what is remarkable the end of the Rod, which I held in my hand, without the wax had the same effect when brought between the strings.1 This effect I also observed many times. The Electricity continued constantly negative near half an Hour, when the balls gradually closed, and all appearances thereof finally ceased.
8. We include this document, although it is not strictly within our rubric, because it was without doubt sent to BF, and is part of a line of experimentation in which he was much interested.
9. Henly was using, with slight modifications, the apparatus and procedure developed by Thomas Ronayne for the observations that he forwarded to BF with his note above, Feb. 15. For a description of the rod see Phil. Trans., LXIV (1774), 428–9.
1. [Henly’s note:] The end of this Rod must therefore be affected in the same manner as the balls themselves.