From William Henly
ALS: American Philosophical Society
Thursday mor[ning, November 26?7, 1772]
I have Received your very obliging favour, and take the first opportunity of returning you my most sincere thanks for the trouble I have given you. I have done very little in Electricity since I had the pleasure of seeing you,8 one appearance however I beg leave to trouble you with, as I have never met with one of the kind before. Having charged my 6 Gallon Jar so as to repel the Index of the Electrometer to about 75 degrees, I insulated it, with a view to discharge it by giving it the contrary Electricity, but finding the Electrometer very damp, and much disposed to throw off the Electric matter, I applied the Ball of a long Rod (which I had in my hand) to the top of the stem, when the whole charge seem’d to fly off in a very fine stream almost instantaneously, and the index came directly into contact with the Ball of the Jar wire. As this happen’d so suddenly and every thing was kept at such a distance from the coating, I suspected that the outside could not have Received its natural quantity so fast as it had been drawn off from the inside, and on presenting my finger towards the Coating, when it had come within an Inch distance of it, a most beautiful and large pencil flew off from my finger, finely diverging towards the coating of the Jar much resembling one of those towards a very large Excited stick of sealing Wax. Probably this Sir may be no new appearance to you, but should that be the case, I flatter myself you will excuse the trouble I give you by laying before you an Account of it.9
I now venture to give my 2 large Jarrs their full charge, even so as to cause a Cascade of Fire to flow over the Neck of the thickest of them which repels the Index of the Electrometer, the whole thickness of the Rod above right Angles or 90 degrees.
With this charge, I have procured a large quantity of those Balls Mr. Canton was so obliging as to shew me, and a great number of large vitrified pieces, perfectly visible to the naked Eye; I have preserved both, for your inspection. I have never fail’d with this charge to fire a quil of Gunpowder, fill’d with the whole grains, and without at all drying or preparing them, tho the weather had made them very damp.1 Mr. Nairne thinks to finish my new Machine by the latter end of this week, when I receive it, I intend myself the pleasure of waiting on you, to request the Honour and favour of your Company to spend another Evening with me in the Electrical way. I beg your excuse for this long interruption, and that you would believe me most sincerely your obliged, and very Humble Servant
Addressed: To / Dr. Franklin / Craven Street / Strand
7. The letter mentions a machine that will be finished within a few days; it was probably the new conductor described by Henly below, Nov. 28. We assume that he was writing on the previous Thursday.
8. Presumably a month before: see Henly to BF above, under Oct. 28.
9. The jar, Henly is saying, had been discharged in such a way that the electricity inside had returned to its natural level while the outer surface retained a strong negative charge. This was contrary to BF’s fourth observation on the electrical action in a jar, above, III, 158. The second part of the experiment conformed to a recognized principle, however, that a negatively charged conductor (the exterior of the jar) induced a positive charge in a grounded conductor (Henly’s finger) to the point of producing a spark: Priestley, History, I, 359; II, 7. See also Edward W. Gray, “Observations on the Manner in Which Glass Is Charged with the Electric Fluid, and Discharged,” Phil. Trans., LXXVIII (1788), 121–4.
1. For Canton’s “glass” balls see above, XVIII, 182 n. BF had pioneered in exploding gunpowder by an electric spark: above, IV, 145, 202.