From William Henly
ALS: American Philosophical Society
Tuesday Morn Jan. 29—712
I think myself highly honour’d by your very obliging favour, and return you my sincerest thanks for your improvement of my Electrometer. I shall take the first opportunity to make that addition to my Apparatus,3 and am well satisfied ’twill remove the objection at once.
I will now beg leave to assure you Sir that if I have been able to produce any Experiments in Electricity, which Dr. Franklin can vouchsafe to bestow the epithet curious4 upon, my highest ambition, and vanity in that Science is satisfied, and fully so.
Since I wrote last, I have insulated my Jar laying under the bottom of it a pretty long pointed wire, this while the Jar is charging positively, continues to throw off a fine diverging pencil of Rays. When I charge it negatively I place a pointed bent wire in a Cork stuck in the Knob of the Jar wire, which during the charge continues to throw off the pencil in the manner before described. These apperances oblige me when I speak of the Theory of the Leiden phyal, to lay aside the Terms Dr. Franklins Hypothesis &c and always to mention its phenomena as agreeable &c to Dr. Franklins Laws of the Leiden phyal.5 I beg leave to present the phyal I mention’d, and am Worthy Sir most sincerely yours
Addressed: To / Dr. Franklin / with a phyal
Endorsed: Recd. Tuesday Jan. 29—71
2. The date, penciled in another hand, may be that when the letter arrived; see the endorsement.
3. For Henly’s electrometer see above, XVII, 259–61. His surviving correspondence with BF gives no clue to the improvement that the latter suggested.
4. Presumably in the now obsolete sense of skillful or expert in contrivance.
5. For BF’s epoch-making work on the Leyden jar see in particular above, III, 157–64, 352–65; IV, 9–34, 65–7. For a helpful discussion of the use of the terms hypothesis and law in eighteenth-century science see I. Bernard Cohen, Franklin and Newton... (Philadelphia, 1956), pp. 575–89; we are most grateful to Professor Cohen for criticizing our annotation of this and subsequent documents relating to electricity. Henly’s experiment, one of many that he devised during the next few years to vindicate the single-fluid theory, was based largely on BF’s second observation above, III, 157–8, and was published in a more sophisticated form in Phil. Trans., LXIV (1774), 400–1. Henly was demonstrating visually BF’s hypothesis that a jar, like all bodies, contains an unvarying amount of “natural” electricity, and therefore that if a positive charge is applied to one surface it will drive to earth from the opposite surface an equivalent positive charge, leaving a “bound” negative charge equal in magnitude to what was lost. In Henly’s sketch, when the interior surface of the insulated jar on the left is charged positively, positive electricity is seen to escape from the exterior surface in the form of sparks or rays to the experimenter’s hand; when the exterior of the jar on the right is charged, the electricity escapes from the interior.