To Ebenezer Kinnersley
Printed in Benjamin Franklin, Supplemental Experiments and Observations on Electricity, Part II. … (London, 1753), pp. 103–6.3 (Yale University Library)
March 16, 1752
Having brought your brimstone globe to work, I try’d one of the experiments you proposed,4 and was agreeably surprized to find, that the glass globe being at one end of the conductor, and the sulphur globe at the other end, both globes in motion, no spark could be obtain’d from the conductor, unless when one globe turned slower, or was not in so good order as the other; and then the spark was only in proportion to the difference, so that turning equally, or turning that slowest which work’d best, would again bring the conductor to afford no spark.
I found also, that the wire of a phial charg’d by the glass globe, attracted a cork ball that had touch’d the wire of a phial charged by the brimstone globe, and vice versa, so that the cork continued to play between the two phials, just as when one phial was charged through the wire, the other through the coating, by the glass globe alone. And two phials charged, the one by the brimstone globe, the other by the glass globe, would be both discharged by bringing their wires together, and shock the person holding the phials.
From these experiments, one may be certain that your 2d, 3d, and 4th proposed experiments, would succeed exactly as you suppose, though I have not tried them, wanting time. I imagine it is the glass globe that charges positively, and the sulphur negatively, for these reasons, 1. Though the sulphur globe seems to work equally well with the glass one, yet it can never occasion so large and distant a spark between my knuckle and the conductor when the sulphur one is working, as when the glass one is used; which, I suppose, is occasioned by this, that bodies of a certain bigness cannot so easily part with a quantity of electrical fluid they have and hold attracted within their substance, as they can receive an additional quantity upon their surface by way of atmosphere. Therefore so much cannot be drawn out of the conductor, as can be thrown on it. 2. I observe that the stream or brush of fire appearing at the end of a wire connected with the conductor, is long, large, and much diverging, when the glass globe is used, and makes a snapping (or rattling) noise: but when the sulphur one is used, it is short, small, and makes a hissing noise; and just the reverse of both happens, when you hold the same wire in your hand, and the globes are work’d alternately: the brush is large, long, diverging and snapping (or rattling) when the sulphur globe is turn’d; short small, and hissing, when the glass globe is turn’d. When the brush is long, large, and much diverging, the body to which it joins, seems to me to be throwing the fire out; and when the contrary appears, it seems to be drinking in. 3. I observe, that when I held my knuckle before the sulphur globe, while turning, the stream of fire between my knuckle and the globe, seems to spread on its surface, as if it flow’d from the fingér; on the glass globe ’tis otherwise. 4. The cool wind (or what was call’d so) that we used to feel as coming from an electrified point, is much more sensible, when the glass globe is used, than when the sulphur one. But these are hasty thoughts. As to your 5th paradox, it must likewise be true, if the globes are alternately work’d; but if work’d together, the fire will neither come up nor go down by the chain, because one globe will drink it as fast as the other produces it.
I should be glad to know whether the effects would be contrary, if the glass globe is solid, and the sulphur globe hollow; but I have no means at present of trying.
In your journeys,5 your glass globes meet with accidents, and sulphur ones are heavy and inconvenient. Query. Would not a thin plane of brimstone, cast on a board, serve on occasion as a cushion, while a globe of leather stuff’d (properly mounted) might receive the fire from the sulphur, and charge the conductor positively? Such a globe would be in no danger of breaking.6 I think I can conceive how it may be done; but have not time to add more than that I am, Yours, &c.
3. Letter IX in Supplemental Observations, 1753, and in 1754 and 1760 editions; Letter X in 1769 and 1774 editions.
4. See above, pp. 263, 269.
5. A reference to Kinnersley’s lecture tours (see above, 192 n).
6. Footnote in 1769 and 1774 editions: “The discoveries of the late ingenious Mr. Symmer, on the positive and negative Electricity produced by the mutual friction of white and black silk, &c. affords hints for farther improvements to be made with this view.” Robert Symmer (d. 1763), an English experimenter, borrowed from BF apparatus for his investigations; he informed BF of his discoveries as he made them; “and, in return, met with an ingenuity and candour, that render him as estimable in private life, as the improvements he has introduced into electricity, and particularly his discovery in relation to thunder and lightning, will render his reputation lasting in the learned world.” I. Bernard Cohen, Franklin and Newton (Phila., 1956), pp. 543–6. No correspondence between BF and Symmer has been found.