From William Henly7
ALS: American Philosophical Society
Sunday Eve. [April?,8 1772]
I have this day repeated in the most accurate manner I was able, my experiment with the Bladder gilded with leaf brass, and suspended on an arm of wood turning freely on a needles point. The following is the result of many trials. After giving the bladder a strong spark from the knob of a positively charged bottle; on presenting towards it a smooth round ball 2 Inches diameter, I found it would come towards it, at upwards of two inches distance, and when within an inch would throw off its electricity in a full strong spark, (in short as the air was dry, and the silk perfectly good and smooth by which it was suspended,) as great an one as it had received from the bottle. After giving the spark as before; upon presenting a fine point towards it, it never acceeded to the point in the least, and when it was pushed nearly into contact, (unless it was done very hastily,) no sensible quantity remained in it. I should have observed that if the ball was presented towards the bladder, even if it was swinging in a contrary direction, it would come back and meet it, throwing its electricity upon it at an inch distance as above described.9
I suppose Mr. Nairne told you of our ill success, with his batteries, and their having little or no effect on my Electrometer. To prove the goodness of that measure, I have this day tried it with your battery setting it upon the prime conductor. I gave the battery 100 turns of my small machine before the index10 began to move, but it then went on very regularly till it goot ⅒ Inch above right angles, when it made an effort to discharge itself but ended in hissing. At this instant I saw every one of the wooden tops to the bottles beset with loose filaments, I counted 7 upon one of them, and I believe in the whole there might be some 100. Think Sir how this must dissipate that atmosphere which ought to have been confined to the index, and to have regulated its action. On discharging it, the report was amazing, and much melted all the connecting wires. I saw plainly Mr. Nairnes error, and had he been advised by me, he might have prevented it. I have however put him in an easy, and simple way to correct it.1
I do still aver, that if the apparatus be properly made, every thing dry, and well connected, the Electrometer is a just and accurate measurer, a satisfactory and most useful instrument, as with this you are always sure of your force before the discharge.
Some friends of mine being strongly of opinion that the matter of light, and the Electric matter are the same thing, I made an experiment to determine the question (not for my own) but for their satisfaction. I insulated my rubber and collecting the Solar Rays in a burning glass brought it so as to let the rubber be in the focus, I burnt my rubber, (which is wood,) several times, but without producing any Electricity.2 Believe me Dear Sir to be with real regard yours
Addressed: To / Dr: Franklin / Craven-street / Strand
7. For the electrical experimenter see above, XVII, 259 n.
8. Lightning had struck the Tottenham Court Road Chapel in March, and Henly had been commissioned to fit the building with lightning rods; see Ronayne to BF above, under March 27. Henly began to test the efficacy of pointed as opposed to blunt rods, and the experiment here described was one of a series: Phil. Trans., LXIV (1774), 133, 142–3. We conjecture that Henly reported his findings to BF during the month after the lightning struck.
9. BF had concluded that a pointed rod, unlike a blunt one, would gradually discharge the electricity in an approaching cloud. Above, III, 126–8; IV, 16–19, 23–4; X, 51–2; see also BF’s experiments below, Aug. 18. Henly devised his metal bladder to simulate a cloud, and this and other experiments buttressed BF’s position when it was later challenged by Benjamin Wilson; see the headnote on the Purfleet committee’s report below, Aug. 21.
10. On his electrometer, which measured the intensity of the charge.
1. Nairne found the electrometer faulty when placed at the usual height, but effective when sufficiently elevated. Phil. Trans., LXIV, 80. Henly may have suggested to him this simple cure, though why it worked we cannot even guess.
2. The rubber or cushion was a wooden block, usually covered with leather, against which a glass globe or cylinder was revolved to generate a static electrical charge; the rubber was customarily grounded. Priestley, History, II, 4. Henly insulated the rubber, to determine whether the sunlight could produce a charge. He published a revised version of this experiment in Phil. Trans., LXIV (1774), 419–21. BF, in a reverse experiment, had determined that sunlight would not discharge a charged body: above, III, 128–9.