This Benjamin Franklin letter is from a series of letters he wrote about his electrical studies in the late 1740s and early 1750s. He sent most of the letters to Peter Collinson, a member of the Royal Society in London, who had sent various electrical apparatuses, books and articles to Franklin's "Library Company of Philadelphia," a lending library he helped found.
Many of Franklin's discoveries were groundbreaking and Collinson recognized their importance. He published the letters together in a book called "Experiments and Observations on Electricity Made at Philadelphia in America," which became so popular that it was the most reproduced book of the 18th century, making Franklin famous throughout Europe and America.
In this particular letter, Franklin discusses his discovery that pointed objects conduct electricity better than blunt objects, a key insight that led to his creation of the lightning rod. He also talks about how electrical charge passes from one item to another and uses, for the first time, the terms "positive" and "negative" and "plus" and "minus" in relation to electricity.
You can read Franklin's other letters about electricity at our Benjamin Franklin and Electricity letters page here.
Philada. May 25. 1747
In my last I informed you that In pursuing our Electrical Enquiries, we had observ'd some particular Phaenomena, which we lookt upon to be new, and of which I promised to give you some Account; tho' I apprehended they might possibly not be new to you, as so many Hands are daily employed in Electrical Experiments on your Side the Water, some or other of which would probably hit on the same Observations.
The first is the wonderful Effect of Points both in drawing off and throwing off the Electrical Fire. For Example,
Place an Iron Shot of three or four Inches Diameter on the Mouth of a clean dry Glass Bottle. By a fine silken Thread from the Ceiling, right over the Mouth of the Bottle, suspend a small Cork Ball, about the Bigness of a Marble: the Thread of such a Length, as that the Cork Ball may rest against the Side of the Shot. Electrify the Shot, and the Ball will be repelled to the Distance of 4 or 5 Inches, more or less according to the Quantity of Electricity. When in this State, if you present to the Shot the Point of a long, slender, sharp Bodkin at 6 or 8 Inches Distance, the Repellency is instantly destroy'd, and the Cork flies to it. A blunt Body must be brought within an Inch, and draw a Spark to produce the same Effect. To prove that the Electrical Fire is drawn off by the Point: if you take the Blade of the Bodkin out of the wooden Handle, and fix it in a Stick of Sealing Wax, and then present it at the Distance aforesaid no such Effect follows; but slide one Finger along the Wax till you touch the Blade, and the Ball flies to the Shot immediately. If you present the Point in the Dark, you will see, sometimes at a Foot Distance and more, a Light gather upon it like that of a Fire-Fly or Glow-Worm; the less sharp the Point, the nearer you must bring it to observe this Light: and at whatever Distance you see the Light, you may draw off the Electrical Fire, and destroy the Repellency. If a Cork Ball, so suspended, be repelled by the Tube, and a Point be presented quick to it, tho' at a considerable Distance, tis surprizing to see how suddenly it flies back to the Tube. Points of Wood do as well as those of metal, provided the Wood is not dry.
To shew that Points will throw off, as well as draw off the Electrical Fire: Lay a long sharp Needle upon the Shot, and you can not electrise the Shot, so as to make it repel the Cork Ball. Fix a Needle to the End of a suspended Gun Barrel, so as to point beyond it like a little Bayonet, and while it remains there, the Gun Barrel can not be electrised (by the Tube applied to the other End) so as to give a Spark; the Fire is continually running out silently at the Point. In the Dark you may see it make the same Appearance as it does in the Case before mentioned.
The Repellency between the Cork Ball and Shot is likewise destroy'd; 1. By sifting fine Sand on it; this does it gradually: 2. By breathing on it: 3. By making a Smoke about it from burning Wood: 4. By Candle Light, even tho' the Candle is at a Foot Distance: These do it suddenly. The Light of a bright Coal from a Wood Fire, and the Light of a red-hot Iron do it likewise; but not at so great a Distance. Smoke from dry Rosin dropt into a little hot Letter Founders Ladle under the Shot does not destroy the Repellency; but is attracted by both the Shot and the Cork-Ball, forming proportionable Atmospheres round them, making them look beautifully; somewhat like some of the Figures in Burnets or Whiston's Theory of the Earth.
n.b. This Experiment should be made [in a closet] where the Air is very still.
The Light of the Sun thrown strongly on both Shot and Cork by a Looking Glass for a long Time together does not impair the Repellency in the least. This Difference between Fire Light and Sun Light is another Thing that seems new and extraordinary to us.
We had for some Time been of Opinion, that the Electrical Fire was not created by Friction, but collected, being an Element diffused among, and attracted by other Matter, particularly by Water and Metals. We had even discovered and demonstrated its Afflux to the Electrical Sphere, as well as its Efflux, by Means of little light Wind-Mill Wheels made of stiff Paper Vanes, fixt obliquely, and turning freely on fine Wire Axes. Also by little Wheels of the same Matter, but formed like Water Wheels. Of the Disposition and Application of which Wheels, and the various Phaenomena resulting, I could, if I had Time, and it were necessary, fill you a Sheet.
The Impossibility of Electrising one's self (tho' standing on Wax) by Rubbing the Tube and drawing the Fire from it: and the Manner of doing it by passing the Tube near a Person, or Thing standing on the Floor &c. had also occurred to us some Months before Mr. Watsons ingenious Sequel came to hand; and these were some of the new Things I intended to have communicated to you: But now I need only mention some Particulars not hinted in that Piece, with our Reasonings thereon; tho' perhaps the latter might well enough be spared.
1. A Person standing on Wax and rubbing the Tube; and another Person on Wax drawing the Fire, they will both of them (provided they do not stand so as to touch one another) appear to be electrised to a Person standing on the Floor; that is, he will perceive a Spark on approaching each of them.
2. But if the Persons standing on Wax touch one another during the exciting of the Tube, neither of them will appear to be electrised.
3. If they touch one another after exciting the Tube, and drawing the Fire as aforesaid, there will be a stronger Spark between them than was between either of them and the Person on the Floor.
4. After such strong Spark, neither of them discovers any Electricity.
These Appearances we attempt to account for thus. We suppose as aforesaid, That Electrical Fire is a common Element, of which every one of the three Persons abovementioned has his equal Share before any Operation is begun with the Tube. A who stands on Wax, and rubs the Tube, collects the Electrical Fire from himself into the Glass; and his Communication with the common Stock being cut off by the Wax, his Body is not again immediately supply'd. B, who stands upon Wax likewise, passing his Knuckle along near the Tube, receives the Fire which was collected by the Glass from A; and his Communication with the common Stock being likewise cutt off, he retains the additional Quantity received. to C, standing on the Floor, both appear to be electrised; for he having only the middle Quantity of Electrical Fire receives a Spark on approaching B, who has an over-quantity, but gives one to A, who has an under-quantity. If A and B touch each other, the Spark between them is stronger, because the Difference between them is greater. After such Touch, there is no Spark between either of them and C; because the Electrical Fire in all is reduced to the original Equality. If they touch while Electrising, the Equality is never destroyed, the Fire only circulating. Hence have arisen some new Terms among us. We say B (and other Bodies alike circumstanced) are electrised positively; A negatively: Or rather B is electrised plus and A minus. And we daily in our Experiments electrise Bodies plus or minus as we think proper. These Terms we may use till your Philosophers give us better. To electrise plus or minus, no more needs to be known than this; that the Parts of the Tube or Sphere, that are rub'd, do, in the Instant of the Friction, attract the Electrical Fire, and therefore take it from the Thing rubbing: the same Parts immediately, as the Friction upon them ceases, are disposed to give the Fire they have received, to any Body that has less. Thus you may circulate it, as Mr. Watson has shewn; You may also accumulate or subtract it upon, or from any Body, as you connect it with the Rubber or with the Receiver; the Communication with the common Stock being cut off. We think that ingenious Gentleman was deceived, when he imagined (Page  of the Sequel) that the Electrical Fire came down the Wire from the Cieling to the Gun Barrel, thence to the Sphere and so electrised the Machine, the Man turning the Wheel &c. We suppose it was driven off and not brought on thro' that Wire; and that the Machine and Man &c. were electrized minus, i.e. had less electrical Fire in them than Things in common.
As the Vessel is just sailing, I can not give you so large an Account of American Electricity as I intended. I shall only mention a few Particulars more. We find granulated Lead better to fill the Vial with than Water, being easily warmed, and keeping the vial warm and dry in damp Air. We fire Spirits with the Wire of the Phial. We light Candles just blown out, by drawing a Spark among the Smoke between the Wire and Snuffers. We represent lightning, by passing the Wire, in the Dark, over a China Plate, that has gilt Flowers; or applying it to gilt Frames of Looking Glasses &c. We electrise a Person 20 or more Times running, with the Touch of the Finger on the Wire, thus; He stands on Wax; give him the electrised Bottle in his Hand; touch the Wire with your Finger, and then touch his Hand or Face, there are Sparks every Time. We encrease the Force of the electrical Kiss vastly, thus; Let A and B stand on Wax; give one of them the electrised Vial in Hand; let the other take hold of the Wire; there will be a small Spark; but when their Lips approach, they will be struck and shockt. The same if C and D (standing also on Wax) and joyning Hands with A and B, salute or shake Hands. We suspend by a fine Silk Thread a Counterfeit Spider, made of a small Piece of burnt Cork, with Legs of Linen Thread, and a Grain or two of Lead stuck in to give him more Weight. Upon the table, over which he hangs, we stick a Wire upright, as high as the Vial and Wire, 2 or 3 Inches from the Spider: then we animate him, by setting the electrified Vial, at the same Distance on the other Side of him: he will immediately fly to the Wire of the Vial, bend his Legs in touching it, then spring off and fly to the Wire in the Table; thence again to the Wire of the Vial; playing with his Legs against both in a very extraordinary Manner, appearing perfectly alive to Persons unacquainted. He will continue this Motion an Hour or more in dry Weather.
We electrify upon Wax, in the Dark, a Book that has a double Line of Gold round upon the Covers, and then apply a Knuckle to the Gilding; the Fire appears every where upon the Gold like a Flash of Lightning; not upon the Leather; nor if you touch the Leather instead of the Gold. We rub our Tubes with Buck Skin, and observe always to keep the same Side to the Tube, and never to sully the Tube by Handling. Thus they work readily and easily without the least Fatigue, especially if kept in tight Pastboard Cases, lined with Flannel and fitting close to the Tube. This I mention, because the European Papers on Electricity frequently speak of rubbing the Tube as a fatiguing Exercise. Our Spheres are fixt on Iron Axels, which pass thro' them. At one End of the Axis there is a small Handle with which we turn the Sphere like a common Grind-Stone. This we find very commode; as the Machine takes up but little Room, is portable, and may be enclosed in a tight Box, when not in Use. 'Tis true the Sphere does not turn so swift as when the great Wheel is used; but Swiftness we think of little Importance, since a few Turns will charge the Vial &c. sufficiently.
I must say no more of Electricity at present, or I shall not leave myself Time to answer your Favour of March last, which came to Hand by Dowers, with the Magazines and LeBlank's Letters, Martin's Electricity and Watson's first Part and Sequel.
We are, as we always have been, extreamly obliged to you for your kind Care in sending us, from Time to Time, what is new and curious, tho' not wrote for.
The rest of the Letter on private Affairs and Matters relating to the Philadelphia Library.
To Peter Collinson Esqr. F.R.S. London
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