The Physics
Opus in profectus

Two bells to give notice

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Letter from Benjamin Franklin to Peter Collinson, September 1753.

In September 1752, I erected an Iron Rod to draw the Lightning down into my House, in order to make some Experiments on it, with two Bells to give Notice when the Rod should be electrified. A contrivance obvious to every Electrician.

I found the Bells rang sometimes when there was no Lightning or Thunder, but only a dark Cloud over the Rod; that sometimes after a Flash of Lightning they would suddenly stop; and at other times, when they had not rang before, they would, after a Flash, suddenly begin to ring; that the Electricity was sometimes very faint, so that when a small Spark was obtained, another could not be got for sometime after; at other times the Sparks would follow extremely quick, and once I had a continual Stream from Bell to Bell, the size of a Crow-Quill. Even during the same Gust there were considerable variations.

Experiments, observations, and facts tending to support the opinion of the utility of long, pointed rods, for securing buildings from damage by strokes of lightning.

Read at the Committee appointed to consider the erecting of conductors to secure the [powder] magazines at Purfleet, August 27th, 1772.

What quantity of lightning a high, pointed rod, well communicating with the Earth, may be expected to discharge from the clouds silently in a short time, is yet unknown; but I reason from a particular fact to think it may at some times be very great. In Philadelphia I had such a rod fixed to the top of my chimney, and extending about nine feet above it. From the foot of this rod, a wire (the thickness of a goose-quill) came through a covered glass tube in the roof, and down through the well of the staircase; the lower end connected with the iron spear of a pump. On the staircase opposite too my chamber door, the wire was divided; the ends separated about six inches, a little bell on each end; and between the bells a little brass ball, suspended by a silk thread, to play between and strike the bells when clouds passed with electricity in them. After having frequently drawn sparks and charged bottles from the bell of the upper wire, I was one night awaked by loud cracks on the staircase. Starting up and opening the door, I perceived that the brass ball, instead of vibrating as usual between the bells, was repelled and kept at a distance from both; while the fire passed, sometimes in very large, quick cracks from bell to bell, and sometimes in a continued, dense, white stream, seemingly as large as my finger, whereby the whole staircase was inlightened as with sunshine, so that one might see to pick up a pin. And from the apparent quantity thus discharged, I cannot but conceive that a number of such conductors must considerably lessen that of any approaching cloud, before it comes so near as to deliver its contents in a general stroke; an effect not to be expected from bars unpointed, if the above experiment with the blunt end of the wire is deemed pertinent to the case.

Memoirs of Benjamin Franklin