Franklin bells (also known as Gordon’s Bells or Lightning bells) are an early demonstration of electric charge designed to work with a Leyden jar. Franklin bells are only a qualitative indicator of electric charge and were used for simple demonstrations rather than research. This was the first device that converted electrical energy into mechanical energy in the form of continuous mechanical motion, in this case, the moving of a bell clapper back and forth between two oppositely charged bells.
History – During one of his experiments with electricity, Benjamin Franklin reportedly invented the “bells” in the 18th Century. However, it should be known that circa 1742 Andrew Gordon, Professor of Natural Philosophy at the University at Erfurt, Germany, created an instrument known as “electric chimes.” This discovery was noted in many scientific books of the age. “Franklin made use of Gordon’s idea by connecting one bell to his pointed lightning rod, attached to a chimney, and a second bell to the ground. One of his papers contains the following description:
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.
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. (Benjamin Franklin’s Lightning Bells)
Excerpted from: The Papers of Benjamin Franklin. Edited by Leonard W. Labaree. New Haven: Yale University Press, 1962. Vol. 5, p. 69, letter from Benjamin Franklin to Peter Collinson dated September 1753.
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 (sic) 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. (Benjamin Franklin’s Lightning Bells)
Design and Operation
The bells consist of a metal stand with a crossbar, from which hang three bells. The outer two bells hang from conductive metal chains, while the central bell hangs from a nonconductive thread. In the spaces between these bells hang two metal clappers, small pendulums, on nonconductive threads. A short metal chain hangs from the central bell.
The central bell’s chain is put in contact with the inner surface of a Leyden jar, while the outside surface of the jar is put in contact with the metal stand. Thus, the central bell takes its charge from the inner surface of the jar, while the outer surface charges the two bells on the conductive chains. This causes the bells to have a difference in electrical potential equal to that between the inner and outer surfaces of the jar. The hanging metal clappers will be attracted to one bell, will touch it, pick up its charge, and be repelled; they will then swing across to the other bell, and do the same there. Each time the clappers touch a bell, charge is transferred between the inner and outer surfaces of the Leyden jar. When the jar is completely discharged, the bells will stop ringing.