University of Virginia Library

Franklin's Theory of Electricity

According to Franklin's theory, electricity exists in all bodies as a "common stock,'' and tends to seek and

remain in a state of equilibrium, just as fluids naturally tend to seek a level. But it may, nevertheless, be raised or lowered, and this equilibrium be thus disturbed. If a body has more electricity than its normal amount it is said to be positively electrified; but if it has less, it is negatively electrified. An over-electrified or "plus'' body tends to give its surplus stock to a body containing the normal amount; while the "minus'' or under-electrified body will draw electricity from one containing the normal amount.

Working along lines suggested by this theory, Franklin attempted to show that electricity is not created by friction, but simply collected from its diversified state, the rubbed glass globe attracting a certain quantity of "electrical fire,'' but ever ready to give it up to any body that has less. He explained the charged Leyden jar by showing that the inner coating of tin-foil received more than the ordinary quantity of electricity, and in consequence is positively electrified, while the outer coating, having the ordinary quantity of electricity diminished, is electrified negatively.

These studies of the Leyden jar, and the studies of pieces of glass coated with sheet metal, led Franklin to invent his battery, constructed of eleven large glass plates coated with sheets of lead. With this machine, after overcoming some defects, he was able to produce electrical manifestations of great force—a force that "knew no bounds,'' as he declared ("except in the matter of expense and of labor''), and which could be made to exceed "the greatest know effects of common lightning.''

This reference to lightning would seem to show

Franklin's belief, even at that thime, that lightning is electricity. Many eminent observers, such as Hauksbee, Wall, Gray, and Nollet, had noticed the resemblance between electric sparks and lightning, but none of these had more than surmised that the two might be identical. In 1746, the surgeon, John Freke, also asserted his belief in this identity. Winkler, shortly after this time, expressed the same belief, and, assuming that they were the same, declared that "there is no proof that they are of different natures''; and still he did not prove that they were the same nature.