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 time, 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.