Electric Current
Electricity and Magnetism

The two electric fluids

Stories from Physics for 11-14 14-16 IOP RESOURCES

Charles François de Cisternay du Fay, an 18th century French nobleman, soldier and scientist, was interested in the attraction and repulsion of objects that occurred after they had been rubbed.

He experimented with small pieces of foil and discovered that, if one piece were charged by glass rubbed with silk, it would be attracted to another charged with amber rubbed with wool. By contrast, two pieces of metal charged from the same source would repel. From these observations, Du Fay concluded that there were two electric fluids: vitreous (from glass rubbed with silk) and resinous (from amber rubbed with wool). He hypothesised that the fluids were separated by frictional forces and neutralised when they recombined.

While many scientists accepted Du Fay’s two fluid model of electricity, Benjamin Franklin was one of the first scientists to propose a single fluid theory. He noted that when objects become charged, for example when glass is rubbed with a cloth, the two bodies gain equal but opposite charges. Franklin therefore hypothesised that all bodies contain a single fluid, or electric fire, which exists in a neutral state unless frictional forces act to cause an imbalance.

Franklin gave Du Fay’s vitreous fluid the label positive and called the resinous fluid negative, leading to the mismatch between conventional current and the direction of flow of electrons that exists today. Franklin also coined the terms conductor and non-conductor.

References

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