Coulomb (C)
Modern equivalent
1 ampere-second
The SI unit of electric charge, defined as the quantity carried by a current of one ampere flowing for one second.
Named for Charles-Augustin de Coulomb, the French military engineer who established in the 1780s that the force between two charges falls away with the square of the distance between them, the coulomb underpins the definitions of both the ampere and the volt. It is an enormous quantity in everyday terms: a single coulomb represents the charge of some six quintillion electrons, which is why practical electrical work deals in currents and potentials rather than in charge itself.