The Ampere

The Ampere is a defined quantity. The Ampere in turn defines the Coulomb - the unit of charge.

A flow of one Ampere is a flow of one Coulomb per second. Since the Ampere defines the Coulomb there must be a way to define the Ampere that does not depend on the number of charges per second flowing in a current of one Ampere.

Early attempts were made to define the Coulomb first by literally counting silver atoms deposited by a current on an electrode; one atom per electron. Uncertainty as to the weight of the silver atom limited the accuracy of the determination. The solution was to define the Ampere in terms of the force between two parallel current carrying wires.

Note: a current of one Ampere flows in parallel wires that, when placed one meter apart, have a force acting between them of two by ten to the minus seven Newtons per meter. .... (A small force but one that can be measured more accurately than the mass of a silver atom.)

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