Previously, since the definition included a reference to force, the SI kg, meter, and second needed to be defined before the ampere could be defined. This definition was difficult to realize with high precision, and as such was changed to be more intuitive, and easier to realize. In terms of the SI unit of charge, the coulomb, one ampere is defined as one coulomb of charge passing through a given point in one second.

Before 2019, the ampere was defined formally as the constant current at which a force of 2 × 10 -7 newtons per meter length would be produced between two conductors, where the conductors are parallel, have infinite length, are placed in a vacuum, and have negligible circular cross-sections. The size of the unit was chosen such that it would fit conveniently within the metre-kilogram-second system of units. In the centimeter-gram-second system of units, the ampere was defined as one tenth of the unit of electrical current of the time, which is now known as the abampere. History/origin: The ampere is named after Andre-Marie Ampere, a French mathematician and physicist. This definition is effective since 2019, and is a significant change from the previous definition of the ampere. The second is defined based on the cesium frequency, Δ νCs.

The ampere is defined formally based on a fixed value for the elementary charge, e, of 1.602176634 × 10 -19 when expressed in the unit C, which is equal to A Definition: The ampere (symbol: A), often referred to as simply amp, is the base unit of electric current in the International System of Units (SI).

1 amp to mamp