The ampere (A) is a fundamental unit in the International System of Units (SI) because it is not derived from any other SI units. It is defined as the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newton per meter of length.

Here's why this makes the ampere fundamental:

* Independent Definition: The definition of the ampere relies on physical phenomena (force between conductors) and fundamental constants (like the vacuum permeability). It is not derived from other SI units.

* Basis for Other Units: The ampere forms the basis for defining other electrical units, such as the volt (V), ohm (Ω), and watt (W). These units are defined in terms of the ampere. For example, the volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.

* Consistency in Measurements: By defining the ampere as a fundamental unit, we ensure that electrical measurements across different systems and locations remain consistent.

In summary: The ampere is fundamental because its definition is independent of other SI units, and it forms the basis for defining many other electrical units.