The ampere is defined using the force between parallel wires: 1 ampere is the constant current which, flowing in two infinite parallel conductors of negligible cross-section placed 1 m apart in vacuum, produces a force of $2 \times 10^{-7}$N per meter of length. This connects the unit of current to mechanical units ($F = \mu_0 I^2/(2\pi d)$). In 2019, the ampere was redefined by fixing the elementary charge$e = 1.602176634 \times 10^{-19}$C.$