Force per unit length between two parallel wires carrying currents $I_1$ and $I_2$ separated by distance $d$:
$$\frac{F}{l} = \frac{\mu_0 I_1 I_2}{2\pi d}$$

Definition of 1 Ampere:
One ampere is that constant current which, when flowing through two infinitely long straight parallel conductors of negligible cross-section placed 1 m apart in vacuum, produces a force of $2 \times 10^{-7}$ N per metre of their length on each other.