Force between Infinite Parallel Straight Conductors carrying Steady Current

Theorem
Let $s_1$ and $s_2$ be wires in a vacuum carrying steady currents $I_1$ and $I_2$.

Let $s_1$ and $s_2$ be parallel and (effectively) infinitely long.

Then the force between $s_1$ and $s_2$ is given by:
 * $\mathbf F = \dfrac {\mu_0 I_1 I_2} {2 \pi r}$

where:
 * $r$ denotes the distance between $s_1$ and $s_2$
 * $\mu_0$ denotes the vacuum permeability.