Two innitely long straight wires of negligible radius are located at a distance $d$. The angle between their directions is $\alpha$. The currents owing in the wires are $I_1$ and $I_2$.

1 Determine the force acting between them. Investigate the limit $\alpha\to0$. How can you get the force between the two parallel wires, which corresponds to the old denition of the unit Amper?