Field of a Wire

Around a long straight wire the field lines are circles, and

$$B = \frac{\mu_0 I}{2\pi r}$$

$\mu_0 = 4\pi\times10^{-7}\ \mathrm{T\cdot m/A}$.

The right-hand grip rule: thumb along the current, fingers curl along the field. Current out of the page: counterclockwise.