$$\require{cancel}$$
In Chapter 6 we showed that when an electric current is situated in an external magnetic field it experiences a force at right angles to both the current and the field. Indeed we used this to define both the magnitude and direction of the magnetic field. The magnetic field is defined in magnitude and direction such that the force per unit length $$\textbf{F}^\prime$$ on the current is given by
$\textbf{F}^\prime = \textbf{I} \times \textbf{B}.$