The iambic trimeter derives its name from its essential shape, which is three metrical units (hence trimeter) which are each basically iambic in form. The iambic metron has the following shape (where the x is an anceps, the - is a longum, and the u is a brevis):
Hence, reversing the current running though a wire will have the affect of reversing the direction of the magnetic force at a point some distance away from the wire. The affect that this has on the overall magnetic field depends on the shape of the wire.
Yes, that is the cross product for magnetic part of the force. Also, if you have an electric field the change in electric potential between two points would be: If the electric field is constant in both direction and magnitude, then the magnitude of the change in electric potential would just be E * s.