 Material surface element

If $\mathbf{A}={\mathbf{t}}_a\times {\mathbf{t}}_{b}dadb$ and each tangent satisfies $D\mathbf{t}/Dt=(\nabla \mathbf{u})\mathbf{t}$, differentiation of the cross product gives $D\mathbf{A}/Dt=(\nabla \cdot \mathbf{u})\mathbf{A}-(\nabla \mathbf{u}{)}^T\mathbf{A}$. Combining this with ideal induction proves pointwise conservation of $\mathbf{B}\cdot \mathbf{A}$.