2). Gauge invariance Potentials are not uniquely determined they differ by a gauge transformation. A>A'=A+Vy q→>9=q at Gauge: Given a set of (A, 9) (A, give identical electric and magnetic fields2)．Gauge invariance t A A A   →  = − →  = +          (A, )  Potentials are not uniquely determined, they differ by a gauge transformation. Gauge: Given a set of give identical electric and magnetic fields ' ' ( , ) A 