"... the original field equations explicitly contain the magnetic vector potential, \overrightarrow{A} ... In Maxwell's original formulaton, Faraday's \overrightarrow{A} field was central and had physical meaning. The magnetic vector potential was not arbitrary, as defined by boundary conditions and choice of gauge as we will discuss; they were said to be gauge invariant. The original equations are thus often called the Faraday-Maxwell theory."