When a current flows through a junction or a contact between two metals of different conductivities a charge is accumulated in the vicinity of contact. The charge accumulated, I've read, is directly proportional to the current. In case current flows from a metal of large conductivity to a metal of small conductivity, charge accumulation is of positive type. Why so? And if there is such an accumulation of charge, won't a potential barrier be set up, that will eventually stop current flow?