  Direct observation of electric potential and field variation near local
scatterers like grain boundaries, triple points and voids in thin platinum
films studied by scanning tunneling potentiometry is presented. The field is
highest at a void, followed by a triple point and a grain boundary. The local
field near a void can even be four orders of magnitude higher than the
macroscopic field. This indicates that the void is the most likely place for an
electromigration induced failure. The field build up near a scatterer strongly
depends on the grain connectivity which is quantified by the average grain
boundary reflection coefficient, estimated from the resistivity.
