Charge flow during metal-insulator contact

The electrification of polymethyl methacrylate films is studied by scanning force microscopy. Charged areas generated by contact electrification are always found to be larger than those generated by corona discharge, and are surprisingly much larger than the area of contact. After each single contact made with the metal tip on the insulator charge was transferred, the sign of which was arbitrary. It is argued that charge already flows and spreads into the insulator at the time of metal-insulator contact.