  It is shown that the conductance relaxations observed in electrical field
effect measurements on granular Al films are the sum of two contributions. One
is sensitive to gate voltage changes and gives the already reported anomalous
electrical field effect. The other one is independent of the gate voltage
history and starts when the films are cooled down to low temperature. Their
relative amplitude is strongly thickness dependent which demonstrates the
existence of a finite screening length in our insulating films and allows its
quantitative estimate (about 10nm at 4K). This metallic-like screening should
be taken into account in the electron glass models of disordered insulators.
