Dynamics of contact electrification

Although the electrical charging of objects brought into contact has been observed for at least 2000 years, the details of the underlying mechanism are still not yet fully understood. The present paper deals with the very basic process of contact electrification between two metals. We have developed an experimental method to follow the charge of a small sphere bouncing on a grounded planar electrode on a time scale down to 1 mu s. It reveals that the sphere is discharged in the moment of contact, which lasts about 6 to 8 mu s. However, at the very moment of disruption of the electrical contact, it regains charge far beyond the expectation according to the contact potential difference. The excess charge rises with increasing contact area.