Contact electrification across metal-dielectric and dielectric-dielectric interfaces

An apparatus is described by means of which a spherical dielectric surface can be compressed under various loads against either a plane metal disk or a plane dielectric disk. A transfer of electric charge across the interface is recorded when the surfaces are separated. The polarity of charging between any two materials depends upon the relative positions of those materials in an electrostatic series; a metal surface does not always assume a positive charge as found for impact electrification. For two given materials the amount of charge is proportional to the contact area between the surfaces and provided that one of the surfaces has a resistivity above a critical value (5 × 1011 Ωcm) then the charge density for various combinations of materials is approximately proportional to the lower dielectric constant in each combination. This result suggests that the charging may be limited by electrical back-discharge between the separating surfaces. It is considered to be unlikely that the electrification originates from a difference in the contact potentials of the two surfaces involved.