Analysis of repeated-contact electrification curves

Samples of different materials were repeatedly contacted with metal plates in high vacuum. The transferred charge after contact and separation was measured. This resulted in individual contact electrification curves. Different behaviour of the materials was observed. High-conductivity samples such as oxidized metals or graphite show charge saturation after few contacts. Low-conductivity samples like quartz, calcite and Teflon show charge accumulation up to a large number of contacts. Reversal of the net charge polarity with increasing number of contacts was observed for calcite and quartz samples.