  Contact charging between insulators is one of the most basic, yet least well
understood, of physical processes. For example we have no clear theory for how
insulators recruit enough charge carriers to deposit charge but not enough to
discharge. In this letter we note that charging and discharging kinetics may be
distinct, and from this observation we develop a mathematical model. The model
surprisingly predicts that charging can decrease as contact frequency
increases: we confirm this prediction experimentally and propose future steps.
