  Here, we study the old problem of why identical insulators can charge one
another on contact. We perform several experiments showing that, if driven by a
preexisting electric field, charge is transferred between contacting
insulators. This happens because the insulator surfaces adsorb small amounts of
water from a humid atmosphere. We believe the electric field then separates
positively from negatively charged ions prevailing within the water, which we
believe to be hydronium and hydroxide ions, such that at the point of contact,
positive ions of one insulator neutralize negative ions of the other one,
charging both of them. This mechanism can explain for the first time the
observation made four decades ago that wind-blown sand discharges in sparks if
and only if a thunderstorm is nearby.
