We report here a novel effect in which the resistance of a semiconducting oxide ceramic increases on application of a small dc bias. The ceramic conducts at high temperatures by an n-type hopping mechanism. On application of a dc bias, conduction electrons are trapped at surface states and the resistance increases. On removal of the dc bias, the trapped electrons are released and the sample regains its original state. This effect is the mirror image of that seen with similar ceramics that conduct by a p-type mechanism whose resistance decreases reversibly on application of a small dc bias. These two phenomena together offer the possibility of novel switching devices and memristive applications, especially if the switching times can be reduced.