In order for the effect to develop the material must be rubbed with a compatible substance, for example, glass rubbed with silk will build up a charge, as will hard rubber rubbed with fur. In the examples given one charge is positive, one negative.
The effect is of considerable industrial importance both in terms of safety and also potential damage to manufactured goods. The spark produced is fully capable of igniting inflammable vapours for example petrol or ether fumes. Means have to be found to discharge hospital trolleys which may carry such liquids.
Even where only high voltage is produced this can result dust particles being attracted to the rubbed surface. In the case of textile manufacture this can lead to a permanent grimy mark where the cloth has been charged