Static electricity. A spark discharge between the dome of a Van der Graaff generator and an electrically earthed sphere. When the generator is switched on, a motor causes a rubber band in the column to run over pulleys at either end. At the base, the band runs past fine points which generate an electrical charge. The band carries the charge up to the dome, where a metal 'brush' transfers the charge to the dome itself. After a short time, the dome accumulates a substantial charge of static electricity which may eventually discharge across the air gap. Potential differences of tens of thousands of volts may be generated in this way.