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Liquids and gases

The generation of static electricity from the relative motion of liquids or gases is well established, with one of the first analyses in 1886 by Lord Kelvin in his water dropper which used falling drops to create an electric generator. Liquid mercury is a special case as it typically acts as a simple metal, so has been used as a reference electrode. More common is water, and electricity due to water droplets hitting surfaces has been documented since the discovery by Philipp Lenard in 1892 of the spray electrification or waterfall effect. This is when falling water generates static electricity either by collisions between water drops or with the ground, leading to the finer mist in updrafts being mainly negatively charged, with positive near the lower surface. It can also occur for sliding drops.