Alessandro Volta invented this instrument to show up electric charges that were so weak they made neither straws nor the most sensitive electrometer fluctuate. The device functioned by considerably varying the capacity of the electrometer. First, you greatly increased the capacity by putting the moveable plate onto the fixed one and then earthing it. In this situation, if you put a lightly-charged Leyden jar or capacious conductor (whose charge is so weak as not to register on the normal electrometer) onto the lower plate, a significant amount of their charge is transferred to the plate. Secondly, the electrometer's capacity is drastically reduced by removing the upper plate.. The charge on the lower plate redistributes itself and some passes to the straws, which then move apart significantly.