The gold-leaf electroscope was invented in the second half of the eighteenth century. It was one of the first instruments that could be reliably used to detect electrostatic charge. To some extent, it could even measure the degree of electrification by the degree of divergence of its leaves.
Still, the electroscope was not sensitive enough to detect weak atmospheric charges, and Alessandro Volta came up with a brilliant idea to overcome this limitation.
Volta's innovation was to add a parallel plate capacitor (in modern terminology) to his electroscope. He fixed one metal plate to the electroscope, attached a flat cake of insulating resin to it and kept another metal plate - this one freely movable - on top.
When he wanted to measure a charge at a low potential, he would connect the source to the lower plate of the capacitor while keeping the upper plate grounded. The large capacitance of the apparatus would cause a greater charge to flow into the lower plate than if it had been a simple electroscope.
The potential of the lower plate would still be too low to show any deflection of the leaves. This is where the ingenuity of the design comes in. If the upper plate is lifted by holding its insulating handle, the capacitance of the apparatus falls as the distance between the plates increases. This causes the potential of the lower plate to rise and force the gold leaves apart.
The device came to be aptly known as the condenser electroscope.
Volta communicated his invention to the Royal Society in a paper in 1782. However, it soon became apparent that the condenser electroscope was too sensitive to stray electrostatic charges and its readings were not reliable. Nevertheless, it would play an important role 20 years later when Volta wanted to demonstrate the polarity of his newly invented electrochemical battery, the pile.