Electroscope

Nollet's device detects and measures electric charge.

Jean-Antoine Nollet (1700-1770) was the first professor of experimental physics at the university of Paris. At that time electrostatics was a topic of great interest.

Nollet's electroscope was designed to detect and crudely measure electric charge. An insulated charge sensor protruded into a cylindrical container, the ends of which were closed by two flat glass windows. The bottom of the sensor (the part in the cylinder) was fitted with two leaves of metal foil ( usually gold). If the sensor's opposite, extruding end was brought into contact with a negatively charged body, electrons were repelled into the two leaves and they separated. The degree of separation was function of the size of he charge. A second negatively charged body contacting the extruding end would cause the leaves to separate more, whereas a positively charged body subsequently applied to the same end would cause the separation to decrease.

Earlier electroscope were mainly used to investigate the amount of charge that could be produced by hand-cranked or foot-treadled frictional electrostatic machines. Here, a globe of glass was charged by rubbing it with a soft material.

Nollet was interested in the causes of electrostatic repulsion and attraction and the speed with which electric current flowed. He once estimated the latter by discharging the charge contained in a large Leyden jar through a line of 200 Carthusian monks who were connected to each other by iron wires 25 feet (7.6m) long. As all the monks leaped with shock at  the same time, he concluded that electricity traveled speedily, and could travel a long distance. 

• A negative charge passing down the electroscope's rod causes the leaves at the end to repel each other.

"The electron.... crystallizes out of Schrödinger's mist like a genie emerging from his bottle."

Sir Arthur Eddington, Nature of the Physical World.