An electroscope is a device used to detect an electric charge. For centuries, it was one of the most popular instruments used by scientists to study electricity. William Gilbert, an English physician and renowned author of De Magnete (“On the Magnet”), built an early form of the electroscope in the early 17th century. His device, dubbed the versorium, consisted of a lightweight needle balanced on a pivot. The presence of electricity in a nearby object caused the needle to move. With the versorium, Gilbert carried out a number of experiments and proved that many materials besides amber acquire a charge when rubbed.
Subsequent scientists made improvements to the electroscope’s design. The repulsion-based electroscope generally supplanted the versorium in the 1700s. These devices owed their usefulness to the principle that like electric charges repel one another (according to what is now known as Coulomb’s law). Benjamin Franklin’s version of a repulsion electroscope employed a pair of linen threads, while some of his contemporaries preferred electroscopes that used dual pith balls. To keep breezes, moisture and other outside influences from disturbing results, some researchers began housing the components of their electroscopes in glass jars or other vessels.
The gold leaf electroscope, like the one illustrated here, first appeared in the latter part of the 18th century. Abraham Bennet, a clergyman and man of science, first described the instrument in Philosophical Transactions in 1787. Bennet’s electroscope consisted of a pair of very thin, narrow leaves of gold hung from a conducting rod surrounded by glass housing. The rod extended out of the top of the housing. That way, whenever an experimenter placed an electrified object close to it, the rod acquired charge via inductance. The charge then flowed down to the gold leaves and evenly distributed across their surfaces. Once electrified, the leaves moved apart because their similar charges triggered repulsion. Some models of gold leaf electroscopes also featured a conductive disc or ball at the apex. Attached to the foot of others, “earthing strips” could be found. The purpose of the metal strips, according to some sources, was to protect the leaves of the device from static charge build-up on the glass, though some scientists claimed they actually improved the precision of the electroscope.
It’s important to note that on its own, the electroscope only signifies the presence of charge, not whether the charge is positive or negative. Researchers quickly realized, however, that the type of charge was simple to determine by following the initial charging of the device with the application of a known charge (from amber, fur or some other familiar, well-studied material). In addition, they found that the electroscope was useful in determining the relative amount of charge present. By equipping their electroscopes with scales, scientists could easily measure the angle between the gold leaves. The greater the angle, they ascertained, the stronger the repulsion and the greater the charge present.
By the late 1800s, researchers such as Marie and Pierre Curie began using advanced electroscopes to study radioactivity. In a charged electroscope, the radiation emitted from radioactive materials ionizes the gas inside the device. The ionization allows the charge of the leaves of the electroscope to dissipate slowly. The rate at which the charge dissipates and the leaves resume their limp, uncharged arrangement is proportional to the radiation intensity.