Electroscopes were the first instruments used to measure electric charge. These first “voltmeters” simply indicated the presence of charge but didn’t provide a calibrated reading. Later devices, which we now call electrometers, did include a calibrated scale for reading the strength of the charge. The first electroscope was a device called a versorium, developed in 1600 by William Gilbert (1544-1603), Physician to Queen Elizabeth I. The versorium was simply a metal needle allowed to pivot freely on a pedestal. The needle is attracted to charged bodies brought near.
Gilbert's Versorium c. 1600
A single hanging thread, called a “Pendulous thread” by Stephen Gray, (1666-1736) was introduced around 1731. The thread would be attracted to any electrified body nearby. In 1753 John Canton improved the electroscope by adding two small pith balls suspended by fine linen thread. When placed in the presence of a charged body, the two balls would become similarly charged, and since like charges repel, the balls would separate. The degree of separation was a rough indicator of the amount of charge.
In 1770, William Henley developed the first portable quadrant electrometer. The device consisted of an insulated stem with an ivory or brass quadrant scale attached. A light rod or straw extended from the center of the arc, terminating in a pith ball which hung touching the brass base of the electrometer. When the brass was electrified the ball would move away from the base, producing an angle which could be read off of the scale.
Cavallo's Pocket Electroscope c. 1779
In 1779 Tiberius Cavallo (1749-1809) designed an improved electroscope, his “pocket electrometer.” The device included several improvements including, for the first time, placing the strings and corks inside a glass enclosure to reduce the effect of air currents.
The first true electrometer came from Horace Benedict de Saussure (1740-1799), who placed the strings and balls inside an inverted glass jar and added a printed scale so that the distance or angle between the balls could be measured. It was de Saussure who discovered that the distance between the balls was not linearly proportional to the amount of charge. However, the exact “inverse square” relationship between charge and distance would be left for Charles Coulomb to discover in 1784.
Abraham Bennet (1750 - 1799) discovered that strips of gold foil were much more sensitive than cork or pith and created the first gold leaf electroscope in 1786