Quadrant-electrometer, after Abraham Brook

In the first century after the discovery of static electricity it appeared to be very difficult to determine the magnitude of an electric charge or an electric potential. The instruments, called electroscopes, developed for this purpose, gave only a rough indication. In most cases, the only thing that could be determined was the existence of an electric charge, provided that the charge was large enough to be observed. One of the first to make an instrument with which the magnitude of an electric charge or voltage could be measured, was Abraham Brooke [See also 548]. The instrument maker John Cuthbertson, working in Amsterdam, made this electrometer based on Brooke’s instrument. The instrument is rather large, because the first director of Teylers Museum, Martinus van Marum, wanted to use the electrometer for his electrostatic generator. On the dial, mounted on the moving arm, he could continuously read the voltage changes, caused by the increasing or decreasing charge.