| About 1814 J. F. G. von Bohnenberger suggested a form
of electroscope in which the single leaf was attracted to one of two conductors
placed symmetrically on either side of it. The conductors were, in turn,
connected to the two poles of a dry pile, thus allowing the sign of the
charge applied to the leaf to be determined.
This example, with the dry pile placed under the electroscope, also includes a spark gap.
In 1975 this instrument was on display at the physics department
of the University of Colorado.
The glass Bohneneberger electroscope at the right below is in the Harvard University Collection.
At the left below is a Bohnenberger electroscope at Depauw
University in Greencastle, Indiana. It was certainly made by the Depauw
professor of physics, Joseph
Naylor, about 1900.
REF: Beryl Clotfelter, "The Bohnedberger Electroscope", Rittenhouse, 10, 65-90 (1996)