This instrument almost certainly was on the Grinnell College campus before the tornado of 1882 which destroyed most of the college's scientific equipment. Its manufacturer is unknown. The Bohnenberger electroscope has a dry pile which produces a constant potential difference of the order of 1000 volts. Charge is placed on the electrode extending up from the top, whence it flows to a single sheet of gold leaf hanging between two plates attached to the dry pile. The charge on the gold leaf, hanging in the electrical field produced by the plates attached to a battery, caused deflection of the gold leaf. Because the polarity of the pile was known, the sign of the charge on the electrode and some indication of its magnitude could be seen.