Exner Electroscope (early 1900s)
This is an Exner electroscope (ca. 8" high) produced by Max Kohl, a German manufacturer of scientific instruments. As a guess, I would say that it dates from 1900 to 1920.
It employs two aluminum leaves, one on each side of a vertical copper plate suspended from the top of the electroscope chamber. The glass on the back of the electroscope has a scale that reads 0 in the middle and goes up to 20 on each side.
Two moveable horizontal rods (K in the following figures), one on each side of the electroscope, control protective cover plates (F in the following figures). When the electroscope was not in use, these plates would be placed up against the leaves (figure below right). This would allow the electroscope to be moved around without risking damage to the leaves.
This type of electroscope was first described in 1887 by Franz Exner who at one time served as the President of the Austrian Academy of Science. Exner developed it for his investigations of atmospheric electricity, but it was also used for radiation measurements.
I would like to express my thanks to Jean-François Loude for his assistance in identifying this electroscope and in providing the above reference.
Exner, F. Uber transportable Apparate zur Beobachtung der atmospharischen Elekricitat. Wien, Akad. Ber. Bd. 95: 1084-1100; 1887.