Modified Exner Electroscope (ca. 1910-1920)

Modified Exner electroscope

The above is a modified version of the Exner electroscope that was produced by Günther & Tegetmeyer in Braunschweig, Germany. Like the Exner electroscope, it employs two aluminum (or gold) leaves attached to the top of a fixed vertical plate in the center of the chamber. The arms projecting out on either side are the controls for the cover plates that can be used to protect the electroscope leaves. If the electroscope is to be moved, the protective plates are moved up against the leaves so that the latter are stabilized. The tube pointing down (4 o'clock position) in the photograph above right would normally be connected to some sort of device containing a desiccant - the latter would be used to dry the air inside the electroscope chamber.

Modified Exner electroscope diagram

The modification, described by Elster and Geitel in 1902, was intended to solve the problem of parallax. Since the two leaves are a couple of centimeters away from the scale on the glass, their observed position on the scale would depend on the distance from which the leaves are observed. To avoid this potential problem, Elster and Geitel mirrored the bottom portion of the glass on the front of the electroscope and mounted a scale (S in the figure to the right) in front of the mirror so that the distance to the scale from the mirror was the same as the distance from the mirror to the leaves. They also attached small telescope (O in the figure) in front of the electroscope so that it would view the mirror (and hence the reflection of the scale) and the leaves.        

Modified Exner electroscope

The second example, shown to the left, has seen better days. Although it is similar to the electroscope featured at the top of the page, there are no markings on it that identify the manufacturer. A personal communication from Rudolf Fricke indicates that it too was produced Günther & Tegetmeyer.

This type of electroscope was also manufactured by Spindler & Hoyer.

I would like to express my thanks to Jean-François Loude for his assistance in identifying this electroscope and in providing the above references, and to Rudolf Fricke for his identification of the manufacturer.


  • Exner, F. Uber transportable Apparate zur Beobachtung der atmospharischen Elekricitat. Wien, Akad. Ber. Bd. 95: 1084-1100; 1887.
  • Elster, J., Geitel, H. Uber eine verbesserung der Ablesung am Exnerschen Elektroskop. Phys. Zeit. 4:137-138; 1902.