Modified Exner Electroscope (ca. 1910 - 1920)

This is a modified (and obviously incomplete) version of the Exner electroscope. Like the latter, 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 hollow tube pointing down towards the lower right in the above photograph would normally be connected to some sort of device containing a desiccant - this would dry the air inside the electroscope chamber.

Its purpose was to measure atmospheric "electricity" and radioactivity (e.g., radon and its decay products).

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.

There is nothing on this instrument that identifies the manufacturer, however these types of electroscopes were known to have been manufactured by both Günther & Tegetmeyer and Spindler & Hoyer. A personal communication from Rudolf Fricke indicates that it was produced by the former, i.e., Günther & Tegetmeyer. The following description is of Gunther & Tegetmeyer's modified Exner electroscope.



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.

Prussia Ministerium fur Vissenschaft, Kunst un Volksbildung. Scientific Instruments. 1904.


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.


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Last updated: 07/25/07
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