From Glossary of Meteorology
A balloon-borne instrument for recording the value of the atmospheric electric field strength within active thunderstorms.
A disk of specially treated paper (pole-finding paper), slowly rotated by an aneroid element as the balloon ascends, lies between two iron electrodes, the upper one being electrically connected to a discharge point attached to the balloon and the lower one being electrically connected to a long trailing wire. Currents passing through the paper under the influence of the potential difference set up in the two electrodes discolor the treated paper in a pattern with a size roughly related to the ambient electric field strength. A thermal element and a humidity element that record in aerograph fashion complete the device.