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Department of Meteorology – University of Reading

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  • Copyright image © Stephen Burt

    Atmospheric Observatory

    Atmospheric electricity measurements

Atmospheric electricity

Atmospheric electricity

The main atmospheric electricity (potential gradient, PG) instrument on our site is a JCI 131 Electrostatic field meter or Field Mill manufactured by John Chubb Instrumentation. This device is based on a ‘field mill’ principle utilising a rotating chopper. The DC output signal from this instrument is proportional to the PG measured at the sensing aperture. Within the Observatory, the unit is currently mounted at 3 m above ground level. The measurements are standardised to the PG at 1 m, as obtained using a long wire antenna.


Chubb field mill March 2016

The Chubb JCI 131 electrostatic field meter (on the left), mounted at 3 m above ground within the Atmospheric Observatory, photographed March 2016. Photograph Copyright © Stephen Burt.

The vertical conduction current is also measured, using two horizontal electrodes mounted to be flush with the Observatory surface. The current flowing into each electrode is determined using an electrometer. Two different collecting electrodes are used, a ‘flat plate’ and a ‘corrugated plate’ (see bottom photograph). The electrodes have the same surface area but different topologies: the differences in the measured currents allow the conduction current to be separated from displacement current induced by changes in the PG.

Atmospheric electricity sensors


Field mill records commenced November 2004.

How to request climatological data from the Observatory

Sensor manufacturer link:

Chubb JCI 131 Electrostatic fieldmeter

Operational notes or changes:

26 Nov 2004
Field Mill Installed at 2 m, north side of site. Recorded on channel B3. Source: Research channel info file
April 2005
Wiring errors detected. Grounding and auto-ranging wrongly configured. New junction box made. Source: RGH notebook and research channel info file
1 May 2005
Concluded to be running correctly. Calibration to use:

PG = 387.0×+12. Source: AJB thesis

17 Jan 2007
Field Mill moved to 3m, south side of site. New calibration experiment performed (6 Jan 2007) giving PG = 248.5×-1. Source: AJB thesis

This page updated January 2018

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