Potential gradient is measured using a JCI Chilworth 131 adverse conditions field meter. It is mounted on a 3m metal pole on the atmospheric observatory, which has a number of metal structures (including 10m tower, fence) within 30m of the senor. Calibration of the field meter alongside a longwire antenna was performed during 2006 to establish the site distortion factor. This correction factor is applied to the raw data to derive the potential gradient at 1m.
Calibration publication: A.J. Bennett and R.G. Harrison, In situ calibration of atmospheric air conductivity measurements Rev Sci Instrum 77, 016103 (2006)

This is used in the Glocaem potential gradient network