Potential gradient is measured using a JCI Chilworth 131 adverse conditions field meter. It is mounted on a 1.5 m metal frame on the roof of the medical teaching labs.
The field mill is positioned on a roof on top of a building that is close to another taller building and does not have a flat surface. Field values at the field mill surface will be distorted by a currently unknown value as relative fields are sufficient for current experimental needs.
Linear response for the instrument was tested within a parallel plate system in the University of Bristol School of Chemistry. NO comparison to calibrated measurements has been undertaken yet, but a comparison to a passive wire antenna nearby in Bristol traceable to a calibrated electrometer is planned for summer 2016.
Measurement frequency is 10Hz.
This is used in the Glocaem potential gradient network