Fluctuations in atmospheric potential gradient caused by the transport of corona ions created by high voltage power lines can be measured at ground level using electric field mills. Measurements of potential gradient at a fixed distance from a 400 and a 275 kV power line were made over one year in all weather conditions. Downwind of the power lines, there is a lower value of potential gradient, with less variation, overnight compared to daytime, implying the production of negative ions in calmer conditions. An average 24 hour cycle showed a typical Carnegie curve when wind directions placed the monitoring site upwind of both power lines. When downwind of the power lines, the Carnegie curve was masked, with the average mean PG inversely correlating with humidity and correlating with wind speed and temperature.