The relation between radio-active collector current potential gradient and wind speed

Several workers have suggested that the current flowing to earth through an earthed radio-active collector could, if wind speed was considered, be used to measure the earth's vertical potential gradient. Without measurement of wind speed such a collector has been used to give an approximate estimate of the potential gradient. The present work sets out to show whether, in fact, an accurate estimate of potential gradient can be got from the simultaneous recording of the current flowing to earth from an earthed radio-active collector and wind speed. Records were taken at the Durham University Observatory of the two variables mentioned and were compared with records of potential gradient as measured with a field mill. The records showed agreement, with the results of other workers in-that there was: a highly significant correlation between the collector current and the potential gradient; and in almost all cases; a significant correlation between collector current and wind speed. However, the results showed that no reliable estimate could be got of potential gradient from the other two records. It was found that the errors in such an estimate were about 6% over short periods (up to three hours) and appreciably greater over longer periods. It is suggested that the apparently random short term errors are due to rapid fluctuations in wind speed which are not accurately recorded on the anemometer because of the smoothing which was necessary. The long term errors are thought to be due to variations in the effective activity of the collector because of contamination on the surface of the radio-active laminae restricting the emission of radiation.