In a previous paper, called E
1
for brevity, I discussed the results obtained for the diurnal variation of the potential gradient of atmospheric electricity at Kew Observatory from 1898 to 1904. The present paper deals with the same subject, but employs data from the fifteen years 1898 to 1912. The earlier period of seven years, though longer than that available at most observatories, was too short to give a satisfactorily representative presentation of some of the phenomena. To obtain results fairly characteristic of the locality many years data are required of some of the meteorological elements, especially barometric pressure and rainfall. For the latter element, in fact, a considerably longer period is desirable than that available even now for potential gradient at Kew. The same may be true of potential gradient itself, but various reasons exist for not waiting longer. Owing to building operations, the electrograph results for 1913 were exposed to special uncertainties. Also the transfer of the electrograph from the position it has occupied since 1898 is now in contemplation. Thus 1912 may be regarded as ending an epoch. Another reason requires fuller explanation. The Kew water-dropper—the earliest it is believed in regular operation—was erected in 1861 under Lord Kelvin’s personal supervision. The original electrometer and batteries as they decayed were replaced by others, but the instrument remained essentially unchanged in its original site until 1896. Of the records obtained prior to that date those of only three years had been discussed, two years, 1862 to 1864, by Prof. J. D. Everett, and one year, 1880, by Mr. G. M Whipple. In both cases the results were expressed in what were really arbitrary units. The relation between the voltage shown by the instrument and the true potential gradient in the open was altogether unknown.
V. Atmospheric electricity potential gradient at Kew Observatory, 1898 to 1912