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V. Atmospheric Electricity Potential Gradient at Kew Observatory, 



1898 to 1912. 



By C. CHREE, Sc.D., LL.D., F.R.S., Superintendent of Kew Observatory. 

 Received December 1, 1914, Read January 21, 1915. 



CONTENTS. 







1. Historical 133 



2, 3. Absolute observations and apparatus ' 134 



4. Standardization of curves 136 



5. Variation of potential gradient with height 137 



6. Correction to old -values of potential gradient 140 



7. Mean monthly and yearly values of potential gradient 141 



8, 9. Diurnal inequality of potential gradient 142 



10. Variation of earth's surface charge 151 



11, 12, 13. Diurnal inequality. Fourier coefficients 152 



14. Annual variation 150 



15. Comparison of Kew and Edinburgh data 157 



1. IN a previous paper,* called E : 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 

 the electrograph from the position it has occupied since 1898 is now in contemplation. 

 ThuX1912 may be regarded as ending an epoch. 



* 'Phil. Trans.,' A, vol. 206, p. 299. 



(527.) [Printed March 4, 1915. 



