"294 Diurnal Variation of Atmospheric Electrical Quantities. 



number of ions per c.c. during the eclipse period. For the 

 1912 eclipse at Kew the conductivity and number of ions 

 showed a decided increase during the first phase, but this 

 was followed by a well-marked depression. For Eskdale- 

 muir, no marked change in potential was found, and this 

 agrees with the results from other eclipses. 



It is well to emphasize that while small instrumental 

 differences may have existed between the three Ebert 

 electrometers used, as we were at that time dependent 

 on values supplied for the capacity and anemometer 

 readings by the makers, these uncertainties cannot affect 

 the character of the diurnal variation described above, 

 because for any one element only one instrument is 

 involved. 



There is some difference of opinion as to the best method 

 of treatment of results from the Wilson electroscope. Some 

 experiments of Gr. Dobson * at Kew appear to show that the 

 conductivity as measured usually with the Wilson instrument 

 on a tripod-stand 1*3 metres from the ground is smaller than 

 that measured at ground -level, owing to the crowding to- 

 gether of the equipotential surfaces above the test-plate and 

 the production of a saturation current. The correction for 

 the conductivity to bring it to ground-level values was 

 -+- 20 per cent. The mean for these corrected values is 

 similar to the total conductivity recorded by the Ebert 

 instrument. As the individual results differed so widely, 

 and owing to the insensitiveness of the Ebert apparatus 

 many negative values for mobility and conductivity were 

 obtained w r ith it, the deduction is not convincing that the 

 corrected conductivity obtained from the Wilson apparatus 

 corresponds with the total conductivity measured by the 

 Ebert apparatus. Under existing conditions, it has been 

 thought advisable to continue the old mode of reduction 

 until a more satisfactory method is devised. The results 

 detailed above are, however, quite comparable, as the 

 observational conditions were uniform. 



In conclusion, I desire to acknowledge my indebtedness 

 to several members of the Observatory staff at Kew and 

 Eskdalemuir for assistance in carrying out the experi- 

 mental work involved, especially to Dr. C. Chree, F.R.S., 

 Superintendent of Kew Observatory, under whose super- 

 vision the work has been carried out, and also to Lieutenant 

 C. D. Stewart, who performed a number of the earlier 

 observations. 



* Proc. Phys. Soc. 1914. 



