658 Dr. A. S. Eve on the Effect of 



the excess of positive electricity in the air near the earth's 

 surface may be expected to be greater than at a considerable 

 altitude. On the other hand, since equal quantities of 

 positive and negative electricity are produced by ionization, 

 such an excess as 17 per cent, of positive ions is surprisingly 

 large and requires explanation. 



It has been found by Smirnov * that the average density of 

 charge (4 km. to the ground) is 9 X 10 -10 E.S.U. per cm. 3 , 

 and this is equivalent to the charge on about 2 ions. Also 

 at 700 m. he found p to be 3*4 x 10~ 9 , and this is equivalent 

 to the charge on about 8 ions. Hence we see that the nega- 

 tive charge on the earth, and the positive charge in the 

 atmosphere, may be accounted for by a small excess of 

 positive over negative ions, something of the order of one 

 per cent, of the total positive or total negative ions present. 



Inasmuch as negative ions diffuse more rapidly than 

 positive, it may be urged that the large observed excess 

 (17 per cent.) of positive ions may be due to the greater 

 loss by diffusion of negative ions to the sides and top of the 

 testing vessel in the Ebert apparatus ; but the arrangement 

 is such as to reduce any such diffusion loss to a minimum. 



Also it will be remembered that there are in the atmo- 

 sphere large ions, first detected by Langevin f, which pass 

 through the Ebert machine mainly without detection by the 

 testing vessel and electroscope. These have a mobility of 

 about 1/3000 cm./sec. per volt/cm. As to the number of 

 these large ions there seems to be much discrepancy among 

 observers, for Langevin found them about fifty times as 

 numerous as the small ions, G-ockel % 1000 per cm. 3 , 

 Daunderer § 6000-7000, and Pollock 600-5500. 



Valuable work has been done at Sydney by Pollock || , who 

 finds three types of ions present, small, intermediate, and 

 large. His results may be summarized thus : — 



Type. 

 Small 



Number of Ions. 

 Oto 157 

 200 to 1000 

 600 to 5500 



Mobility. 

 About 1-0-1-8 



Intermediate . . . 

 Large 



1/15 to 1/150 

 1/1280 to 1/3370 



The conditions at Sydney appear somewhat singular, but 

 it is possible that observers elsewhere have grouped most of 



* Acad. Sci. Petershourg, Bull. ix. p. 759, 10th May, 1908. 

 + Comptes Rmdus, cxl. p. 232 (1905). 

 X Phi/s. Zeit. x. p. 396 (1909,. 

 § Phys. Zeit x. p. 113 (1909). 

 j| Science, 11th Jane. 1909. 



