40 president's address SECTION A. 



present knowledg-e of the existence of the intermediate ions, it can 

 readily be shown that the inner electrode of the instrument is alto- 

 getlier too long. The apjjaratus, as ordinarily employed, catches not 

 only the small ions, liut a pi'oportion of the others as well. Calculat- 

 ing with my own measures of the mobilities and specific numbers, it 

 appears that the determination of the specific number of the small 

 ions from the indications of the Ebert insti'ument must be from two 

 to four times too great. As for the remaining part of the dis- 

 crepancy, liaving used Dolezalek electrometers in my own observa- 

 tions, I may, perhaps, be prejudiced in thinking that the metal leaf 

 electroscope of the Ebert apparatus is an unreliable appliance for use 

 in such detenninations ; in any case the matter must be made the 

 subject of a special inquiry, but in the meantime I have the utmost 

 confidence in my own measures. 



With regard to tlie other ions, from the very limited series of 

 observations which I have as yet made of the intermediate ones, in 

 air in its natural state, what I luive previously called relatively strong 

 ionisation is represented by about 1000 per cubic centimetre, while 

 for the relatively weak ionisation the number is about 200. 



For the specific number of the large ions, a series of 117 obser- 

 vations gives 5,500 as the maxinuun, and GOO as the minimum, the 

 mean for the positives being 1,014, and for tlie negatives 2,228. 



The numbers given, with the exception of those for the inter- 

 mediate ion, are the results of measures with air drawm directly into 

 the testing apparatus without the intervention of any pipes; later 

 observations give, on the whole, mxich higher values for the specific 

 number of tlie large ions in air led through a considerable length of 

 piping. 



It is now well known, since Lord Kelvin's memorable work on the 

 sid^ject, that a potential difference exists between the earth's surface 

 and the upper layers of the atmospliere. In tlie electrical field which 

 is thus indicated the ions in the air move more or less steadily in a 

 Aertical direction, the negatives ordinarily travelling upwards, the 

 |iositives downwards to the earth. Such a movement constitutes a 

 vertical electric current in the air, tlie magnitude at any time depend- 

 ing- on tlie air's specific conductivity and the value of the potential 

 gradient at the moment. The specific conductivity is represented by 

 tlie sum of the continued jiroduct of the specific number, the mobility, 

 and the charge for each class of ion. An instrument designed by Dr. 

 Gerdien, in Avhich an electroscope is used as in the Ebert apparatus, 

 lias been universally employed for such determinations as have been 

 made of this important quantity. It measures the sum of the con- 

 ductivities due to each type of ionisation, and the calculation of the 

 result from ol)servations with the apparatus is not affected by the. 

 discovery of a new class of ions. The conqilexity of tlie natural 

 ionisation, however, prevents the instrument beinir used to accurately 

 determine the specific number of the small ions. The average value of 

 the specific conductivity of the air in other parts of the world, as given 

 by the Gerdien apparatus, is about T0~'^ in electrostatic units*. The 

 magnitude of this quantity can be calculated from the measures of 

 tlie mobilities and specific numbers of the ions, and the averag^e 



Gerdien. Gessell. Wiss. Gottingen, Nachr., Math-Plivs. 



Klas.se, 1, p. 77, 1007. Dike. Terr. Mag-ii. and Atincs. Elect., Sept., 1908. 



