778 Sir Oliver Lodge on the Transmission 



total molecules per c.c. ; and by Professor Townsend's mea- 

 surements the value of N<? for air at ordinary pressure and 

 temperature is 1'22 x 10 10 E.S. units per c.c., or '41 E.M. 

 units. So, since e = 4'65x 10" l0 E.S. units, N = 2*6xl0 19 . 



The speed of ions in air at ordinary pressure, when urged 

 by a force X, has been measured, and is given by 



Mi = y ■ = 420 + 510 = 930 centim. per sec. per unit E.S.F.,.. 

 = 3*1 centim. per second for a gradient of 1 volt per centim. 



It is found that over a considerable range of pressure 

 u x varies inversely with the concentration, or N?^ is constant.. 

 So, from the above data, ^eu 1 = 4*1 X 3*1 = 13 amperes per 

 square centimetre when driven by a volt per centimetre. The* 

 intensity of current i = kNeuX., so the specific conductivity 



i 1 



X~ 



and 



= kNe 



w. 



n = kN = — :: = - x io 27 



1 1 „ amperes 1 volt 



kcr sq. centimetre centimetre 



Hence a = —77 ohm centimetre, or — -c.o'.s. 



16k ' 16k & 



and the number of ions per c.c. is 



2-6 X l0 19 xl0 9 _ 2 

 13cr a 



when a is expressed in absolute c.g.s. units. 



At really low pressures, comparable with a millimetre of 

 mercury or less, the mobility of ions is much greater than 

 what is here dealt with. Besides, all this refers to atomic 

 or molecular ions, i.e. to electrolytic conduction, and does 

 not take into account the far greater mobility of free electrons,, 

 even though they be only momentarily or transitionally or 

 potentially free. It seems possible that some writers, in 

 considering the opacity of air, may have been satisfied with 

 the known kinds of ion for which data were available ; and 

 that the presence of free electrons in the upper regions would 

 give a much higher conductivity than has been suspected. 

 It can hardly be doubtful that electrons are directly supplied 

 to the upper air, in the form of ft rays from the sun. 



Ionization Causes, 



With reference to the likely amount of ionization in the 

 upper atmosphere, I doubt if sufficient attention has been 

 paid to the positive charges presumably received from the 

 sun. If the sun is radioactive it probably emits a. as well as 



