380 Mr Wilson, On the Measurement of the Earth-Air 



proportional to the pressure ; thus the number of ions per c.c. is 

 the same at all heights. The conductivity which is proportional 

 to the mobility and to the number of ions will thus be inversely 

 proportional to the pressure. At 18 kilometres it will be 10 times 

 and at 36 kilometres 100 times the conductivity near the ground. 

 The conductivity near the ground is moreover much diminished 

 by the presence of dust particles to which the ions attach them- 

 selves, the average life of the free ion being very much less than 

 would be deduced from the ordinary recombination constant. 

 The conduction of the higher layers may thus be very much 

 greater than the above estimate. At greater heights than those 

 considered the conductivity might be expected to increase at first 

 at an even more rapid rate than would be given by the above law, 

 as the size of the ions is less at these low pressures. It is of course 

 a somewhat arbitrary assumption which has been made, i.e. that 

 the atmosphere is exposed at all levels to ionising radiation of the 

 same intensity ; but it is likely that the uppermost layers are ex- 

 posed to the action of various ionising radiations at the least as 

 intense as those to which the lower layers are exposed. (As 

 pointed out by Nordmann (0. R. 136, p. 1430, 1903), the diminution 

 of the recombination constant with increasing height is of great 

 importance in another way, for at great heights the ionisation 

 would take some hours to die away after the ionising radiation, e.g. 

 sunlight, had been cut off.) There may thus be above the lower 

 layers a great thickness of air having a conductivity enormously 

 greater than that of the lower atmosphere. The total variations 

 of potential throughout this upper atmosphere even over great 

 distances in a lateral direction may be comparatively small. Since 

 the potential gradient in fine weather regions is of the order of 

 100 volts per metre near the ground and dies away in the course 

 of a few thousand metres above the ground, the potential of the 

 upper atmosphere in such regions is of the order of 100,000 volts ; 

 a very small lateral potential gradient in the conducting layers 

 may be sufficient to maintain the necessary current. According 

 to the condensation theory showers of the appropriate kind act as 

 the batteries or dynamos by which the atmospheric electrical 

 current is maintained. We may suppose that by the falling 

 to the ground of a preponderance of negatively electrified rain 

 there is maintained in the ascending air above the shower a very 

 high positive potential. We know, from the strength of the field 

 developed in the immediate neighbourhood of the shower and from 

 the production of lightning in extreme cases, that the differences 

 of potential developed may be very high, probably many millions 

 of volts between a point above the shower and the ground. If we 

 suppose that owing to the conductivity of the upper layers the 

 potential differences within these layers remain small, we shall also 



