I. THE SIGNIFICANCE OF ATMOSPHE RIC - E LE CTRIC OBSERVATIONS AT SEA 



Why investigate atmospheric electricity at sea? 

 What can be accomplished there that could not be ac- 

 complished more easily ashore? These are questions 

 which no doubt have arisen in the minds of all those who 

 have been concerned in one way or another with such in- 

 vestigations. Certainly when facing the difficulties al- 

 ways attending such measurements but increased at sea 

 by the high average humidity, the salt-laden air, and the 

 rolling and pitching of the vessel, observers must have 

 emphatically propounded questions of equivalent import. 



The answers which may be given today to these ques- 

 tions contain new aspects which could not have been seen 

 fifteen years ago, when the first intensive program of at- 

 mospheric-electric observations at sea was inaugurated 

 on the Carnegie . These years have seen important ad- 

 vances in the science of atmospheric electricity, ad- 

 vances which, to a marked degree, were derived from 

 the data obtained at sea, and which present the phenom- 

 ena of atmospheric electricity in a new and improved 

 perspective. The answers to our questions and the full 

 significance of atmospheric-electric observations at sea 

 will be seen best if viewed in this perspective. It may 

 assist in this respect to review briefly some aspects of 

 the science. It will be recalled that in so far as con- 

 cerns atmospheric-electric phenomena, the earth may 

 be considered a large electrically conducting sphere, 

 charged with negative electricity and surroxmded by an 

 atmosphere which to a small degree conducts electricity 

 because of the presence there of electric carriers, or 

 ions--some positive and some negative. The positive 

 ions are attracted to the earth and tend to neutralize its 

 negative charge, while those of negative sign are re- 

 pelled. Thus neither the charge of the earth nor the 

 conductivity of the atmosphere could continue long with- 

 out some source of replenishment io'r each of them. 

 Calculations show that the charge would fall to one -tenth 

 its initial value in twelve minutes if the conductivity as 

 observed remained undiminished. 



The sources of replenishment of the ions in the air 

 are such ionizing agents as the radioactive matter of the 

 earth and of the atmosphere, together with the penetrat- 

 ing radiation which includes cosmic radiation. The 

 charge of the earth is maintained by some elusive, un- 

 known factor, which we may call the supply current. To 

 ascertain the origin and character of this supply current 

 constitutes one of the most important objectives in pres- 

 ent day atmospheric-electric investigations. 



Although the earth's charge or its measure--the po- 

 tential-gradient at the surface--and the conductivity of 

 the air, are both on the average constant, yet interesting 

 variations occur in both these features. The conductivi- 

 ty which depends both on the number of small ions and 

 on their mobility will be affected by variations in these 

 elements. The variations in the mobility of these ions, 

 however, is of only minor importance. It is the number 

 of ions (small mobile ions) which is the controlling fac- 

 tor in conductivity. This number is dependent both on 

 the rate at which ions are formed and on their rate of 

 decay or diminution. As already mentioned, the radia- 

 tions from the radioactive matter in the earth and the 

 atmosphere, together with the penetrating radiation, are 

 the ion-producing agencies. Over land the radioactive 

 matter of the earth and air is the predominating agency. 



while at sea the only agency known to be active is the 

 penetrating radiation. Ionization produced by the latter 

 cause is nearly constant everywhere, whereas that pro- 

 duced by radioactive matter varies considerably with 

 both time and place. The radioactive matter in the air 

 produces more than half the ions which arise from radi- 

 oactive sources over land. It comes originally from the 

 soil, and hence varies from place to place, depending on 

 the content and porosity of the soil. It varies with time 

 imder the influence of changes in soil temperature, vari- 

 ations in barometric pressure, and depends on the di- 

 rection, velocity, and turbulence of the wind. 



Although ions probably are produced at the average 

 rate of about ten pairs per cubic centimeter per second 

 over land, and at the rate of only one and one -half to two 

 per cubic centimeter per second over the oceans, yet 

 the average number of ion-pairs in a cubic centimeter 

 of air near the earth's surface is roughly 500, not only 

 over land but also at sea. The population of small ions 

 in the air is determined by their birth rate and their 

 death rate, or the rate of formation and rate of diminu- 

 tion. It is apparent then that the rate of diminution over 

 land is considerably greater than over sea. 



The rate of diminution depends in part on the recom- 

 bination which takes place between positive and negative 

 small ions, and in part on the union of these small ions 

 both with large, relatively immobile, ions and with cer- 

 tain electrically neutral particles or molecular com- 

 plexes. These large ions and molecular complexes also 

 serve as nuclei; or starters, in the condensation of 

 water vapor. On this account they are called condensa- 

 tion nuclei. Everywhere in the lower kilometer of the 

 atmosphere these nuclei occur in sufficient abundance to 

 constitute the element which chiefly determines the rate 

 of diminution of small ions. Their number and corre- 

 sponding effect in decreasing the average life of small 

 ions are much greater over land than at sea and are sub- 

 ject to large fluctuations which, especially in the vicinity 

 of cities, often arise from man-made conditions. 



The conductivity of the atmosphere thus affected has 

 in turn an influence on the earth's charge and on its at- 

 tendant electric force, or potential-gradient. Thus, for 

 example, the low and fluctuating conductivity in the vi- 

 cinity of cities gives rise to high and fluctuating values 

 of the potential-gradient. The earth's charge, however, 

 as measured by the potential-gradient varies from other 

 causes. Thus, the electric charges generally connected 

 with dust, smoke, fog, etc., and especially those intense 

 charges which are developed in thunderstorms, all affect 

 the potential-gradient directly. Effects arising from 

 these causes will obviously vary in an irregular manner 

 from place to place and from time to time, especially 

 over land. 



There is also another source of variation in the po- 

 tential-gradient, to which attention is especially invited. 

 From a study of the potential-gradient data obtained on 

 cruises IV, V, and VI of the Carnegie , Mauchly was able 

 to conclude that the regular change in this element dur- 

 ing the day over the ocean proceeds on a universal sched- 

 ule. Thus, for example, high values tend to occur every- 

 where at about 18 to 20 hours (6 to 8 p.m.) in Greenwich 

 meridian time. Also, later he found evidence which sup- 

 ports the view that the same phenomenon occurs over 



