TERRESTRIAL MAGNETISM 6 1 



expeditions. One of the great outstanding questions pertains to the 

 origin and maintenance of the earth's negative electric charge. While 

 the electric conductivity of the atmosphere is extremely small, it 

 is nevertheless sufficiently large to dissipate the earth's entire surface 

 charge in a quarter of an hour if there were no agency operating to 

 replenish that charge. No experiments made in regions remote from 

 the magnetic poles have as yet disclosed that replenishing agency. 

 Special experiments bearing on this matter in regions fairly close to 

 the magnetic poles are therefore highly desirable. 



The need for further observations regarding the diurnal and annual 

 variations of atmospheric electricity in high latitudes should be 

 brought to the attention of polar explorers. The accumulated ob- 

 servations from various expeditions indicate that both of these varia- 

 tions follow extremely interesting laws. The diurnal variation of the 

 atmospheric potential gradient progresses in both polar regions 

 according to universal rather than local time; the potential gradient 

 reaches its maximum value about the time when the sun is in the 

 meridian of the north magnetic pole and its minimum value when 

 the sun is in the meridian of the south magnetic pole. 



The annual variation of the atmospheric potential gradient seems 

 also to follow the same course during the year in both the Arctic and 

 Antarctic, irrespective of local season. Thus for both polar regions, 

 the potential gradient is on the average greater during the period 

 of the year, October to March, when the earth is nearer to the sun 

 than its mean distance for the year, than during the period, April to 

 September, when the earth is farther from the sun than the mean 

 distance. 



The precise laws followed by the variations in the electric conduc- 

 tivity of the atmosphere and of the so-called air-earth current are not 

 definitely known and are of special interest in polar work. 



Earth Currents 



This is a highly important subject for investigation, particularly 

 in polar regions. The precise relations of these currents with magnetic 

 disturbances, polar lights, and even atmospheric electricity remain 

 to be fully elucidated. 



