POTENTIAL GRADIENT AND DRIFT. 309 



19 miles an hour, from the N.N.W. At this time the Sound was nearly free 

 from ice. 



Of these nine records of negative potential gradient four (1), (2), (5) and (7) occurred 

 with a high wind but no drift ; three (4), (8) and (9) occurred during snowfall ; one (6) 

 occurred with a clear sky but very heavy low surface drift ; and one (3) occuried during 

 the passage of heavy low cloud but without wind, drift or snow. 



The explanation of the high potentials during drift proves to be very difficult. I did 

 not realise this at first and considered that the whole phenomenon was due to the ice 

 particles becoming charged positively by collision amongst themselves and with the snow- 

 covered ground. Rudge has experimented on the well-known electrical effects accompanying 

 the raising of clouds of dust, and shown that when dust is blown into the air or even 

 let fall through the air it becomes highly charged. Treating air full of snow-drift as a 

 dust cloud of great extent, it is reasonable to suppose that the snow would, like the dust, 

 become highly charged. Thus, when the whole lower atmosphere is full of positively charged 

 snow, there would be a high positive potential gradient. It follows as a consequence that 

 if the positively charged snow was all or mainly below the level of the collector of the 

 potential gradient apparatus the registered potential gradient would be reversed. This is easily 

 seen, for the normal positive potential gradient is caused by the negative charge on the earth, 

 and if this charge is replaced temporarily by a la3'er of drifting snow highly charged with 

 positive electricity, the gradient must be reversed. By far the heaviest surface drift I observed 

 in the Antarctic occurred on December 8, 1911, when a high northerly wind carried along 

 the recently fallen loose snow. This drift was not only very heavy, but it was also very low) 

 and all well under the collector ; it seemed, therefore, very strong support to the theory of 

 positively charged snow to find that during this period the potential gradient was negative 

 and high. 



The first difiiculty encountered was, when preparing this chapter, I searched for other 

 cases of surface drift accompanied by negative potential gradient. I then found that instead 

 of being accompanied by negative potential gradient surface di'ift was nearly always accom- 

 panied by very high positive potential gradient. At first it occurred to me that this might 

 be due to the clouded sky which usually accompanied the drift, surface or otherwise, while 

 the surface drift of December 8th occurred under a perfectly clear sky. I therefore searched 

 the records for cases of surface drift with a clear sky and found seven cases beyond that 

 of December 8th. In all these new cases the potential during the surface drift was positive 

 and very high. This disposes at once of the explanation that the drift snow has a liigh 

 positive charge, for if it had, so long as it was mainly below the collector, the potential 

 should be reversed and only when it extended more above than below the collector should 

 the positive potential gradient be increased. Stated in other words it is quite impossible to 

 explain the observed facts by considering only the charge on the driven snow, for charged 

 snow would cause the sign of the potential gradient to be reversed when the distribution 

 of the snow changed from being mainly below the collector to being mainly above ; while 

 the observations show that the potential gradient is nearly always positive and high both 

 when there is only a little surface drift and when the drift is so great and extends so high 

 in the atmosphere that there must be more drift above than below the collector. One is 

 forced, therefore, to seek the electrification elsewhere than on the driven snow. It is incon- 

 ceivable that the surface of the earth can become electrified, for this would necessitate the 

 surface being highly insulating, otherwise the charge would be neutralised at once. There is 

 no doubt that a snow surface is a bad conductor of electricity, for we used for our telephone 

 from Cape Evans to Hut Point, a distance of 15 miles, a bare aluminium wire laid over 



