POTENTIAL GRADIENT AND DRIFT. 311 



containing it, there tends always to be an accumulation of positive electricity in the air of 

 the upper part of the cloud. The field set up in this way is again parallel to the earth's 

 normal field and therefore increases it, thus there is no reversal of the field when the drift 

 snow extends from the ground to heights well above the collector. In other words the 

 potential gradient -will be positive and high whenever there is drift quite independently of 

 whether the drift is suiface drift or true drift throughout the lower atmosphere. 



We have now explained the vast majority of the observations and it is only necessary 

 to prove the rule by explaining also the exceptions. The most important case is the high 

 negative potential observed during heavy surface drift on December 8th. It has been found 

 that when dust is raised in the air the resulting charge is affected by the character of the 

 dust. Fine dust has been found to be charged with the opposite kind of electricity from 

 that associated with coarser dust of the same material. Thus the loose snow in the drift of 

 December 8th might have the opposite charge from the hard grains of ice of which surface 

 drift in th^. vast majority of cases consisted. This assumption seems, however, imnecessary. 

 On December 8th the surface drift was unusually heavy on account of the large amount of 

 loose snow lying on the ground after the recent heavy fall of large flake snow. In such a 

 drift the separation of electricity would be excessive, therefore the air immediately above 

 the drift would be very highly charged with positive electricity. If at the same time there 

 was little tendency to the formation of ascending air currents, the concentration of positive 

 electricity would remain in the air near the upper hmit of the drift. It is not difficult to 

 conceive that under such conditions there would be more positive electricity below the collec- 

 tor, which was 10 feet above the ground, than above it. This would fully account for the 

 reversed potential gradient. It also explains why later on the potential became positive, but 

 very unsteady, with occasional returns to the inverted direction ; for then the positively charged 

 air extended higher into the atmosphere and the collector had generally more positive 

 electricity above it than below it, but occasionally the original conditions were re-established 

 for short p?riods. 



That the potential should be occasionally reversed during periods of snowfall, as in cases 

 (4), (8) and (9) above, is not difficult to explain. Snowfall is a sign of ascending currents, 

 for they are necessary to supply the water vapour necessary to the production of snow 

 The air of these ascending currents must constantly pass out of the top of the cloud and take 

 with it positive electricity. If this air then enters an air cirrrent from a different direction 

 from the lower wind, it will be removed from the region of the cloud. The whole cloud will 

 then have a residual charge of negative electricity associated with the snow. This being above 

 the collector will reverse the field and a negative potential gradient will be recorded. 



The five cases of negative potential gradient without appreciable drift cannot be ex-plained 

 by the above consideration which are based on the separation of electricity associated with 

 drift, and as no explanation of the reversed gradient is obvious it will serve no useful purpose 

 to make assumptions, which cannot be tested, to provide an explanation. 



The above discussion leads to the conclusions — 



(a) The electricity which affects the recorded potential gradient dming drift is not 



associated with the driven snow ; 

 (6) but with, the air in and above the drift ; 



(c) the separation of electricity takes place when ice crystals collide, the ice becoming 

 negatively charged and the air positively charged. 

 These conclusions are based entirely on the observed potential gradient, it is ob\-ious 

 that the only satisfactory test would be to examine the sign of the charge on the drift 



