10 Dr. G. C. Simpson on the 



negative charge. Now Rudge * has shown that when two 

 solids are nibbed together, the air near them becomes 

 charged and in consequence the sum of the charges left on 

 the solids is the complement of the charge given to the air. 

 In other words, the collision and friction of solids of the 

 same kind produces a charge on the solid and a charge of 

 the opposite sign in the air. 



When the collisions are sufficient in number the electrical 

 separation can be very large, and by blowing dust into the 

 air a large separation of electricity can be effected. It is not 

 difficult to imagine that a snow storm is a repetition of 

 Rudge's experiments on an enormous scale in which the 

 snow-flakes take the place of the dust particles. The constant 

 collision and rubbing together of the snow-flakes causes them 

 to be charged and the air to receive the opposite charge. As 

 the snow falls it leaves the charged air behind it, so pro- 

 ducing a separation of electricity which accounts for the 

 observed electrical effect. 



Rudge did not make any experiments with snow or ice, 

 therefore his work does not indicate the sign of electricity 

 we might expect on the snow. The following observation 

 made in the Antarctic, however, confirms his general conclu- 

 sion and gives the sign desired. 



On December 6th, 1911, there was a heavy fall of snow. 

 Two days later, when the sky was cloudless, a wind sprang 

 up from the north causing a very heavy drift of loose snow 

 along the surface. The driven snow did not extend more 

 than four or five feet above the ground. The collector of the 

 potential gradient apparatus was well above the top of the 

 drift. As soon as the wind arose and the drift commenced, 

 the potential gradient was reversed and high negative values 

 were recorded as long as the surface drift continued. This 

 observation showed that the drift snow had a high positive 

 charge. The drift was blowing over the frozen sea and 

 therefore came into contact with nothing but snow ; hence 

 the charge was produced by the friction of snow on snow. 



Returning now to the conditions durino- snowfall, we have 

 shown how the snow can obtain a positive charge. Assuming 

 an ascending air current (this is necessary for snowfall as for 

 rainfall) the charged air will ascend as the snow descends, so 

 that there will be an accumulation of negative electricity 

 towards the top of the cloud. Snow formed here will start 

 with a negative charge, which, under suitable conditions, it 

 might carry down to the ground ; thus accounting for the 

 negatively charged snow. 



The high charges found by Rudge on dust would lead one 



* Rudge, Proc. Roy. Soc. A. xc. pp. 25G-272 (1914). 



