523 



It will be seen that in calm weather the potential with snow or with ice crystals in the air did not 

 sensibly exceed the mean for normal conditions, which amounted to 92 volts, while the potential with 

 ice particles in the air increases continuously with the strength of the wind. 



The connection between abnormally high potentials, when there were ice particles in the air, and wind 

 is equally well shown in the next table, where, for the same limits of wind strength as in the previous 

 table, is given in each case the number of occasions when the potential lay between given limits ; as 

 before, only those observations being included for which there is a record of drift, snow or ice particles 

 in the air. 



Wind alone without ice particles in the air does not give abnormally high potentials. 



The mean value of the potential on the occasions (of which there are 46) on which the wind strength 

 exceeded 3 without any record of ice particles in any form in the air amounts to 93 ; or if we exclude 

 three occasions when the potential was negative, to 109. These values do not differ to any appreciable 

 extent from the mean for normal conditions. The absence of any connection between abnormally high 

 potentials and wind unaccompanied by drift is equally well shown in the following table, in which the 

 above-mentioned 42 cases of wind without drift are classified according to the potentials observed. 



High positive readings of the electrometer are thus undoubtedly associated with drifting snow ; snow 

 unaccompanied by high wind, and high winds unaccompanied by drifting snow, do not give abnormally high 

 values. With snow in the air the potential is higher the greater the wind velocity. Cases of high 

 potential without drift or snow are comparatively rare ; of the 19 occasions on which the potential 

 exceeded 400 volts there was drift or snow on all except two. 



It is not impossible that these high positive values of the potential associated with driving snow are in 

 a sense spurious, that the potential indicated by the electrometer may not under such conditions be 

 related in any immediate way to the undisturbed air potential at the level of the collector. With the air 

 full of dry ice particles blown along by the wind there must be a certain amount of electrification by 

 friction of the ice against the instrument and the observer's clothing. The high potential indicated by 

 the electrometer might be due to electrification by friction of the ice against the collector, supporting 

 wire or electrometer terminal, or again the potential in the neighbourhood of the observer might be 

 raised by the acquisition of a positive charge by his clothing. It is improbable, however, that either of 

 these processes is the cause of the high potentials observed, for the sign of the effect is the opposite 

 of what such explanations would lead us to anticipate. According to SoilNCKE ('Wied. Ann.,' vol. 28, 

 p. 550, 1886), ice appears to acquire a positive charge by friction against other substances. A negative 

 charging up of the observer's clothing or of the electrometer terminal and its connections, and a consequent 

 lowering of the potential readings, might therefore rather have been expected. The observer faced the 

 wind in reading the instrument; ice particles which had become electrified by striking against his clothing 

 would not therefore be blown in the direction of the instrument, and any effect of a charge acquired by 

 the ice particles in this way would be slight. 



3x2 



