392 



DK. G. C. SIMPSON ON THE ELECTEICITY OF 



tendency was for negative electricity to be brought down by the rain more generally 

 in the second than in the first half of the storms. 



Other characteristics of negatively charged rain were noted in the course of the 

 work, namely, that such rain nearly always fell from a lightly clouded sky, and fell at a 

 very uniform rate, without the rapid changes in the rate of falling which accompanied 

 the positively charged rain. 



The chief characteristics of positively charged rain were that it was always 

 associated with the heavy rainfall which accompanied the centre of a thunderstorm, 

 and with nearly every case in which the rainfall suddenly increased in violence. Both 

 light and heavy rain were more often charged positively than negatively, and on the 

 average light rain was more highly charged than heavy rain. 



The relationship between potential gradient and rain electricity will now be 

 considered. The potential gradient record did not commence until July 18, and 

 owing to spiders spinning their webs from the collector to the surrounding objects 

 a good many days' records were lost. In spite of these facts many data were 

 collected, and can be used to show the relationship between the signs of the electricity 

 of the rain and the potential gradient. Defining the sign of the fine weather potential 

 gradient as positive, it may be taken as a general rule that negative potential gradient 

 only occurs during periods of disturbed weather. It is a well-known fact that during 

 thunderstorms the potential gradient undergoes violent and rapid changes, and these 

 changes must in some way be associated with the same electrical effect that causes the 

 thunder and lightning. Thus one might expect that a close relationship would exist 

 between the sign of the potential gradient and the sign of the rain electricity, but this 

 consideration was not borne out by the measurements. 



From the records of the three instruments, potential gradient electrometer, rain 

 electricity electrometer, and rain-gauge, it has been possible to pick out 1950 two 

 minutes' intervals during which rain fell and the potential gradient was measured. 

 From these data the following table lias been constructed : 



TABLE VII. 



From this it will be seen : 



(1) That during rain, whether charged or uncharged, the potential gradient was 

 more often negative than positive. 



