394 DR. G. C. SIMPSON ON THE ELECTRICITY OP 



water has been taken as the limit of the accuracy of the electrical measurements in 

 this work, hence it may be concluded that the results are not materially affected by 

 the Lenard effect. 



The more important results of this work may now be summed up as follows : 



(1) During 71 per cent, of the time that charged rain fell the charge was positive. 



(2) 75 per cent, of the electricity brought down by the rain was positive. 



(3) Light rain was more highly charged than heavy rain. 



(4) All rainfall which occurred at a greater rate than a millimetre in two minutes 

 was positively charged. 



(5) The proportion of negative electricity brought down by the rain was slightly 

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



(6) The potential gradient was more often negative than positive during rain. 



(7) No relationship between the sign of the potential gradient and the sign of the 

 electricity of the rain could be detected. 



PART II. Laboratory Experiments. 



While the measurements of the rain electricity described in the previous part of 

 this paper were being made, a second investigation was undertaken in the laboratory 

 of the Simla Meteorological Office, with the object of finding the physical process by 

 which electrical separation takes place during thunderstorms. The scheme of the 

 research was to imitate as far as possible in the laboratory each process which takes 

 place in a thunderstorm and to note any electrical effects. 



A large number of experiments was made with vortex rings composed of air in 

 different physical states to see if any electrical separation accompanies the friction 

 and mixing of masses of air having different temperatures and humidities ; the 

 freezing and thawing of water were examined and a number of other experiments 

 made, but all with negative results. A series of experiments was then undertaken 

 which was based on the following considerations. 



LENARD* has shown that if air ascends with a greater velocity than 8 metres a 

 second no water can fall through the current, for if the drops are below a certain size 

 they are carried upwards with the air, while if they are above that size they are 

 unstable and quickly break up into smaller drops, which are then carried upwards. 

 Now, as will be seen later, it is exceedingly probable that during thunderstorms 

 ascending currents much greater than 8 metres a second come into play, and these 

 must therefore hold a considerable quantity of water in suspension. This water will 

 be constantly going through the process of growing from small drops to large drops, 

 only to be broken up into small drops again. If, therefore, the breaking of large 

 drops into small drops is accompanied by a separation of electricity, thunderstorm 

 electricity might owe its origin to such an effect. 



* LENARD, 'Met. Zeit.,' vol. 39, p. 249, 1904. 



