Electricity of Atmospheric Precipitation. 5 



found that there was perfect insulation between two con- 

 ductors only half a sodium wave-length apart. 



A theory built on such questionable foundations would 

 not be considered strong even if it appeared capable of 

 explaining the phenomena, but it will now be shown that, 

 granting these premises of Elster and Geitel, the theory 

 tails to explain the chief electrical phenomena during pre- 

 cipitation. 



The influence theory as revised in 1913 is as follows : — 



Consider a drop of water falling through the atmosphere. 

 It is then in an electrical field and in consequence becomes 

 electrically polarized. If we assume the earth's normal 

 field, then the drop will have a positive charge on its under 

 half and a negative charge on its upper half. As this drop 

 falls through a cloud which is raining it will constantly 

 strike, on its under side, cloud particles and smaller drops. 

 These small drops make electrical contact, take on to them- 

 selves part of the induced positive charge from the large 

 drop, and then rebound. In consequence the large drop 

 becomes negatively charged and the cloud particles posi- 

 tively charged. 



Now the most important point to notice in this theory is 

 that with any given field the rain brings down the same 

 sign of charge us that below it which is causing the field. 

 Thus in the earth's normal field the charge on the earth is 

 negative, and the rain would bring down a negative charge 

 leaving the corresponding positive charge above. This would 

 intensify the field. In fact, according to the theory, the 

 rain would always intensify a field ; it could under no 

 circumstances weaken, much less reverse, it. 



Let us follow the process in some detail. Imagine that 

 the clouds on a given day gradually increase in density. 

 We know from observation that this does not disturb the 

 earth's normal field. Now let rain commence. The drops 

 have a positive charge induced on their lower halves, which 

 is lost by impact with cloud particles and smaller drops ; 

 hence the rain will arrive at the ground with a negative 

 charge. This intensifies the field and causes subsequent 

 rain to be more highly charged. And so the process goes 

 on with an ever-increasing field. But observation shows 

 that this practically never occurs. With non-thunderstorm 

 rain it almost always happens that as soon as the rain starts 

 the field is reversed and the potential gradient becomes 

 negative. As the rain cannot reverse the field some other 

 factor must have come into play. What is it ? We must 

 pause to consider how the field can be reversed. The field 



