384 DE. G. C. SIMPSON ON THE ELECTKICITY OF 



was very often subjected to this trouble, and many records of the potential gradient 

 were lost in consequence. The only means which could be devised to diminish the 

 difficulty was to take every opportunity of visiting the instruments and sweeping 

 away the webs. 



After the instruments had been set up, the question had to be decided how 

 sensitive to make the electrometer. In view of the fact that the work was being 

 undertaken mainly with a view to the investigation of thunderstorms, it seemed 

 desirable that the sensitiveness should be so arranged that the deflection should 

 never be greater in two minutes than would bring the boom of the electrometer to 

 the edge of the paper on which the record was being made. This necessitated a 

 somewhat low sensitiveness, and, as a consequence, the electricity of rain with very 

 low charges could not be measured. There was the possibility of altering the 

 sensitiveness of the instrument according to the kind of rain which was expected, 

 but this was considered impracticable in view of the fact that the instrument could 

 not be watched continually, but had to be left to itself to take the records entirely 

 automatically. In reality, this difficulty was not serious. The conditions during 

 thunderstorms are entirely different from those during gentle rain, and to investigate 

 the two cases is really a matter of two distinct researches. The present research 

 was concerned with highly charged rain, and so the fact that the instrument did not 

 also record very small charges was not of any great consequence. 



By good fortune, after the first one or two storms, a degree of sensitiveness was 

 found which proved to be satisfactory during the rest of the rains. On only two 

 or three occasions did the boom pass to the edge of the paper, and on each of these 

 occasions the instrument was under observation, so that a rough estimate could be 

 made as to what would have been the extent of the deflection from the time taken 

 for the boom to reach the stop. This sensitiveness gave a deflection of 1 cm. for 

 40 volts, and, as a deflection of 0'2 mm. could be recognised with certainty, a 

 potential of 1 volt could be measured. The capacity of the receiving system was 

 determined to be 141 cm., and hence the smallest charge which could be measured 

 with certainty was 0'47 electrostatic unit. The rain entered the receiver through 

 an opening 29 cm. in diameter, so that when a charge of 7 X 10~ 4 electrostatic unit 

 fell on each square centimetre of surface in two minutes it was sufficient to be 

 recorded by the instrument. 



In order to determine the charge brought down by each cubic centimetre of rain 

 it was only necessary to divide the charge registered by the electrometer during any 

 two minutes by the amount of rain recorded by the rain-gauge during the same 

 interval of time. On account, however, of the limitation of the method of recording 

 the rainfall already pointed out, it was not possible to find the value of the charge 

 per cubic centimetre of rain from electrometer and rain-gauge records for every 

 single two minutes' interval ; but experience showed that it was exceptional not to 

 be able to do so, and during the greater part of the rains this value was obtained. 



