﻿24 EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 



give like deflections for like voltages were emplo3^ed. The collectors 

 were frequently interchanged and other checks applied. The following 

 mean values were obtained : 



Height. Potential at beginning. 



6.1 m. 20 volts. 



7.7 38 



9.1 48 



12.0 60 



16.8 121 



After five minutes. 



30 volts. 

 51 



72 

 141 



Notice that a time element comes into the discussion, for it appears 

 that the collector requires some little time to come to the j^otential of 

 the air. Again, it appeared that by varying the rate of flow the values 

 would be somewhat altered, and it would therefore be necessary in any 

 extended survey to use not only similar instruments but similar times. 

 Pellat (see " Comptes Rendus," March, 1885, p. 375) found while study- 

 ing the means emplo3^ed to get the potential of the air that with a flow 

 of eight litres in twelve hours about six minutes were required for the 

 electrometer to attain the proper value, while with a flow of twelve litres 

 only five minutes were needed. Water-drojDpers are the collectors most 

 generally used, but there are other forms, and in some waj^s preferable. 

 The paper match (blotting paper soaked in nitrate of lead) is slower than 

 the water-dropper and gives somewhat lower values. 



Assuming that a proper collector can be designed and the error due to 

 bending corrected, we have as the first problem the determination of the 

 potential gradient at any height. Exner (see " Ursache und Gesetze der 

 Atmos. Elec") experimented with balloons carrying insulated water- 

 droppers. Three sets of observations were made at 400, 550, and 660 

 metres, with the hope of getting an approximate value for 500 metres. 

 A constant value of 193 volts was obtained, but the constancy is perhaps 

 due to the fact that the balloon traversed the distance in a few minutes. 

 From measurements made with small balloons he obtained for the poten- 

 tial in free air — 





Metres. 



Volts. 



17 



100 



110 



120-140 i 



130 



160 



160 



18 . 



20 



21 



22 



25 





25. 

 27. 

 30. 

 34. 

 40. 



Metres. 



Volts. 



160 



170 



195-210 



250 



280 



d'v 



and from these -r- = 6.8 volts per metre. 



