Gerdien has more recently repeated the experiments, employing an apparatus devised by him for the purpose.
Gerdien (29) obtained o 86 for I + and 1.09 for L.
Gerdien (61), near the ground a mean value for d 2 V/dh 2 is -(I/io) volt/(metre) 2.
Gerdien himself makes I + -I_ considerably larger than Simpson, and concludes that the observed value of p is from 30 to 50 times that calculated.
The presumption is either that d 2 V/dh 2 near the ground is much larger numerically than Gerdien supposes, or else that the ordinary instruments for measuring ionization fail to catch some species of Ion Whose Charge Is Preponderatingly Negative.
Gerdien (61) has made some calculations as to the probable average value of the vertical electric current in the atmosphere in fine weather.
As to the convection current, Gerdien supposes-as in § 25-p =2-7 X 10° electrostatic units, and on fine days puts the average velocity of rising air currents at 10 cm.