Linke's mean value for dV/dh at the ground was 125.
For practical purposes we may treat r 2 as constant, and replace d/dr by d/dh, where h is height in centimetres above the ground.
In section, and suppose it cut by equipotential surfaces at heights h i and h 2 above the ground, we have for the total charge M included in the specified portion of the tube 47rM = (dV/dh)h i - (dV/dh)h2.
Gerdien (61), near the ground a mean value for d 2 V/dh 2 is -(I/io) volt/(metre) 2.
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.