Note concerning the measurement of ionic density on the top of a tower

On pages 35–37 of the March issue of the Journal for the current year, P. L. Mercanton gives some very interesting results of measurements of the atmospheric ionization made by him on a parapet near the top of a tower, and in the interior of the tower at its base. He finds that if n + and n − refer, respectively, to the positive and negative ionic‐densities, n + / n − is much larger for the measurement made in the strong electric field near the top of the tower than in the zero electric field in the interior of its base. Thus, for n + / n − as measured at the top of the tower, values 4.33, 2.16, 4.46, and 1.70 were obtained on four different occasions, while the corresponding values for the foot of the tower were, respectively, 1.31, 1.09, 1.24, and 1.19. I wish to call attention to the confirmation, afforded by these observations, of the view put forward in my former communication 1 as to the effect of the charge induced on the Ebert ion‐counter by the potential‐gradient in modifying the values obtained in ionic‐density measurements. The theory of the phenomenon shows that, for a potential‐gradient of the normal sign, measurements of n + should be unaffected by the induced charge, while those of n − should be too small. The effect of the negative induced charge on the instrument is to retard the velocity of the approaching negative ions. Thus, these ions move slower than the incoming stream of air, with a result that a smaller number of them enter the instrument per second than would enter if there were no induced charge. The positive ions are accelerated by the induced charge, but theory shows that the excess number of ions entering the instrument is just equal to the number which are captured by the outer cylinder of the apparatus, so that the measurements of n + are unaffected by the induced charge.