The mobility‐spectrum of atmospheric ions

The mobility‐spectrum of atmospheric ions has been measured in the range of mobilities between 2.0 cm/sec/volt/cm and 10−3 cm/sec/volt/cm using a divided‐electrode air‐blast method. A continuous recording method was employed and the entire spectrum covered in about one hour. The spectrum shows the ordinary small ions, a group of ions at a mobility of about 0.5, and then a practically continuous distribution out to the lowest mobilities measured. The group having a mobility of 0.5 may be correlated with laboratory experiments of Chapman in which ions of this character were formed in the spray of dilute salt solutions. Simultaneous continuous measurements of the variation of density of condensation‐nuclei lead to a lower limit for the size of the nuclei measured in the cloud‐chamber of diameter 3.3 × 10−6 cm.