Condensation-Nucleus Counter

a device for determining the concentration (number per unit volume, usually per cm³) of condensation nuclei in the atmosphere. The most common types of condensation-nucleus counters are based on the principle of an adiabatic chamber. The volume of air to be investigated is introduced into a small chamber filled with moistened air, and the chamber is then adiabatically expanded. As a result, the air is cooled, and the water vapor becomes supersaturated and condenses on the condensation nuclei. The droplets formed settle to the bottom of the chamber and are counted with the aid of a magnifying glass. This principle underlies the counters developed by J. Aitken (1887) and I. Scholz (1932). The Scholz counter permits determination of the concentration of nuclei over almost the entire range of concentrations encountered in the atmosphere; one model is capable of measuring separately the number of charged and neutral nuclei.

Photoelectric condensation-nucleus counters have been developed that are based on the measurement of the intensity of the light passing through the chamber after the chamber has been expanded in order to form drops (fog) on the condensation nuclei. The greater the concentration of drops and, consequently, of condensation nuclei, the greater the attenuation of a light beam directed through the chamber onto a photoelectric cell. The pho-tocurrent is measured with a galvanometer. To arrive at the concentration of condensation nuclei, the device must first be calibrated. These counters have the advantage that their readings are objective and their operation can be automated.