A cloud chamber is described which measures either the absolute or relative concentration of nuclei in an aerosol. Water droplets are condensed on aerosol particles in a small sample, and the resulting water droplets are photographed on 16‐mm film by flashtube illumination and then counted in a microprint reader. Two types of expansion cloud chambers were found to work satisfactorily: the volume‐defined and the pressure‐defined. The apparatus is completely automatic and will operate continuously on a one‐minute cycle. It can measure aerosol concentrations that range from 200 particles/cm3to 2×106particles/cm3.