The electrical conductivity of the atmosphere at any part of the earth's surface depends on the local concentration of small ions. This in turn depends 1. on their rate of production and 2. on their rate of removal. The removal is due principally to attachment to condensation nuclei. Methods are described for counting the nuclei and for determining their sizes and masses. The ratios of charged to uncharged nuclei, when they are in equilibrium with the small ions, are determined for nuclei of various sizes. Observations on nuclei in the free atmosphere show that they are usually not in equilibrium. The difficulty of establishing any simple relation between atmospheric conductivity, rate of ion-production and nucleus concentration is increased by the discovery that the air-borne radioactive matter which contributes to the small-ion production tends to increase with increase of nucleus concentration.