Small water droplets with diameter d < 8 pm are allowed to fall in a settling chamber in the presence of a quasi-uniform electric field varying from zero t o 600 V/cm. The trajectories of these droplets are photographically recorded through an incident-light microscope with dark field illumination. Results from 2 000 photographs, each one containing about 25 droplet pairs, show that the coalescence efficiency is almost unity. The coalescence frequency is almost zero in the absence of an electric field and increases rapidly at fields stronger than 400 V/cm. The coalescence frequency increases about linearly with electric fields from 100 to 400 V/cm. A survey of coalescences of droplets with parallel trajectories gives some order of magnitude for the collision efficiency, which indicates that the influence of the electric field is very pronounced for small droplets.

Small ice crystals with average diameter of about 30 µm are produced in a large cold room and allowed to fall in a settling chamber in the presence of a quasi-uniform electric field. Aggregates (flakes) of ice crystals are collected by permanent replicas. Results show that an electric field above a threshold value of about 4 × 10 4 V m -1 rapidly increases the growth of flakes by the capture of small ice crystals. The influence of the electric field upon the growth of ice aggregates is maximum at a field strength of about 1.5 × 10 5 V m -1 . Comparison of the results with Jiusto's mathematical model of the growth rate gives values of the collection efficiency at different field strengths. It is very likely that the electric field increases the adhesion (aggregation) efficiency rather than the collision (cross-section) efficiency. RÉSUMÉ Des cristaux de glace d'un diamètre moyen d'environ 30 µm, sont produits à l'intérieur d'une grande enceinte refrigérée et tombent en chute libre à travers un champ électrique vertical et uniforme. Les agrégats des cristaux sont recueillis par des lamelles induites de formvar. Les résultats montrent qu'au-dessus d'un champ seuil de 4 × 10 4 V m -l , les agrégats (flocons) croissent rapidement avec l'intensité du champ appliqué. L'influence du champ électrique sur la croissance des flocons est maximale pour un champ de l'ordre de 1.5 × 10 5 V m -1 . En comparant les résultats expérimentaux avec le modèle mathématique de Jiusto sur la croissance des flocons, des valeurs du coefficient de captation sont données en fonction de l'intensité du champ appliqué. Il semble que le champ électrique a pour effet d'augmenter le coefficient d'adhésion des cristaux plutôt que le coefficient de collision.

Small water droplets with diameter d ≤ 8 μm are allowed to fall in a settling chamber in the presence of a quasi‐uniform electric field varying from zero to 600 V/cm. The trajectories of these droplets are photographically recorded through an incident‐light microscope with dark field illumination. Results from 2 000 photographs, each one containing about 25 droplet pairs, show that the coalescence efficiency is almost unity. The coalescence frequency is almost zero in the absence of an electric field and increases rapidly at fields stronger than 400 V/cm. The coalescence frequency increases about linearly with electric fields from 100 to 400 V/cm. A survey of coalescences of droplets with parallel trajectories gives some order of magnitude for the collision efficiency, which indicates that the influence of the electric field is very pronounced for small droplets.