On the Wall-effect of a Nucleus-counter

[摘要] It was found out that a thin boundary layer in which no fog is formed exists along the wall surface of the cloud chamber, when dusty air is taken into the chamber and expanded with expansion-ratio of about 1.2. The mean thickness of this boundary layer, which was measured with a microscope in various ways (see figure), was 0.25mm.The thickness seemed to be related to the speed of the expansion. The above figures were observed when the speed of expansion was about 0.03 sec. When the speed was slow as in the case of nucleus counting, the thickness increased becoming 0.4_??_0.8mm.The formation of this layer seemed to be explained by the thermal conduction from the wall and also by the decrease of density of water vapour in the chamber due to expansion.The above phenomena, the wall-effect, have a great important role in the theory of m_??_asurement of nuclei. Due to the existence of such a layer, it is true that nuclei fallen on a glass plate are not the entire nuclei that exist in the chamber, i.e. nuclei included in the layers near the glass plates may be left unchanged into drops.Thus we have to repeat expansions successively on the same sample of air in order to find out the total number of nuclei in the chamber.Assuming the thickness of the layer, the number of drops that falls at each stage of expansion was theoretically calculated and the result was compared with the observed values obtained by J. J. Nolan, G. R. Wait and other workers. It was proved that number of drops caught in each expansion seemed to be explained mostly by the existence of the layer.

[学科分类] 大气科学