On the electric charge collected by water-drops falling through a cloud of electrically charged particles in a vertical electric field

Abstract

The experiments described in this paper are a continuation of work described in a former paper, and have for their object the examination of a mechanism suggested by Wilson in connection with the theory of thunder-clouds. In the former work the interaction of large water-drops with ions produced by X-rays was investigated. In the present work the interaction of large water-drops with electrically charged cloud particles is investigated, and the mechanism suggested by Wilson takes the following form. Consider an uncharged water-drop falling vertically through a cloud of very small water-droplets, each of which has an electric charge either positive or negative. Let there be a vertical electric field which will be taken to be of positive potential gradient so that positively charged cloud particles move down and negatively charged cloud particles move up. The electric field induces equal charges of opposite signs on the upper and lower halves of the drop. In the case considered the upper charge is negative and the lower one positive. A charged cloud particle has a definite small mobility depending on its radius and the charge it carries. Suppose now that the mobility is so small that in strong electric fields, such as occur in thunder-clouds (up to 10,000 volts/cm), the velocity with which the positively charged cloud particles move down is less than the velocity of the falling drop. Under these conditions, those positive cloud particles which are above the drop cannot overtake the drop and so do not reach it, although attracted by the negative charge on its upper half. Those positive cloud particles, which are below and which the drop over-takes, are first repelled by the lower positive charge on the drop before being attracted by the upper negative charge and, since these charges are equal in the neutral drop, these cloud particles do not reach it. Negative cloud particles coming up to meet the falling drop are attracted to its lower positively charged half and give the drop a net negative charge. This destroys the equality of the induced charges, and some of the positive cloud particles which the drop overtakes are now attracted to it. In the presence of equal numbers of positively and negatively charged cloud particles a limiting condition is approached in which the drop collects equal numbers of positive and negative cloud particles per second and has a net negative charge equal to some fraction of the induced charge.