Growth of Drops Formed in a Wilson Cloud Chamber

Abstract

Water drops and water‐alcohol drops formed in a Wilson cloud chamber were photographed with periodic illumination and their velocity was measured. The drop size was determined by means of Stokes' law, and the square of the radius of the drop was found to vary linearly with time as soon as the drop was large enough to register photographically. A 60‐cycle neon light that moved with constant velocity during the exposure was turned on by the expansion valve. This light was photographed on the same frame, and its flashes were used to establish the zero of the time scale for the drop images. It was observed that the straight line obtained from the experimental points intersected the time axis about 0.1 sec. after the zero. This fact seems to indicate that the rate of growth of the drops is smaller during the first 0.1–0.2 sec. and then reaches a constant value. The slope of the straight part of the curve was found to be about 7.5×10⁻⁶ cm²/sec. for water and 4.4×10⁻⁶ cm²/sec. for water‐alcohol.