On the falling cloud-chamber and on a radial-expansion chamber

Abstract

When cloud-track photographs are taken with the cloud-chamber fixed between the poles of a magnet, it is difficult to fulfil the conditions for maximum efficiency of either the magnetic or optical parts of the apparatus. If it were possible to remove the cloud-chamber from between the pole pieces (or from any confined space) after the expansion and then to obtain undistorted photographs of the resulting cloud-tracks, the possibilities of the cloud-chamber method might be considerably extended. The simplest method of removing the cloud-chamber from its original position would be allow it to fall. And dropping the cloud-chamber would seem to be not only the easiest but also the best possible method of displacing it. For if the cloud-chamber is released at the moment of expansion, then so long as it is falling freely the disturbing effects of gravity will be eliminated; the droplets will not fall relatively to the gas in the chamber, and convection currents will not be set up in spite of the differences of temperature which arise in the gas owing to contact with the walls and to condensation on tracks. Thus, dropping the cloud-chamber instead of causing distortion of the tracks will tend to prevent it, if the photograph is taken while the chamber is still falling freely. The falling cloud-chamber may thus have considerable advantages quite apart from the particular application which suggested it. Before making experiments with a falling cloud-chamber, it seemed desirable to endeavour to design one which should have both the form must suitable for insertion between the poles of a magnet and also admit of the maximum efficiency of the optical arrangements. The form of expansion apparatus which suggested itself as likely to be good for both purposes was one in which the cloud-chamber is a shallow cylinder with fixed parallel glass ends and in which the motion of the air during expansion is radial. It was decided to try a modification of the expansion apparatus described a year ago in which the pressure remains constant after expansion, not the volume as in the older type of apparatus.