...ion streamers of the cloud of positive ions (Fig. 1). In order to obtain a separate picture of the positive and negative clouds, the authors used as an electrode a disk connected to ground, placed perpendicular to the earth. In this way the photographs of Fig. 2 were obtained (a—negative cloud; b—positive cloud).

The photographs obtained by the authors are of great interest. By further developing the same method and, accordingly, varying and measuring the conditions under which the discharge arises, it will undoubtedly be possible to give a clear quantitative picture of the course of the spark discharge, which is very important for a whole series of physical problems.

P. Pavlov

WILSON CHAMBER WITH AN EXTENDED DURATION OF THE SUPERSATURATED VAPOR STATE *

As is known, an ionizing particle entering a Wilson chamber is detected only if the vapor present in the chamber is in a supersaturated state. The time during which such a particle is detected in the form of a track is not large for ordinary chambers—it ranges from 0.1 to 0.2 sec., depending on the type of chamber and the nature of the ionizing particle. The shortness of this interval of time is a complicating circumstance in the study of phenomena (especially those rarely encountered) in the region of the atomic nucleus.

O. Frisch succeeded in constructing a Wilson chamber in which the duration of the supersaturated-vapor state is increased to 1 sec. This is achieved by the fact that the necessary supersaturation of the vapor, obtained as a result of a rapid primary expansion, is then maintained by a slow expansion over a longer interval of time. The slow expansion is necessary to compensate for the heat supplied to the gas owing to the thermal conductivity of the surrounding bodies, as well as the heat released during condensation of the vapor.

The very simple construction of Frisch’s chamber consists essentially of the following.

A glass vessel with a flat glass plate glued on top (for observing the tracks) is placed on supports in a second vessel turned upside down. All this is filled with liquid (in the author’s experiments, a 50% mixture of water and alcohol) in such a way that between the upper glass and the surface of the liquid in the first vessel there remains a layer of air, which is the chamber in which tracks appear. The vessel is filled with liquid by means of a tube passing through the bottom of the first vessel. Another tube connects the air layer with the atmosphere and serves to establish the initial volume of the chamber. To expand the chamber, the air from under the second vessel is released, by means of a third tube, into special vessels that have been evacuated in advance. In this process the liquid enters the second vessel, lowering its level in the first. To obtain the initial rapid expansion and the subsequent slow removal of air from under the inverted vessel, it is passed through tubes with stopcocks into two separate vessels; in doing so, the positions of the stopcocks are selected in such a way that one vessel is filled with air rapidly, thereby causing supersaturation of the vapor in the chamber, while the other is filled slowly, thereby ensuring the preservation of the previously obtained supersaturation of the vapor. By selecting the initial volume of the chamber, the magnitudes of the vessels for expansion, and the resistance of the tubes leading to these vessels, Frisch obtained the possibility of observing the trajectories of ionizing particles (unfortunately, it is difficult to judge the quality of the tracks obtained, owing to the absence of photographs).

L. Groshev

* O. Frisch, Naturwiss. No. 10, p. 166, 1935.