showed that, other conditions being equal, the greatest intensity of oscillations is obtained in the temperature range from 200 to 350° C.

The highest frequency at which it was still possible to observe a diffraction pattern reached \(1.35\cdot 10^8\) hertz (excitation of the 510th harmonic of quartz).

Fig. 5.

a  b

In this case it was possible to establish a gradual increase in the propagation velocity of ultrasonic oscillations in quartz as the frequency increased (in the interval from \(0.26\cdot 10^8\) to \(1.3\cdot 10^8\) hertz the velocity increased by approximately 20%). Sokolov does not consider the reasons for this increase in velocity to be entirely clear.

N. Malov

INVESTIGATION OF A SPARK DISCHARGE USING A WILSON CLOUD CHAMBER*

In the usual “electrical” or optical methods of investigating a spark discharge, we deal either with electrical signals or with optical signals emitted by the spark that has already formed. To clarify the process of the formation of an electrical discharge in its very first stages, which are not accompanied by any light effect, the only means at present is the Wilson cloud chamber.

The first attempts in this direction belong to Wilson himself, who as early as 1899,** using his chamber, observed the formation of clouds of positive and negative ions during a discharge from a point.

At the present time, the Japanese physicists Nakaya and Yamasaki have succeeded, by means of the Wilson chamber, in obtaining the excellent photographs reproduced here of the very earliest stages of a spark discharge.

Fig. 1. Positive and negative ion clouds.

For this purpose they used a spark gap between nickel wires (1.7 mm in diameter), placed in the Wilson chamber, the ends of which were ground into hemispheres. The spark gap under study was connected into a rather complex oscillatory circuit,

* See Nakaya and Yamasaki, Proc. Roy. Soc. A, 148, No. 864, 446, 1935.
* Wilson, Phil. Transactions, A, 192*, 439, 1899.

...which was supplied with electrical energy from a Wommelsdorf induction machine. By means of a special pendulum and the corresponding contacts, the chamber was first expanded, after which, into the spark gap under investigation, a short electrical impulse was sent, the front of which was cut off by a spark jumping across a second spark gap connected in parallel with the one under investigation. By changing the magnitude of this second gap, it was possible to regulate the electrical voltage in the gap under investigation.

Fig. 2b — positive cloud.

Fig. 2a — negative cloud.

By means of the same pendulum, an electrical spark was made to jump in mercury vapor in the second circuit; this spark served for illumination. Owing to careful adjustment of the pendulum and of all the contacts, mounted on a common stand, the time between the transmitted impulse and the illuminating spark amounted to only a few hundredths of a second. Thanks to this, the authors succeeded in photographing the ions arising at the electrodes at the very place of their formation.

With a gradual increase of the voltage on the electrodes, there appears first a spindle-shaped cloud of negative ions, and then a branched-

... jets of a cloud of positive ions (Fig. 1). In order to obtain a separate picture of the positive and negative clouds, the authors used as the electrode a disk grounded in quality. 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 developing the same method further and correspondingly 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 range of physical problems.

P. Pavlov

A WILSON CHAMBER WITH AN INCREASED 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 a particle of this kind 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 hindering circumstance in the study of phenomena (in particular, those rarely encountered) in the region of the atomic nucleus.

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

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

A glass vessel with a flat glass cemented on top (for observing tracks) is placed on supports in a second vessel open at the top. 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 where the tracks appear. Filling the vessel with liquid takes place through a special 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 let out, by means of a third tube, into special vessels evacuated in advance. In doing so, the liquid enters the second vessel, lowering its level in the first. To obtain the initial rapid expansion and the subsequent slow suction of air from the space harmful to the vessel, it is passed through tubes with stopcocks into two separate vessels; in this case the positions of the stopcocks are selected in such a way that one vessel is filled with air rapidly, thereby producing 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 sizes 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.