From Current Literature

STUDY OF THE PROPAGATION OF ULTRASONIC OSCILLATIONS IN SOLIDS

To study the reflection and refraction of ultrasonic oscillations, Bez-Bardili (Bez-Bardili, Phys. Z. 36, 20–24, 1935) made use of the Schlieren method with a large number of apertures, proposed by Bär and Meyer, which makes it possible to survey at once a large portion of the ultrasonic field (see Uspekhi fizich. nauk 15, 1935). In the path of the ultrasonic beam various refracting and reflecting systems were placed. The diffraction optical pattern was projected onto a screen and photographed. Bez-Bardili’s work, while introducing nothing fundamentally new into the experimental technique, is of interest because of the distinctness of the pictures he obtained, to which we shall now turn. Fig. 1 shows the refraction of an ultrasonic beam \((f = 1.89 \cdot 10^6\) cycles), propagating in xylol, upon incidence on a prism made of aluminum (the beam is incident from the right). In addition to the refraction of the beam, clearly

Fig. 1.

Fig. 2.

Fig. 3.

one sees also its absorption in aluminum, causing a decrease in the number of diffraction bands.

Fig. 2 depicts the propagation of a beam through a plane-parallel aluminum plate 10 mm thick. On the right-hand side of the figure the incident and reflected beams are visible, and on the left the beam displaced by the plate. In Fig. 3 (the ultrasonic beam of oscillations falling from the left), the action of a lens, also made of aluminum, is visible. Finally, Fig. 4 shows the action on a parallel ultrasonic beam of a regular grating. Here one can clearly see the diffracted transmitted beams; moreover, it is apparent that such a grating acts as a reflecting one (the beam falls from the left).

Fig. 4.

Measurements of the propagation velocity of the ultrasonic beam, whose accuracy the author estimates at 2%, gave on average the following results (in m/sec):

Sokolov* used the diffraction of light rays in an ultrasonic grating for purposes of defectoscopy. He placed an oscillating quartz at one end of the metallic part under investigation, and at the other end of the part he placed a vessel with a liquid (the most suitable liquid turned out to be turpentine). In the absence of defects in the part, the energy of the ultrasonic beam entering the liquid was large, so that a diffraction pattern with a large number of spectra was obtained (Fig. 5a). The presence of inhomogeneities in the metal caused weakening of the beam and a decrease in the number and sharpness of the spectra (Fig. 5b). In exciting the quartz with a voltage of the order of several kilovolts, suitable frequencies proved to be those from \(3\cdot 10^6\) to \(6\cdot 10^6\) hertz (at higher frequencies strong damping arose; at lower frequencies the spectra obtained were not very distinct). Passing a ray of light through the oscillating quartz, Sokolov investigated the behavior of the quartz, observing the corresponding diffraction pattern. The following results of these investigations are the most interesting. When the quartz was excited in air, it was possible to obtain spectra of very high orders (up to the 18th, and in one case even up to the 34th). A change in the temperature of the quartz

* Phys. Z. 36, 142—143, 1935.

showed that, other conditions being equal, the greatest intensity of the 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).

a                          Fig. 5.                          b

At the same time it was possible to establish a gradual increase in the propagation velocity of ultrasonic oscillations in quartz as the frequency was 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

STUDY OF THE SPARK DISCHARGE BY MEANS OF A WILSON CHAMBER *

With the usual “electrical” or optical methods of studying a spark discharge, we are dealing either with electrical signals or with optical signals emitted by the spark that has already formed. For elucidating the process of the origin 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 chamber.

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

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

Fig. 1. Positive and negative clouds of ions.

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 had been ground in the form of hemispheres. The spark gap under investigation was included in a rather complicated oscillatory circuit,

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