摘要:

Single crystals of lead telluride measuring up to 1¼‐cm diameter and 6 cm long have been grown in sealed silica crucibles by the Bridgman‐Stockbarger method of lowering a melt slowly through a freezing level. The crystals have been grown for the purpose of testing the semiconducting and allied properties of lead telluride in that form. Extreme purity of the materials and cleanliness of the crucible have been found essential for successful growth. X‐ray tests have shown that the specimens are single crystals. The purest crystal grown so far had a conductivity corresponding to an electron concentration of 5×1014per cc. Attempts to introduce excess of one of the constituents—lead or tellurium into the crystal lattice during growth have not so far been successful.

ISSN:0021-8979    

DOI:10.1063/1.1699890

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Exact equations for the propagation of electromagnetic waves in a perfectly conducting horn of arbitrary shape are given. They take the form of an infinite set of simultaneous ordinary linear differential equations, and can be interpreted as the equations of propagation of a system of coupledE‐ andH‐waves. If the coupling is neglected, we need only consider a single differential equation for eachE‐ andH‐wave, which can be solved approximately by the W.K.B. method. This approximate solution brings out the distinction between ``transmission regions'' and ``attenuation regions'' of the horn, as found by Barrow and Chu for the sectoral horn.It is shown that the error due to neglect of coupling is, in general, of the order of the square of the flare angle as far as the variation of the field along the horn is concerned, but is of the first order in the flare angle as regards the variation of the field over the cross section. The coupling cannot, however, be neglected between modes of propagation which have the same cut‐off frequency for all cross sections. The propagation characteristics of several special shapes of horn are discussed in detail.

ISSN:0021-8979    

DOI:10.1063/1.1699891

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Exact equations are given for the propagation of acoustic waves in horns of arbitrary shape. These equations are similar to, though simpler than, the equations previously found for electromagnetic horns, and can be regarded as giving rise to an infinite number of coupled modes of propagation. If the coupling is neglected, the equation for the fundamental mode is the familiar one, but the theory also furnishes equations for the higher modes. The error involved in neglecting coupling is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1699892

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

In connection with a study of the emissivity of metals at low temperatures, a quantitative analysis has been made of the factors involved in the design of storage containers for liquid helium. Experiments confirm the analysis, which has resulted in an exceedingly simple liquid‐nitrogen‐protected liquid helium container having a helium evaporation rate of 1 percent per day.The hemispherical emissivity of mechanically polished copper at 4.23°K was determined for blackbody radiation. For radiation corresponding to that emitted by a blackbody at 77.1°K and 297.1°K, the measured emissivities are 6.9×10−3and 1.29×10−2, respectively. These data are discussed in relation to the recent theoretical work of Reuter and Sondheimer.The heat of vaporization of helium at 4.228°K was determined to be 4.93 cal g−1, in substantial agreement with the data of Dana and Kamerlingh Onnes. Kistemaker's thermodynamic considerations questioning the accuracy of these data are evaluated in terms of the new determination.

ISSN:0021-8979    

DOI:10.1063/1.1699893

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

In this paper it is shown that the viscosity of the liquid normal paraffins can be accurately defined as a simple function of relative free‐space except for values in the neighborhood of the freezing points of each compound. A novel method of extrapolating the specific volumes of this family of compounds to absolute zero is described which permits the calculation of reliable values of the relative free‐space from density data.An expression of the same form as the author's function, but in which temperature rather than free‐space is the primary variable (the so‐called Andrade equation), fails to reproduce the viscosity ofn‐heptadecane over the same range of temperatures within the limits of the known accuracy of the measurements.

ISSN:0021-8979    

DOI:10.1063/1.1699894

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Differential equations for critical stability of a fluid in a porous medium and heated from below are solved by an approximate method not hitherto used. A single formula is obtained for the thermal environment at critical stability, apparently valid over a wide range of conditions, which allows for temperature dependence of viscosity and nonlinear temperature distributions. New experiments are reported in which initial convection has been detected by visual, thermal, and radioactive‐trace means. Experimental mean thermal gradients for critical stability deviated from values predicted by the new solution of theory by the following average amounts: visual, 150 percent; thermal, 32 percent; radioisotopic, 11 percent.

ISSN:0021-8979    

DOI:10.1063/1.1699895

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The effects of high energy radiation on the luminescence properties of solids are surveyed. Of the four effects considered—radiophotoluminescence, radiophotostimulation, radiothermoluminescence, and the ``killing'' of luminescence by x‐rays or gamma‐rays—the first effect is shown to have advantages in principle over the others as a basis for dosimetry.The absorption, excitation, emission, sensitivity, energy dependence, and stability characteristics of a radiophotoluminescent Ag‐activated phosphate glass are described. It is shown that dosage measurements can be made with this material, employing a simple fluorophotometer, from 10 roentgens to a few thousand roentgens of gamma‐rays. By proper shielding, such a dosimeter element can be made reasonably independent of energy. The dosage indication obtained is quite stable with time and is not seriously affected by exposure of the glass to visible or ultraviolet light or to temperatures in the range of −70° to +100°C. Because of its sensitivity range and its stability over a wide range of ambient conditions, the dosimeter appears to be particularly suited for monitoring personnel that may be exposed to rather high doses of gamma‐rays, such as may be encountered in A‐bomb explosions.

ISSN:0021-8979    

DOI:10.1063/1.1699896

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

A new method for determining the ratios of the critical shear stresses has been applied to data taken from the literature and to original results. Measurements of yield strength were found to be not too satisfactory for the investigation of the slip behavior of the body‐centered cubic materials. On the other hand, any method which determined the slip plane and the orientation of the tensile axis gave useful results. In all instances where the data were available, the ratio of the critical shear stresses on {123} and {112} was very close to one, and did not change with temperature. The ratio of the critical shear stresses on {123} and {110} changes with temperature and composition, with the value of S123generally being higher than that for S110.

ISSN:0021-8979    

DOI:10.1063/1.1699897

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

In a random solid solution, if the two atoms have appreciably different sizes, the nearest‐neighbor distances and to a lesser extent the higher neighbor distances will be of three kinds, &ggr;AA, &ggr;AB, and &ggr;BB. The effect produces modulations in the diffuse intensity similar to those produced by short‐range order. The size effect is important when the difference in scattering power is large, the difference in size is large, and the short‐range order is small. The size effect is illustrated by a single crystal pattern of Cu3Au and a powder pattern of Ni3Au2. An asymmetry in the wings about a fundamental reflection is a result of the size effect.

ISSN:0021-8979    

DOI:10.1063/1.1699898

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The propagation of small sinusoidal modulations in the infinite, parallel‐plane diode is studied. A second‐order differential equation is obtained for the ac convection current. Solutions are found in closed form for the cases of a drift‐stream, space‐charge‐limited acceleration, and acceleration with an arbitrary current density less than the space‐charge‐limited value.

ISSN:0021-8979    

DOI:10.1063/1.1699899

出版商:AIP    

年代:1951

数据来源: AIP