摘要:

The theory of x‐ray extinction in diffraction studies is reviewed, and application to small crystals completely bathed in the incident beam is considered. Using a suitable approximation to a sum of Bessel functions, a correction formula is obtained, valid in the region of small Bragg angle &thgr; where the extinction is likely to be greatest. The formula is discussed; and it is shown that when the crystal structure is essentially known, as in later stages of refinement, it is possible to separate the effects of primary from secondary extinction and to correct the experimental data accordingly.

ISSN:0021-8979    

DOI:10.1063/1.1721871

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

A matrix method is used to solve the difference equations which describe the zone refining of a bar. The matrix method is designed for direct numerical calculation of the solute distribution, after any number of passes, with any initial solute distribution. The method is illustrated with a simple example.

ISSN:0021-8979    

DOI:10.1063/1.1721872

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Streaming birefringence was studied in the annular space between a fixed outer and a rotating inner cylinder at velocities on both sides of transition to Taylor vortex flow. With a well‐collimated light beam, quantitative measurements were possible as close as 0.1 mm from a wall. For an aqueous colloidal solution of bentonite, shear stress could be evaluated from birefringence measurements only when, for a given birefringence, the angle of extinction was the same in the unknown flow as in a known laminar flow. In turbulent shear flow this is not generally the case. For a pure liquid, ethyl cinnamate, the angle of extinction remained at 45° to the streamlines, within experimental error, under all flow conditions obtainable; thus even in turbulent shear flow the birefringence appears to arise primarily from the principal stresses. Assuming birefringence proportional to mean shear stress permits construction of a velocity profile across the gap, yielding the correct value of velocity of the moving wall within limits of experimental error. Thus in this type of two‐dimensional shear flow the mean shear stress can be measured by means of the streaming birefringence of a pure liquid.

ISSN:0021-8979    

DOI:10.1063/1.1721873

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The mechanism of material transport in sintering can be elucidated in some cases by direct observation of the rate of interface growth and approach of centers between spherical particles. Measurements with glass, sodium chloride, and copper indicate that with these materials viscous flow, evaporation‐condensation, and self‐diffusion are the rate‐determining mechanisms. Values of viscosity, vapor pressure, and diffusion constants have been determined, but calculations of diffusion constants from these data are subject to uncertainties of interpretation. A model is presented for the behavior of copper during the initial stages of sintering, which is in agreement with available experimental data, and which requires vacancy elimination at dislocations or grain boundaries. Data for refractory oxides indicate the importance of purity and fabrication pressure, but the sintering mechanism for these materials is not determined by the present data.

ISSN:0021-8979    

DOI:10.1063/1.1721874

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

A theory of steady‐state creep is developed using Mott's mechanism of dislocation climb. It is assumed in the analysis that the rate‐controlling process is the diffusion of vacancies between dislocations which are creating vacancies and those which are destroying them. The concentration of vacancies along a dislocation line is determined by setting the change in the free energy caused by a decrease or increase in the number of vacancies equal to the change in the elastic energy occuring during dislocation climb. The creep equation that results from the analysis iscreep rate=const (&sgr;&agr;/kT)exp(−Q/kT)where &agr; is a constant(&agr;∼3 to 4),Qis the activation energy of self‐diffusion,kThas its usual meaning, and &sgr; is the stress. This equation is valid in the stress range from the critical shear stress to a stress about equal to 108to 109dynes/cm2. At larger stresses the creep rate increases much more rapidly with stress.

ISSN:0021-8979    

DOI:10.1063/1.1721875

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

It is concluded from torque measurements on single crystals of Alnico 5 that the high coercive force of these crystals depends upon the shape anisotropy of the fine precipitated plates and that the crystal anisotropy of the plates is negligible. The present physical picture of the alloy is that it consists of plates of precipitate of high magnetic saturation separated by a matrix of comparatively low magnetic saturation. The plates are made up of rods of precipitate (approximately 75 A in diameter ×400 long), but they act magnetically like single domain plates. The magnetization vector turns in the plane of the plates, and it is the length‐to‐width dimension ratio of the plates which determines the effective shape anisotropy and the resulting coercive force of the alloy.

ISSN:0021-8979    

DOI:10.1063/1.1721876

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The frequency‐dependence of internal friction of four &agr; brass (62.5% Cu) reeds was measured at room temperature under conditions where contributions to the internal friction other than that arising from transverse thermal currents were small and assessable. Measurements were made by the free‐decay method, at maximum strain amplitudes of 5×10−6. By using specimens of two different thicknesses, frequencies differing from the frequency of peak damping by nearly a factor of 20 were obtained by measurements in the range 7–370 cy/sec. The results, in conjunction with measurements of Young's modulus (1.06×1012dynes/cm2±0.5%), coefficient of linear expansion (18.7×10−6/°C±0.5%), density (8.42 g/cc±0.2%), specific heat (0.088 cal/g/°C±2%), and thermal diffusivity (0.34 cm2/sec±5%), indicate that Zener's theory of damping by transverse thermal currents is probably valid exactly. From the position of the peak, the thermal diffusivity is found more precisely as 0.346 cm2/sec±1.5%.

ISSN:0021-8979    

DOI:10.1063/1.1721877

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

A hysteresis effect in multiplier phototube noise as a function of tube gain has been found for both RCa 5819 and DuMont 6292 tubes. The effect is a function of both temperature and the length of time that a tube has ``rested.'' The hysteresis is seen to be a noise buildup with time but not a change in dynode multiplication. The reasons for this effect are not clear. Several possible mechanisms are discussed. Conditions are determined for obtaining reproducible noise counts for liquid scintillation counting of low‐energy beta emitters.

ISSN:0021-8979    

DOI:10.1063/1.1721878

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

This paper deals with a new kind of electron microscopy in which the specimen constitutes an electron mirror. The low velocity of the electrons in front of this mirror‐specimen enables pictorial representations of potential distributions on surfaces. A resolving power comparable to that of the light microscope has been obtained. Sample micrographs depicting surface reliefs, potential distributions on surfaces, and the distribution of electrical conductivity are shown. The peculiar kind of image formation is briefly discussed and reference is made to the potentialities of such a research tool for surface physics and metallurgy.

ISSN:0021-8979    

DOI:10.1063/1.1721879

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

In this paper we illustrate by a simple example that when we wish to express the admittance of a cavity excited through a hole in terms of natural frequencies of oscillations which can exist when the cavity is short‐circuited, it is important to remember that the short‐circuit must conform to the impressed field. When this is done, one need not fear that the ``obvious'' set of natural modes may be incomplete.

ISSN:0021-8979    

DOI:10.1063/1.1721880

出版商:AIP    

年代:1955

数据来源: AIP