Volume 20, Issue 1, 01 January 1949
Index of content:
20(1949); http://dx.doi.org/10.1063/1.1698232View Description Hide Description
In a commercial electron diffractioncamera some alkaline earth metals (from Be to Sr) or Al, Ni, or Mo were evaporated onto outgassed metal sheets or glass slides, using the starting pressure of 0.1μ of mercury. The diffraction patterns of the thinnest layers (to about 103A) presented, regardless of the evaporation speed used, the pattern of the oxide of the evaporated metal exclusively. The diffraction pattern of the metal space lattice itself was growingly observed with Be, Al, Ni, and Mo only after uninterruptedevaporation until layer thicknesses of about 1μ were reached. When Mg, Ca, and Sr were evaporated, no patterns without, at least, the most intensive oxide rings could be attained under the vacuum conditions given in the apparatus.
The results are in accord with some earlier findings of Langmuir, Beeching, Darbyshire, and Beischer, and clear up some hitherto not fully explained effects with stepwise evaporated layers. The appearance of oxide patterns is always accompanied by strong improvements of the vacuum in the evaporation compartment by the arising metal vapors, i.e., by the well‐known gettering effect.
20(1949); http://dx.doi.org/10.1063/1.1698242View Description Hide Description
Some layers presenting the space lattice pattern of the metal (Be, Mg, Al, Mo, or Ni) were heated for constant periods. Beginning with a distinctly marked threshold temperature, the interference rings of the oxide space lattice appeared while, simultaneously, the metal space lattice pattern diminished, entirely disappearing with another higher lying temperature. The oxidation temperature ranges (for ten‐minute heating periods) of Be, Mg, and Al were measured. With glucinum it is equal to that within which, Kollath stated, there was a strong increase of the emissivity of berylliumsecondary emission cells.
An elementary calculation gives an inferior time limit to oxidation of the observable surface layer. The surface layer of polished metals is considered; the high secondary emissivity in the case of glucinum, together with the usually supposed Beilby layer giving only two broad diffuse diffraction rings, points to the existence of a random liquid‐like network of BeO molecules on the surface of polished berylliumsecondary emission cells.
20(1949); http://dx.doi.org/10.1063/1.1698231View Description Hide Description
A simple theory for stress‐strain rate analyses during plastic flow of anisotropicsheet metals is proposed for the work hardening range. The effect of orientation on the yield strength in simple tension and biaxial tension was discussed for various types of symmetry. The theory appears to be approximately correct for mild steel plates that exhibit planar isotropy.
20(1949); http://dx.doi.org/10.1063/1.1698233View Description Hide Description
The resolving power of the electron microscope and the contrast in the image are calculated for different conditions of focusing, illumination and aperture. These conditions can change the limit of resolution by a factor of about 3. The contrast in the image of an atom is appreciably increased by defocusing and spherical aberration. Nevertheless, the contrast improves when the numerical value of the aberration constant is diminished. The effect of different methods of spherical correction is discussed briefly.
20(1949); http://dx.doi.org/10.1063/1.1698234View Description Hide Description
This paper presents typical data on the internal friction of twelve single crystals of zinc oscillating longitudinally. The measurements of decrement were made at frequencies of 41 kc and 57 kc. (a) Zinc of three grades of purity was used. The behavior of decrement as a function of stress amplitude for these different materials was found to depend less on the purity of the metal than has been reported previously (see references 1 and 2). (b) For one crystal the decrement showed little rise from its initial value of 2×10−5 at very low stress amplitudes up to stress amplitudes nearly three times the static elastic limit. Considerable similarity of behavior of the decrement as a function of stress amplitude resolved into the basal plane has been found for a number of crystals with a wide range of orientation. The obtaining of these consistent results depended on the use of appropriate annealing and etching techniques. (c) The decrement of a well annealed crystal increased with rising temperature at all stress amplitudes throughout the temperature range −75°C to 150°C. (d) The prior application of static stress far below the static elastic limit has been found to increase the decrement at high stress amplitudes without affecting that at very low stress amplitudes. This increase was a function of the stress amplitude at which the decrement was measured, and of the prior static stress, L. The observations fitted a relation of the form logΔ=C 1+C 2 L, where C 1 depended on the amplitude of measurement and C 2 was approximately constant.
20(1949); http://dx.doi.org/10.1063/1.1698235View Description Hide Description
Certain mathematical representations of a limiter and discriminator are postulated. On this basis, and on the basis of the statistics of narrow‐band random noise, the output signal voltage and r.m.s. noise output voltage are calculated for a limiter and discriminator fed by an f‐m carrier and narrow‐band random noise. The output signal‐to‐noise ratio can thus be expressed as a function of the input signal‐to‐noise ratio. The former is plotted versus the latter in Fig. 5, for several different degrees of limiting. A sharp increase in the output signal‐to‐noise ratio from far below the input signal‐to‐noise ratio to only 0.9 db below it is found to occur as the latter increases through the range 0 db to 6 db. For the sake of comparison, Fig. 5 also shows the corresponding curve for square law detection of an a‐m carrier, as well as that for f‐m without a limiter. It is to be emphasized that these results pertain to the unfiltered discriminator output; consideration of filtering, which is generally present in a receiver to a considerable extent, requires a determination of the spectral distribution of the discriminator output noise.
20(1949); http://dx.doi.org/10.1063/1.1698236View Description Hide Description
The application of ultrasonic methods to the determination of the elastic constants of solids is considered in some detail. It is shown that a rotating plate technique in which ultrasonic transmission is plotted as a function of the angle of incidence of the waves allows determination of the velocities of dilatation and shear waves in the plate. From these data, Poisson's ratio and the mechanical moduli may be determined. Details of an apparatus for making such measurements are given. The elastic constants of several metals have been measured with this equipment and the values obtained are shown to be in agreement with previous published data. In addition, measurements of a number of thermoplastic and thermosetting resins have been made successfully. It has been found that in the case of Melmac Resin 26–8B there is no variation in the elastic constants as a function of cure time.
The Effect of Current Magnitude upon the Behavior of a Superconducting Bolometer in Its Transition Region20(1949); http://dx.doi.org/10.1063/1.1698237View Description Hide Description
A simple theoretical analysis of the superconductingbolometer in transition is shown to be useful in predicting phenomena such as the shape of the transition curve and the dependence of the bolometer time constant upon the current magnitude. The analysis holds for bolometer currents smaller than a critical value which is equal to the square root of the ratio of the bolometer cooling constant to the bolometerdR/dT. For bolometers constructed in this laboratory this critical current value falls between 15 and 30 ma.
Qualitative extension of the theory to the case of large bolometer currents is made by considering the presence of regions at different temperatures within the bolometer and by summarizing the possible effects of the magnetic field due to the bolometer current upon the transition. Experimentally obtained transition curves are given for both small and large bolometer currents.
20(1949); http://dx.doi.org/10.1063/1.1698238View Description Hide Description
This paper gives calculations and computations, based on simple ideal conditions, of the possible disturbances of underground temperatures caused by uplift and denudation. The effects of the speed of denudation, the rate of uplift, and the time during which these two processes may have been operating, are illustrated by numerical tables applying to some specific ideal situations. The resulting disturbance of the heat flow in the earth (which is evaluated by making use of underground temperature gradients) is discussed. It is shown that uplift and denudation increase the heat flow above its equilibrium value, but that no very large increase is likely to occur unless the equilibrium heat flow is small and the speed of denudation is large.
20(1949); http://dx.doi.org/10.1063/1.1698239View Description Hide Description
The E‐plane radiation pattern has been measured at short distances and both E and H‐plane patterns at relatively great distances from the mouths of several electromagnetic horns. Measured values are compared with values calculated by the Kirchhoff formula and in addition the distant patterns are compared with values calculated by the corrected formula due to Stratton and Chu.
For the range of horn length and aperture considered, the Kirchhoff formula gives satisfactory agreement in the E‐plane for angles as great as twenty degrees and at distances as short as one foot from the mouth of the horn; the H‐plane predictions are not as satisfactory.
20(1949); http://dx.doi.org/10.1063/1.1698240View Description Hide Description
In investigations of lubricated sliding friction between two crossed cylinders it is found that for hard, smooth surfaces the friction force is a straight‐line function of the applied load, although this line does not generally pass through the origin. The slope of this line defines a friction coefficient, designated as the incremental frictionf Δ, which is thought to have certain advantages in the study of non‐fluid lubricated friction. It is obviously independent of the load. It is found to be independent of viscous fluid effects and hence permits investigation of non‐fluid friction at reasonable speeds. In spite of being devoid of any viscous component, it is found in a great many cases to decrease with increasing speed, U, over at least part of the range, and this variation often appears to be a linear function of the logarithm of the speed. The curve of f Δ vs. ln U seems to be characteristic of the chemical structure of the lubricant and of the rubbing surfaces. It is not contended that viscous effects are absent under the conditions of these experiments but simply that the value of f Δ, which is not the totalfriction coefficient, does not, because of its method of determination, include such effects.
20(1949); http://dx.doi.org/10.1063/1.1698241View Description Hide Description
The fundamental optical problem encountered in three‐dimensional photoelasticity is that of the propagation of light in a heterogeneous, aeolotropic medium. Although there is an extensive literature on the homogeneous, aeolotropic medium and the heterogeneous, isotropic medium, only a few special cases of the combined problem have been studied from the point of view of Maxwell'selectromagneticequations. With regard to approximate treatments, the most general is that of F. Neumann (1841), derived from purely kinematical considerations. In the present paper the passage from Maxwell's to Neumann's equations is executed and the nature and order of magnitude of the necessary approximations are disclosed. During the passage, there are discovered a pair of equations, of relatively simple form, which open the way to a variety of new solutions in closed form and reveal the essential parameter upon which the optical phenomena depend.
20(1949); http://dx.doi.org/10.1063/1.1698243View Description Hide Description
Measurements of the absorption coefficients are made with a double crystal spectrometer in the parallel position, placing the sample between the two crystals, and between the second crystal and the chamber. In the first position the beam is cut down by the ordinary absorption and also by the power going into small‐angle scattering; in the second position it is cut down only by the ordinary absorption. The difference in the two measuredabsorption coefficients gives directly the total power in small‐angle scattering. From the latter, an average spherical particle size can be computed.
20(1949); http://dx.doi.org/10.1063/1.1698244View Description Hide Description
The lower limit of size of biological objects which can be photographed with the electron microscope, by the aid of shadow‐casting, is shown to depend upon the smoothness of substrate upon which they can be mounted, and upon the continuity of structure of the thin films with which they are shadowed. Numerous attempts to improve the existing deficiencies are reported, both with respect to producing smoother substrate films, and to producing films for shadow‐casting of high efficiency and continuity of structure.
No success has been encountered in producing a usable substrate film perceptibly smoother than the collodion and Formvar films commonly used. It is found that the best shadow‐casting material for this type of film is uranium or uranium oxide.
Verification has been obtained of the severe granulation of goldfilms previously used in the pre‐shadowed replica process, when subjected to the electron current of a biased‐beam electron gun. Attempts to reduce the granulation to a satisfactory level have failed. Various methods of preparing pre‐shadowed replicas are reported. The factors affecting the tenacity of evaporated films to glass surfaces are discussed. It is found that elements which oxidize readily are relatively adherent to glass, while the elements gold,palladium,platinum, and rhodium are not. Uranium sulfide can be used as a pre‐shadowed replica material, but only with some uncertainty, owing to its chemical instability. It has been found that a palladium‐platinum mixture is the most satisfactory material for use in the pre‐shadowed replica technique, and that films of this mixture in a thickness of about 6A produce adequate shadows in which there is no sign of granulation. The surface of clean glass is again found to have the smoothest structure of any material known, with practically no sharp discontinuities in elevation as great as 10A.
An appendix is given, in which technical details of shadow‐casting and replica production are described.
20(1949); http://dx.doi.org/10.1063/1.1698224View Description Hide Description
A simple numerical‐graphical method for the calculation of stresses in plastic zones adjacent to boundaries of arbitrary shape is outlined, applicable to problems of plane plastic flow in ideally plastic materials. For illustration, the method is applied to determine the stresses in plastic zones near elliptic free boundaries. The yield load of a tensile bar containing elliptic notches is obtained for a series of notches of varying sharpness. Some related problems are discussed briefly.
20(1949); http://dx.doi.org/10.1063/1.1698225View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698227View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698228View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698229View Description Hide Description