Volume 11, Issue 8, 01 August 1940
Index of content:
A Graphical Method for Selecting Suitable Targets for Precision Determination of Cubic Lattice Constants and for Solving Cubic Powder Patterns11(1940); http://dx.doi.org/10.1063/1.1712805View Description Hide Description
Graphical charts from which the angles of diffraction lines may be obtained as a function of lattice constant and radiation have been prepared for cubic powder patterns. The charts are especially useful in predicting positions of back‐reflection lines in order that suitable targets for precision determinations of lattice constants may be chosen. Moreover, cubic powder patterns may be quickly indexed, beta‐lines identified, and lattice constants read to better than one percent. No calculations are necessary after the angles of diffraction have been measured.
11(1940); http://dx.doi.org/10.1063/1.1712806View Description Hide Description
In the steady‐creep stage in metals, stress (or resistance to deformation) is related to the velocity of deformation, in accordance with the hyperbolic‐sine law, or the exponential law. These laws, however, fail completely to describe the initial stage of plastic deformation in lead. The present series of tests is an investigation of this initial stage in compression. An optical lever system and high speed kymograph have been employed. The photographic records obtained show the progress of deformation with time. The duration of the initial stage increases with the compressing load. For a load of 120.0 kg the duration is about 0.16 sec. It is found that a unique, initial deformation, independent of time, is associated with each load. From measurements on the deformation curves, resistances as functions of time have been calculated. These data indicate a linear relationship between the resistance R and time t:where l=a parameter of small value, evidently a correction term, and m=rate of increase of resistance. Using this equation and the dynamic constants of the apparatus, deformation curves in good agreement with experiment have been derived. Resistance in the initial stage appears to be only approximately a function of deformation alone
11(1940); http://dx.doi.org/10.1063/1.1712807View Description Hide Description
A rigorous treatment is given of the problem of wave propagation in an elastic continuum when the influence of the initial stress is taken into account. After a short review of the theory various cases of initial stress are considered. It is shown that a uniform hydrostatic pressure does not change the laws of propagation. A hydrostatic pressure gradient produces a buoyancy effect which causes coupling between rotational and dilatational waves. Bromwich's equations for the effect of gravity on Rayleigh waves are derived from the general theory and the physical transition from Rayleigh waves in a very rigid medium to pure gravity waves in a liquid is discussed. The case of the vertical uniform stress is also considered and it is shown that the effect of the initial stress on the waves in this case cannot be accounted for by elasticanisotropy alone. Reflections may be produced by a discontinuity in stress without discontinuity of elastic properties.
11(1940); http://dx.doi.org/10.1063/1.1712808View Description Hide Description
From the viewpoint of torsional oscillations an internal combustion engine with a long crankshaft is generally considered to be equivalent to a uniform shaft carrying equidistant identical disks. It is here shown that advantage can be taken of the regularity of such a system to simplify the calculation of torsional oscillations. This is done by applying a mathematical method known as the calculus of finite differences. The procedure leads to a frequency equation (2.7) of remarkable symmetry in which appear as parameter the number n of cylinders in line and two simple functions K 1 and K 2 of the frequency which characterize the dynamical properties of the machines coupled at both ends of the crankshaft. These characteristic functions are of the nature of mechanical impedances, but due to their physical interpretation as a spring modulus (or spring constant) generalized to dynamic phenomena, the appellation dynamic modulus is being preferably used in the present paper. The concept of dynamic modulus is briefly introduced in the first section, while the second deals with the establishment of the frequency equation and an artifice for its rapid graphical solution avoiding the necessity of plotting an oscillatory function. Numerical applications to Diesel engines are treated in the last section. An example is also given of an extreme case where the fundamental frequency has a very low value and a special method is used for the calculation of this frequency.
11(1940); http://dx.doi.org/10.1063/1.1712809View Description Hide Description
A solution of the oscillatorequation is obtained in which is indicated the basic mechanism involved in the frequency control of oscillators. The well‐known fact that control of an oscillator is possible when the ratio of controlling frequency to oscillator frequency is m/n, where m and n are integral, is theoretically demonstrated.
11(1940); http://dx.doi.org/10.1063/1.1712810View Description Hide Description
The coefficient of friction has been experimentally determined in the thick film region of lubrication for both fixed angle and pivoted plane sliders as a function of the lubricant viscosity μ, the relative speed of sliding U, and the load W. Three steel, brass, or bronze sliders of one centimeter length and width constituted the stationary surfaces; the moving surface was of steel. Deviations between experiment and theory for the fixed angle wedges at low values of μN/W may be explained on the basis of variations and inaccuracies in the sliders themselves. However, in the case of the pivoted sliders the experimental results are in excellent agreement with theory, and, in fact, provide for the first time an absolute and quantitative direct empirical check of the hydrodynamic theory of lubrication.
11(1940); http://dx.doi.org/10.1063/1.1712815View Description Hide Description
Electrodes are devised by means of which rectilinear electron flow according to well‐known space charge equations can be realized in beams surrounded by charge‐free space. It is shown how these electrodes can be used in the design of electron guns having desirable characteristics.
11(1940); http://dx.doi.org/10.1063/1.1712831View Description Hide Description