Volume 21, Issue 2, 01 February 1950
Index of content:
21(1950); http://dx.doi.org/10.1063/1.1699625View Description Hide Description
Expressions are obtained for the instantaneous dependence of pressure upon radius within a collapsing cylindrical shell of incompressible liquid when subjected to (a) neither external nor internal pressure or to (b) an external pressure but no internal pressure. Dynamical considerations suggest that very high pressures and radial velocities should be reached.
21(1950); http://dx.doi.org/10.1063/1.1699626View Description Hide Description
The temperature distribution in an electric coil winding is determined by solving the differential equations which apply to the system approximately. The maximum and average temperature rises are given for several types of windings. A distinction is made between small and large temperature rise; the treatment of the former neglects local changes in resistivity and thermal conductivity caused by the increase in temperature. A correction factor is computed to obtain the actual temperature rise from the much simpler calculations for the maximum temperature rise which would occur if the resistance were independent of temperature. It is found that non‐uniform windings, wound of several different wire sizes to minimize the resistance, exhibit the same temperature rise characteristics as conventional uniform windings of zero curvature.
21(1950); http://dx.doi.org/10.1063/1.1699627View Description Hide Description
Experiments on the evaporation outgassing of antimony in vacuum and in an inert atmosphere show that an inert atmosphere is advantageous for outgassing antimony if loss of antimony by evaporation is intollerable. A simple theory based on well‐established kenetic theory considerations is given which may explain and did predict the results.
21(1950); http://dx.doi.org/10.1063/1.1699628View Description Hide Description
A discussion of deflection defocusing is given to point out the two‐dimensional character of the focusing action of deflection fields. A photograph of spot distortion actually observed is shown. A brief theory of combined spherical and cylindrical lenses is given, and a method of spot correction based on the principle of ``beam predistortion'' is described. An electron gun used for such correction is shown and experimental results obtained with this gun are exhibited.
21(1950); http://dx.doi.org/10.1063/1.1699629View Description Hide Description
The transmission of a plane elastic wave at oblique incidence through a stratified solid medium consisting of any number of parallel plates of different material and thickness is studied theoretically. The matrix method is used to systematize the analysis and to present the equations in a form suitable for computation.
21(1950); http://dx.doi.org/10.1063/1.1699630View Description Hide Description
The integral‐equation theory of coupled antennas developed by King and Harrison, Tai and Bouwkamp for two‐ and three‐coupled antennas is generalized to any number of units symmetrically arranged around a circle. The case of four antennas at the corners of a square is discussed in detail. Application to cage and corner‐reflector antennas is indicated.
The analysis of N parallel antennas arranged in line is carried out to determine the driving voltages required to maintain specified currents. It is shown that the inverse problem cannot be carried out in general except as a problem in simultaneous integral equations, but that the special conditions obtaining for half‐wave dipoles permit an approximate analysis for the currents with given voltages. This is carried through.
21(1950); http://dx.doi.org/10.1063/1.1699607View Description Hide Description
Dimensional analysis is used in a study of the behavior of metal when it is cut. Present theories are described and some additional significant variables are found. The analysis provides a basis for the interpretation of known experimental results, and should serve as a guide for future experimental and theoretical investigations.
21(1950); http://dx.doi.org/10.1063/1.1699608View Description Hide Description
A theoretical study has been made of the initial electron flow when a signal voltage V(t) is applied across an idealized planar diode. The analysis is based upon three fundamental postulates: Poisson's equation, Newton's equation, and the continuity equation. This quasi‐stationary study includes both the growth and motion of the space‐charge cloud of electrons across the interelectrode space and the transient build‐up of the current in the external circuit. The analytical forms and numerical plots of the current build‐up under space‐charge‐limited and temperature‐limited operation are compared for various ``large'' and ``small'' signal voltages.
21(1950); http://dx.doi.org/10.1063/1.1699609View Description Hide Description
The thermoelectric power of several refractorythermocouples was investigated, W‐Ta for temperature measurements up to 3000°C, W‐Mo up to 2600°C, Ta‐Mo up to 2600°C and W‐W/Mo (alloy 50 percent Mo/50 percent W) up to 2900°C.
Basic calibration was carried out using several melting points and the results were verified and interpolated by means of an optical pyrometer.
The couples W‐Ta and W‐Mo have been applied in this laboratory to the temperature measurement requirements in high temperature vacuum furnaces and determinations of spectral emissivity.
21(1950); http://dx.doi.org/10.1063/1.1699610View Description Hide Description
The interaction between the electromagnetic field in a cavity and an electron beam projected along the axis of the cavity is examined. The particular cavity considered here is of the cylindrical TE11nz type in a steady axial magnetic field. If the cavity is excited in a linearly polarized mode, the electromagnetic field will drive the electrons in a helical trajectory with an expanding radius, and the electrons will excite and transfer energy to a degenerate mode oriented spatially at right angles to the driving field. In the driving plane of polarization (both planes of polarization if the cavity is excited in a circularly polarized mode), the electron beam will excite a field in phase opposition to the driving field in a manner analogous to the counter e.m.f. in an electromechanical generator. The converse case of a TE11nz cavity excited by a spiral beam of electrons is also considered.
Geometrical Factors Affecting the Contours of X‐Ray Spectrometer Maxima. II. Factors Causing Broadening21(1950); http://dx.doi.org/10.1063/1.1699611View Description Hide Description
By applying Stokes' Fourier transform method for the analysis of diffraction maxima it is shown that the pure diffraction contour generated by a crystallite size distribution is apt to be approximated rather closely by the function 1/(1+k 2φ2). In the case of the x‐ray spectrometer this pure diffraction contour is broadened significantly by the action of the following five geometrical factors: (I) the x‐ray source width, (II) flat rather than curved sample surface, (III) vertical divergence of the x‐ray beam, (IV) penetration of the sample by the beam, and (V) the receiving slit width.
The broadening of the pure diffraction contour due to the action of each of the five factors and the breadth of the final contour generated by the instrument can be deduced by employing the convolution approach suggested by Spencer. The effect of each instrumental factor is expressed by a convolution equation of the form,in which φ is the angular displacement from twice the ideal Bragg angle, 2θ, f i−1 is the contour before the action of the ith geometrical factor, Wi is the form of the ith geometrical factor, and fi is the contour after the action of Wi on f i−1. Starting with a pure diffraction contour of the form 1/(1+k 2φ2), generalized broadening curves are derived for the effect of each of the five geometrical factors. Using these curves it is possible to predict the breadth of the final diffraction contour generated by the spectrometer from an initial contour of any breadth.
21(1950); http://dx.doi.org/10.1063/1.1699612View Description Hide Description
The accuracy of the Scherrer crystallite size equation is limited in part by the uncertainty in β, the experimentally deduced pure diffraction broadening. Currently used procedures for estimating β from the observed breadth of a Debye‐Scherrer line are not, in general, applicable to the x‐ray spectrometer.
By making use of a scheme of convolution analysis for analyzing the effect of geometrical factors in broadening the pure diffraction contour, a correction curve is developed for determining β from the experimentally measured line breadths b and B (Jones' notation). The degree of reliability of this correction procedure is ascertained by applying Stokes' direct Fourier transform procedure for determining the form of the pure diffraction contour free of instrumental effects.
Suggestive procedures are given for crystallite size determination with the x‐ray spectrometer in different size ranges, and several examples are described.
21(1950); http://dx.doi.org/10.1063/1.1699613View Description Hide Description
A mass spectrometer tube using velocity selection is described. The tube has parallel plane grids made with knitted wire nets with a large percentage of open area. The grids are arranged in groups of three, and radiofrequency alternating potential is applied to the middle grid of each group. Stopping potentials are used to turn back all ions except those with the selected mass. The spectrometer has sufficient resolution for ordinary gas analysis requirements and is simpler, more compact, and more rugged than magnetic beam deflection devices.
21(1950); http://dx.doi.org/10.1063/1.1699614View Description Hide Description
A method has been developed for the study of grain boundary migration in high purity aluminum, which allows positive determination of the direction of grain boundary movements. By means of this method it was confirmed that the migration of grain boundaries resulting from surface energy takes place in the direction toward the centers of curvature of the boundaries. On the other hand, strain induced grain boundary migration, such as occurs in recrystallization, proceeds in a direction away from the centers of curvature of the moving boundaries. It was also found that certain strained grains serve as nuclei for strain‐free grains of the same orientation which grow at the expense of neighboring strained grains. Most of the strain‐free grains formed during the annealing of high purity aluminum cold rolled up to about 40 percent are produced by this mechanism of strain‐induced boundary migration, without the formation of new nuclei.
21(1950); http://dx.doi.org/10.1063/1.1699615View Description Hide Description
Expressions are derived for the external field produced by a slot of arbitrary shape in the wall of a circular wave guide (of infinite extent and infinite conductivity), the tangential components of the electric field in the slot being assumed to have been prescribed. This is accomplished by matching a Fourier representation of the external field, built up by superposition of basic sets of cylindrical waves, to a Fourier expansion of the prescribed field over the surface of the cylindrical wave guide. The far‐zone field is obtained by applying the method of steepest descent to the Fourier integrals that constitute the coefficients in the expansion for the external field. The results satisfy the radiation conditions for far‐zone fields.
21(1950); http://dx.doi.org/10.1063/1.1699616View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699617View Description Hide Description
This paper presents an approximate method of determining the pressure distribution during impact on the surface of a body which strikes a horizontal water surface at an arbitrary angle.
The effect of the splash is neglected and the pressure on the free boundary is assumed proportional to the potential, as if the process were an impulse. The shape of the submerged portion of the striking body is approximated by a semi‐ellipse (two dimensions), by a hemisphere, half an ellipsoid of revolution and half a general ellipsoid. Under those conditions, explicit results for the pressure distribution are found. As a special example, the impact pressures on a sphere striking at 45° angle are computed in detail.
21(1950); http://dx.doi.org/10.1063/1.1699618View Description Hide Description
Contrary to current belief, cavitation has been found responsible for the depolymerizing effect of intense ultrasonic waves. This was demonstrated by irradiating two portions of a 1 percent polystyrene solution in toluene under conditions identical except for the following. The first portion was given no special prior treatment, showed many cavitation bubbles during irradiation, and decreased in molecular weight (as measured by the intrinsic viscosity) to about one‐tenth of the initial value. The second portion was given a preliminary treatment of degassing by boiling under vacuum, showed no cavitation bubbles during the irradiation, and underwent no appreciable change in molecular weight.
Similar experiments with solutions of hydroxyethyl cellulose in water showed that, in this case also, cavitation is necessary for depolymerization. The opposite conclusion of earlier investigators is attributed to their inadequate method for eliminating cavitation.
Oxidants known to be produced by ultrasonic waves in solutions containing dissolved oxygen or nitrogen cannot be responsible for the degradation, because substantially the same amount of depolymerization occurs even when helium is the only gas present.
A Three‐Stage Electron Microscope with Stereographic Dark Field, and Electron Diffraction Capabilities21(1950); http://dx.doi.org/10.1063/1.1699619View Description Hide Description
An electron microscope is described in which a range of magnifications between 1000 and 100,000 times can be covered continuously. This is achieved through the use of an intermediate projector lens.
The principles of design are briefly discussed and the construction of the microscope and the high frequency‐operated 100‐kv power unit is dealt with in some detail. The instrument is provided with objective apertures to enhance image contrast and to permit operation with dark field illumination.
Data is given relating to the electron optical system and it is shown how the instrument can be used to obtain electron diffraction patterns characteristic of small areas of a specimen under microscopic observation. The alignment procedure is briefly described and some typical micrographs are shown.
21(1950); http://dx.doi.org/10.1063/1.1699620View Description Hide Description