Volume 20, Issue 10, 01 October 1949
Index of content:
20(1949); http://dx.doi.org/10.1063/1.1698249View Description Hide Description
This is a quantitative experimental study of the primary ionization of paraffin wax by beta‐radiation. Electrons emitted by uranium and carbon 14 were used. Interpretation, by means of a simple theory, of experimental data on the electrical conductivity permits evaluation of the ionic mobility in paraffin and the efficiency of ion‐pair production. Only small primary ions of relatively long mean life are detected by the conductivity method. Dipolar admixtures promote secondary reactions and lower the level of conductivities attained. No ionization was detected in two hydrocarbon polymers.
20(1949); http://dx.doi.org/10.1063/1.1698250View Description Hide Description
The open two‐wire line is investigated as a measuring instrument and difficulties in its operation are overcome. These include, in particular, the problem of balance. The impedance of antennas is shown to depend on the nature of the driving structures. A comparison of measured impedances and corresponding theoretical values shows good agreement.
20(1949); http://dx.doi.org/10.1063/1.1698251View Description Hide Description
The relation between elastic and plastic strains and the martensite transformation from face‐centered to body‐centered structure has been investigated in Fe‐Ni 71–29 alloy. Transformation begins at about −30°C and is of the martensite type except that it is not all completed instantaneously but continues at decreasing rate at constant temperature.
It was found that (1) There is no anomalous decrease in elastic moduli as the Ms temperature is approached, nor is Ms affected by applied elastic stresses. (2) Plastic deformation in a temperature range above Ms causes partial transformation in amounts decreasing to zero at temperature Md≈Ms +75°C, but stabilizes against further transformation by cooling. (3) Plasticyield stress increases sharply from Md to Ms then drops suddenly to a low value just below Ms . (4) The Ms temperature depends on grain size; a factor of ten increase in grain size reduces Ms about 30°C.
These results are contrary to expectations on the basis of the usual homogeneous shear model for the transformation mechanism. They indicate that the martensite phase becomes thermodynamically stable at Md but that transformation does not begin until Ms because of surface and strain energy considerations in growth of a martensite plate from a small nucleus.
A High Temperature Precision X‐Ray Camera: Some Measurements of the Thermal Coefficients of Expansion of Beryllium20(1949); http://dx.doi.org/10.1063/1.1698252View Description Hide Description
A high temperature precision x‐ray camera has been designed and constructed by modification of the familiar back‐reflection symmetrical focusing type of camera. The camera, specimen, and furnace are used within a vacuum chamber to avoid oxidation. The apparatus in its present form has been employed at temperatures up to 1000°C, but higher temperatures could be easily attained by making a few minor changes in the furnacematerials.
The lattice parameters of beryllium have been measured up to 1000°C. The derived coefficients of thermal expansion, both linear and bulk, are presented as a function of temperature. In addition, the data present strong evidence that the hexagonal close packed form of beryllium which exists at room temperature is stable up to at least 1000°C, contrary to several reports in the literature.
20(1949); http://dx.doi.org/10.1063/1.1698253View Description Hide Description
At very high frequencies, the effect of losses in coupling devices makes itself felt in the determinations of the various Q's and of the circuit efficiency of resonant systems. It is shown that the neglect of these losses (which appear in equivalent circuits in the form of series resistance), can lead to appreciable error. Formulas are derived and curves are plotted which permit an unambiguous determination of the Q's and of the circuit efficiency when the following items are known:
1. the standing wave ratio at resonance,
2. the standing wave ratio far from resonance, and
3. knowledge as to whether the resonant system under study is ``under'' or ``overmatched'' to the external load.
20(1949); http://dx.doi.org/10.1063/1.1698254View Description Hide Description
There are a number of important potential problems having axial symmetry and involving a conical boundary whose solutions require the use of zero‐order Legendre functions of real but non‐integral degree. The numerical evaluation of such problems is relatively simple once the zeros of these functions have been determined and when certain integrals have been evaluated. This paper presents tables and curves giving this information, together with a number of approximate expressions for these quantities. A number of potential distributions involving conducting cones, spheres, and rings are worked out to illustrate the application of these functions.
20(1949); http://dx.doi.org/10.1063/1.1698255View Description Hide Description
An apparatus is described which produces homogeneous mists or clouds of solid particles of any desired size. As in Walton's original model liquid is sprayed by a Beams high speed air‐driven top, but an improved performance has been achieved by utilizing a property of high speed air films so that automatic extraction of unwanted satellite droplets, better running characteristics over a wider range, simplified construction, and low air consumption are obtained.
20(1949); http://dx.doi.org/10.1063/1.1698256View Description Hide Description
The current and temperature distributions in a conductor are affected not only by the magnetic effect of the current, i.e., skin effect, but also by the variation of resistivity with temperature. Consequently the two are interdependent, and since the resistance change amounts to between three and four percent for every 10°C rise in temperature in metals, and is still greater in insulators, its effect can be quite appreciable. A complete solution for the steady state is given herewith.
20(1949); http://dx.doi.org/10.1063/1.1698257View Description Hide Description
The formation of precipitates from the solid solutions of C and N in α‐iron has been studied by means of the internal friction peak associated with the stress‐induced interstitial diffusion of the solute atoms. A transformation law has been found to fit the experimental data. The data are interpreted as showing the following: (1) Fe3C precipitates in the shape of spheres; (2) an intermediate phase in the precipitation of nitrogen forms in the shape of disks; and (3) within the experimental error there is indicated no continuous nucleation. Using the results of a derivation made by Zener, the mean distance between nuclei is calculated for Fe3C. For carbon concentrations of about 0.016 wt. percent, this distance is about 800, 2000, and 7000 Angstroms at 27°C, 102°C, and 312°C, respectively. With appropriate experimental conditions, the temperature dependence of the precipitation rate is found to be about the same as the temperature dependence of the diffusion rate of the solute atoms.
20(1949); http://dx.doi.org/10.1063/1.1698258View Description Hide Description
The radius of a spherical precipitate particle growing in a solid solution of initially uniform composition may be shown to be equal to α(Dt)½, where D is the atomic diffusion coefficient, t the time of growth, and α, the growth coefficient, is a dimensionless function of the pertinent compositions. In this paper the precise dependence is found of this function upon the pertinent concentrations. A similar computation is made for the growth coefficient corresponding to the one‐dimensional growth of a plate.
20(1949); http://dx.doi.org/10.1063/1.1698259View Description Hide Description
The infra‐red absorption spectra of the purified rubber hydrocarbon fractions of several naturally occurring elastomers (hevea, guayule, kok‐saghyz, gutta‐percha, balata, and chicle) have been determined at room temperature over the range 2–15μ and it has been shown that the elastomers can be classified on the basis of the spectral data as belonging to one or the other of two different types, namely the hevea type or the gutta type. Both types of elastomers have also been studied over the temperature range −25 to +80°C and reference spectra for both the amorphous and the solid (crystalline) phase(s) occurring in this range have been determined for each type. At room temperature the spectral differences between the hevea (amorphous state) and gutta (α‐ or β‐crystalline modifications) types are much more pronounced than is the case when the spectra of both types in the same (amorphous) states are compared.
The spectra of two pair of cis‐trans‐isomers containing the structural unit of polyisoprene have been determined and correlated with the elastomerspectra. The results indicate but do not prove that hevea‐ and gutta‐type elastomers have the molecular structure of cis‐ and trans‐polyisoprene, respectively.
The infra‐red absorption spectra of oriented crystalline films of hevea rubber and of gutta‐percha in both the α‐ and β‐forms have also been obtained over the range 2–15μ using polarized radiation. The observed dichroic effects lead directly to the conclusion that hevea‐ and gutta‐type elastomers are cis‐ and trans‐polyisoprene, respectively. Although it has not been possible to deduce the exact spacial configuration of the molecules in the crystallites, it has been indicated that the planar isoprene models are unsatisfactory and that the results are in close agreement with the twisted models proposed on the basis of x‐ray diffraction.
Further use has been made of the polarization data in proposing nearly complete but tentative interpretations of the spectra of both hevea‐ and gutta‐type elastomers. The possibility of making use of the vibrational assignments for the interpretation of spectral changes produced by oxidation or other forms of degradation is pointed out.
20(1949); http://dx.doi.org/10.1063/1.1698260View Description Hide Description
Single crystal magnetic properties have been closely approached in a polycrystalline 50 percent nickel‐iron by the development of a favorable grain orientation. The method is adapted to commercial production by careful control of processes effecting the fundamental magnetization process in the magnetic domain. Elimination of steps I and III of the normal magnetization has produced a nearly geometrically true rectangular shape of the hysteresis loop.
20(1949); http://dx.doi.org/10.1063/1.1698261View Description Hide Description
The minimum tilt at which bearing balls roll down an inclined plate of stainless steel is greater for rough than for smooth surfaces. The roughest surfaces (finish No. 1, average height of elevations 3×10−4 cm, and finish No. 2D, average height 0.9×10−4 cm) gave an almost quantitative agreement between this height of elevations and the height of hills calculated from a theory attributing rolling friction to surface roughness. No similar agreement was observed for fine finishes, presumably because the balls used (radii between 0.159 and 0.635 cm) were too big for the irregularities on these surfaces. The elastic deformation of the surfaces in contact was irrelevant for coarse finishes. Capillary attraction presumably did not interfere as superficial drying of the system had no effect.
Surface roughness which is the cause of Coulomb's sliding friction thus seems to cause also rolling friction at low pressures.
20(1949); http://dx.doi.org/10.1063/1.1698262View Description Hide Description
With the aid of an idealized representation of the discriminator, Middleton has shown that a frequency‐modulated carrier remains intelligible for smaller carrier strengths when narrow‐band f‐m is used and the limiter is omitted than with a limiter and/or wide‐band f‐m. It is, therefore, of some interest to treat the demodulation process as it actually occurs, in the absence of a limiter. Thus, the discriminator has been taken to consist of two selective circuits, both fed by the output of the intermediate‐frequency amplifier, but peaked at different frequencies, feeding rectifiers whose outputs are subtracted. Each half of the device is treated in the manner used by Rice to determine the result of passing random noise and a sine wave through a rectifier; however, there is a correlation between the noise voltages fed to the two rectifiers. The signal output and the spectral distribution of the noise output are obtained first for quadratic rectification, then in the general case, which is then specialized to linear rectification. The results are applied to a case of rectangular i‐f noise spectrum, and the signal‐to‐noise ratio is determined for the cases of narrow‐band and wide‐band f‐m. These results are found to be very much like those for the idealized representation of the discriminator; all are compared, along with amplitude modulation, in Table I. The optimum signal‐to‐noise ratio for f‐m without a limiter is found to obtain with narrow‐band f‐m when the discriminator is designed for no wider a band than necessary; this optimum signal‐to‐noise ratio differs very little from that for a‐m.
20(1949); http://dx.doi.org/10.1063/1.1698263View Description Hide Description
The use of simple, aperture‐collimated cameras in x‐ray diffraction studies at small angles is of value in investigating the molecularly large structural features of colloidal systems, particularly when pinhole collimation is required. To make effective use of available x‐radiation one must (1) consider the initial limitations imposed on a camera by the purpose for which it is to be employed, and (2) choose remaining camera dimensions so that the registering film may receive as much radiation as possible.
The initial conditions are three in number, involving the desired pattern magnification, the necessary angular or large‐spacing resolution, and the requisite guarding against beam spill‐over, which together essentially limit the specimen‐to‐film distance, the beam diameter at the film, and the diffraction area permitted to be unguarded but covered by a beam stop. Other camera dimensions are chosen so that the film center is as close to, while viewing as much of, the radiation source as possible. In accomplishing these aims it is useful to align the camera at near grazing incidence to the x‐ray tube target, though there are limitations to this procedure.
Rules for systematically accomplishing the above purposes are derived herein for the extremes of symmetrical (pinhole) and unsymmetrical (slit) aperture systems, as well as for ones employing a simple class of rectangular (unsymmetrical pinhole) aperture. Criteria are also given for evaluating the relative speeds of cameras which may depart from optimum because of mechanical limitations. Finally is described the construction of a series of cameras, found useful in studying the fibrous proteins, particularly collagen, and typical diffraction patterns obtained therefrom are reproduced.
20(1949); http://dx.doi.org/10.1063/1.1698264View Description Hide Description
Bethe's dynamical theory of electron diffraction in crystals is developed using the approximation of nearly free electrons and Brillouin zones.
The use of Brillouin zones in describing electron diffraction phenomena proves to be illuminating since the energy discontinuity at a zone boundary is a fundamental quantity determining the existence of a Bragg reflection. The perturbation of the energy levels at a corner of a Brillouin zone is briefly discussed and the manner in which forbidden reflections may arise at a corner pointed out. It is concluded that the kinematic theory is inadequate for interpreting electron images of crystalline films.
An electrolytic method for preparing thin metal sections for electron microscopy and diffraction is introduced and its application to the structure of cold‐worked aluminum and an aluminum‐copper alloy demonstrated. It is concluded that cold‐worked aluminum initially consists of small, inhomogeneously strained and disoriented blocks about 200A in size. These blocks are not revealed by etching but would contribute to line broadening in the conventional diffraction experiments. By means of a reorientation of the blocks through a nucleation and growth process, larger disoriented domains about 1–3μ in size found experimentally could be accounted for. It is suggested that such a nucleation and growth reorientation phenomenon is responsible for self‐recovery in cold‐worked metals.
The formation of CuAl2 precipitate particles is demonstrated with both electron micrographs and diffraction patterns. A fine lamellar structure found in the quenched Al‐4 percent Cu alloy is at present unexplained.
20(1949); http://dx.doi.org/10.1063/1.1698245View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698246View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698247View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698248View Description Hide Description