摘要:

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 2–15&mgr; 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 (&agr;‐ or &bgr;‐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 ofcis‐trans‐isomers containing the structural unit of polyisoprene have been determined and correlated with the elastomer spectra. The results indicate but do not prove that hevea‐ and gutta‐type elastomers have the molecular structure ofcis‐ andtrans‐polyisoprene, respectively.The infra‐red absorption spectra of oriented crystalline films of hevea rubber and of gutta‐percha in both the &agr;‐ and &bgr;‐forms have also been obtained over the range 2–15&mgr; using polarized radiation. The observed dichroic effects lead directly to the conclusion that hevea‐ and gutta‐type elastomers arecis‐ andtrans‐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.

ISSN:0021-8979    

DOI:10.1063/1.1698259

出版商:AIP    

年代:1949

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1698260

出版商:AIP    

年代:1949

数据来源: AIP

摘要:

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 3×10−4cm, and finish No. 2D, average height 0.9×10−4cm) 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.

ISSN:0021-8979    

DOI:10.1063/1.1698261

出版商:AIP    

年代:1949

数据来源: AIP

摘要:

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 itactuallyoccurs, 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.

ISSN:0021-8979    

DOI:10.1063/1.1698262

出版商:AIP    

年代:1949

数据来源: AIP

摘要:

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 availablex‐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.

ISSN:0021-8979    

DOI:10.1063/1.1698263

出版商:AIP    

年代:1949

数据来源: AIP

摘要:

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 1–3&mgr; 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 CuAl2precipitate 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.

ISSN:0021-8979    

DOI:10.1063/1.1698264

出版商:AIP    

年代:1949

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1698248

出版商:AIP    

年代:1949

数据来源: AIP