Index of content:
Volume 19, Issue 8, 01 August 1948
19(1948); http://dx.doi.org/10.1063/1.1698193View Description Hide Description
The methods of observation are described both for ``bursts'' of signal strength and ``whistles'' caused by reflection of radio waves from meteoric ionization. Consideration is given to the geometry of reflection, and it is shown that meteoric velocity and range can be accurately determined by combined pulse and whistle observations. The possibility of determining the direction and position of a meteor path by observations at three receiving locations is examined, and a method for interpreting such data is developed. An investigation is made of the fraction of meteors capable of producing bursts, and the important effect of the meteoric radiant is emphasized.
19(1948); http://dx.doi.org/10.1063/1.1698194View Description Hide Description
Three techniques have been found particularly useful in the analysis of the electron diffraction patterns produced in the scattering of electrons by gases. The first is the use of a continuously rotating shutter which is placed directly in front of the photographic plate used to record the patterns. The shutter has an angular opening proportional to the third power of the distance from the center of the pattern and serves to reduce greatly the high background intensity gradient which is characteristic of electron scattering. The second is the application of a Fourier computing scheme to the transformation from intensity curves, derived from observed patterns, to the ``radial distribution function'' which is of value in selecting a model for the molecule whose structure is being determined by the analysis. The third technique is the computation of patterns corresponding to various molecular models by means of a graphical synthesizer in (sinx)/x. This synthesizer has advantages over the usual numerical computing schemes in that it takes account of the angular variation of atomic structure factor for electron scattering and also makes evident the contributions of scattering by specific atom pairs to the complete synthesized intensity curve.
19(1948); http://dx.doi.org/10.1063/1.1698195View Description Hide Description
A study of the capabilities and limitations of the Formvar replica method has been made with more particular reference to shadow cast Formvar replicas. Observations have been made on the dependence of the deduced surface structure on the method of replication of the surface and the method of shadow casting the replica. The most suitable thickness of Formvar and evaporated metalfilm has been studied for various surfaces. Other replica methods have been summarized and compared with Formvar replica methods.
The structure of glass surfaces, both polished and fractured, is briefly discussed on the basis of observations made from Formvar replicas.
19(1948); http://dx.doi.org/10.1063/1.1698196View Description Hide Description
Pseudostructures, which are not real properties of electron microscope specimens but are introduced by the action of the electron beam, are described. Examples are given from samples of tetracopper calcium oxychloride, precipitates from slowly hydrolized ferric chloride solutions, and tungsten oxide to illustrate some of the effects which may occur. The polymorphism of these substances, particularly of tungsten oxide, is discussed.
The major effects which contribute to the pseudostructure formation are melting, evaporation, change of crystalline states, increase in degree of crystallinity, and migration of material. Reasons are given for concluding that the primary cause of specimen changes is to be found in the heating effect of the beam. The possibilities of secondary causes are also considered.
Artifacts may also be introduced into bacterial specimens. Observed changes in such samples are described briefly and examples are given.
19(1948); http://dx.doi.org/10.1063/1.1698197View Description Hide Description
A laboratory for testingoptical range finders constructed under the sponsorship of the Ordnance Department, U. S. Army is described. The central piece of the equipment, specially designed and constructed for the purpose, is a testing instrument that optically simulates targets at ranges from 855 yards to infinity, introducing the corresponding angular parallax between two beams of light at a separation of 15 feet, with angular errors not exceeding one second. This instrument is installed in a room with temperature control equipment and also provision for applying directed radiation to simulate extremes of temperature substantially as great as those encountered under service conditions. Such a laboratory provides convenient means of rapidly acquiring exact information regarding the behavior of a range finder and localization of the sources of error. It also enables one to predict how a given range finder will perform at any point on the earth's surface without the necessity of maintaining laboratories in the different temperature zones.
19(1948); http://dx.doi.org/10.1063/1.1698198View Description Hide Description
The errors of a range or height finder are divided, on the basis of origin, into four groups, namely: 1. External errors that originate from the lack of homogeneity in the air column between the instrument and target. 2. Instrumental errors that arise from defective performance of the instrument. 3. Physiological errors that are caused by physiological limits upon the performance of the observer's eye. 4. Psychological errors that have their origin in the psychological limitations of the ability of the observer to judge the range setting correctly.
Of these classes of errors, the designer can exert direct control, within limits, upon the instrumental errors. The three other classes cannot be controlled directly, but, given a qualitative and quantitative knowledge of their characteristics, it is possible to design a range finder or height finder in such manner as to lessen their detrimental effects. A brief discussion is given of the different known sources of error in each of the four classes, with typical values of the resulting errors as obtained by measurements made in the range‐finder testing laboratory established at the National Bureau of Standards under the sponsorship of the Ordnance Department of the Army.
19(1948); http://dx.doi.org/10.1063/1.1698199View Description Hide Description
We have developed a method for the evaporation of alloys in high vacuum which from the nature of the procedure used should be applicable to all alloys which are not refractory. The method has been tested on α‐brass and β‐brass and on a gold‐cadmium alloy. These alloys have components whose vapor pressures are very different and yet chemical and electron diffractionanalyses on the evaporated brass alloys and chemical analysis on the evaporated cadmium‐gold alloy show that the composition of the original alloy is held to within close limits.
19(1948); http://dx.doi.org/10.1063/1.1698200View Description Hide Description
The Laplacian probability equation can be used to predict the reproducibility of the intensities of x‐rays diffracted by crystalline powders. If absorption is neglected, and N is the number of particles irradiated, p the proportion of particles which are oriented so as to diffract, and q=1 − p, the relative mean deviation to be expected in repeated intensity measurements is given by Um =0.798 (q/Np)½. When absorption and the actual distribution of particle sizes are taken into account, the expression becomeswhere μ is the linear absorption coefficient of the material, ve the effective volume per particle, and A the cross‐sectional area of the incident beam.
The theory is confirmed by measurements on four size fractions of quartzpowder using a Norelco Geiger‐counter x‐ray spectrometer. For typical experimental conditions and powders of medium absorbing power for x‐rays, it is demonstrated that good reproducibility can be expected only if the effective particle dimension is less than approximately five microns.
19(1948); http://dx.doi.org/10.1063/1.1698201View Description Hide Description
Five methods are described for measuring the complex differential dynamic Young's modulus of rubber‐like materials, under conditions of very small sinusoidal strain variations, at frequencies from 10−1 to 105 cycles per second and in a temperature range which at its widest limits extends from minus 60 to plus 100 degrees centigrade. These methods are (1) measurement of the frequency of oscillation and the decrement of a ``rocking‐beam oscillator'' in which the restoring force is supplied by rubber (0.1 to 25 c.p.s.); (2) measurement of the frequency of resonance and the band width of a rubber reed which executes forced oscillation (10 to 500 c.p.s.); (3) measurement of the phase velocity and the attenuation of longitudinal waves in a thin rubber strip in the absence of standing waves (1 to 40 Kc); (4) measurement of the frequency of longitudinal‐wave resonance and the band‐width of a half‐wave transmission line (below 300 c.p.s.); (5) measurement of the change in the frequency of resonance and the change in the band width of a magnetostrictive rod against whose end a rubber sample is pressed (12 to 120 Kc). Equations are developed for calculating the complex modulus from the results of each experiment, and the practical advantages and limitations of the methods are described.
19(1948); http://dx.doi.org/10.1063/1.1698202View Description Hide Description
Based on Turnbull's concept of the importance of transient nucleation during phase transformations in solids, a general theory of nucleation has been developed. The nucleation mechanism for reactions where stable nuclei appear only during cooling or plastic deformation has been clarified. The general theory makes possible quantitative description of the rates of nucleation of all phase transformations as well as of fracture cracks, slip bands, and magnetic transformations in solids.
19(1948); http://dx.doi.org/10.1063/1.1698203View Description Hide Description
The construction of pole figures of crystal orientation data obtained by means of x‐rays can be made quickly and accurately if the angular relationships between planes or ``poles'' are known and if all of the data from a diffraction pattern are used. A flexible set of equations is derived from which these relationships can be found starting with customary crystallographic information. Data for an hypothetical case of diffraction are computed to illustrate the method. Although the examples refer, for the most part, to cubic crystals, the method can be extended to cover other types.
19(1948); http://dx.doi.org/10.1063/1.1698204View Description Hide Description
Gold smoke deposits have been examined at high resolution with the electron microscope, and the size and distribution of the unit particles determined. The smokes were produced by evaporating gold from a hot tungsten filament in a partial atmosphere of nitrogen, and the samples were collected on thin collodion films. It was found that the size of the unit colloidal gold particles varied with the pressure of the atmosphere and the rate of evaporation. The manner of aggregation of the particles was found to be independent of the nitrogen atmosphere pressure and rate of evaporation over a relatively wide range. The aggregations, however, changed markedly when a small amount of oxygen was present during the evaporation. Measurements in the visible and infra‐red regions of the spectrum were correlated with the electron microscope investigations, and it was found that the optical properties of the deposits depend strikingly upon the manner in which the smoke particles aggregate and how the aggregates are distributed.
19(1948); http://dx.doi.org/10.1063/1.1698205View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698208View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698209View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698210View Description Hide Description