Showing papers in "Journal of Applied Physics in 1942"

An X‐Ray Study of Carbon Black

Abstract: X‐ray studies have been made of a number of carbon blacks, prepared under different conditions, and subject to various heat treatments. The patterns were made in evacuated cameras, using CuKα radiation monochromated by reflection from rocksalt. The patterns consist of crystalline reflections (00l), and two‐dimensional lattice reflections (hk). The structure is one of true graphite layers arranged roughly parallel and equidistant, but otherwise completely random. The dimensions within a layer are the same as in graphite; the layer separation is somewhat larger than in graphite. The effect of heat treatment is to increase the size of the parallel layer groups. At graphitization the material changes discontinuously to the crystalline graphitestructure. The usual carbon black is not finely divided graphite. Small angle scattering studies indicate the existence of clusters a few hundred angstroms in size. It is these clusters which are measured by microscope counts, by the electron microscope, and by surface areas, rather than the much smaller parallel layer groups.

Demagnetizing Factors of Rods

Abstract: A chart is constructed for converting the apparent magnetic permeability to the true permeability of cylinders of any given ratio of length to diameter The curves are based on previous calculations in which account was taken of the variation of the demagnetizing factor N with permeability, but in which the permeability was assumed constant over any one cylinder The flux distribution has been determined experimentally in several cylinders, and as the field acting on the specimen is increased from zero, the positions of the effective poles have been found to move toward the middle of the rod until (if the permeability is sufficiently high) they are located about 07 of the distance from the middle to the ends, and then to move toward and approach the ends as limits as the permeability declines in high fields

An Electron Microscope Study of Some Structural Colors of Insects

Abstract: The electron microscope has been used to study two types of structures responsible for the physical colors of insects. The iridescence of the beetle Serica sericea is due to a line grating on its wing‐covers, 0.8μ between lines. The structure giving rise to the brilliant blue color of the tropical butterfly Morpho cypris is quite elaborate, consisting essentially of hundreds of vanes on each wing scale, the vanes possessing linear thickenings 0.2μ apart which reinforce the reflection of blue light. Numerous exceedingly fine markings on parts of the wing scales go down in dimensions to 60A, and some of them may be related to the chemical structure of the scales. Chemical tests show that the scales are not composed of chitin, the principal component of the external parts of most insects. It is interesting that bombardment of the scales with an intense electron beam (1000 electrons per square angstrom per second) for a matter of minutes does not appreciably affect the swelling properties of the scales in organic liquids.

Specific Surface Measurements on Compact Bundles of Parallel Fibers

Abstract: The flow of air through compact bundles of parallel textile fibers has been studied in order to obtain values of the shape factor k0 for the channels through which the flow takes place. When the fibers are cylinders and the flow is parallel to their axes, k0 is an increasing function of the porosity e. Tables and curves showing this relationship are given. For fibers such as cotton, where the shape and size of cross section changes along the fiber length, k0 changes less rapidly with e. Data are given for this relationship.

Surface Replicas for Use in the Electron Microscope

Abstract: A simple method is described for making replicas of surfaces for study in the electron microscope. The surface to be studied is cleaned and then coated with a very thin slightly wedge‐shaped film of polyvinyl formal having a range in thickness from about 500 to 750A in 2.5 cm. This film is formed by dipping the prepared surface into a 0.5‐percent solution of the resin dissolved in dioxane or ethylene dichloride and permitting the solvent to evaporate while the surface is held in a vertical position. The replica film is removed by placing the coated surface under water and peeling the film from it. A description of this technique is given in detail. A graph is presented which shows the thickness of resin films obtainable from various concentrations of resin in dioxane, and a table gives the interference effects which may be used for determining approximate thicknesses of thin resin films. Several easy procedures are described for checking the thickness of such films. A technique is described for preparing strong films having a thickness of 100A and an area of 3 cm2 when mounted in air. Such films are very useful when working with the electron microscope and several methods are described for mounting specimens on them, including a conditioning treatment—useful in mounting water suspensions—which makes one surface very hydrophilic. Five micrographs show the results obtainable with the techniques described.

Fatigue of Fabrics

Abstract: The effect of temperature and time of application of load on the growth and the apparent ``tensile strength'' of fabrics was studied with a number of new tests. The results gave considerable information about the mechanism of failure of mechanical fabrics at low loads in service which could not be obtained with standard textile tests.A ``fatigue'' test was developed which measured the lives of samples at elevated temperatures when subjected to a constant average load and a superimposed cyclic stress. The change in life with temperature on this test is enormously greater than the change of tensile strength with temperature. Over a considerable range the logarithm of the life under a given load is a linear function of the reciprocal of the absolute temperature, showing that the failure depends on a viscous or plastic flow within the fibers. Calculated activation energies for this flow process increase for different fibers in the order rayon, cotton, Nylon. The construction of the fabric affects the absolute...

A Theoretical Approach to the Problem of Yarn Strength

Abstract: An idealized yarn composed of fibers with specified properties is treated analytically with the aim of determining the yarn strength at any degree of twist. The results are presented in the form of equations and curves which relate the yarn strength to the fiber properties and the degree of twist. Two cases are studied: (1) All fibers alike; (2) fiber properties variable from fiber to fiber. In the latter case the mathematical expectation of the yarn strength at any yarn cross section is obtained. It is found that the optimum twist multiplier is largely determined by the fiber length, fiber fineness, and coefficient of friction, whereas the maximum yarn strength (corresponding to the optimum twist multiplier) is more strongly dependent upon the intrinsic fiber strength than upon the other fiber properties studied.

Energy and Energy Flow in the Electromagnetic Field

Abstract: A new method for deriving the usual Poynting vector is given. This method also yields other equally valid energy flow vectors with the same or other equally valid postulated electromagnetic energy densities. Examples of such alternative Poynting vectors with their associated energy densities are given. A definition is given which distinguishes between the conduction and displacement currents in matter. It is shown that by addition of a term, the postulated energy flow widely used by electrical power engineers and based on wattmeter readings becomes a valid alternative Poynting vector.

Variation of the Axial Aberrations of Electron Lenses with Lens Strength

Abstract: The variation of refractive power and spherical aberration with electrode voltages and field strengths is studied for two characteristic unipotential lenses, an immersion lens, and a magnetic lens. Conclusions are drawn herefrom regarding the variation, with lens strength and applied voltage, of the resolving power obtainable with the lens as an electron‐microscope objective. Scattered measurements by other authors agree satisfactorily with the results. The ``relativistic aberration'' of the electrostatic unipotential lenses, i.e., the effect on the image of fluctuations in the over‐all applied voltage, is calculated and shown to be of significance in the electrostatic electron microscope. Furthermore, the axial chromatic aberrations are computed for the four systems and the question of upper limits of the last two aberrations is discussed.

Calculation of Axially Symmetric Fields

Abstract: A practical method is developed for calculating the distribution of scalar fields in axially symmetric systems. To obtain a complete solution a knowledge of the potential distribution along a cylinder of constant radius is required; in many cases the distribution along the cylinder can be estimated with sufficient accuracy by an inspection of the electrode configuration. The solution is first obtained in the form of an integral. This is evaluated for the class of boundary potential functions of the form Vn(1, z)= zn,z>00,z<0. In this expression, Vn(1, z) is the potential along a cylinder of unit radius, z is measured along the axis of the cylinder, and n is a positive integer (zero included). Numerical values are given in tabular form for the cases n=0, 1, 2 and 3. It is then shown how the solution for the above boundary potential functions can be used to obtain the solution for an arbitrary distribution of potential along a cylinder of constant radius. In particular, the potential distribution of a simpl...

On Relaxation Effects in Amorphous Media

Abstract: The Maxwell relaxation theory leading to the concept of a viscous‐elastic body is briefly stated. Extensions to systems built up from several such viscous‐elastic components with different relaxation times are discussed. In the case of high molecular weight materials, it is more appropriate to assume a continuous distribution of relaxation rates. Equations are formulated whose solution gives the distribution function in terms of the rate of deformation and of the applied stress or the elongation in terms of the two other quantities. The connection with the corresponding distribution of dielectric relaxation times, molecular inhomogeneity and chain branching, and effects at mechanical frequencies large compared with the relaxation rates, are pointed out.

The Cause of High Voltage Surges in Rectifier Circuits

Abstract: It is shown that high voltage surges are produced in apparatus associated with gas discharge devices when, and only when, the demand for current exceeds the current‐carrying capacity of the gas (or vapor). Cathode spot extinction and sputtering of hot cathodes cause only slight disturbances which do not produce high voltage in practical circuits. Experimental data on surge limits, i.e., the maximum currents that can be carried without surges, are given for four types of mercury vapor tube. These data lead to a single set of values of maximum current density as a function of vapor pressure, which may be used for the safe design of apparatus.

Bending settlement of a slab resting on a consolidating foundation

Abstract: The calculation of the settlement and bending of an elastic slab resting on a consolidating foundation under the action of a load concentrated on a line. Two cases are considered: first, when the slab is perfectly pervious to water, and second, when it is impervious. The problem is two‐dimensional.

Diffusion of Heat Through a Rectangular Bar and the Cooling and Insulating Effect of Fins. I. The Steady State

Abstract: Exact solutions for temperature distribution and heat flow through a rectangular fin are rederived in more general and simple form than previously given. These are studied numerically and graphically and shown to yield new results not contained in the approximate formulas hitherto used. Except for the factor of thermal conductivity of the fin material, the cooling effect is a function only of the height‐width ratio β of the fin, and of a dimensionless ``relative boundary resistivity'' μ governing the heat loss to the environment. It is shown that, for any value of this boundary resistivity, there is a fairly sharp optimum value of the height‐width ratio beyond which further increase in height will not lead to much improvement in cooling effect. This optimum β decreases with decreasing μ until, at a certain critical value μ* in the neighborhood of 1, it becomes zero. At the critical μ*, the cooling effect is altogether independent of β, and the presence of the fin is a matter of indifference. Below μ*, the cooling effect decreases with increasing β, so that the presence of the fin serves merely to insulate the hot plate. This analysis of a single fin is the basis upon which a study of the conditions for optimal cooling by an array of fins will be carried out in a subsequent publication.

An Extension of Saint‐Venant's Principle, with Applications

Abstract: Simple energy considerations, which have previously been employed to provide a rational basis for the principle of Saint‐Venant, are shown to lead to the conclusion that forces applied in the neighborhood of a rigidly fixed portion of an elastic solid can cause only local stress and strain This principle indicates useful features of certain problems encountered in engineering, and is used here in the calculation of the stiffness of a thin layer of elastic material confined between rigid plates, under any kind of load applied to the plates

Measurement of Relative Abundance with the Mass Spectrometer

Abstract: This paper covers the general methods employed in mass spectrometry, machine design, and possible sources of error in relative‐abundance measurements which are inherently associated with instrument design. Also included are graphs for determining the critical constants of the usual types of instruments.

Electrostatic Electron Lenses with a Minimum of Spherical Aberration

Abstract: A method is suggested for the construction of electrostatic electron lenses by building equipotential surfaces whose position in space is calculated from an assumed axial potential. The calculations are carried through for examples of symmetrical and immersion lenses. The symmetrical lens chosen is the well‐known one with minimum spherical aberration represented by an axial potential V(z) = V0+A exp (−Bz2) and it is shown that this lens can actually be constructed. An axial potential formed by a sum of two exponential terms is found, for suitable values of the constants, to reduce the spherical aberration to less than one‐half that of the above symmetrical lens. An expression is proposed for the immersion lens with minimum spherical aberration. The aberration for this lens is approximately one‐half that of the above symmetrical lens. It is shown that the usual formula for the focal length of a strong lens is merely the first term in an infinite series and that serious inaccuracies occur by not considering...

The Mechanism of the Electric Spark

Abstract: Meek's criterion for the formation of a spark discharge in a gas is criticized. An error in the derivation of his formula for the radial field about an electron avalanche channel is noted, and the value adopted for this field is shown to be too small to draw the postulated number of electrons into the channel from the space outside. The importance attached to the radial field is found to be misplaced because of the above and because a criterion based solely on the total number of ions produced in an avalanche gives results for sparking potentials for different spark lengths and for a gas at different pressures in agreement with those obtained by Meek's method. Both methods give results which, using the values assigned to the constants, fall more and more below the experimental values of the sparking potentials as the length of the spark gap, δ, is increased until at 20 cm the deviation is over 7 percent. To bring the computed value for δ=20 cm into agreement with the experimental value the constants in th...

The Photographic Action of Electrons in the Range Between 40 and 212 Kilovolts

Abstract: The Hurter and Driffield curves and the absolute sensitivities of Eastman medium lantern slide plates are determined for electrons accelerated through differences of potential from 40 to 212 kilovolts. Within experimental error the H and D curves are identical in shape for the different voltages. The sensitivity reaches a peak near 100 kv and then drops off rapidly. This phenomenon may be utilized to increase the effective sensitivity of plates at high voltages by applying a thin, high density intensifying screen to the surface of the emulsion.

A Rapid Infra‐Red Analytical Method for Hydrocarbon Mixtures and A Routine Spectrophotometer for Plant Control

Abstract: A rapid method has been developed for the infra‐red spectroscopic analysis of simple mixtures of hydrocarbons. Through the construction of special equipment this method has been reduced to a routine procedure which can be carried out by comparatively inexperienced operators. The instruments built have proven sufficiently sturdy to be used for control work in plant operation. By basing the analytical method on an empirical calibration, the calculations involved have been reduced to a minimum. The instruments are relatively simple and permit the use of comparatively large spectral slit widths. This has eliminated the necessity for amplification of the thermopile output energy and for the determination of any continuous absorption curve. However, small spectral slit widths can be attained with the more versatile spectrophotometer if this should be required.

Frictional Phenomena. VIII

Abstract: GEL formation in colloidal systems causes the viscosity of such systems to vary with the stress: these are called non‐Newtonian liquids. Because the physical cause of such behavior is related to the gel structure, the phenomenon is called structural viscosity. Three particular features are observed with such systems: a non‐linearity of the rate of shear vs. stress relation in the case of stationary flow; a finite relaxation process for suddenly applied stresses or deformations; and a frequency dependent dynamic viscosity in the case of alternating processes. The chief experimental facts in these three fields are presented, together with certain semitheoretical equations best suited to describe the experiments.

Rate of Crystal Growth in Drawn Tungsten Wires as a Function of Temperature

Abstract: Large single crystals can be grown in ``218'' and similar tungsten wires by vacuum heating at a constant temperature in the range 1900°K to 2200°K. The growth can be followed by observation of the thermionic emission pattern, using a cylindrical electron‐projection tube with a fluorescent screen. The rate of growth is found to increase with temperature according to an exponential law. Crystal growth is slower in wires drawn to smaller diameter; this can be explained by the small grain hypothesis. The perfection of crystals is discussed.

Physics in Industry

Abstract: To-day the nation’s industries are working together as never before to do their part in the battle for freedom. It is fitting at such a time that we look over that part of civilized life for whose cultivation we as physicists are responsible, that we try to see how our science has been developing and in what direction we may cause it to grow in the future. We want to see clearly the job ahead so we can set about doing it earnestly and cheerfully; for we believe that, with effort, we can determine the future in peace as in war.

A Diffraction Adapter for the Electron Microscope

Abstract: An adapter has been developed which allows a conventional electron microscope to be used interchangeably as an electron diffraction camera or an electron microscope. The adapter comprises a unit which takes the place of the projection lens unit of the microscope, and includes a newly designed microscope projection lens, a specimen holder, and a focusing lens. To transform the instrument from a microscope to a diffraction camera (or vice versa) it is necessary only to transfer the specimen from the regular object chamber to the adapter. Diffraction patterns may be obtained by either reflection or transmission. As a result of the excellent reproducibility of voltages and currents from the regulated power supplies used in the electron microscope, the diffraction camera holds its calibration to within 0.1 percent over long periods. Using a calibration determined by measurements of gold patterns, lattice spacings of a number of common materials were determined and found to agree with x‐ray values to within 0.5...