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This paper reports on the observation of substantial rotation and reflection enhancements with thin magnetic films evaporated on the base of high index prisms with films of suitable index on only the air side of the magnetic thin film. Angles of incidence at the prism‐magnetic thin‐film interface were near the critical angle for total internal reflection. The magnetic film thickness and the thicknesses and indices of the dielectrics were obtained with the help of a computer program which calculated the reflection coefficients as well as the Kerr rotations and energy mode transfer. Enhancements of rsp up to a factor of 20 were obtained.

This paper presents a new theory of failure for materials subjected to a combined state of stress. The theory is developed for brittle materials, such as concrete, in which the compressive and tensile strengths are of different magnitudes. It is a semi‐empirical theory which has some experimental support. Based on it working stresses are defined for various loading conditions. The use of these proposed working stresses in some cases makes considerable difference in design as compared to the present specifications requirements. These working stress values are conveniently represented by a diagram to facilitate the computations in design. The application of this information can be made simply in most cases where there are stresses acting in two right‐angled directions—such as slabs in buildings, roads and bridges, retaining walls, dams and other hydraulic structures of concrete and reinforced concrete.

This paper describes experiments on the birefringence of a strip of plastic material, first when the stretched strip is maintained at constant length and second when it carries a constant load. The results indicate that in neither case is the birefringence proportional to the tension. The data are further examined, and the argument is presented to show that in the constant length experiment the ratio of tension T at any time after the initial tension, T1, is established should be the same function of the time as is the expression [1−2μlogeLL1] for the constant load experiment. Finally it is shown that the tension ratio data can be represented by a curve of the type, T′T1=Ae−at+Be−bt+Ce−ct.

The anomalous flow of certain suspensions through narrow tubes, known as the ``sigma'' phenomena, for which no fully satisfactory explanation has yet been given, is discussed. If it be assumed that the streamlining layers are not infinitely thin, so that a summation treatment takes the place of an integration, a series of equations can be derived depending on the relation assumed to exist between velocity gradient and shearing stress and on the nature of the friction between the layers. These equations reduce, in the limiting case of infinitely thin layers to the well‐known forms of Poiseuille, Buckingham‐Reiner, etc., and the general equation of Schofield‐Scott Blair can also be derived. For finite thicknesses of layer, the equation derived adequately explains the sigma‐phenomenon, and this is shown quantitatively by applying it to the data given in the original paper.

This paper is a report on an experimental investigation of the resistance to the uniform motion of a solid through a viscous liquid, the results of which appear to indicate that the liquid slipped on the surface of the solid. The apparatus was a straight, closed, glass tube containing a steel ball and the liquid being studied. The tube was arranged so that it could be inclined at various angles with the horizontal, and the only motion of the ball that was considered in working up the observations was one of pure rolling. The liquids employed were mixtures of glycerine and methyl alcohol. The analysis of the data is based upon Stokes' and Oseen's forms of Rayleigh's law of resistance, and the results of the investigation are interpreted in the light of theories due to A. B. Basset and to H. Brillie.

The general survey which has been presented here shows that, while a number of the optical properties of rubber have been investigated, the measurements of most of them have been more or less preliminary or exploratory in nature and often directed towards some immediately practical application. There appears to be ample opportunity for extended, systematic research on the optical properties for their own sake. The results to be expected from such study may lead in two directions, each of them illustrated in a very preliminary fashion, by examples given in the present paper. The one direction is that of more fundamental knowledge of the constitution of rubber, especially when optical methods are used in conjunction with other types of measurement. An illustration already mentioned is the use of molecular refractivity to confirm the existence of a C5H8 unit containing one double bond as the unit group in rubber. Such work might well be extended as assistance in elucidating the constitution of vulcanized rubber. Another illustration is the use of infrared spectra or Raman spectra to determine the types of chemical bonds present in rubber. The other direction in which a study of the optical properties of rubber may lead is toward the direct solution of practical problems without regard for the theoretical significance of the measurements. Problems of control or analysis might be approached in this manner, since optical methods usually require only small amounts of material, and are often easily adaptable to rapid and precise measurements. Such applications have already been illustrated in preliminary form by the use of refractive index to determine amounts of combined or dissolved sulfur. The solution of practical problems by optical methods is also illustrated by the use of the photoelastic method for the analysis of stresses in rubber. Optical measurements in the past have been restricted in most cases to samples having fairly high light transmission. On this account the work has included only unvulcanized rubber and vulcanized compounds of the “pure gum” type. By the use of very thin sections or by employing methods which are based on the reflection of light rather than its transmission, it should be possible to extend the measurements to many practical rubber compounds which appear to be opaque, but which in reality are dispersions of filler particles in a matrix of transparent rubber.

An experimental investigation has been made of the initial focusing of ions from capillary sources of the diffusion type described in the previous paper. The lens system used consisted of a 90‐degree cone at the ion source followed by three coaxial cylinders which were arranged at various potentials up to 25 kilovolts. Since the beams are rendered slightly luminous by traces of gas in the high vacuum region, they have been studied by a photographic method. The beams have also been studied by an electrical scanning method. Both types of investigation were made as functions of the electrode potentials and arc variables and are in good agreement. Based on these observations a focusing and canal arrangement suitable for differential pumping has been tested. This arrangement has been embodied in a design for a projected ion gun assembly.

Straight analytic determination of the heat exchange by radiation in a device is possible only if the emitting and absorbing surfaces are very simple in shape and configuration. In most practical cases, and particularly if the radiation is partly absorbed on its way by gases, approximative calculations and graphical methods have been used in order to evade the mathematical difficulties. In the present paper an experimental model method with the following characteristic features is described. A model, similar to the device under investigation, is constructed. Light is used instead of the long wave heat radiation, and its absorption by photographic films which cover the surfaces of the model, is measured by the methods of photometry. The case of gases which absorb heat radiation is imitated by using liquids which absorb light rays. The application of the method to radiation through a non‐absorbing medium is shown by experiments on a tube bank model. The adaption of the method to heat radiation through absorbing gases is demonstrated by photos of the spectrum of a special combination of light filter, absorbing liquid, and photographic film which are in tune in a narrow wave‐length range.

The paper gives a description of some tests carried out in preparation for the construction and development of an apparatus for artificial atomic disintegrations. The problems involved are (i) the focusing of a beam of 10–30 kv hydrogen ions from a canal‐ray ion source (glow discharge type) and (ii) the behavior of a cascade acceleration tube of the Coolidge type. In particular it was necessary to know the efficiency of the latter, i.e., the ratio of output to input currents. The general properties of electrostatic focusing systems are first treated briefly, and adequate references are given to enable those interested in the problem to pursue the matter further. It is pointed out that in view of the aberrations present, the constants of our lenses cannot be accurately calculated and we have therefore concentrated our attention on presenting the results in a concise form. Several lens systems were used to focus the canal‐ray beams and details are given in tables. It was found possible to focus at least 50 ...

This paper, consisting of three parts, describes investigations made with the objective of developing a simplified, practical microscope of the type which yields magnified images of transparent specimens with a resolving power superior to that of the best light microscopes. The first part deals with the general problem of design, including the electron gun and the imaging lenses. A later part will describe a completed instrument embodying many of the results of the investigations.

The rate of removal of mechanical strain should follow Maxwell's first‐order law, but for glasses it has been found that the rate is proportional to the square of the birefringence. The present paper explains this in terms of the structural picture of glass. During annealing the stress optical constant of the glass, as well as its viscosity, undergoes a continuous change. Birefringence, therefore, is not a correct measure of the stress, and the Adams‐Williamson findings are only apparently in contradiction to the classical derivation of Maxwell.

The cyclotron is in process of a metamorphosis from an academic luxury for nuclear physicists to an indispensable tool for applied science and industry. Tracer experiments with induced radioactivities have become invaluable for basic studies in other sciences and as a guide to engineering development in many industries. The radioactivities and neutron radiations themselves are being put to use in industry and in medicine on an ever increasing scale. The time is near when the use of the cyclotron as a production unit may even outweigh its further scientific usefulness. Cyclotron physicists, well aware of this trend, are eager to push the development of the instrument to engineering perfection. To further this effort, this paper evaluates available information which may aid in improving the efficiency, reliability, and output and in determining most economical sizes and designs of cyclotrons for large scale applications.

This paper deals with considerations concerning the reduction in amplitude of unwanted vibrations of machinery parts through the use of materials of high damping capacity. The problem is to reconcile the requirement for high damping with that for high mechanical quality (strength, fatigue resistance, etc.). Two ways are suggested: (1) the use of a material whose decrement generally is low but rises rapidly as the stress increases; (2) the use of a material whose damping capacity is low but rises to high peaks in certain frequency ranges. It is shown by numerical computation in two instances, namely, turbine blade vibrations and crankshaft oscillations in engines, how the suggested methods would work out in practice.

Interference patterns resulting from a system of orderly arranged sources, whether they be radio, light, or sound waves, can be conveniently analyzed by the use of the exponential operator ej. This paper illustrates the application of the exponential operator in the development of the equation for the general antenna array.

This paper describes a new and simpler method for the determination of orientations of cubic single crystals from back reflection x‐ray photographs by use of vectors in the reciprocal lattice. An example is given to illustrate the usefulness of the method.

In this paper a transmission type electron microscope with magnetic lenses is described. The electron speed can be varied between 30 ekv and 100 ekv. The magnification of the instrument is produced in three stages. The instrument has improved air locks, hydraulically operated stage movement, and a stage tilting device up to ±15½°. It is also provided with means for bright and dark field illumination and for conversion into a diffraction camera. Improved electrical circuits provide the necessary stability of the power supplies.

This paper describes a new apparatus for measuring electrostatic contact potentials on various materials at elevated temperatures. In this apparatus, the electrostatic charge, acquired by rolling a steel ball down the surface of a rubber test specimen on a heated inclined plane, is measured when the ball drops into the cup of a suitable measuring device such as our electrostatic modulator. With this instrument the contact potentials of both rubber and GR‐S was found to become highly negative at elevated temperatures. This apparent ``boiling off'' of electrons and resultant disruption of electrostatic attractive forces within the material is much greater for GR‐S than for rubber and probably accounts for the much greater decrease in tensile of GR‐S over rubber at elevated temperatures, and is further confirmation of the electrostatic contact potential theory of reinforcement. By the further application of this theory suitably dispersed compounding materials, which are in effect highly positive at elevated ...

In 1940, dielectric measurements in the centimeter range were considered as difficult and not very accurate. The authors, therefore, developed a ``hollow‐pipe'' method which overcame these objections and required only a weak oscillator and small amounts of the dielectric material. The theory and its practical applications, as perfected by March 1941, are presented in this paper.

Dr. Darrow has suggested that a short introduction may enhance the usefulness of the more specialized papers to be presented to the Society by members of the old Theoretical Physics group of the Metallurgical Laboratory (Plutonium Project). Because the presentation of these papers will have to be very brief and because some of our collaborators could not present abstracts in time for the meetings, I was most happy to follow our Secretary’s invitation to summarize our work in general terms.

This paper presents a method of using sand in building models of two‐dimensional Fourier projections in crystal structure analysis. The machine for producing these models is described and the results are illustrated by projections of known structures. The mathematical principle of this method of summing Fourier series is identical to that which Bragg developed in England. The chief difference is in the use of sand instead of photographic film as the medium for producing contrasts in electronic density throughout the projection. The method herein described has a number of features which contribute to its usefulness: A two‐dimensional Fourier projection comprising the addition of one hundred and fifty terms may be made in about four hours by a single worker. The time required is not determined by the size of the unit cell. The subdivisions of the projection face are sufficiently fine to assure an accuracy as great as the x‐ray data from copper radiation will warrant when a scale of one angstrom to two centi...

The determination of column deflections and column buckling loads has been considered for many years. The available theories, however, do not provide for materials which creep with time under constant loads. For the design of structural members made of these materials, a consideration of creep may be of practical importance. Plastics, concrete, and some metals creep at normal temperatures while other metals creep only at high temperatures and at stress values beyond the yield point. A consideration of creep in the design of some structures appears appropriate in view of the modern developments in plastics and the presence of high stress values which are sometimes beyond the yield stress. This paper gives a rational theory for predicting creep deflections in columns. A special case using this theory is applied to the interpretation of some preliminary tests of an aluminum alloy.

In this paper are presented the approximate formulas for the components of radiation vectors of a short‐circuited circular loop with non‐uniform current distribution. The formulas are valid for the ratio of loop perimeter to wavelength of the order of 0.5 or less, and assume the current distribution of the hyperbolic cosine form. These formulas lead to the radiation intensity formula from which the expressions for the horizontal and vertical field patterns are derived. The latter formulas are further simplified assuming that the attenuation constant is much smaller than the phase constant. From the expression for horizontal field pattern, it follows that the pattern is symmetrical about the loop axis of symmetry. Moreover, this horizontal pattern exhibits a directional effect with maximum field in the direction of the feeder end of the loop. This directional effect is a function of loop dimensions. The theoretical horizontal pattern agrees very closely with the experimental one. From the expression for vertical field pattern, it follows that the non‐uniform current distribution produces a pattern intermediate between that for horizontal dipole and horizontal small loop with uniform current distribution. The field intensity in the zenithal direction is again a function of loop dimensions. Using the expression for radiation intensity, formulas for a radius of equivalent circular horizontal field pattern, power gain, average power gain, and radiation resistance are derived. It is shown that the average power gain is essentially a function of loop radius and decreases with the increase of the latter. Finally, the approximate expression for the attenuation constant of the transmission line equivalent to the loop is derived.

The paper is concerned primarily with the electron guns employed in electron microscopy.The position of the image of the cathode, the magnification, and the divergence of the beam leaving the anode are calculated for a three‐electrode electron gun with plane electrodes and circular apertures both for zero and battery bias. The simplifying assumption is made that the electrostatic lenses are thin. The results are compared with the experiments of Johannson. The extension of the results to the gun used in the transmission electron microscope is considered with the aid of electrolytic trough measurements. The case of a gun biased by a cathode resistor is also discussed.

In this paper an electromagnetic field theory is developed for unstrapped magnetron anode blocks of rather general symmetry, from which the wave‐length spectrum and associated electromagnetic fields can be computed. The method is a generalization of that developed by Clogston (A. M. Clogston, Ph.D. thesis, Massachusetts Institute of Technology, 1941) for the simple case in which all of the side resonators are alike. The characteristics of the rising sun type anode block are discussed in some detail. In addition, the general features of anode blocks having any kind of symmetry are discussed in the limiting case of small anode diameter.

Two techniques of radiography by means of secondary photoelectrons liberated by hard x‐rays (180 kv), one by reflection and one by transmission, are described. Special fine‐grained photographic emulsions of the Lippmann type are required which will not be affected by x‐rays traversing the film, but are sensitive to photoelectrons liberated from the surface of a specimen in intimate contact with the photographic film or paper. In this way photographic negatives may be reproduced (from the silver of the image), microradiographs prepared by enlargement of any area of the radiograph, surface topography studied, ores analyzed, etc. For electronic radiography by transmission, instead of the specimen being its own electron generator, a thin metal foil (usually lead) pressed against the specimen generates the electrons which then traverse the thin specimens, and register on the photographic film or paper. Applications are papers, insect wings, varnish or rubber films, plant leaves, tissue sections, plastics, and alloys.

The problem of devising a rapid scanning focusing antenna based on wave‐guide principles has been solved only approximately and not too satisfactorily. In the present paper, a known property of a plane optical system with variable index of refraction is shown to satisfy the requirements of such an antenna system. By a suitable transformation, the plane variable refractive index system can be transformed into a surface of revolution of constant refractive index which reproduces the optical properties of the plane system. This surface provides an apparently practicable solution to the scanning problem. It is shown that the diameter of the feed circle can be made arbitrarily small by judicious use of an ellipsoidal reflector.

The paper is devoted to a discussion of the properties of electromagnetic waves propagating with a velocity W

The investigation contained in this paper gives a general treatment of the theory of biconical antennas based upon Smith's formulation of the problem. For the case of small‐angle cones, an expression of the effective load admittance is obtained which is the same one that Schelkunoff derived based upon two rather ingenious methods, both of which, however, are not as rigorous as the present one. As an illustration of another application of the analysis, a treatment is given of the characteristics of biconical antennas where the space between the cones is filled with a dielectric other than air.

Image contrast in electron microscope images may arise from absorption, scattering, or phase changes impressed on the illuminating electron wave. In the present paper the intensity distribution in the in‐focus and out‐of‐focus image of an edge of a transparent thin film introducing a prescribed phase change in the incident beam is calculated. It is found that the resulting ``phase contrast'' increases both with the film thickness (i.e., magnitude of phase change) and with the degree of defocusing and, for thin specimens, exceeds other sources of contrast in magnitude. The model is too schematic to permit a quantitative comparison with measured intensity distributions.

Analysis of the non‐radiative modes propagated along a circular dielectric rod is accompanied by a discussion of dielectric loss. The attenuation for the three lowest modes in polystyrene is computed and agrees with the experimental data of Chandler given in the preceding paper.

At radio wave‐lengths the use of mirrors and lenses in the measurement of radiators, by methods which are customary in physical optics, is possible only if errors, due to incidental diffraction from the relatively small apertures of such apparatus, can be corrected by calculation. Theory is developed in this paper from which these calculations can be derived for linear radiators. A new relation which expresses the Fresnel field in terms of the distant field is fundamental to the method. Satisfactory agreement has been obtained between the predicted and measured distant fields of several electromagnetic horns whose near fields were obstructed; the dispersion between measured and predicted values is given as under one‐half decibel but the details of the experiment are not reported.

We present numerical investigations for high-order harmonic generations by a high-intensity chirped-laser interaction with vacuum electrons in the presence of an axial static magnetic field. Combined role of frequency chirp of the laser and a magnetic field enhances the electron acceleration and leads electron energy gain after passing of the laser pulse that generates high-harmonic radiations until the magnetic field diminishes. The result shows that high-harmonic radiations in the range of soft x-rays up to about 10 nm can be generated with common experimental parameters by the proposed scheme of this paper.

Analytical computation of the detailed lobe structure of an elevated meter or centimeter band antenna for a standard atmosphere is quite tedious. In case there are discontinuities in the gradient of refractive index the analysis becomes hopelessly involved.This paper presents a straightforward method of graphical analysis for the optical region, equally applicable to a standard or stratified atmosphere.

In this paper it is shown how the low frequency index of refraction of a metallic delay‐lens medium may be calculated from electrolytic‐tank measurements on individual metallic elements of the medium. The proximity between adjacent elements is taken exactly into account for low frequencies. The test apparatus is described, and measured results are presented for three kinds of delay‐lens structures.

This paper describes an experimental model of a combination Kerr cell camera and flash illumination unit designed specifically for the photography of projectiles under conditions of strong ambient illumination. Specific cases wherein such conditions are found are considered and the applicability of the Kerr cell as a secondary protecting shutter is described. A description is given of a pulse forming network—pulse transformer circuit designed to apply an operating voltage pulse to a Kerr cell of thirty‐six (36) kilovolts amplitude and one point nine (1.9) microseconds duration. Limitations inherent in the present equipment are considered together with proposals for overcoming these defects in future models. Typical results are presented, including (a) a picture taken with the unit showing a twenty‐millimeter shot at the time of emergence from the muzzle of a gun, and (b) a picture of a thirty‐seven millimeter shot taken shortly after penetration of a metal target plate.

In a previous paper, the author reached a consideration that the endurance limit of a material should be determined by the amount of the mean stress in a certain small area around a point, instead of the stress at the point, and the extent of the area was determined for several kind of metals, by using the results of tension-compression fatigue tests. In this paper, the extent of area where the mean value of stress to be taken is determined for a mild steel in another way, viz., by a torsion fatigue test. The extent of the area for mild steel almost agrees with the one obtained in the previous tests, and the conclusion that the extent of area chiefly depends on the kind of metal and is independent of the course of the experiment was reached.

In this paper the applicability of non‐radiating surface waves for transmission lines is investigated. Two types of waves are considered. The first one, originally studied by A. Sommerfeld, is guided by a cylindrical conductor of finite conductivity. Although this wave type has (under comparable conditions) much lower attenuation than the waves in coaxial cables or rigid wave guides, its practical application is restricted by the fact that the extension of the field is very large. Efficient excitation and undisturbed propagation of this wave mode are feasible only for very high frequencies. The other wave type considered in this paper has not been treated in the literature. It is guided by a conductor which is coated with a dielectric layer or the surface of which is otherwise modified; for example, by being threaded. The field of this wave type has a structure similar to that of Sommerfeld's wave, but the extension of the field can be controlled by the surface modification. Thus low loss transmission lines on the basis of this wave type become feasible for frequencies above 100 megacycles. The information necessary for the design of such lines is given and the agreement between the theoretically expected transmission losses and the measured transmission losses is checked.

Two wind tunnels recently completed at the Ames Laboratory of the NACA—the 10‐ by 14‐in. supersonic wind tunnel and the supersonic free‐flight wind tunnel—are capable of providing test Mach numbers well in excess of 5. Since at this time such facilities are relatively uncommon, this paper is primarily devoted to a description of these wind tunnels and their associated equipment.The treatment of the 10‐ by 14‐in. supersonic wind tunnel is concerned for the most part with the design and operation of the nozzle and diffuser which provides Mach numbers of 3.5 to about 8. Some mention is also made of the pressure measuring and flow visualization equipment. In the case of the supersonic freeflight wind tunnel high test Mach numbers are obtained by firing models at high speed into an oncoming supersonic air stream. Consequently the main emphasis is placed upon the launching techniques, the available range of test conditions, and the methods of obtaining data.

In conical flows the velocity, pressure, and density do not vary on lines through the vertex of the cone. In this paper it is shown that for interferograms of general conical flows δ(y, z)/z=f(y/z), where y and z are any set of Cartesian coordinates with origin at the image of the vertex of the cone, and δ is the fringe shift at y, z. Thus, for strictly conical flow, a graph of δ(y, z)/z versus y/z should be a single curve. This suggests a test for approximate conicity that requires very little computation. This test is applied to interferograms obtained from a number of approximately axisymmetric flows at various Mach numbers about cone‐cylinders in free flight. Plotted fringe shift data from the region near the nose fall into a narrow band, an indication of approximate conicity. They also closely check the corresponding theoretical fringe shift curve calculated for Taylor‐Maccoll flow.

In dealing with numerical values for relative intensities derived from the Debye‐Scherrer technique, the alteration of diffracted intensities produced by absorption in the sample cannot be ignored. Claassen has shown how this effect can be allowed for in the case of homogeneous cylindrical samples. This paper deals with a sample consisting of a cylinder surrounded by another annular cylinder both when the core is inert (i.e., absorbing and producing only extraneous scattering) and when the annulus is inert. The method is applied to show how the diffraction pattern for a liquid enclosed in a glass container may be corrected for the diffuse diffraction of the container as well as for absorption in the liquid and in the glass.

A method of measuring the reflection coefficient of metal‐plate media at 3 cm is described. The technique differs from that presented in the preceding paper in that the measurements are made in a closed system and back reflections are eliminated by an absorbing wedge. Angle of incidence and the plate thickness are varied. The agreement with the Carlson‐Heins theory is excellent for very thin plates. For plates of moderate thickness near normal incidence the measured reflection is higher than that predicted for an infinitely thin set of plates with identical index of refraction.

Existing calculations on the tunnel effect through thin films in contacts apply to very weak and very strong electric fields. It is the main purpose of the present paper to complete the picture by the treatment of intermediate cases, which are important for many contact applications.

The dielectric constant and permeability of a metallic‐obstacle medium are obtained in this paper by various static methods. The equivalent shunt capacitive susceptance and series inductive reactance of the individual obstacles are also determined. In the first part of the paper, a correspondence is established between an infinitely thin conducting obstacle and an aperture in an infinitely thin conducting wall. With the aid of this correspondence, delay medium formulas are given for several obstacle shapes. Formulas are then derived for the effect on the magnetic field of moderate obstacle thickness. Finally, it is shown how the electric and magnetic constants may be evaluated by electrolytic tank measurements in the case of obstacles having completely arbitrary shape and thickness. This has been done for a typical delay medium, and the effect of obstacle thickness on the index of refraction is shown.

A new double spectrometer method for the study of polycrystalline materials is developed. From intensity data of the diffracted spots and known angular settings of the specimen, the angular range of reflection of the misaligned scattering regions in a crystallite is determined statistically.The instrument and the technique of the multiple exposure method is described. The statistical evaluation of the data is discussed and a mathematical treatment introducing correction terms as functions of the azimuth and of the Bragg angle is presented.A commercial silicon powder specimen was investigated. The average angular range of reflection was found to be nearly the same for all (hkl)‐reflections with exception of (111), (333), and (444).The study of broadening effects with the same instrument has been started.The new double spectrometer method for polycrystalline specimens described in this paper is based on the examination of numerous microscopic spots reflected by individual crystallites. The small spots on th...

In the performance of antenna measurements the antenna is usually placed over a large conducting screen. The assumption is then made that the measurements are in close agreement with those that would be obtained if the screen were of infinite size. Recent experimental work, however, indicates that the finiteness of the ground screen may influence the measurements to a considerable extent.It is therefore desirable to solve this problem theoretically in order to determine the extent of the change introduced by the use of a finite screen. This paper treats the problem of an antenna erected vertically in the center of a circular screen. An integral equation is obtained for the electric field in the plane of the screen, outside the screen. This equation is solved approximately by the use of a variational principle, and a formula is obtained for the change of antenna impedance as a function of the diameter of the screen. The formula may be used to give a simple and accurate estimate of the changes introduced by...

This paper presents the input admittance of a symmetric biconical antenna of an arbitrary angle as the limit of a certain sequence of functions. The first term of this sequence approaches the exact expressions for the input admittance as the cone angle approaches either zero or 90°. For this reason our conjecture is that this term represents a good first approximation for all angles.

In a previous paper [The Input Impedance of an Antenna Over a Finite Ground Screen, Technical Report No. 119 (Cruft Laboratory, Harvard University, November 30, 1950)] the author treated the problem of the input impedance of an antenna erected over a large circular screen. Using the formulation developed therein, an expression is obtained for the entire radiation pattern of an antenna over a large circular screen.The most important difference between the pattern of an antenna over a large screen and an antenna over an infinite screen is the presence of lobes over the antenna. From the expression for the radiation pattern, simple formulas are obtained for: (a) the number of lobes, (b) the angular position of the lobes, and (c) the angle, βmax, within which the lobes occur. One important result obtained is that θmax∼(1/d12), where d is the diameter of the screen. It is apparent that the effects of a finite screen decrease very slowly as the diameter of the screen is increased.

It is shown in this paper that exact expressions may be derived for the absorption factor for the intensity of Bragg reflection by a small euhedral crystal completely bathed in a parallel x‐ray beam. The expression is a sum of terms evaluated for subdivisions of the crystal determined according to discontinuities in boundary conditions imposed by the distribution of edges between the plane faces of the crystal. For a prismatic crystal of constant cross section, general expressions are derived for five general types of subdivisions in terms of the linear absorption coefficient and certain linear dimensions. Other expressions are also given for other types of subdivisions which arise in certain cases. The application of the method, including corrections for secondary extinction, is illustrated by calculations on tetragonal barium titanate.

In several types of applications of microradiography, particularly for histochemical and microchemical analysis by x‐ray absorption spectroscopy, it is desirable to use monochromatic radiation of a wavelength selected specifically with reference to an absorption edge of the element of interest. An x‐ray tube having as its target a material whose Kα characteristic wavelength is suitable is the most usual source of monochromatic x‐radiation. In many cases, however, an otherwise desirable target is physically unsuitable for use as a target inside a vacuum tube. Hence, the possibility of producing fluorescent characteristic radiation with intensities sufficient for microradiography becomes of interest. The very high intensity of the primary x‐ray beam made available by the newly developed beryllium‐window x‐ray tubes suggests new possibilities for the excitation of fluorescent radiation.This paper reports the results of exploratory work attempting to establish the factors governing the intensity and degree of monochromaticity of fluorescent x‐radiation. Several materials, ranging from titanium (atomic number 22) to molybdenum (42) were studied comparatively, with exciting radiations of various characteristics; titanium was studied in greater detail. It is concluded that a considerably higher degree of monochromaticity is obtainable by this method than by direct bombardment and that intensities useful for some aspects by microradiography are obtainable.

This paper describes both a traveling‐wave method and a standing‐wave method for determining complex dielectric constants of liquids from measurements made at 10‐cm wavelength with a traveling probe immersed in a dielectric‐filled slotted line. An interferometric null technique was developed for improving the accuracy of the measurements, especially in highly absorptive media. In the traveling‐wave method, absolute measurements of both wavelength and absorption index can be made simultaneously; from these two quantities the complex dielectric constant is readily computed. In the standing‐wave method, the absorption index is determined graphically from the power‐standing‐wave ratio while the wavelength is obtained by a modified null technique. For low loss media, a simplified standing‐wave method is practical whereby the absorption index is obtained from measurements of the breadths of successive minima of the standing wave. Corrections for the finite resistance of the short circuit used in the standing‐wave method and for conductor losses along the slotted line are fully treated.