Index of content:
Volume 21, Issue 10, 01 October 1950
21(1950); http://dx.doi.org/10.1063/1.1699537View Description Hide Description
It is known that the configuration of the input driving system may be of importance to the impedance of antennas at high frequencies. In this paper, a region in the vicinity of the input is considered as a transducer between the TEM mode on the driving transmission line and the spherical nearly‐TEM mode on the antenna system, and the constants of this transducer are found both by calculation and measurement for a particular case. The calculation is based upon quasi‐static concepts, utilizing an electrostatic field map obtained from an electrolytic tank to give the approximate form of the electric field lines. The measurement is made by finding three input reactances corresponding to the input region closed by three spun aluminum hemispheres of different radii. The three sets of data, taken at each frequency, are sufficient to determine the constants of the transducer. They are expressed here as a pi‐network for comparison with calculated values.
The main portion of the antenna is analyzed substantially as in Schelkunoff's approach, with the TM modes in space and in the antenna region expressed as a shunt admittance at the end of the antenna in the TEM mode equivalent circuit. This admittance is then transformed to the desired reference on the antenna by a perturbation calculation. From there it is transformed to a desired reference on the feed line by means of the input network constants. Values calculated in this way are compared with measured values over the range 200 to 1000 mc/sec. for the particular monopoles studied. Agreement is not perfect, but appears considerably better than without consideration of the specific input configuration.
Microwave Techniques for the Measurement of the Dielectric Constant of Fibers and Films of High Polymers21(1950); http://dx.doi.org/10.1063/1.1699538View Description Hide Description
A resonant cavity method is described for the determination of the dielectric constant of polymeric solids in the form of fibers, films, and thin cylinders. Methods for investigating electrical anisotropy are outlined. In the case of fibers, for the electric field parallel to the fiber axis, the dielectric constant is determined directly. For the electric field perpendicular to the fiber axis, the problem arises of calculating the dielectric constant of the fiber from that of an air‐fiber mixture. For the latter purpose the Lichtenecker equation was found to be applicable. Illustrative measurements of the dielectric constant of wool, Nylon, and Cellophane are presented and discussed briefly.
Determination of Residual Stresses of Quenching Origin in Solid and Concentric Hollow Cylinders from Interferometric Observations21(1950); http://dx.doi.org/10.1063/1.1699539View Description Hide Description
A theoretical procedure for the determination of the stresses of quenching origin in solid and concentric hollow cylinders has been derived. The method relies on interferometric data and the assumptions discussed in Section I, whose validity for the technically important case of glass is supported by a number of experimental investigations.
The results should be of interest for the analysis of stresses in quenched glass cylinders, so great in magnitude that the usual cutting techniques are precluded, because of the explosive characteristics of highly stressed objects of this material.
21(1950); http://dx.doi.org/10.1063/1.1699540View Description Hide Description
It was earlier established that the lens field between the cathode and crossover can be represented by a simple function V=A sinhkz, so that the main properties of the paraxial ``crossover'' (or minimum section of the beam) can be determined as a first approximation in terms of ``A'' and ``k.'' The distance of the crossover works out at 8π/3k√3; its potential is A exp(8π/3√3); and the angle of the beam is simply proportional to ``k.'' The radius of the crossover is then 2/k exp[−(2π/3√3)]·tanθ for electrons of initial velocity ``A,'' where θ is the angle of emission within the paraxial limit. The differential analyzer was also used to obtain the correct solution for the trajectory equation from the field plots for different systems, and shows that the simple theory gives an adequate approximation for design purposes.
21(1950); http://dx.doi.org/10.1063/1.1699541View Description Hide Description
Some surface structure characteristics of a metalsingle crystal are considered and the conclusions are applied to the preparation of single crystal plates of copper. A successful preparation is described as well as suitable methods for evaluating the effectiveness of the preparation. Using physical microadsorption isotherms of nitrogen at 78.1°K, electron diffraction, and x‐ray diffraction methods, the conclusion is tentatively asserted that the surfaces are essentially planar on a molecular scale, and that the surface atoms are arranged in an approximately undistorted lattice.
21(1950); http://dx.doi.org/10.1063/1.1699542View Description Hide Description
Transient and steady state solutions of the parallel plane diode are obtained. The limitations on a single valued velocity theory are investigated. These general results are applied to the space charge limited diode with the a.c. components small compared to the d.c. Power consumption is discussed and it is shown that hitherto neglected terms are of importance.
21(1950); http://dx.doi.org/10.1063/1.1699543View Description Hide Description
A simple theory of the lagging effect of the positive ions in neutralizing the space charge near the cathode of a gaseous diode is developed. By means of bridge measurement up to a frequency of 350 or 1000 kc per second, depending upon the type of tube used, the theory is checked experimentally in terms of the high frequency impedance of the diodes. Several constants of interest, such as the transit time and the lifetime of a positive ion, its effectiveness in neutralizing space charge, etc., can be deduced from the theoretical and experimental investigations.
21(1950); http://dx.doi.org/10.1063/1.1699544View Description Hide Description
The stress wave caused by a longitudinal impact at the end of a cylindrical bar has been analyzed in the case where the impact velocity is large enough to produce plastic strain. The theory gives a method for computing the stress distribution along the bar at any instant during impact. It is shown that for a given material, there is a critical impact velocity such that when subjected to a tension impact with a velocity higher than the critical, the material should break near the impacted end with negligible plastic strain.
An experimental investigation was made concurrently with the theoretical study. Some of the most significant experimental results are presented in this paper.
21(1950); http://dx.doi.org/10.1063/1.1699545View Description Hide Description
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.
21(1950); http://dx.doi.org/10.1063/1.1699432View Description Hide Description
The field of a quarter‐wave‐length antenna above a circular conducting disk of zero thickness—the groundplane—is calculated theoretically by use of the wave functions of the oblate spheroid. Assuming a sinusoidal distribution of current on the antenna, we compute the currents on the groundplane, the radiation resistance and the radiation pattern of the system for various values of the radius a. With these results the distortion of antenna radiation by finite groundplanes can be studied. We find good agreement with recent experimental data.
21(1950); http://dx.doi.org/10.1063/1.1699433View Description Hide Description
The different apparatus used by some thirty investigators, who have induced double refraction in a liquid contained in the annular gap between two concentric cylinders, are examined with particular reference to the onset of turbulence. In the case where the outer cylinder alone rotates, curves are given which easily enable the critical gradients corresponding to the dimensions of the apparatus used to be found. The results of the examination are so tabulated that present and future workers in this field can quickly decide on the significance of the measurements obtained by any of the investigators. An explanation is put forward of the surprising fact that with either the inner or outer cylinder rotating measurements of double refraction apparently show no very sharp break between regions of laminar and turbulent flow. The value of making simultaneous measurements of streaming double refraction and viscosity is pointed out and the advantages of using an apparatus with rotating outer and stationary inner cylinder are briefly summarized. Mention is made of optical design.
21(1950); http://dx.doi.org/10.1063/1.1699434View Description Hide Description
The correlation of experimental data and the proper description of the state and history of deformation of work‐hardening materials has received much attention. An often desired objective is the plotting of a variable involving stress alone, a so‐called effective stress, against a variable depending on the instantaneous strain or the history of strain, effective strain. Quantities such as maximum or octahedral shearing stress and strain or better an integrated effective strain increment have been employed. Also, simple effective stress definitions have been given for anisotropic as well as isotropic metals by Dorn, by Jackson, Smith, Lankford, and by previous investigators.
It is shown that the stress‐strain relations of a recent theory of plasticity provide a very convenient means of defining and studying effective stress and effective strain. The strong theoretical and practical limitations of such concepts are indicated and the calculation of specific plastic work is discussed for both isotropic and anisotropic metals.
One of the major objectives of the paper is to stimulate critical experimental examination of as yet unproved assumptions in the theory of plasticity and in the correlation of experimental data.
21(1950); http://dx.doi.org/10.1063/1.1699435View Description Hide Description
The known facts about nucleation phenomena in liquid metals are interpreted satisfactorily on the basis of the critical size and interfacial energy concepts. In large continuous masses nucleation is almost always catalyzed by extraneous interfaces. However, in very small droplets the probability that a catalyticinclusion is present is so much less that their minimum nucleation frequencies are reproducible and form a consistent set of values.
Interfacial energies, σ, between crystal nuclei and the corresponding liquids have been calculated from nucleation frequencies of small droplets on the basis of the theory of homogeneous nucleation. Energies of interfaces, σ g , one atom thick and containing N atoms were calculated from the σ's. The ratio of σ g to the gram atomic heat of fusion, ΔHf , was approximately 0.45 for most metals but ∼0.32 for H2O, Bi, Sb, and Ge.
The effect of relative complexity of crystal structure upon the supercooling behavior of pure metals apparently is a reflection of its effect upon ΔHf .
21(1950); http://dx.doi.org/10.1063/1.1699436View Description Hide Description
A hot‐cathode arc in cesium vapor has been studied by the use of a plane Langmuir probe to investigate the potential distribution, electron temperature and electron density in the plasma. The range of arc currents and vapor pressures over which satisfactory probe measurements may be made are determined.
In some cases it was possible to maintain the discharge with a total arc drop of less than the ionization potential of cesium (3.87 volts). Potential ``humps'' along the arc are shown to be absent, and ionization by the high energy electrons of a Maxwellian electron velocity distribution is found unsatisfactory as a possible mechanism for positive ion production in the arc. The mechanism of successive electron collisions can account for the observed ionization on the assumption that the effective lifetime in the excited state is increased by the process of the imprisonment of resonance radiation.
21(1950); http://dx.doi.org/10.1063/1.1699437View Description Hide Description
The allotropy of beryllium has been confirmed by employing different alloying properties of allotropic forms of metallic elements. Alloys of beryllium and gold, studied by x‐ray diffraction methods, showed that the limit of solubility of gold in beryllium is between two and three atomic percent, and practically all of it is dissolved in the β‐form. The lattice parameters of the two forms observed here are: a 0=2.286A, C 0=3.588A, and C 0/a 0=1.570 for the α‐form, and a 0=6.93A, C 0=11.35A, and C 0/a 0=1.638 for the β‐form. The volume of the unit cell of the β‐form is about 29 times that of the α‐form.
21(1950); http://dx.doi.org/10.1063/1.1699438View Description Hide Description
WL α radiation filtered through a Cu–Zn foil is essentially monochromatic and has the advantage over filtered CuKα or MoKα radiation in that the WL α1 line is approximately ten times as intense as the WL α2 line. Moreover the comparatively wide Δλ‐spread (WL α2−WL α1) results in very sharp powder lines at those Bragg angles where the CuKα doublet or MoKα doublet is unresolved.
21(1950); http://dx.doi.org/10.1063/1.1699439View Description Hide Description
The shielding properties of the four‐foot concrete wall of the M.I.T. cyclotron have been studied, using foils for neutrons and x‐ray films for gammas. Fast neutrons were measured by counting the beta‐active products of Al27(n, p)Mg27, Fe56(n, p)Mn56, and Al27(n, α)Na24 reactions, thermal activation being eliminated by Cd holders. Resonance (1.44 ev) and thermal neutron distributions, measured by Cd‐In‐Cd and In, with suitable corrections, were reduced to absolute intensities by comparison with a known flux. Measurements were made with cylindrical concrete blocks fitted into a 4½″ diameter hole which extended through the concrete wall. The results indicate an attenuation factor of 8.3 per foot for the fast neutron component and 10 per foot for thermal neutrons. An estimate of the fast neutron flux above 4.5 Mev showed a transmitted intensity slightly higher than the presently accepted tolerance value when bombarding Be. The absolute thermal neutron flux was below the tolerance level. The r.m.s. distance and the diffusion length for thermal and resonance neutrons in concrete were estimated by considering the distributed source to be equivalent to a point source at the inner face of the wall. The gamma‐ray measurements were not convertible to roentgens. A mean energy of 2.7 Mev was obtained for the gamma‐radiation within the wall. The attenuation factor for total gamma‐radiation in the wall at large distances from the inner face was 5.9 per foot.
21(1950); http://dx.doi.org/10.1063/1.1699440View Description Hide Description
A mathematical analysis is presented which is believed to be applicable to a general class of storage tubes where signal storage is accomplished by depositing through secondary emission a charge pattern on a dielectricsurface. The assumptions made to linearize and simplify the problem are outlined and plots are given of the predicted output signals for writing, reading, and cancellation operations when the input signal is a step function. Experimental evidence is presented to substantiate the analytical results.
21(1950); http://dx.doi.org/10.1063/1.1699441View Description Hide Description
A graphical method is derived for the analysis of microwaveelectron tubes, ion accelerators etc., which refers particle positions and velocities to a moving reference system. If the forces are dependent on time only, the trajectories are transformed into straight lines. For inhomogeneous fields an approximation procedure applies. To demonstrate the capabilities of this method a brief treatment of the transit time phenomena in cavity triodes is outlined.
21(1950); http://dx.doi.org/10.1063/1.1699442View Description Hide Description
An experimental study was made of the orienting effects of silver on sodium chloride crystallized from solution. Pronounced epitaxy was observed in the following cases: (1) On oriented films of silver with (100) and also with (111) planes parallel to the surface; and (2) on polycrystalline high purity silver sheet.