Volume 19, Issue 2, 01 February 1948
Index of content:
19(1948); http://dx.doi.org/10.1063/1.1698377View Description Hide Description
High speed motion‐picture studies were made of the dependence of air‐water entry cavities on the relevant physical parameters—in particular, on the atmospheric pressure over the water surface. Experiments on the vertical entry of spheres ¼ in. to 1 in. in diameter, with entrance velocities between 15 ft./sec. and 100 ft./sec., and air pressures between 1/60 atmosphere and 3 atmospheres, show that surface sealing of the cavity is a major factor in controlling cavity formation, and is the factor most responsible for non‐Froude scaling of cavity phenomena. Surface sealing is a function chiefly of the atmospheric density and projectile velocity, although surface tension is also important in its effect on splash formation. Froude's scaling is found to hold true in the region of low Froude's numbers and low atmospheric pressures. The measured average drag coefficient of the projectiles is independent of cavity size and shape. Jets are found to occur regularly and with great strength at both deep and surface closures of the cavity. The observed presence of jets in finite cavities is predicted by an extension of the Kirchhoff‐Helmholtz theory of the infinite (two‐dimensional) cavity to the case of finite cavities with finite cavitation numbers. Some of the results of this theory are mentioned.
19(1948); http://dx.doi.org/10.1063/1.1698380View Description Hide Description
In recent years there has been an increasing interest in the development of statistical theories of strength. A main aim of these theories is to explain in a reasonable way such things as the dependence of the strength of specimens on their volume or length. In this paper it is pointed out that the problems posed by these models are equivalent to an important problem in mathematical statistics, namely, the distribution of the smallest value in samples of size n drawn from a population having some probability density functionf(x). The calculations made by mathematical statisticians give a far more complete description of the results to be expected than do the estimates to be found up to now in the technical literature.
19(1948); http://dx.doi.org/10.1063/1.1698382View Description Hide Description
An electrical device is designed which will solve ordinary, non‐linear, non‐homogeneous differential equations or the boundary value problems based on such differential equations. The device is based on the fact that the charge on the condenser in a series resonant circuit varies with the time in a manner described by a linear second‐order differential equation. The desired variations of the coefficients with time are fed into the apparatus by using a variable voltage generator. This generator provides a small voltage which varies with time in accordance with any given curve. The variable voltage generator has been constructed and operates satisfactorily. The resulting solution is drawn by an oscillograph. The instrument is designed to furnish a solution which does not deviate from the true solution by more than four percent.
19(1948); http://dx.doi.org/10.1063/1.1698384View Description Hide Description
A method for separating the π‐mode from neighboring resonant modes in a multi‐cavity magnetron without the use of ``straps'' is described. The new resonant circuit design known as the ``rising sun'' design utilizes two different resonator sizes arranged symmetrically around the cylindrical cathode‐anode space, with resonators alternately larger and smaller. Some of the essential features of the mode spectrum and the electromagnetic field in the interaction space in this design are derived from a simple lumped‐constant theory. The advantages of the rising sun over the strapped anode block become important in the short wave‐length part of the microwave region, because of simpler construction and smaller radiofrequency losses. For very high power magnetrons the advantages of the rising sun structure apply to wave‐length as long as 10 cm.
19(1948); http://dx.doi.org/10.1063/1.1698386View Description Hide Description
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.
The Palletron, A New Electron Resonator and Its Proposed Application to the Generation of Potentials in the Million‐Volt Range19(1948); http://dx.doi.org/10.1063/1.1698388View Description Hide Description
The kind of electrostatic field in which an electron or ion will follow simple harmonic motion is shown to have a parabolic potential distribution. A gap at the center of this field provides a means for exchange of energy between the electrons and the associated circuit. If energy is taken from the electrons the device provides an oscillator of high efficiency; if energy is given to them the device may be used to generate a high d.c. voltage. Both kinds of device have been operated successfully.
Experimental results on a small model of the voltage generator are given and a proposed design for the million‐volt range is briefly described.
19(1948); http://dx.doi.org/10.1063/1.1698394View Description Hide Description
Experimentally determined impedance characteristics are presented for helical antennas. At frequencies too low for the axial or beam mode, the terminal impedance changes greatly with small variations in frequency. However, in the frequency range of the fundamental axial mode, the impedance is relatively constant, especially if the number of turns and the pitch angle of the helix are not too small. This characteristic is an advantage in wide frequency‐band applications. For example, the resistance variation of a 5‐turn, 18‐degree helix over a frequency band of about 1.7 to 1 is only 105 to 145 ohms, and the reactance variation only 0±15 ohms. The effect of the pitch angle and number of turns is discussed.
19(1948); http://dx.doi.org/10.1063/1.1698396View Description Hide Description
Dark‐field images in the electron microscope were studied by means of an objective aperture system fixed to the object rather than to the objective lens. The resolution in images produced by the diffusely scattered component is in the range 100 to 200A, but the resolution in images produced by Bragg reflections approaches that obtainable in bright‐field operation. A resolution of about 50A was obtained with test objects consisting of evaporated films containing small crystallites, but it is estimated that crystallites having dimensions down to about 20A can be recorded with conditions prevailing in this work. Substances studied in dark‐field include evaporated films of metals and compounds and finely divided substances. It is concluded that the dark‐field method is of value for studying the size, location, and other characteristics of crystalline components in a specimen. Some of the theoretical factors influencing resolution in dark‐field are discussed.
19(1948); http://dx.doi.org/10.1063/1.1698398View Description Hide Description
Tensile tests were made at 25° and 93°C on specimens taken from a rolled silver chloride sheet which was prepared from a single crystal. Tensile strengths of approximately 2500 and 1500 lb./in.2 were obtained at 25° and 93°C, respectively; the corresponding elongations at failure were about 15 and 40 percent, respectively. For specimens annealed at 200°C and tested at 25°C, the tensile strength was 75 percent and the elongation about twice that of the corresponding values for unannealed specimens. The stress‐strain diagram of the rolled material was determined at 25°C with optical strain gauges while loading and unloading two specimens to maximum strains of 0.0015 and 0.0037, respectively; no effect of this moderate strain on the infra‐red transmissivity was observed.
19(1948); http://dx.doi.org/10.1063/1.1698400View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698404View Description Hide Description