Volume 21, Issue 3, 01 March 1950
Index of content:
21(1950); http://dx.doi.org/10.1063/1.1699632View Description Hide Description
This paper describes the results of radio angle‐separation measurements on a 2.50‐mile path over Lake Buchanan near Austin, Texas. The measurements were made to test the angle‐separation equipment developed at The University of Texas, and to compare these results with measurements obtained by rocking a 20‐foot parabolic antenna.
21(1950); http://dx.doi.org/10.1063/1.1699633View Description Hide Description
When an electric arc is driven by a magnetic field, under special conditions, curious phenomena are observed. An electric arc cannot be driven by the magnetic field when it is established at very small gaps between the electrodes. Neither can it be driven by the magnetic field for the instant just after the contacts have been broken. These phenomena are called immobility phenomena. In the present work, the immobility time and the driving velocity of the electric arc were measured under various air pressures. As the air pressure decreases, the immobility time increases and the driving velocity decreases. Under a certain low air pressure, the arc cannot be driven by the magnetic field. When the air pressure is lower than this critical value, the arc can be driven again by the magnetic field; but in this case, the arc moves in a direction that is opposite to the direction of the electromagnetic force. This phenomenon is called the reverse driving phenomenon. The causes for these phenomena are considered.
21(1950); http://dx.doi.org/10.1063/1.1699634View Description Hide Description
Apparatus has been developed for measuring the dynamic viscosity and rigidity of soft rubber‐like solids in small oscillating deformations. A plate rigidly attached between two identical coils in two permanent magnets shears a pair of disk‐shaped samples when a driving current is passed through one coil. The open circuit voltage from the other coil is compared in amplitude and phase with the driving current by a method in which all measurements are in the form of settings of a potential divider. The apparatus has two advantages over the more familiar resonance devices: (a) the amplitudes of motion, which need not be measured directly, are extremely small, thus minimizing any non‐linear effects, or temperature change due to heat dissipation; (b) a continuous range of frequencies, spaced as closely as desired, is available without adjusting masses. The dynamic rigidity and viscosity of two samples of polyisobutylene, of molecular weights 1.2 and 0.47 million, have been measured at 15, 25, and 35°C at frequencies from 20 to 600 cycles/ sec., with a reproducibility of five percent or better. The rigidity increases and the viscosity decreases with increasing frequency. Both decrease with increasing temperature, and are almost independent of molecular weight. The temperature dependence is quantitatively explained by the assumption that stresses relax by flow processes whose apparent activation energies are all identical with that characterizing the steady flow viscosity.
21(1950); http://dx.doi.org/10.1063/1.1699635View Description Hide Description
The sixty‐cycle dielectric strengths of some gaseous fluorocarbons have been measured between three differently shaped electrode pairs at pressures up to three atmospheres. The breakdown potentials for propforane, butforane, and pentforane were found, in most instances, to be equal to or greater than those for sulfur hexafluoride under comparable conditions, and to be far greater than those for nitrogen. Fluorocarbons thus have possible uses as gaseous insulators in high voltage apparatus.
21(1950); http://dx.doi.org/10.1063/1.1699636View Description Hide Description
Polyethylene of such molecular weight and structure that it readily fibers or cold draws to 300–600 percent elongation by usual uniaxial, tensile stressing may react quite differently under biaxial tension. When biaxial tension in 1:1 ratio is applied to a diaphragm, some polymers show brittlefracture with <20 percent elongation at break. However, if the average molecular weight of such polyethylenes is shifted upward by crude fractionation, or an initially higher average is used, the polymers orient under complex stresses. Then, they usually elongate several hundred percent before rupture. Variations in crystallinity are also significant, although most technical polyethylene soon attains at room temperature enough crystallinity so that this factor does not cause big differences.
Although the whole study is so far preliminary, x‐ray scattering of stressed samples suggests that preferred glide on certain crystallite planes tends to occur as the yield point approaches. These are such as to inhibit smooth alignment of the long chain axis in the direction of stressing. This could lead to brittleness.
Apparatus for complex stressing of sheets and tubes is described. Strains are taken from coordinates printed on the sample by the silk screen process. High speed stressing was also observed. The speed of retraction of amorphous polyethylene chains approaches that of rubber.
D.C. Characteristics of Silicon and Germanium Point Contact Crystal Rectifiers. Part I. Experimental21(1950); http://dx.doi.org/10.1063/1.1699637View Description Hide Description
Typical d.c. current‐voltage characteristics obtained for Si and Ge crystal rectifiers are described. A survey of the published theories of the rectifier shows that none of them will account for the principal features of the observed characteristics. The most obvious discrepancy is in the low resistance direction of flow where the logarithm of the current rises with increasing voltage at only a fraction of the rate indicated by the theories. The need for a more flexible theory is pointed out and the conditions which it must meet, both as a function of voltage and temperature are outlined.
21(1950); http://dx.doi.org/10.1063/1.1699638View Description Hide Description
The electric breakdown in CO2 has been investigated from low pressures through the critical point into the liquid state. Paschen's similarity law is verified for low pressures. At high pressures small departures are observed for long gap‐lengths and large departures for small gap‐lengths. Simultaneously the scatter of the breakdown voltage becomes independent of illumination and the breakdown strength dependent on the cathodematerial; this appears due to the onset of field emission.Measurements of prebreakdown currents have yielded values for Townsend's first coefficient as well as for the field emission constants. For small gap‐lengths the prebreakdown currents are higher than the normal field emission equation predicts, indicating some new process effective at short gap‐lengths. The transition from the gaseous to the liquid state does not produce a discontinuous change of the breakdown voltage.
21(1950); http://dx.doi.org/10.1063/1.1699639View Description Hide Description
The equations of hydrodynamics are modified by the inclusion of additional terms which greatly simplify the procedures needed for stepwise numerical solution of the equations in problems involving shocks. The quantitative influence of these terms can be made as small as one wishes by choice of a sufficiently fine mesh for the numerical integrations. A set of difference equations suitable for the numerical work is given, and the condition that must be satisfied to insure their stabilty is derived.
21(1950); http://dx.doi.org/10.1063/1.1699640View Description Hide Description
By elementary and obvious methods a differential equation is obtained, which in general is non‐linear and usually affords a solution which is a simple sinusoidal function of the independent variable (i.e., simple harmonic motion). Two particular examples are studied briefly although no physical realizations are presented. One is led to speculate that new types of oscillators having a sinusoidal output are described by these equations, and that practical applications, such as to frequency metering, may eventually be evolved. However, the writer's speculations along these lines have as yet borne no fruit and his primary object in publishing this material is to bring to the attention of those interested in non‐linear phenomena, about which our understanding is so essentially imperfect, an analysis which, though itself possibly trivial, conceivably may prove of service in setting someone else off on a productive investigation.
21(1950); http://dx.doi.org/10.1063/1.1699641View Description Hide Description
The theory of the hydraulic jump is presented briefly, and the analogy between this phenomenon and the compression shock wave in gases is pointed out. The results of experimental measurements of hydraulic‐jump intersections on a water table are reported. Considerable disagreement between theory and experiment is found. Other investigators have noted a disagreement between theory and experiment for compression‐shock intersections in gases. The discrepancy in the aerodynamic case appears unlike that found in the hydraulic case. Possible reasons for the discrepancy in the hydraulic case are discussed; some sources of error are peculiar to hydraulic jumps and do not apply to compression shocks. Such factors limit the utility of the water table as an analog device.
21(1950); http://dx.doi.org/10.1063/1.1699642View Description Hide Description
In the sealing and processing of electron tubes residual stresses are introduced into the glass parts. This paper formulates a practical optical method for locating and measuring the maximum tension in the walls of glass bulbs.
The basic principles of photoelasticity in three dimensions are applied to the case of a cylindrical shell. Theoretical results are verified experimentally for a tube in which the stress distribution is axially symmetrical.
Experimental results on tubes in which the stress distribution was unsymmetrical lead to the conclusion that the maximum tensile stress is proportional to the maximum retardation, the factor of proportionality being simply related to the form of the fringe pattern.
A quick and simple technique is described for making the necessary optical measurements.
The method presented herein has been used and found to be practical for routine laboratory testing of tubes in quantity.
21(1950); http://dx.doi.org/10.1063/1.1699643View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699645View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699646View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699649View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699650View Description Hide Description