Volume 19, Issue 9, 01 September 1948
Index of content:
19(1948); http://dx.doi.org/10.1063/1.1698211View Description Hide Description
The instantaneous rate of decay or intensification of a shock front in any fluid for which dissipative effects are localized in a shock zone is shown to be directly related to the coexisting pressure gradient in the wake of the wave adjacent to the front. The higher time derivatives of the shock velocity are also explicitly related, although in a more complicated fashion, to the higher spatial derivatives of wake pressure. Thus the experimental observation of the velocity or some related variable of a blast or ballisticshock wave can be used to ascertain properties of the fluid motion in the wake of the shock. Conversely, a knowledge of the wake profile at any instant can be employed to determine the corresponding rate of decay of the shock front. The general theory is applicable to shocks of all strengths where an equation of state can be applied graphically or analytically.
19(1948); http://dx.doi.org/10.1063/1.1698212View Description Hide Description
This paper treats the problem of determining the current distribution on the surface of a perfectly conducting sphere when driven by a dipole antenna erected on its surface. Curves of the real and imaginary parts of the surface currents are given for the case of a half‐wave dipole and various radii of the sphere.
19(1948); http://dx.doi.org/10.1063/1.1698213View Description Hide Description
The expected number of peaks/sec. (nσ+N ), caused by noise or by a signal and noise, in excess of a given amplitude, or triggering level, is determined for broad‐band noise (a) and for the envelope of a narrow‐band disturbance (b). The signals considered here are pulses of various shapes (rectangular, triangular, trapezoidal), or a carrier modulated by these pulses. The expected number of peaks per second is directly proportional to the band width of the noise, and depends in some degree (20–30 percent) on filter shape, the gaussian response yielding a larger value of nσ+N than the rectangular, for the same total input power. Further, nσ+N is greatest when the triggering level is the most probable amplitude. Pulse shape is not significant when the pulses are on only a small fraction of the total period of observation, but becomes so if (1) the time of observation is comparable to the pulse length, and (2) the pulses have sharp leading edges. The latter gives rise to a transient effect which is not ignorable. This transient contribution does not appear in the envelope of narrow‐band waves, as long as the maximum time‐rate of change of the wave form of the envelope is much less than the central or resonant frequency of the IF, i.e., as long as the concept of ``envelope'' is meaningful. Rectification does not change nσ+N , provided that there is no appreciable distortion of the envelope by video‐ or audio‐filters and that the triggering level of the input is passed by the detector.
Improvement, defined in ways appropriate to the systems in question, of one circuit which can tolerate A 1 spurious signals/sec. in excess of a given triggering level over another circuit, which can tolerate only A 2 such signals (A 1>A 2), is calculated for broad‐band signal and noise, and for narrow‐band waves following half‐wave linear or quadratic rectification. In general, the improvement is slight (a few db) if the trigger levels of the two circuits are not close to the most probable value of the amplitude and if the ratio of the number of spurious signals allowed/sec. to the band width of the disturbance is small. Significant improvement follows only when the maximum tolerable number of triggering peaks/sec. is the same order of magnitude as the band width of the noise. Tables and figures illustrating the above are included in Part I; the mathematical derivations are contained in Part II.
19(1948); http://dx.doi.org/10.1063/1.1698214View Description Hide Description
The design of both narrow band high stabilization and broad band lower stabilization spectrum line discriminators is developed in a generalized form applicable to various spectrum lines and cavity designs. Comparison with experiment is made for two broad band discriminators utilizing the NH3 3,3‐absorption line at 23,870 megacycles as the frequency determining element. These discriminators used with an amplifier of 2000 gain, reduce drifts by factors of 250 and 1000, respectively, compared to an unstabilized tube.
19(1948); http://dx.doi.org/10.1063/1.1698215View Description Hide Description
A previous paper described the radiation field of a dielectric rod used as a termination for a wave guide. The theory therein made use of a standing wave distribution of equivalent currents over the surface of the rod to describe its radiation effect. It is shown in this paper that the equivalent currents may be expressed in terms of traveling waves without appreciably changing the computed radiation patterns.
19(1948); http://dx.doi.org/10.1063/1.1698217View Description Hide Description
Interferometric methods have been used to measure uniform cylindrical silica and glass fibers as small as 5μ in diameter with better than 1 percent accuracy. Such fibers yield sharp images when examined in the electron microscope and constitute a convenient and rugged specimen for accurate direct calibration of the instrument for magnifications up to 10,000 diameters. Higher magnification ranges are then readily determined by means of an internal calibration obtained by taking micrographs of glass spheres about 0.5 micron in diameter. Such spheres are also convenient specimens for determining the image distortions which must be known to correct the widths of the fiber images. This method appears to be capable of greater accuracy than any employed hitherto.
19(1948); http://dx.doi.org/10.1063/1.1698218View Description Hide Description
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.
19(1948); http://dx.doi.org/10.1063/1.1698219View Description Hide Description
The Absorption of Incident Quanta by Atoms as Defined by the Mass Photoelectric Absorption Coefficient and the Mass Scattering Coefficient19(1948); http://dx.doi.org/10.1063/1.1698220View Description Hide Description
Evidence presented indicates that the absorption of quanta as defined by the mass photoelectric absorption coefficient τ/ρ and the mass scattering coefficient σ/ρ of any element may be represented by a simple expression.
Tabulated values of the calculated mass absorption coefficients μ/ρ are given for wave‐lengths from 0.01 to 40 angstroms for hydrogen and carbon and from 0.01 to λ K and λ K to λ L for aluminum,copper,tin, and lead. Graphs are given showing agreement with observed values.
19(1948); http://dx.doi.org/10.1063/1.1698221View Description Hide Description
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.
19(1948); http://dx.doi.org/10.1063/1.1698223View Description Hide Description