Volume 20, Issue 5, 01 May 1949
Index of content:
The Triangulation of a Two‐Dimensional Continuum for the Purpose of the Approximate Solution of Second‐Order Partial Differential Equations20(1949); http://dx.doi.org/10.1063/1.1698395View Description Hide Description
It is shown how a non‐homogeneous, non‐isotropic, electrically conductive, two‐dimensional continuum can be replaced by a network of resistors whereby the nodes of the network are situated at any arbitrarily selected points, and how Southwell's relaxation method can be applied to this network for the solution of potential problems.
20(1949); http://dx.doi.org/10.1063/1.1698397View Description Hide Description
The electronic transadmittances, transmodulation functions, and other characteristics of a planar, multigrid high frequency tube are expressed in terms of four fundamental admittance and modulation functions which are sufficient for the description of the low level r‐f properties of the electron beam. A matrix representation is introduced based on the successive velocity moments of the beam which leads to the formulation of the characteristics as products involving excitation, grid transmission, modulation, and modulation admittance matrices. The electronic equations obtained are a formulation in which the Llewellyn approximation that the second and all higher velocity moments are zero is not made. Thus the theory allows for higher order velocity effects but is dependent on the knowledge of the fundamental admittance and modulation functions.
20(1949); http://dx.doi.org/10.1063/1.1698399View Description Hide Description
An analysis is made of the operation of magnetic amplifiers or non‐linear transformers. Open circuit output voltages are calculated for transformers driven by a sinusoidal primary current, and also for sinusoidal primary current plus a direct current bias. The case of a loaded secondary is also considered and it is shown that there exist conditions for instability or resonance. A subsequent paper will consider Barkhausen noise as a limiting condition on the sensitivity of these devices.
20(1949); http://dx.doi.org/10.1063/1.1698401View Description Hide Description
Using the method of steepest descents the far field and the asymptotic form of the current distribution is obtained for an infinitely long, perfectly conducting cylindrical antenna excited by a localized electromotive force. The low frequency value of the radiation conductance is determined by integrating the radiated energy flux over a large sphere.
20(1949); http://dx.doi.org/10.1063/1.1698403View Description Hide Description
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.
20(1949); http://dx.doi.org/10.1063/1.1698405View Description Hide Description
A procedure is outlined for calculating the maximum changes in the exhaust velocity of gases which can be introduced during flow through a rocket nozzle as the result of vibrational time lags. Representative calculations have been carried out for a number of propellant systems. These calculations indicate that the vibrational time lags cannot produce changes in exhaust velocity which exceed a few percent. The numerical values of the changes are found to depend not only on the nature of the gases but also on the occurrence or non‐occurrence of chemical reaction during flow.
20(1949); http://dx.doi.org/10.1063/1.1698406View Description Hide Description
The Substitution Principle established by Munk and Prim for flows of a gas having constant specific heats is extended to the wider class of fluids having a state equation of the form ρ=P(p)S(s). The equations of motion for this class of fluids is reduced to a canonical form involving only the reduced velocity vector and the pressure. It is shown that explicit elimination of the pressure from these equations is possible only for those fluids whose state equation has the form ρ=pkS(s) with k<1. Relations between the Mach number and stagnation pressure and the reduced velocity field are discussed for the class of fluids under investigation.
20(1949); http://dx.doi.org/10.1063/1.1698407View Description Hide Description
This paper describes a special technique by which it is possible to stabilize the breakdown voltage of a glow tube having zirconiumelectrodes in pure argon at a pressure of 8.5 mm Hg. A brief account is also given of some experimental methods by which it is possible to find the order of magnitude of two critical currents, whose concepts have been used in the theory of sparkdischarge at low pressures. One of these critical currents is the ``threshold current,'' which is defined as the maximum steadystate current in a glow tube just before a spark. The other critical current is the maximum electronic current (attained during the development of a spark) at cathode before space‐charge distortion becomes appreciable. The methods described below were confined to two tubes having zirconiumcathodes of area 0.3 sq. cm, the electrode spacing being 3.5 mm. One of these two tubes had a zirconiumanode, and the other one had a thorium anode. The tubes contained pure argon at a pressure of 8.5 mm Hg. For both the tubes, the order of the ``threshold current'' was found to be 10−8 amp., and the order of the other critical current (electronic current at cathode) was estimated as 10−8 amp.
20(1949); http://dx.doi.org/10.1063/1.1698408View Description Hide Description
An expression is derived for the total emission conductance produced by the electrons returned to the cathode of a diode operating with a retarding off‐cathode field. For the retarding voltage and diode spacing used, the assumption of a linear potential distribution is justified. Examples of the variation of the total emission conductance with retarding voltage and frequency are illustrated.
20(1949); http://dx.doi.org/10.1063/1.1698409View Description Hide Description
A visual method for studying the path of ultrasonicwaves through thin plates of material has been devised, and a set of photographs showing the complexity of ultrasonic reflection and transmission by metal plates has been obtained. An attempt has also been made to estimate the critical angles of total reflection for dilatation and shear waves by visual examination of the reflection of a supersonic beam. Using these data, Young's modulus and the shear modulus for a aluminum sheet have been determined and the values are compared with those obtained using a previously described electrical method and with published data.
20(1949); http://dx.doi.org/10.1063/1.1698410View Description Hide Description
The high current carbon arc with maximum crater brightness of 2000 candles per mm2, axial intensity of 1.2×106 candles, power inputs up to 200 kw, light efficiencies exceeding 90 lumens per watt, radiation efficiency of 73 percent, crater temperatures up to 8000°K, and arc stream temperatures of the order of 12,000°K is the most powerful and one of the most interesting radiation sources known. The rising voltagecharacteristic, the anodic vapor stream which causes its excellent radiation properties, and the contracted arc stream distinguish the high current carbon arc from the well‐known normal low current carbon arc. The properties of the contracted arc stream are discussed and a general theory of arc streams is presented. Starting from the anodic mechanism for the low current carbon arc, the anodic mechanism of the high current arc is developed. The rising voltagecharacteristic, the anodic vapor stream, and the high crater brightness are explained as a consequence of an abnormal voltage drop in front of the anode which increases with increased current density and is caused by a very rapid evaporation of the anodic material. The important role which the magnetic field of the arc current plays in the stabilization of all high current arcs is pointed out. The author believes that this unique high temperature arc has bearing on other arc and spark discharges, and also will play an important role in future developments of high temperature physics and chemistry.
20(1949); http://dx.doi.org/10.1063/1.1698411View Description Hide Description
A continuous‐wave Doppler method of determining meteoric velocities and heights was tested during the 1948 Perseid meteor shower. The accuracy of the technique was found to compare very favorably with optical and other radio methods. Doppler measurements have the advantage of relatively great sensitivity.
20(1949); http://dx.doi.org/10.1063/1.1698412View Description Hide Description