Volume 18, Issue 8, 01 August 1947
Index of content:
18(1947); http://dx.doi.org/10.1063/1.1697828View Description Hide Description
The characteristics of certain bridged‐T filter structures have been investigated theoretically and an experimental confirmation of this theory at u.h.f. has been obtained. In theory it is possible to obtain infinite attenuation at a designated frequency, while retaining essentially the same band width as that for the uncompensated filter. A model has been constructed which has a band width of ½ Mc/sec. at 3000 Mc/sec. with an attenuation at center frequency of over 70 db. This is to be compared with the attenuation from a similar uncompensated filter of 20 db. Certain other known filter expressions have also been reduced to forms readily utilizable at these frequencies. Their utility for u.h.f. applications is discussed. The distortion produced in otherwise rectangular pulses of short duration has been investigated both theoretically and experimentally. Curves are presented which show the resultant wave form as a function of the tuning of the filter.
18(1947); http://dx.doi.org/10.1063/1.1697829View Description Hide Description
The theory of robots or reproducing power‐amplifying systems is considered. After a general terminological orientation the study in the first part is confined to simple robots , the theory of which is equivalent to that of non‐linear springs with inertia and friction. The alacrity and fidelity of control are well exhibited by studying the behavior of a robot under constant tracking. Special emphasis is placed on the specification of conditions for optimum performance, i.e., minimum deviation or maximum fidelity after any given time. Curves are given for both proportional and on‐off robots, from which the parameter values for optimum performance may be read.
In Part II the theory of proportional robots is analyzed in detail, especially from the point of view of conditions for optimum performance. Two subsequent papers will contain the theory for the on‐off case and the general non‐linear robot case. The paper is organized as follows:
Part I. Descriptive account
A. Terminology and kinematic description
B. Dynamical equation of robots with usual load
C. Optimum performance curves. Practical examples
Part II. Theory of simple robots with proportional control
A. Complete solutions as functions of time. Optimum performance curves
B. Path curves in the position‐velocity plane
18(1947); http://dx.doi.org/10.1063/1.1697830View Description Hide Description
This equation pertains to a loss‐free frequency modulation circuit. Floquet's theory is applied to obtain stable solutions when θ, ε are outside the range of the approximation solution given by Carson in 1922. A method is described by means of which solutions may be computed to any degree of accuracy. The necessary formulae associated with the solution are derived, and convergence is discussed. A numerical example, illustrating the procedure to be followed, is worked out in detail. An appropriate normalization of the solutions is suggested in order to obtain standard frequency modulation functions. It is shown how the analysis may be extended to cover the case of the equation,which applies to a frequency modulation circuit having constant resistance R.
18(1947); http://dx.doi.org/10.1063/1.1697831View Description Hide Description
Synthetic manganese‐activated calcites are shown to be practically inert to ultraviolet excitation in the range 2000–3500A, while they are luminescent under cathode‐ray excitation. The incorporation of small amounts of an auxiliary impurity along with the manganese produces the strong response to ultraviolet radiation hitherto ascribed to CaCO3:Mn itself. Three such impurities have been studied: lead, thallium, and cerium. The first two induce excitation in the neighborhood of the mercury resonance line, while the cerium introduces a response principally to longer wave ultraviolet. The strong response to 2537A excitation shown by some natural calcites is likewise found to be due to the presence of lead along with the manganese, rather than to the manganese alone. The data do not warrant ascribing the longer wave‐length ultraviolet‐excited luminescence of all natural calcites to the action of an auxiliary impurity. The essential identity of the cathode‐ray excited luminescencespectra of CaCO3:Mn, CaCO3: (Pb+Mn), CaCO3:(Tl+Mn), and CaCO3:(Ce+Mn) with the 2537A‐excited spectra of the latter three is evidence that the luminescent center in all cases is the manganese ion or the MnO6 group. It is shown that a ``cascade'' mechanism for the action of the auxiliary impurities, lead, thallium, and cerium, is incorrect; and that the phenomenon must be considered as a case of sensitized luminescence. Owing to the nature of cathode‐ray excitation, the manganese activator can be excited by this agent even in the absence of a second impurity. For optical excitation, however, an absorption band for the ultraviolet must be established by building into the CaCO3:Mn a second impurity or ``sensitizer.''
18(1947); http://dx.doi.org/10.1063/1.1697832View Description Hide Description
Two general mathematical methods are discussed which may be used to study the effects of electric and/or magnetic deflection fields on electron beams. The basic equations for the ``path method'' are the equations of motion of Newton or Euler‐Lagrange. The ``iconal method'' makes use of certain properties of the Hamiltonian function.
These methods are then applied to describe the action of balanced, two‐dimensional electric deflection fields on electron beams. It is shown that both methods yield essentially the same results. Expressions are derived describing the magnitude of deflection and the distortions of an electron beam.
Only the path method is used in a similar investigation of magnetic‐type deflection fields.
18(1947); http://dx.doi.org/10.1063/1.1697833View Description Hide Description
Methods of discharging static electricity from aircraft in flight so as to reduce or eliminate corona interference, commonly known as precipitation static, are discussed. These methods have been classified into those employing either gaseous ions (high mobility), or those employing charge carriers such as water spray or dust particles (low mobility). Devices within the first classification offer the better solution to the problem from a practical standpoint. A discharger invented and developed at the Naval Research Laboratory is described.
18(1947); http://dx.doi.org/10.1063/1.1697834View Description Hide Description
The fundamental physical phenomenon upon which linear electron accelerators and traveling beam tubes depend is the fact that the phase velocity of guided TMwaves can be reduced to a fraction of the velocity of light. There are various practical ways of achieving these reduced velocities, such as diaphragms or spiral grooves in the guide wall. An open helix has been used by Pierce.
An accurate knowledge of the field pattern would greatly facilitate an understanding of these devices. It is of particular importance to have exact information about the field in the region in which the charge actually travels, namely, in a region located at least a considerable fraction of a wave‐length from the guide walls. Because of the complex shapes assumed by actual guide walls calculations of the field are inevitably somewhat inexact and complicated.
The authors propose that, for the purposes of theory, the well‐established proposition concerning the equivalence of true and simulated dielectrics in producing a slow field be used. We propose to think of slowing down the phase velocity by lining the guide walls with a natural dielectric. The essential properties of the slow field, particularly in the important region referred to above, will not be affected by this method of obtaining it while, on the other hand, the field calculation becomes quite easy and exact.
Although this procedure is intended in the main as a device to facilitate the theoretical calculation of the field, the authors believe that such a step—replacement of periodic metallic structures by dielectrics—might prove useful in some actual applications.
18(1947); http://dx.doi.org/10.1063/1.1697835View Description Hide Description
Because of the positive coefficient of resistivity of tungsten, incandescent lamps permit a current considerably greater than the normal lamp current to flow for a short initial period. When lamps are used as load for testing electrical apparatus, the factors affecting this inrush must be considered. Size of power source, its distance from the load (lamps), and frequency of switching the lamps (employing repetitive operations for endurance testing) are evaluated.
From the given curve the necessary cooling time for the desired current inrush may be determined, and thus the frequency with which the lamps could be switched ``on'' and ``off'' may be calculated. Practical means for obtaining the required cooling time are described.
The Capacity per Unit Length and Characteristic Impedance of Coaxial Cables with One Slightly Non‐Circular Conductor18(1947); http://dx.doi.org/10.1063/1.1697836View Description Hide Description
It is possible to calculate approximately the electrostatic field in a coaxial line with one slightly non‐circular conductor as a perturbation of that existing in the usual coaxial line. General expressions are then derived for the capacity per unit length and the characteristic impedance of such lines. In particular, these results are applied to the coaxial line with outer square conductor.
18(1947); http://dx.doi.org/10.1063/1.1697837View Description Hide Description
Thermal conductivity measurements on lead, tin, and zinc extending over the liquid as well as the solid states were reported by one of the authors in 1940. The present report extends this work to cover aluminum in both the liquid and solid phases. An improved technique is described. The law k/ρC=K(1/T) + K′ is found to hold for aluminum also, with the intercept, K′, the same for both states. The thermal conductivity (k) decreases with rise of temperature as it does for lead, tin, and zinc. On this point previous workers have published very conflicting data.
- LETTERS TO THE EDITOR
18(1947); http://dx.doi.org/10.1063/1.1697838View Description Hide Description
18(1947); http://dx.doi.org/10.1063/1.1697840View Description Hide Description
18(1947); http://dx.doi.org/10.1063/1.1697842View Description Hide Description