Volume 19, Issue 4, 01 April 1948
Index of content:
19(1948); http://dx.doi.org/10.1063/1.1715068View Description Hide Description
This paper deals with a phenomenon of self‐excitation observed in an electromechanical system. It is shown that this phenomenon is expressible in terms of a difference‐differential equation, having a frequency spectrum that depends on a parameter. The linear theory fails to give an answer to the question of stability of oscillations. It is necessary for this purpose to make use of the equations of the first approximation of the non‐linear theory. Expressions for a stable stationary amplitude and frequency obtained on that basis show an agreement with the observed features of these self‐excited oscillations.
19(1948); http://dx.doi.org/10.1063/1.1715070View Description Hide Description
An analogy device for directly solving simultaneous linear equations (without resorting to successive approximation methods) is described in this paper. It utilizes a number of high gain amplifiers interconnected by networks whose elements bear definite relationships to the known coefficients and constants of the system of equations. The stability criterion for such a system, and its application to an equation solver are described. Since the device produces the answers to a system of equations without delay, it is readily adaptable to problems of synthesis as well as those of analysis.
19(1948); http://dx.doi.org/10.1063/1.1715071View Description Hide Description
A general expression has been derived theoretically for the pressure due to surface waves produced by an underwater explosion. The integrals involved have been evaluated in terms of dimensionless variables over a range of practical interest. A specific formula is given for the pressure on the bottom of the water due to the first pulsation of the gas globe from a TNT explosion originating also on the bottom.
Calculation of the Temperature Development in a Contact Heated in the Contact Surface, and Application to the Problem of the Temperature Rise in a Sliding Contact19(1948); http://dx.doi.org/10.1063/1.1715072View Description Hide Description
I. A circular area on a semi‐infinite body is assumed to be heated at a constant rate from the time t=0 on. The development of the temperature in this area and in its surroundings is calculated with certain simplifications.
II. In a sliding contact the heat generated by friction is limited at every point to the time of contact. The temperature reached by the contact surface is calculated by aid of Part I and with assumptions about the size of the contact surface, which are based on earlier investigations of the author.
Observations on the temperatures in bimetallic sliding contacts, indicating the contact temperature rise by means of a thermoelectric current, are compared with calculations partly according to the deduced theory partly according to an earlier published theory by Jaeger. A fairly good agreement is found with one exception.
Measurement of Carbon Black Particles by the Electron Microscope and Low Temperature Nitrogen Adsorption Isotherms19(1948); http://dx.doi.org/10.1063/1.1715073View Description Hide Description
Electron micrographs and nitrogen adsorption isotherms have been compared on six commercial carbon blacks as methods for obtaining particle size and surface area values of finely divided materials. Good agreement between the two procedures was obtained for four of the backs: Grade 6, P 33, acetylene black, and lampblack. For two blacks known to be porous, Mogul and after‐treated lampblack, the areas, as measured by the nitrogen isotherms, were 5 to 7 times as large as one would calculate from the electron microscope results.
The Visco‐Elastic Behavior of a Highly Plasticized Nitrocellulose in Compression under Constant Load19(1948); http://dx.doi.org/10.1063/1.1715074View Description Hide Description
The behavior of a highly plasticized nitrocellulose in compression under constant load has been investigated over a rather wide range of load and temperature. The load dependence appears to require the assumption of non‐Newtonian viscosity. Though the observations are not quantitatively accounted for by the Tobolsky‐Eyring model, the load and temperature dependence of the rate of compression appear to have some relation to the predictions of these authors.
An Oxide Replica Technique for the Electron Microscope Examination of Stainless Steel and High Nickel Alloys19(1948); http://dx.doi.org/10.1063/1.1715075View Description Hide Description
An oxide replica method has been developed which is suitable for the electron microscope examination of the surfaces of stainless steel, nickel, and high nickel alloys. The oxide film is produced on the metal surface by oxidation in a molten nitrate solution and is chemically stripped from the base metal by a bromine‐methanol solution. Replicas of this type have been used to study deep‐etched structures, preferential etch attack, and secondary phases in austenitic and ferritic stainless steel and in nickel.
19(1948); http://dx.doi.org/10.1063/1.1715076View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1715077View Description Hide Description
A simple and accurate method of determining pole figures by using a special sample mount with a Geiger counter x‐ray diffraction goniometer is described and examples given. Also, a correction formula is derived which takes into account absorption change and change in diffracting volume as the sample changes position with respect to the x‐ray beam.
19(1948); http://dx.doi.org/10.1063/1.1715078View Description Hide Description
The temperature rise of powder grains in different rocket motors, resulting from the absorption of radiant energy, has been computed by the method of Avery. The calculated results show that more radiant energy is absorbed by the powder grains in large service rockets than in small experimental test motors. The increase in powder temperature before burning, associated with the absorption of radiant energy, leads to an increase in the average rate of burning of the solid fuel charge. The differences in burning rate produced by the absorption of radiant energy amount to less than four percent for the rocket motors commonly employed. Thus burning rate results obtained in small test motors may be applied to predict the approximate behavior of the same propellant in a large service rocket.
19(1948); http://dx.doi.org/10.1063/1.1715079View Description Hide Description
It has been observed in these laboratories (J. Slepian and E. J. Haverstick, Phys. Rev. 33, 52 (1929); J. Slepian, W. E. Berkey, and M. J. Kofoid, J. App. Phys. 13, 113 (1942)) that in a certain range of pressures, of the order of millimeters, short arcs burn with low apparent cathodecurrent densities. The melting of the electrodes is then greatly reduced.
In this paper are described spectrographic tests to determine the presence of electrodematerial in the arc. It was found that as the gas pressure was increased, for an arc of given current and length, at a critical pressure the amount of cathode metal in the arc, as indicated by spectral line density, suddenly increased. Presumably, boiling of the cathode metal took place at this pressure. This critical pressure was found to increase as the arc length was reduced.
In this range where melting of the cathode occurred, it was found that the molten spot was at the center of the cathode for magnetic cathodematerial but at the edge of the cathode for non‐magnetic material.
19(1948); http://dx.doi.org/10.1063/1.1715080View Description Hide Description
This paper deals with the stresses in a plate containing a hole whose rim is composed of two equal circular arcs. The stress function concerned is constructed by means of an even integral solution of biharmonic equation expressed in bipolar coordinates. The parameters involved in the solution are determined from the given boundary conditions with the aid of Fourier transforms. Three fundamental stress systems are discussed. Expressions of the stress along the rim of hole are derived and, in particular, values of the maximum stress are calculated. The results are then plotted in conjunction with those of a plate containing two equal circular holes, which are obtained by the present author in a previous paper. It may be interesting to compare the the methods used in both solutions.
Minimum Detectable Absorption in Microwave Spectroscopy and an Analysis of the Stark Modulation Method19(1948); http://dx.doi.org/10.1063/1.1715081View Description Hide Description
It is shown that the minimum detectable absorption coefficient for an isolated spectral line as measured in a matched wave guide of optimum length iswhere N is the noise factor and Δf the band width of the receiver, α c is the attenuation constant for the wave guide, and P 0/e is the average microwave power available at the receiver. The equivalence between cavity and wave guide methods is developed. The Stark modulation method of Hughes and Wilson is analyzed, assuming a quadratic Stark effect, and expressions are developed for the modulation factor of a microwave at twice and four times the frequency of the Stark field. For a matched guide the modulation factor is α gl/4. Also the shift in line position is calculated, and finally, the directions in which improvement in sensitivity may be expected are indicated.
19(1948); http://dx.doi.org/10.1063/1.1715082View Description Hide Description
The beat‐frequency method for measuring the dielectric constant and dipole moment of gases is critically investigated. It appears that the wide discrepancies in the data of previous observers are due in part, at least, to the neglect of certain stray components in the circuits involved. An apparatus taking these factors into consideration is described, and a method for the ``absolute'' standardization of the equipment is given. Results of dielectric constant and dipole momentmeasurements for several gases are presented and compared with previous data.