Volume 16, Issue 11, 01 November 1945
Index of content:
16(1945); http://dx.doi.org/10.1063/1.1707518View Description Hide Description
If a mass spectrometer is to be used successfully for gas analyses, the gas flow through the system must meet a number of requirements. The three modes of gas flow—molecular, intermediate, and viscous—are reviewed, and their working ranges are discussed in terms of the pressure of the system. The upper limit for molecular flow, through capillaries or through small openings, is found to lie at a pressure where the mean free path of the molecules is between 15 and 40 times the diameter of such a gas leak. Expressions are developed to show that, for molecular flow, the ion beam intensity is independent of molecular weight but is a linear function of the sample pressure, and that the principle of linear superposition applies to mixture peaks. Since the composition of a gas mixture in a reservoir is found to change with time (a numerical example is worked out), the necessity for analyses of short duration becomes evident. The practical aspects of designing the gas system for molecular flow and for efficient ion production are discussed in detail. The various flow requirements are best met by a gas leak consisting of a small hole in a diaphragm, or several holes in parallel. A relatively high gas pressure within the electron beam boundaries and resulting large ion currents are produced by introducing the gas as a molecular beam into a gas‐tight ionizing region.
16(1945); http://dx.doi.org/10.1063/1.1707519View Description Hide Description
The application of a direct‐current bias in the magnetic circuit of induction accelerators is described. In its simplest form this bias reduces power consumption to approximately 30 percent; in especially designed machines a reduction of the weight to 50 percent or less is an additional advantage. A machine with a closed central core is discussed and the use of orbit contraction by saturation over a range of x‐ray energies is described.
16(1945); http://dx.doi.org/10.1063/1.1707520View Description Hide Description
Experience shows that in the course of the induction melting process occasional local overheating of the crucible takes place, which in extreme cases leads to perforation of the latter. An explanation of this occurrence is presented on the basis that it is essentially a phenomenon of thermal instability. The theory shows that overheating of the crucible is more probable the steeper the electrical resistivity—temperaturecharacteristic of the crucible material, a condition brought about by impurities present. Numerical computations, showing the transition from stable to unstable operating conditions, are carried out for a small experimental and a large industrial furnace. The equations developed show how the various operating conditions, particularly the frequency of the current in the coil, have to be selected in order to insure operation in the stable region.
16(1945); http://dx.doi.org/10.1063/1.1707521View Description Hide Description
16(1945); http://dx.doi.org/10.1063/1.1707522View Description Hide Description
Newton's image relations are satisfied for all electron lenses if object and image are located outside the region of the electromagnetic fields. It will be shown that a special class of electron lenses exists for which these relations hold for any object and image position. The results will be compared with those obtained by W. Glaser.
Rigorous Treatment of the Electrostatic Immersion Lens Whose Axial Potential Distribution Is Given by: φ(z)=φ0 e K arctan z16(1945); http://dx.doi.org/10.1063/1.1707523View Description Hide Description
Investigations have shown that the electrostatic immersion lens, given by the axial potential distribution φ(z)=φ0 exp(K arctan z), is the simplest electrostatic lens for which Newton's image equations are satisfied throughout the field. The general solution of the paraxial ray differential equation is derived. Based on this solution and the fact that Newton's image equations are satisfied, exact expressions are derived for all important optical quantities such as the focal lengths, the location of the four cardinal points, magnification, the object‐image relation, and the spherical and chromatic aberration for any object position. The optimum positions of an object are determined so that the chromatic and spherical aberrations are reduced to a minimum. All quantities are represented graphically. A plot of the equipotential lines in space is given for a special value of the voltage ratio.
16(1945); http://dx.doi.org/10.1063/1.1707524View Description Hide Description
Seven methods (Voigt model,Maxwellmodel,operator equation, mechanical impedance function, creep curve, relaxation curve, and dynamic modulus function) of specifying viscoelastic behavior are discussed. A number of exact relations between these methods of specification are worked out in detail. The majority of these relations are simple enough to be of practical value although a few are too cumbersome. Approximate relationships between the creep curve, the relaxation curve, Maxwellmodel, and Voigt model are discussed; and numerical examples show the magnitude of errors introduced by the approximation to be small even in quite unfavorable cases. A consideration of the practical utility and physical meaning of the various methods of specification distinguishes between (1) those of general descriptive value and those of direct experimental value; (2) those useful in a phenomenological study of mechanical behavior and those more suited to a formulation of molecular theory. A summary of the present molecular theories is presented together with their interpretation in terms of the Voigt and Maxwell specifications.
16(1945); http://dx.doi.org/10.1063/1.1707525View Description Hide Description
A double‐yoke permeameter, with which both normal and ideal permeabilities of moderately large steel plates can be measured, is described. The technique for making these measurements is also given. The permeameter is suitable for measuring plates several feet long, up to a foot wide, and from ⅛″ to 1½″ thick; two approximately identical test plates are required.
16(1945); http://dx.doi.org/10.1063/1.1707526View Description Hide Description
An electron diffractioncamera is described for the study of physical and chemical reactions occurring on the surfaces of metals and alloys in situ at elevated temperatures. The specimen is mounted in a metal block and shielded from the surrounding cold surfaces by a series of metal shields. The metal block contains an internal electrical heater which allows the temperature of the specimen to be accurately controlled at temperatures of 800°C. Gas purification chains are included for the preparation of pure oxygen, hydrogen, etc. Several typical electron diffraction reflection studies of the oxidation of iron and stainless steel are included.
16(1945); http://dx.doi.org/10.1063/1.1707527View Description Hide Description
The following is a report on experimental results obtained for the virtual masses of thin rectangular plates and rectangular parallelepipeds moving in water. Experimental data, together with an empirical equation, showing the manner in which the virtual mass of a rectangular plate varies with dimensions and direction of motion are given. Data and an empirical equation giving the virtual mass of a rectangular parallelepiped as a function of dimensions and direction of motion, for the special case in which the motion is parallel to one face, are also listed.
16(1945); http://dx.doi.org/10.1063/1.1707528View Description Hide Description
The problem of examining opaque surfaces in the electron microscope is discussed and it is shown that none of the methods heretofore described is completely satisfactory for the examination of organic surfaces. Two new methods, a low pressure polystyrene‐silica technique and a silver‐silica double evaporation method, are described and it is demonstrated that they allow observation of organic as well as metallurgical surfaces. Electron micrographs of fiber surfaces, biological material, paint films, ores, and metals are included to illustrate the utility of these techniques.
16(1945); http://dx.doi.org/10.1063/1.1707529View Description Hide Description
Using the method of Copeland and Sparing, the duration of an arc has been observed under fixed operating conditions a sufficient number of times to give a satisfactory estimate of the average life. The results show that the average life increases as the area of the cathode pool is reduced, and that the rate of change is particularly rapid for low arc currents.