Volume 13, Issue 4, 01 April 1942
Index of content:
13(1942); http://dx.doi.org/10.1063/1.1714858View Description Hide Description
GEL formation in colloidal systems causes the viscosity of such systems to vary with the stress: these are called non‐Newtonian liquids. Because the physical cause of such behavior is related to the gel structure, the phenomenon is called structural viscosity. Three particular features are observed with such systems: a non‐linearity of the rate of shear vs. stress relation in the case of stationary flow; a finite relaxation process for suddenly applied stresses or deformations; and a frequency dependent dynamic viscosity in the case of alternating processes. The chief experimental facts in these three fields are presented, together with certain semitheoretical equations best suited to describe the experiments.
13(1942); http://dx.doi.org/10.1063/1.1714860View Description Hide Description
Magnetic measurements on ring samples of siliconiron, commercially pure nickel, and low carbon steel, in the thickness range 3.8×10−3 to 38.1×10−3 cm, indicate that it is possible to detect surface stray flux contributions to the total magnetization by comparing hysteresis loops obtained at a maximum applied field of 4 oersteds. The magnetic quality appears to be insensitive to changes of specific surface though a progression of properties with sample thickness suggests the presence of a low permeability surface layer.
13(1942); http://dx.doi.org/10.1063/1.1714861View Description Hide Description
A Model Method of Photographic and Photometric Determination of Heat Radiation Between Surfaces and Through Absorbing Gases13(1942); http://dx.doi.org/10.1063/1.1714862View Description Hide Description
Straight analytic determination of the heat exchange by radiation in a device is possible only if the emitting and absorbing surfaces are very simple in shape and configuration. In most practical cases, and particularly if the radiation is partly absorbed on its way by gases, approximative calculations and graphical methods have been used in order to evade the mathematical difficulties. In the present paper an experimental model method with the following characteristic features is described. A model, similar to the device under investigation, is constructed. Light is used instead of the long wave heat radiation, and its absorption by photographic films which cover the surfaces of the model, is measured by the methods of photometry. The case of gases which absorb heat radiation is imitated by using liquids which absorb light rays. The application of the method to radiation through a non‐absorbing medium is shown by experiments on a tube bank model. The adaption of the method to heat radiation through absorbing gases is demonstrated by photos of the spectrum of a special combination of light filter, absorbing liquid, and photographic film which are in tune in a narrow wave‐length range.
Diffusion of Heat Through a Rectangular Bar and the Cooling and Insulating Effect of Fins. I. The Steady State13(1942); http://dx.doi.org/10.1063/1.1714863View Description Hide Description
Exact solutions for temperature distribution and heat flow through a rectangular fin are rederived in more general and simple form than previously given. These are studied numerically and graphically and shown to yield new results not contained in the approximate formulas hitherto used. Except for the factor of thermal conductivity of the fin material, the cooling effect is a function only of the height‐width ratio β of the fin, and of a dimensionless ``relative boundary resistivity'' μ governing the heat loss to the environment. It is shown that, for any value of this boundary resistivity, there is a fairly sharp optimum value of the height‐width ratio beyond which further increase in height will not lead to much improvement in cooling effect. This optimum β decreases with decreasing μ until, at a certain critical value μ* in the neighborhood of 1, it becomes zero. At the critical μ*, the cooling effect is altogether independent of β, and the presence of the fin is a matter of indifference. Below μ*, the cooling effect decreases with increasing β, so that the presence of the fin serves merely to insulate the hot plate. This analysis of a single fin is the basis upon which a study of the conditions for optimal cooling by an array of fins will be carried out in a subsequent publication.
13(1942); http://dx.doi.org/10.1063/1.1714864View Description Hide Description
The generally employed methods of investigating arcback in mercury arc rectifiers are incapable of uncovering some of the important characteristics of the phenomenon. A new method of study has shown that arcback probability varies with time after the appearance of negative voltage, in a surprising and hitherto unsuspected manner. Theoretical implications of this effect are discussed.